MICREL MIC2085/MIC2086 Single Channel Hot Swap Controllers handbook
Contents
1. 120 115 110 E E 105 D amp 100 a amp 95 gt 90 85 80 40 20 0 20 40 60 80 100 TEMPERATURE C ON Pin Threshold Rising vs Temperature 1 30 O 1 25 o N W jam E 1 20 Oo 119 5 20 0 20 40 60 80 100 TEMPERATURE C FB Power Good Threshold vs Temperature 1 30 ES Voc 16 5V Q 1 25 5 3 id x Voo 2 8V 1 20 eo 5V Et m LL 145 COMPARATOR OFFSET VOLTAGE V 40 20 0 20 40 60 80 100 TEMPERATURE C Comparator Offset Voltage vs Temperature o Dm e P o to o iS e En 0 045 20 0 20 40 60 80 100 TEMPERATURE C January 2004 mV ON THRESHOLD V Von PSLOW Az Kei Current Limit Threshold Slow Trip vs Temperature a a a a A Az XN 4549 20 0 20 40 60 80 100 TEMPERATURE C ON Pin Threshold Falling vs Temperature 1 20 1095 20 0 20 40 60 80 100 TEMPERATURE C Overvoltage Pin Threshold vs Temperature w e Dv a OVERVOLTAGE PIN THRESHOLD V Rois 9 zi ip o 11945 20 0 20 40 60 80 100 TEMPERATURE C2. Figure 4 Foldback Current Limit Response January 2004 7 M0235 121903 MIC2085 2086 Micrel Typical Characteristics Supply Current Power On Reset Timer Current Power On Reset Timer Off Current m vs Temperature T vs Temperature 4G vs Temperature D 9 Zz 35 24 5 E30 __ Voge 16 5V_ E Voc 16 5V 22 Voc 5V 7 Voc 16 5V g 2 5 H z z Ul Im z A 6 L c Veo DN EE E LIL E 20 cc c 2 0 a ERE c 5 2 kene 9 a x 1 5 F E 6 4p n Voo SV Se LEE Voc 2 3V 3 umm TEE 2 oc e mu Voc 2 3V 1 6 0 0 14 0 40 20 0 20 40 60 80 100 40 20 0 20 40 60 80 100 40 20 0 20 40 60 80 100 TEMPERATURE C TEMPERATURE C TEMPERATU
3. Finally the external gate drive of the MIC2085 86 requires a low voltage logic level MOSFET when operating at voltages lower than 3V There are 2 5V logic level MOSFETs avail able Please see Table 4 WOSFET and Sense Resistor Venaors tor suggested manufacturers MOSFET Steady State Thermal Issues The selection of a MOSFET to meet the maximum continuous currentis a fairly straightforward exercise First arm yourself with the following data question see Sense Resistor Selection The manufacturer s data sheet for the candidate MOSFET The maximum ambient temperature in which the device will be required to operate Vin 12v H VCC SENSE MIC2085 Micrel Any knowledge you can get about the heat sinking available to the device e g can heat be dissipated into the ground plane or power plane if using a surface mount part Is any airflow available The data sheet will almost always give a value of on resis tance given forthe MOSFET ata gate source voltage of 4 5V and another value at a gate source voltage of 10V As a first approximation add the two values together and divide by two to get the on resistance of the part with 8V of enhancement Callthis value Roy Since a heavily enhanced MOSFET acts as an ohmic resistive device almost all that s required to determine steady state power dissipation is to calculate I R The one addendum to this is that MOSFETs have a slight increase in Roy with incre
4. PomZinsa 4 Surface Mounted pop titiri 1 10 3 10 2 10 1 1 10 30 Square Wave Pulse Duration sec Figure 12 Transient Thermal Impedance M0235 121903 24 January 2004 MIC2085 2086 So our original approximation of 61 1 C was very close to the correct value We will use Tj 61 C Finally add 11 25W 50 C W 0 08 45 C to the steady state Tj to get T jrgANsiENT max 106 C This is an accept able maximum junction temperature for this part PCB Layout Considerations Because of the low values of the sense resistors used with the MIC2085 86 controllers special attention to the layout must be used in order for the device s circuit breaker function to operate properly Specifically the use of a 4 wire Kelvin connection to measure the voltage across Rsgysg is highly recommended Kelvin sensing is simply a means of making sure that any voltage drops in the power traces connecting to the resistors does not get picked up by the traces themselves Additionally these Kelvin connections should be isolated from all other signal traces to avoid introducing noise onto these sensitive nodes Figure 13 illustrates a recommended Current Flow Micrel multi layer layout for the Rgense Power MOSFET timer s overvoltage and feedback network connections The feed back and overvoltage resistive networks are selected for a 12V application from Figure 5 M
5. 15A lout 2 20A Vcc 5V Jour 20A Vcc 5V Jour lt 10A louT lt 10A lout 10A 15A Vcc lt 5V lout 2 20A Vcc 5V lour lt 10A lout lt 10A Vcc lt 5V www irf com 310 322 3331 www fairchildsemi com 207 775 8100 Philips PH3230 SOT669 LFPAK lout 2 20A www philips com Hitachi HAT2099H LFPAK lout 2 20A www halsp hitachi com 408 433 1990 These devices are not limited to these conditions in many cases but these conditions are provided as a helpful reference for customer applications Sense Resistors WSL Series Resistor Vendors Vishay Dale OARS Series LR Series second source to WSL Table 4 MOSFET and Sense Resistor Vendors M0235 121903 26 Contact Information www vishay com docswsl 30100 pdf 203 452 5664 www irctt com pdf files OARS pdf www irctt com pdf files LRC pdf 828 264 8861 January 2004 MICZ085 2086 Micrel Package Information a PIN 1 0 157 3 99 DIMENSIONS 0 150 3 81 INCHES MM Ge 0 2286 JL gt Peer ae 0 025 0 635 0 012 0 30 8 0 0098 0 249 BSC 0 008 0 20 0 0098 0 249 8i Ur e 0 102 mmmn 0 0075 0 190 190 d i gl 0 196 4 98 0 050 127 4 SEATING 0 0688 1 748 0 189 4 80 0 016 0 40 PLANE 0 0532 1 351 0 244 6 20 0 229 5 82 Rev 04 16 Pi
6. Carrying the numbers through for the case where the value of the sense resistor is 3 high yields 40mV 38 8mV RSENSE MAX 1 03 Loap cont kLospeon C Once the value of Rgense has been chosen in this manner itis good practice to check the maximum ILOAD CONT which the circuit may let through in the case of tolerance build up in the opposite direction Here the worst case maximum cur rent is found using a 55mV trip voltage and a sense resistor that is 3 low in value The resulting equation is 55mV 56 7mV 0 97 Rsense Nom PsENsE NOM ILOAD CONT MAX 12 As an example if an output must carry a continuous 6A without nuisance trips occurring Equation 11 yields 38 8mV D bm 6A RSENSE MAX M0235 121903 The next lowest standard value is 6 0mW At the other set of tolerance extremes for the output in question 56 7mV ILOAD CONTMAX pomo 9 454 almost 10A Knowing this final datum we can determine the necessary wattage of the sense resistor using P I R where will be ILOAD CONT MAX and R will be 0 97 RsENsE NOM These numbers yield the following Pmax 10A 5 82mQ 0 582W In this example a 1W sense resistor is sufficient MOSFET Selection Selecting the proper external MOSFET for use with the MIC2085 86 involves three straightforward tasks Choice of a MOSFET which meets minimum voltage requirements Selection of a device to handle the maximum continuous current s
7. Higher UVLO Setting January 2004 M0235 121903 MICZU85 2086 Fast Output Discharge for Capacitive Loads In many applications where a switch controller is turned off by either removing the PCB from the backplane or the ON pin is reset capacitive loading will cause the output to retain voltage unless a bleed low impedance path is in place in order to discharge the capacitance The MIC2086 is equipped with an internal MOSFET that allows the discharging of any load capacitance to ground through a 5509 path The dis charge feature is configured by wiring the DIS pin to the output source ofthe external MOSFET and becomes active Rsense 0 0072 Vin 41996 12VB ON Signal o MIC2086 CPOR GND 3 9 10 VCC ON Signal 0 MIC2086 CPOR 3 C3 0 01uF Undervoltage Output 11V POR START UP Delay 6ms Circuit Breaker Response Time 620us R5 of Figure 8 a is optional to combine in series with internal 5509 Additional pins omitted for clarity POR PWRGD CFILTER 2 Micrel DIS pin output is low once the ON pin is deasserted Figure 8 a illustrates the use of the discharge feature with an optional resistor R5 that can be used to provide added resistance in the output discharge path For an even faster discharge response of capacitive loads the configuration of Figure 8 b can be utilized to apply a crowbar to ground through an external SCR Q3 that is triggered when the DIS pin go
8. When the voltage at FB rises above Veg capacitor Cpop begins to charge again When the voltage at CPOR rises above the power on reset delay threshold V7 the timer resets by pulling CPOR to ground and POR is deasserted If Cpop 0 then tstapry defaults to 20us M0235 121903 2 January 2004 MIC2085 2086 Micrel Pin Description Cont Pin Number Pin Number MIC2086 MIC2085 ON Input Active high The ON pin an input to a Schmitt triggered compara tor used to enable disable the controller is compared to a Vru reference with 100mV of hysteresis Once a logic high is applied to the ON pin Von gt 1 24V a start up sequence is initiated as the GATE pin starts ramping up towards its final operating voltage When the ON pin receives a low logic signal Voy 1 14V the GATE pin is grounded and FAULT is high if VCC is above the UVLO threshold ON must be low for at least 20us in order to initiate a start up sequence Additionally toggling the ON pin LOW to HIGH resets the circuit breaker Power On Reset Output Open drain N Channel device active low This pin remains asserted during start up until a time period tpog after the FB pin voltage rises above the power good threshold Veg The timing capacitor Cpog determines tpor When an output undervoltage condition is detected at the FB pin POR is asserted for a minimum of one timing cycle tpor The POR pin has a weak pull up to VCC Power Good Output Open drain N Channel
9. ground The following equation is used to determine the overcurrent timer period tocs oy Vom TIMER where Vrp the CFILTER timer threshold is 1 24V and Loun the overcurrent timer current is 204A Tables 2 and 3 provide a quick reference for several timer calculations using select standard value capacitors tocstow CriLrER X 0 062x Cry teR HF 7 M0235 121903 MIC2085 2086 0 01nF 0 02uF 0 033uF Table 2 Selected Power On Reset and Start Up Delays 1800pF 0 020uF Table 3 Selected Overcurrent Timer Delays Applications Information Output Undervoltage Detection For output undervoltage detection the first consideration is to establish the output voltage level that indicates power is good For this example the output value for which a 12V supply will signal good is 11V Next consider the tolerances of the input supply and FB threshold Vg For this example the 12V supply varies 5 thus the resulting output voltage may be as low as 11 4V and as high as 12 6V Additionally the FB threshold has 50mV tolerance and may be as low as 1 19V and as high as 1 29V Thus to determine the values of the resistive divider network R5 and R6 atthe FB pin shown in Figure 5 use the following iterative design procedure 1 Choose R6 so as to limit the current through the divider to approximately 100A or less VEB MAX a 1 29V 100u0A 100uA R6 is chosen as 13 3kQ 1 2 Next determine R5 usi
10. For input overvolt age protection the first consideration is to establish the input voltage level that indicates an overvoltage triggering a sys tem output voltage shut down For this example the input value for which a 12V supply will signal an output shut down is 13 2V 41094 Similarly from the previous example 1 Choose R3 to satisfy 100A condition tovcr 0 01x C4 uF 9 Vov MIN x 1 19V 100uA 100uA R3 is chosen as 13 7kQ 1 2 Thus following the previous example and substituting R2 and R3 for R5 and R6 respec tively and 13 2V overvoltage for 11V output good the same formula yields R2 of 138 3kQ The next highest standard 1 value is 140kQ Now consider the 12 6V maximum input voltage Vcc 5 the higher tolerance for R3 and lower tolerance for R2 13 84k and 138 60kO respectively With a 12 6V input the voltage sensed atthe OV pin is below Vov MiNy and the MIC2085 86 will not indicate an overvoltage condition until Vag exceeds at least 13 2V R32 211 9kQ January 2004 Micrel MIC2085 2086 Rsense Q1 0 0120 IRF7822 Vin 1 2 2 SO 8 Vout Ct 12V 3A MIC2085 Downstream R3 Q2 13 7kQ 2N4401 Q3 1 TCR22 4 C3 0 05uF Overvoltage Input 13 3V Undervoltage Output 11 0V lI L POR START UP Delay 30ms E R7 needed when using a sensitive gate SCR Additional pins omitted for clarity Figure 5 Undervoltage Overvoltage Circuit 17 M0235 121903 January 2004 MIC2
11. Output Voltage Slew Rate dVoyr dt LIM Ci oap where l 4 is the programmed current limit value Conse quently the value of Coen must be selected to ensure that the overcurrent response time tocs gw exceeds the time needed for the output to reach its final value For example given a MOSFET with an input capacitance Cigg Ceate 4700pF C gap is 2200uF and lj yr is set to 6A with a 12V input then the load capacitance dominates as determined by the calculated INRUSH gt lw Therefore the output voltage slew rate determined from Equation 4 is DA L 7 KA 2200uF ms and the resulting tos ow needed to achieve a 12V output is approximately 4 5ms See Power On Reset Start Up and Overcurrent Timer Delays section to calculate tocs ow GATE Capacitance Dominated Start Up In this case the value of the load capacitance relative to the GATE capacitance is small enough such that the load current during start up never exceeds the current limit threshold as determined by Equation 3 The minimum value of Ce that will ensure that the current limit is never exceeded is given by the equation below Output Voltage Slew Rate dVgy7 dt X Ci oap 5 January 2004 MIC2085 2086 where Ca4rg is the summation of the MOSFET input capacitance Cigg and the value of the external capacitor connected to the GATE pin of the MOSFET Once Care is determined use the following equation to determine the output slew rate for gate capaci
12. current source If the voltage at CFILTER reaches the overcurrent timer threshold V of 1 24V then CFILTER immediately returns to ground as the circuit breaker trips and the GATE output is immedi ately shut down For the second level if the voltage sensed across VCC and SENSE exceeds 95mV at any time the circuit breaker trips and the GATE shuts down immediately bypassing the overcurrent timer period To disable current limit and circuit breaker operation tie the SENSE and VCC pins together and the CFILTER pin to ground January 2004 Micrel Output Undervoltage Detection The MIC2085 86 employ output undervoltage detection by monitoring the output voltage through a resistive divider connected at the FB pin During turn on while the voltage at the FB pin is below the threshold Veg the POR pin is asserted low Once the FB pin voltage crosses Veg a 2uA current source charges capacitor Cpop Once the CPOR pin voltage reaches 1 24V the time period tpop elapses as the CPOR pin is pulled to ground and the POR pin goes HIGH If the voltage at FB drops below Veg for more than 10us the POR pin resets for at least one timing cycle defined by tpop see Applications Information for an example Input Overvoltage Protection The MIC2085 86 monitors and detects overvoltage condi tions in the event of excessive supply transients at the input Whenever the overvoltage threshold Voy is exceeded at the OV pin the GATE is pulled low and
13. gt 1V Vec 16 5V 50 mA Voc 2 3V 20 mA Von ON Pin Threshold Voltage ON rising 1 19 1 24 1 29 V ON falling 1 09 1 14 1 19 V VSTART Undervoltage Start up Vcpon ising 1 19 1 24 V Timer Threshold VoL FAULT POR PWRGD Output lout 1 6mA V Voltage PWRGD for MIC2086 only IPULLUP Output Signal Pull up Current FAULT POR PWRGD GND uA FAULT POR PWRGD COMPOUT PWRGD for MIC2086 only VREF Reference Output Voltage lLoap OMA Ogge 0 1uF V AV ius Reference Line Regulation 2 8V lt Veg lt 16 5V 15 17 mV Rpis Discharge Pin Resistance ON pin toggles from HI to LOW 1000 AC Electrical Characteristics Symbol locFAST Fast Overcurrent Sense to GATE Voc 5V Low Trip Time Voc Vsense 100mV Coate 10nF See Figure 1 tocsLow Slow Overcurrent Sense to Gate Voc 5V Notes 1 Exceeding the absolute maximum rating may damage the device 2 The device is not guaranteed to function outside its operating rating 3 Specification for packaged product only 4 Specification for packaged product only M0235 121903 6 January 2004 MIC2085 2086 Micrel Timing Diagrams VraiPFAST Vcc Vsense dm 0 Vaate M ES CFILTER POR Figure 2 Power On Reset Response Loupe sy det Arm Fast Comparator Arm Slow Comparator L Leners m lpog 1 24V de Figure 3 Power On Start Up Delay Timing Current Limit Threshold mV 0 200 400 600 800 1000 FB Voltage mV
14. 0 Ci oap 5700uF 2 ES TIME 10ms div Turn Off Normal Discharge Kei E N N a ae WEE d a f zo ARS AL Aie MAR e eA nad cap AM n RAI rgd IAEA dee oz gt F ws ft og f 52 yt eng a ZW Vin 12V Rpis External 0 Rioap 4 82 C oap 1000uF L2 SW2 HIGH 3 gt 0 TIME 2 5ms div January 2004 11 12V Turn On Response Va 12V Rioap 4 82 Ci gab 1000uF TIME 20ms div Power Good Response Va 2 12V D cap 4 89 Croan 1000uF 2 mo LS H E EE EECH os S TIME 10ms div Turn Off Crowbar Discharge z E o OS GATE 20V div 5 gt OS LC eye ee AE AN BT p i Mass 12V RLoap 4 89 Ci oap 1000uF SW2 LOW Vour 5V div TIME 2 5ms div Micrel M0235 121903 MIC2085 2086 Micrel Functional Characteristics continued Turn On Into Short Circuit 1V div t 1Vidiv Y FAULT CFILTER ON 10V div Viy 12V Rioap 9 Ci oap 10004F Vour 5V div y TIME 10ms div M0235 121903 12 January 2004 MIC2085 2086 Micrel Functional Block Diagram MIC2086 Charge Pump B Circuit Breaker Response or UVLO DIS and PWRGD are not available on MIC2085 Pin numbers for MIC2085 are in parenthesis where applicable MIC2086 Block Diagram January 2004 13 M0235 121903 MICZ085 2086 Function
15. 085 2086 PCB Connection Sense There are several configuration options for the MIC2085 86 s ON pin to detect if the PCB has been fully seated in the backplane before initiating a start up cycle In the typical applications circuit the MIC2085 86 is mounted on the PCB with a resistive divider network connected to the ON pin R2 is connected to a short pin on the PCB edge connector Until the connectors mate the ON pin is held low which keeps the GATE output charge pump off Once the connectors mate the resistor network is pulled up to the input supply 12V in this example and the ON pin voltage exceeds its threshold Voy Micrel of 1 24V and the MIC2085 86 initiates a start up cycle In Figure 6 the connection sense consisting of a logic level discrete MOSFET and a few resistors allows for interrupt control from the processor or other signal controller to shut off the output of the MIC2085 86 R4 keeps the GATE of Q2 at Vin until the connectors are fully mated A logic LOW at the JON OFF signal turns Q2 off and allows the ON pin to pull up above its threshold and initiate a start up cycle Applying a logic HIGH at the ON OFF signal will turn Q2 on and short the ON pin of the MIC2085 86 to ground which turns off the GATE output charge pump Backplane PCB Edge oy Rsense OI Connector Connector rg 0 0080 Si7860DP ViN ee 2 PowerPAK SO 8 V 12V 5A VCC MIC2085 ON_OFF Downstream Signals GND E Long ER Undervoltage Outp
16. 6 5 5V H ON SIGNAL O D1 1N914 FAULT OUTPUT H C3 Undervoltage Output 4 27V 0 02uF POR START UP Delay 12ms Circuit Breaker Response Time 290us Auto Retry Duty Cycle 2 5 Additional pins omitted for clarity MIC2085 Micrel The circuit in Figure 10 distributes 12V from the backplane to the MIC2182 DC DC converter that steps down 12V to 3 3V for local bias The pass transistor Q1 isolates the MIC2182 s input capacitance during module plug in and allows the backplane to accommodate additional plug in modules withoutaffecting the other modules on the backplane The two control input signals are VBxEn L active LOW and a Local Power Enable active HIGH The MIC2085 in the circuit of Figure 10 performs a number of functions The gate output of Q1 is enabled by the two bit input signal VBxEn L Local Power Enable 0 1 Also the MIC2085 limits the drain current of Q1 to 7A monitors VB In for an overvoltage condition greaterthan 16V and enables the MIC2182 DC DC converter downstream to supply a local voltage rail The uncommitted comparator is used to monitor VB In for an undervoltage condition of less than 10V indicated by a logic LOW at the comparator output COMPOUT COMPOUT may be used to control a downstream device such as another DC DC converter Additionally the MIC2085 is configured for auto retry upon an overcurrent fault condition by placing a diode D1 between the FAULT and ON pins ofthe co
17. Discharge Pin Resistance vs Temperature 1000 900 800 700 600 500 400 300 200 40 20 0 20 40 60 80 100 TEMPERATURE C UVLO Threshold vs Temperature UVLO THRESHOLD 49 20 0 20 40 60 80 100 TEMPERATURE C ON Pin Input Current vs Temperature T E z DI tr E 2 o E 2 a zZ z a z O oL LL sauneja 40 20 0 20 40 60 80 100 TEMPERATURE C Output Signal Pull Up Current vs Temperature 26 10 20 0 20 40 60 80 100 TEMPERATURE C Micrel M0235 121903 MIC2085 2086 Micrel Test Circuit Rsense _Q1 SE 0 012 Si7892DP E e Vin 5 PowerPAK SO 8 12VH Vour Rioap Cioap MIC2086 Downstream Signal GND CFILTER Q2 ZTX788A Q3 0 047uF 0 047uF TCR22 4 Not all pins shown for clarity C5 R8 0 033 uF i 8 S d M0235 121903 10 January 2004 MICZ085 2086 Functional Characteristics 12V Hot Insert Response Voc ON 1Vidiv 1V div 5V div Lor Cpon lour 1A div 3 lin Rioap 4 82 Ci oap 1000uF TIME 20ms div Inrush Current Response SEET Vour FAULT Vec 10V div 10V div 5V div vy Vn 12V Rioap 34
18. RE C Overcurrent Timer Current Overcurrent Timer Off Current Gate Pull Up Current vs Temperature 5 vs Temperature vs Temperature 30 4 25 _ 26 E T Voc 16 5V i m 22 cc w E Tepr ERT 18 x EL WA 2 3V f Va 5V T Voc 2 3V Van 2 3V V 5V b perm Ven 25V 4 14 GG 1H cc 5 cc 5 CC 10 0 0 40 20 0 20 40 60 80 100 40 20 0 20 40 60 80 100 40 20 0 20 40 60 80 100 TEMPERATURE C TEMPERATURE C TEMPERATURE C Gate PUP Current External Gate Drive External Gate Drive S vs Temperature cc 25 cc ig vo emperalure 22 14 Voc 5V 20 20 A 18 120 331 16 14 z 15 an Voc 16 5V S us L A g 8 EE EE E 8 10 5 10 wE Er 8 M 4 6 5 4 Voc 2 3V P 2 2 0 03 4 6 8 10 12 i4 16 18 0 20 0 20 40 60 80 100 2 4 6 8 10 12 14 16 18 Voc TEMPERATURE C Voc V POR Delay Overcurrent Gate Sink Current Gate Sink Current Timer Threshold vs Temperature vs Gate Voltage vs Temperature 100 600 _ _ ______ _ 1 25 90 80 390 1 24 oP 400 e zl EE k 12Voc A 123 e geg JL O Ur UI p 50 LT E gt 122 cr V 5V L ec 30r Man 2 8V Z 1 21 20 eL 10 0 1 20 40 20 0 20 40 60 80 100 0 2 4 6 8 10 12 14 40 20 0 20 40 60 80 100 TEMPERATURE C Vaar DI TEMPERATURE C M0235 121903 8 January 2004 MIC2085 2086 Typical Characteristics Current Limit Threshold Fast Trip vs Temperature
19. SIM Ent aen MIC2085 MIC2086 Single Channel Hot Swap Controllers General Description The MIC2085 and MIC2086 are single channel positive voltage hot swap controllers designed to allow the safe insertion of boards into live system backplanes The MIC2085 and MIC2086 are available in 16 pin and 20 pin QSOP packages respectively Using a few external components and by controlling the gate drive of an external N Channel MOSFET device the MIC2085 86 provide inrush current limiting and output voltage slew rate control in harsh critical power supply environments Additionally a circuit breaker function will latch the output MOSFET off if the current limit threshold is exceeded for a programmed period of time The devices array of features provide a simplified yet robust solution for many network applications in meeting the power supply regulation requirements and affords protection of critical downstream devices and components All support documentation can be found on Micrel s web site at www micrel com Typical Application Backplane PCB Edge Connector Connector Rgense 0 0070 Lon Pin Features MIC2085 Pin for pin functional equivalent to the LTC1642 2 3V to 16 5V supply voltage operation Surge voltage protection to 33V Operating temperature range 40 C to 85 C Active current regulation limits inrush current independent of load capacitance Programmable inrush current limiting Analog foldback cur
20. V T4 25 C unless otherwise noted Bold indicates specifications over the full operating temperature range of 40 C to 85 C Symbol loc Supply Current UV Lockout Hysteresis VUVHYST ME Vuv Undervoltage Lockout Threshold Voc rising 2 18 2 28 V Voc falling 2 0 2 10 V mA el Iw VEBHYST OV Pin Threshold Voltage 2 3V lt Voc lt 16 5V Line Regulation Vov OV Pin Threshold Voltage OV pin rising 139 124 1 29 mV Vou POR Delay and Overcurrent CFILTER Vepor Vcg rgg rising 1 19 V Timer Threshold IcPoR Power On Reset Timer Current Timer on 1 5 uA Timer off mA Lous Current Limit Overcurrent Timer on uA Timer Current CFILTER Timer off mA Ven CRWBR Pin Threshold Voltage 2 3V lt Voe lt 16 5V mV AN cp CRWBR Pin Threshold Voltage 2 3V lt Voc lt 16 5V mV Line Regulation lor CRWBR Pin Current CRWBR On Verwer OV 30 uA CRWBR Off Vorwegr 1 5V mA Vom Circuit Breaker Trip Voltage VrniP Voc 7VsENSE Vraegow 40 48 5 mV Current Limit Threshold 2 3V lt Voc lt 16 5V VrRpFAST X J 95 110 mV K 150 mV X 200 mV Vas External Gate Drive VaATE Nee January 2004 9V lt Veg lt 15 0V ale 5V lt Voc 9V 11 12 4 5 X l lt o o lt lt lt M0235 121903 MIC2085 2086 Micrel Electrical Characteristics Cont lGATE GATE Pin Pull up Current Start cycle Va Arg OV Voc 16 5V 16 8 uA Vec 2 3V 14 8 uA IGATEOFF GATE Pin Sink Current FAULT 0 Veate
21. a start up sequence i e start up delay times out Once the start up delay terAn elapses CPOR is pulled immediately to ground and a 15uA current source begins charging the GATE outputto drive the external MOSFET that switches Vi to Mou The programmed start up delay is calculated using the following equation V TH_ 0 62x Cpop HF 1 CPOR where Vu the POR delay threshold is 1 24V and lcpog the POR timer current is 2uA As the GATE voltage contin ues ramping toward its final value Voc Ves at a defined slewrate See Load Capacitance Gate Capacitance Domi nated Start Up sections a second CPOR timing cycle begins if 1 FAULT is high and 2 CFILTER is low i e not an overvoltage undervoltage lockout or overcurrent state This second timing cycle tpopr starts when the voltage at the FB pin exceeds its threshold Veg indicating that the output voltage is valid The time period tpop is equivalent to tern and sets the interval for the POR to go Low to High after power is good See Figure 2 of Timing Diagrams Active current regulation is employed to limit the inrush current transient response during start up by regulating the load current at the programmed current limit value See Current Limiting and Dual Level Circuit Breaker section The fol lowing equation is used to determine the nominal current limit value tstart Cpon T VrripsLow _ 48mV Rsense FRsense 2 M0235 121903 Mi
22. aker Threshold Discharge Output MIC2085 xBQS x J 95mV NA 16 pin QSOP x K 150mV x L 200mV MIC2086 xBQS 20 pin QSOP Contact factory for availability Pin Configuration VCC VCC CRWBR 1 VCC SENSE CFILTER SENSE GATE CPOR GATE REF ON REF Bis POR COMP COMP FAULT 6 COMP COMP FB COMPOUT COMPOUT GND 8 9 Ov OV MIC2085 16 Pin QSOP QS MIC2086 20 Pin QSOP QS Pin Heo Overvoltage Timer and Crowbar Circuit Trigger A capacitor connected to this pin sets the timer duration for which an overvoltage condition will trigger an external crowbar circuit This timer begins when the OV input rises above its threshold as an internal Abu A current source charges the capacitor Once the voltage reaches 470mV the current increases to 1 5mA Current Limit Response Timer A capacitor connected to this pin defines the period of time tocs oy in which an overcurrent event must last to signal a fault condition and trip the circuit breaker If no capacitor is connected then tocsi ow defaults to Sus 3 3 CPOR Power On Reset Timer A capacitor connected between this pin and ground sets the start up delay Ural and the power on reset interval tpop When VCC rises above the UVLO threshold the capacitor connected to CPOR begins to charge When the voltage at CPOR crosses 1 24V the start up threshold Vern a start cycle is initiated if ON is asserted while capacitor Cpog is immediately discharged to ground
23. al Description Hot Swap Insertion When circuit boards are inserted into live system backplanes and supply voltages high inrush currents can result due to the charging of bulk capacitance that resides across the supply pins of the circuit board This inrush current although transient in nature may be high enough to cause permanent damage to on board components or may cause the system s supply voltages to go out of regulation during the transient period which may result in system failures The MIC2085 86 acts as a controller for external N Channel MOSFET devices in which the gate drive is controlled to provide inrush current limiting and output voltage slew rate control during hot plug insertions Power Supply VCC is the supply input to the MIC2085 86 controller with a voltage range of 2 3V to 16 5V The VCC input can withstand transient spikes up to 33V In order to help suppress tran sients and ensure stability of the supply voltage a capacitor of 1 0uF to 10uF from VCC to ground is recommended Alternatively a low pass filter shown inthe typical application circuit can be used to eliminate high frequency oscillations as well as help suppress transient spikes Start Up Cycle When the voltage on the ON pin rises above its threshold of 1 24V the MIC2085 86 first checks that its supply Vcc is above the UVLO threshold If so the device is enabled and an internal 2uA current source begins charging capacitor Cpog to 1 24V to initiate
24. any hot swap applications will require load currents of several amperes Therefore the power Vcc and Return trace widths W need to be wide enough to allow the current to flow while the rise in tempera ture for a given copper plate e g 1 oz or 2 oz is keptto a maximum of 10 C 25 C Also these traces should be as short as possible in order to minimize the IR drops between the input and the load For a starting point there are many trace width calculation tools available on the web such as the following link http www aracnet com cgi usr gpatrick trace pl Finally plated through vias are utilized to make circuit con nections to the power and ground planes The trace connec tions with indicated vias should follow the example shown for the GND pin connection in Figure 13 Current Flow to the Load to the Load M9 SENSE RESISTOR POWER MOSFET X 2512 SO 8 B G w B w D S R4 100 Via to GND Plane CaATE LE A R3 E 137k0 Bj 196 mm R2 140kQ e e x 9 sl z 9 o 1 Via to POWER VCC OH H LL gy t E 2 Plane 77 Im W D SBS e SS sen D c O Oo Lo n o O S Ee Q o o N o ke tc m H H m d 5 z S 2 2 2 o om 5 ip m B d d N e Ut wo o N oo R5 m 100kQ Goen Cpor 195 B rc 13 3kQ Current Flow from
25. arrying just about 2 5A for some time When performing this calculation be sure to use the highest anticipated ambient temperature Taiyaxy in which the MOSFET will be operating as the starting temperature and find the operating junction temperature increase AT d from that point Then as shown next the final junction temperature is found by adding TA MAX and AT j Sincethis is not a closed form equation getting a close approximation may take one or two iterations but it s not a hard calculation to perform and tends to converge quickly Then the starting steady state T is Ty Tamax AT x I x Daat Ty 55 C 17MQ 55 C 25 C 0 005 17mQ2 x 2 5A x 50 C W Ty 55 C 0 122W 50 C W 61 1 C Iterate the calculation once to see if this value is within a few percent of the expected final value For this iteration we will start with T equal to the already calculated value of 61 1 C Ty T 17mQ 61 1 C 25 C 0 005 17mMQ x 2 5A x 50 C W Ty 55 C 0 125W 50 C W 61 27 C Normalized Thermal Transient Impedance Junction to Ambient Notes 0 1 RN Pom Thermal Impedance DEEL el Normalized Effective Transient o ty 1 Duty Cycle D ma o 2 Per Unit Base Rinya 50 C W Single Pulse 3 Tum Ta
26. asing die temperature A good approximation for this value is 0 5 increase in Roy per C rise in junction temperature above the point at which Roy was initially specified by the manufacturer For instance if the selected MOSFET has a calculated Roy of 10mO at a T 25 C and the actual junction temperature ends up at 110 C a good first cut at the operating value for Roy would be Ron 10mO 1 110 25 0 005 14 3mO The final step is to make sure that the heat sinking available to the MOSFET is capable of dissipating at least as much power rated in C W as that with which the MOSFET s performance was specified by the manufacturer Here are a few practical tips 1 The heat from a surface mount device such as an SO 8 MOSFET flows almost entirely out of the drain leads If the drain leads can be sol dered down to one square inch or more the copper will act as the heat sink for the part This copper must be on the same layer of the board as the MOSFET drain o Vout 12V 5A Downstream Signals Undervoltage Output 11 0V POR START UP Delay 60ms Recommended for MOSFETs with gate source breakdown of 20V or less IRF7822 Vas MAX 12V for catastrophic output short circuit protection Additional pins omitted for clarity Figure 11 Zener Clamped MOSFET GATE January 2004 M0235 121903 MIC2085 2086 2 Airflow works Even a few LFM linear feet per minute of air will cool a MOSFET down sub stantial
27. crel where Vrpips ow S the current limit slow trip threshold found in the electrical table and Rsgwsg is the selected value that will set the desired current limit There are two basic start up modes for the MIC2085 86 1 Start up dominated by load capacitance and 2 start up dominated by total gate capaci tance The magnitude of the inrush current delivered to the load will determine the dominant mode If the inrush current is greater than the programmed current limit 1 wl then load capacitance is dominant Otherwise gate capacitance is dominant The expected inrush current may be calculated using the following equation C LOAD LOAD EN 5uA x C 3 ATE GATE where La is the GATE pin pull up current Cj gap is the load capacitance and Cae is the total GATE capacitance Cigg of the external MOSFET and any external capacitor connected from the MIC2085 86 GATE pin to ground Load Capacitance Dominated Start Up In this case the load capacitance C gap is large enough to cause the inrush current to exceed the programmed current limit but is less than the fast trip threshold or the fast trip threshold is disabled M option During start up under this condition the load current is regulated at the programmed current limit value wa and held constant until the output voltage rises to its final value The output slew rate and equivalent GATE voltage slew rate is computed by the following equation INRUSH Igare X S
28. device active high When the voltage at the FB pin is lower than 1 24V the PWRGD output is held low When the voltage at the FB pin is higher than 1 24V then PWRGD is asserted A pull up resistor connected to this pin and to VCC will pull the output up to VCC The PWRGD pin has a weak pull up to VCC 7 FAULT Circuit Breaker Fault Status Output Open drain N Channel device active low The FAULT pin is asserted when the circuit breaker trips due to an overcurrent condition Also this pin indicates undervoltage lockout and overvoltage fault conditions The FAULT pin has a weak pull up to VCC T FB Power Good Threshold Input This input is internally compared to a 1 24V reference with 3mV of hysteresis An external resistive divider may be used to set the voltage at this pin If this input momentarily goes below 1 24V then POR is activated for one timing cycle tpor indicating an output undervoltage condition The POR signal de asserts one timing cycle after the FB pin exceeds the power good threshold by 3mV A 5ys filter on this pin prevents glitches from inadvertently activating this signal 11 OV OV Input When the voltage on OV exceeds its trip threshold the GATE pin is pulled low and the CRWBR timer starts If OV remains above its threshold long enough for CRWBR to reach its trip threshold the circuit breaker is tripped Otherwise the GATE pin begins to ramp up one POR timing cycle after OV drops below its trip threshold COMPOUT U
29. es low which turns on the PNP transistor Q2 See the different Functional Characteristic curves for a comparison of the discharge response configurations Q1 Si7892DP PowerPAK SO 8 Vour 12V 5A DIS Downstream Signals Q1 Si7892DP PowerPAK SO 8 Downstream Signal 4 4kQ Q3 TCR22 4 Figure 8 MIC2086 Fast Discharge of Capacitive Load M0235 121903 20 January 2004 MIC2085 2086 Auto Retry Upon Overcurrent Faults The MIC2085 86 can be configured for automatic restart after a fault condition Placing a diode between the ON and FAULT pins as shown in Figure 9 will enable the auto restart capability of the controller When an application is configured for auto retry the overcurrent timer should be set to minimize the duty cycle of the overcurrent response to prevent thermal runaway of the power MOSFET See MOSFET Transient Thermal ssues section for further detail A limited duty cycle is achieved when the overcurrent timer duration tocs ow is much less than the start up delay timer duration Gap and is calculated using the following equation t Auto Retry Duty Cycle 9CSLOW x100 10 START An InfiniBand Application Circuit The circuit in Figure 10 depicts a single 50W InfiniBand module using the MIC2085 controller An InfiniBand backplane distributes bulk power to multiple plug in modules that employ DC DC converters for local supply requirements Vin 1 97
30. ly If you can position the MOSFET s near the inlet of a power supply s fan or the outlet of a processor s cooling fan The best test of a surface mount MOSFET for an application assuming the above tips show it to be a likely fit is an empirical one Check the MOSFET s temperature in the actual layout of the expected final circuit at full operating current The use of a thermocouple on the drain leads or infrared pyrometer on the package will then give a reasonable idea of the device s junction temperature MOSFET Transient Thermal Issues Having chosen a MOSFET that will withstand the imposed voltage stresses and the worse case continuous 2R power dissipation which it will see it remains only to verify the MOSFET s ability to handle short term overload power dissi pation without overheating A MOSFET can handle a much higher pulsed power without damage than its continuous dissipation ratings would imply The reasonforthis is that like everything else thermal devices silicon die lead frames etc have thermal inertia In terms related directly to the specification and use of power MOSFETS this is known as transient thermal impedance or Zei Au Almost all power MOSFET data sheets give a Transient Thermal Impedance Curve For example take the following case Viy 12V tocsLow has been set to 100msec ILOAD CONT MAX is 2 5A the slow trip threshold is 48mV nominal and the fast trip threshold is 95mV If the out
31. n QSOP QS 0 344 8 74 0 0575 REF 0 337 8 56 Sa ra f 0 0787 x 0 0 005 DETAIL A PIN 1 77 E ID MARK 0 157 3 99 i 0 150 3 81 Se ut 0 244 6 20 d 70 229 5 82 0 009 0 229 0 007 0 178 0 025 BSC RN kK 0 012 0 305 0 635 0 008 0 203 Rev 04 Note 1 All Dimensions are in Inches mm excluding mold flash 2 Lead coplanarity should be 0 004 max 0 010 0 254 3 3 Max misalignment between top and bottom 0 004 0 102 4 The lead width B to be determined at 0 0075 from lead tip Mattie tats 0 068 1 73 0 053 1 35 0 004 GAUGE PLANE SEATING PLANE 0 050 1 27 0 016 0 40 20 Pin QSOP QS January 2004 27 M0235 121903 MICZ085 2086 Micrel MICREL INC 1849 FORTUNE DRIVE SAN JOSE CA 95131 USA TEL 1 408 944 0800 Fax 1 408 944 0970 wes http www micrel com The information furnished by Micrel in this data sheet is believed to be accurate and reliable However no responsibility is assumed by Micrel for its use Micrel reserves the right to change circuitry and specifications at any time without notification to the customer Micrel Products are not designed or authorized for use as components in life support appliances devices or systems where malfunction of a product can reasonably be expected to result in personal injury Life support devices or systems are devices or system
32. ncommitted Comparator s Open Drain Output COMP Comparator s Non Inverting Input COMP Comparator s Inverting Input 15 NA DIS Discharge Output When the MIC2086 is turned off a 550Q internal resistor at this output allows the discharging of any load capacitance to ground ae ae Reference Output 1 24V nominal Tie a 0 1uF capacitor to ground to ensure stability Gate Drive Output Connects to the gate of an external N Channel MOSFET An internal clamp ensures that no more than 13V is applied between the GATE pin and the source of the external MOSFET The GATE pin is immediately brought low when either the circuit breaker trips or an undervoltage lockout condition occurs January 2004 3 M0235 121903 MIC2085 2086 Micrel Pin Description Cont Pin Number Pin Number Pin Function MIC2086 MIC2085 18 15 SENSE Circuit Breaker Sense Input A resistor between this pin and VCC sets the current limit threshold Whenever the voltage across the sense resistor exceeds the slow trip current limit threshold VtpRips_ oy the GATE voltage is adjusted to ensure a constant load current If Vrgips oy 48mV is exceeded for longer than time period toca oy then the circuit breaker is tripped and the GATE pin is immediately pulled low If the voltage across the sense resistor exceeds the fast trip circuit breaker threshold Vrpiprast at any point due to fast high amplitude power supply faults then the GATE pin is immediately brought low withou
33. ng the output good voltage of 11V and the following equation R6 212 9kO R5 R6 VouT Good va oe M0235 121903 Micrel Using some basic algebra and simplifying Equation 8 to isolate R5 yields GER ss E VEB MAX where VEB MAX 1 29V VouT Good 11V and R6 is 13 3kQ Substituting these values into Equation 8 1 now yields R5 100 11kO A standard 100kQ 1 is selected Now consider the 11 4V minimum output voltage the lower tolerance for R6 and higher tolerance for R5 13 17kQ and 101kQ respectively With only 11 4V available the voltage sensed at the FB pin exceeds Vega thus the POR and PWRGD MIC2086 signals will transition from LOW to HIGH indicating power is good given the worse case tolerances of this example Input Overvoltage Protection The external CRWBR circuit shown in Figure 5 consists of capacitor C4 resistor R7 NPN transistor Q2 and SCR Q3 The capacitor establishes a time duration for an overvoltage condition to last before the circuit breaker trips The CRWBR timer duration is approximated by the following equation 8 1 C4x Von CR where Vcg the CRWBR pin threshold is 0 47V and lcg the CRWBR pin current is 45uA during the timer period see the CRWBR timer pin description for further description A similar design approach as the previous undervoltage detec tion example is recommended for the overvoltage protection circuitry resistors R2 and R3 in Figure 5
34. ntroller Figure 9 Auto Retry Configuration January 2004 M0235 121903 MIC2085 2086 InfiniBand Application Micrel InfiniBand InfiniBand MIC2182 Backplane MODULE DC DC Converter Long Rsense Q1 N 0 0072 IRF7822 VB_In 1 11995 SO 8 ax E e Vin R2 165k0 196 R4 16 15 78 7KQ VCC SENSE Short R3 1 X 13 3kQ 1 VBxEn_L Oo RUN SS me 1 CPOR Mic2085 COMP DS ERU I 3 3V G 4A K ze 5 REF 0 01nF T O 033uF CFILTER GND CRWBR Local Power Enable Overvoltage Input 16 0V Undervoltage Input 10 0V Undervoltage Output amp Power Good Output 10 0V Circuit Breaker Response Time 1 2ms POR START UP Delay 18 5ms Auto Retry Duty Cycle 6 5 Figure 10 A 50W InfiniBand Application Sense Resistor Selection The MIC2085 and MIC2086 use a low value sense resistor to measure the current flowing through the MOSFET switch and therefore the load This sense resistor is nominally valued at 48mV li GAp CONT To accommodate worst case tolerances for both the sense resistor allow 3 over time and temperature for a resistor with 1 initial tolerance and still supply the maximum required steady state load current a slightly more detailed calculation must be used The current limit threshold voltage the trip point for the MIC2085 86 may be as low as 40mV which would equate to a sense resistor value of 40mV li oAp cONT
35. put is accidentally connected to a 3Q load the output current from the MOSFET will be regulated to 2 5A for 100ms tocs ow before the part trips During that time the dissipation in the MOSFET is given by P Exl Eposrfer 12V 2 5A 3Q 4 5V Puosrer 4 5V x 2 5A 11 25W for 100msec At first glance it would appear that a really hefty MOSFET is required to withstand this sort of fault condition This is where the transient thermal impedance curves become very useful Figure 12 shows the curve for the Vishay Siliconix Si4410DY a commonly used SO 8 power MOSFET Micrel Taking the simplest case first we ll assume that once a fault event such as the one in question occurs it will be a long time 10 minutes or more before the fault is isolated and the channel is reset In such a case we can approximate this as a single pulse event thatis to say there s no significant duty cycle Then reading up from the X axis at the point where Square Wave Pulse Duration is equalto 0 1sec 100msec we see that the Zg J A of this MOSFET to a highly infrequent event of this duration is only 8 of its continuous Da Ai This particular part is specified as having an Dou A of 50 C W for intervals of 10 seconds or less Thus Assume T 55 C maximum 1 square inch of copper at the drain leads no airflow Recalling from our previous approximation hint the part has an Roy of 0 0335 2 17m9O at 25 C Assume it has been c
36. rent limiting Electronic circuit breaker Dual level overcurrent fault sensing Fast response to short circuit conditions lt 1s Programmable output undervoltage detection Undervoltage lockout protection Power on reset MIC2085 86 and power good MIC2086 status outputs FAULT status output Driver for SCR crowbar on overvoltage Applications RAID systems Cellular base stations LAN servers WAN servers InfiniBand Systems Industrial high side switching Q1 Si7884DP 2 P PAK SO UN i Powerl SO 8 zs bg 12V 5A Croan 220uF R11 Mie Output Signal Power Good PWRGD LOGIC dch CONTROLLER R3 i RESET cec uu 1829 Power On Reset D Pin Output Q3 GND CFILTI TCR22 4 T s z 0 ie 0 we sang T GND Ju Ju p Du Long Overvoltage Input 13 3V Undervoltage Lockout 10 8V Undervoltage Output amp Power Good Output 11 4V InfiniBand is a trademark of InfiniBand Trade Association PowerPAK is a trademark of Vishay Intertechnology Inc Micrel Inc 1849 Fortune Drive San Jose CA 95131 USA January 2004 POR START UP DELAY 60ms Circuit Breaker Response Time 500us R6 is an optional component used for noise filtering R9 needed when using a sensitive gate SCR e tel 1 408 944 0800 fax 1 408 944 0970 1 http www micrel com M0235 121903 MIC2085 2086 Micrel Ordering Information Part Number Fast Circuit Bre
37. s that a are intended for surgical implant into the body or b support or sustain life and whose failure to perform can be reasonably expected to result in a significant injury to the user A Purchaser s use or sale of Micrel Products for use in life support appliances devices or systems is at Purchaser s own risk and Purchaser agrees to fully indemnify Micrel for any damages resulting from such use or sale 2003 Micrel Incorporated M0235 121903 28 January 2004
38. t delay To disable the circuit breaker the SENSE and VCC pins can be tied together The default Vrpiprast for either device is 95mV Other fast trip thresholds are available 150mV 200mV or OFF VrnipeAsT disabled Please contact factory for availability of other options 19 20 16 VCC Positive Supply Input 2 3V to 16 5V The GATE pin is held low by an internal undervoltage lockout circuit until VCC exceeds a threshold of 2 18V If VCC exceeds 16 5V an internal shunt regulator protects the chip from VCC and SENSE pin voltages up to 33V M0235 121903 4 January 2004 MIC2085 2086 Absolute Maximum Ratings All voltages are referred to GND Supply Voltage Vog acsectieseatearacetammerdadurarects 0 3V to 33V SENSE Pn eite ns 0 3V to Vcc 0 3V GATE PIM e ees 0 3V to 22V ON DIS POR PWRGD FAULT COMP COMP COMPOUT 0 3V to 20V CRWBR FB OV BEE sese 0 3V to 6V Maximum Currents Digital Output PINS iris eir ens 10mA POR FAULT PWRGD COMPOUT BID T 30mA ESD Rating Human Body Model 2kV Machine Model 200V Electrical Characteristics Micrel Operating Ratings Supply Voltage Vog centres 2 3V to 16 5V Operating Temperature Range 40 C to 85 C Junction Temperature Tj ssrcccnssecasscessascversenscaneanceases 125 C Package Thermal Resistance Hg A 16 pin QSOP eie eene ite cotidie 112 C W 20 PIN OSOP eorr peciit tet eee ge DEED 91 C W Voc 5 0
39. tance dominated start up dVour dt output ATE 6 GATE Table 1 depicts the output slew rate for various values of Cc Arc laate 19HA dVour dt 0 001 uF 15V ms 0 01 uF 1 5V ms 0 1 uF 0 150V ms Table 1 Output Slew Rate Selection for GATE Capacitance Dominated Start Up Current Limiting and Dual Level Circuit Breaker Many applications will require that the inrush and steady state supply current be limited at a specific value in order to protect critical components within the system Connecting a sense resistor between the VCC and SENSE pins sets the nominal current limit value of the MIC2085 86 and the current limit is calculated using Equation 2 However the MIC2085 86 ex hibits foldback current limit response The foldback feature allows the nominal current limit threshold to vary from 24mV up to 48mV as the FB pin voltage increases or decreases When FB is at OV the current limit threshold is 24mV and for FB 2 0 6V the current limit threshold is the nominal 48mV See Figure 4 for Foldback Current Limit Response charac teristic The MIC2085 86 also features a dual level circuit breaker triggered via 48mV and 95mV current limit thresholds sensed across the VCC and SENSE pins The first level of the circuit breaker functions asfollows Once the voltage sensed across these two pins exceeds 48mV the overcurrent timer its duration set by capacitor Coup starts to ramp the voltage at CFILTER using a 2uA constant
40. teady state thermal issues Verify the selected part s ability to withstand any peak currents transient thermal issues MOSFET Voltage Requirements The first voltage requirement for the MOSFET is that the drain source breakdown voltage of the MOSFET must be greater than Vinmax For instance a 16V input may reasonably be expected to see high frequency transients as high as 24V Therefore the drain source breakdown voltage ofthe MOSFET must be at least 25V For ample safety margin and standard availability the closest minimum value should be 30V 22 January 2004 MIC2085 2086 The second breakdown voltage criterion that must be met is a bit subtler than simple drain source breakdown voltage In MIC2085 86 applications the gate of the external MOSFET is driven up to a maximum of 21V by the internal output MOSFET At the same time if the output of the external MOSFET its source is suddenly subjected to a short the gate source voltage will go to 21V OV 21V Since most power MOSFETs generally have a maximum gate source breakdown of 20V or less the use of a Zener clamp is recommended in applications with Vcc 2 8V A Zener diode with 10V to 12V rating is recommended as shown in Figure 11 At the present time most power MOSFETs with a 20V gate source voltage rating have a 30V drain source break down rating or higher As a general tip choose surface mount devices with a drain source rating of 30V or more as astarting point
41. the Load gt DRAWING IS NOT TO SCALE See Table 4 for part numbers and vendors Optional components Trace width W guidelines given in PCB Layout Recommendations section of the datasheet 1 Via to GND Plane Figure 13 Recommended PCB Layout for Sense Resistor Power MOSFET and Feedback Overvoltage Network January 2004 25 M0235 121903 MIC2085 2086 MOSFET and Sense Resistor Vendors Device types and manufacturer contact information for power MOSFETs and sense resistors is provided in Table 4 Some of the recommended MOSFETs include a metal heat sink on the bottom side of the package The recommended trace for Micrel the MOSFET Gate of Figure 13 must be redirected when using MOSFETs packaged in this style Contact the device manufacturer for package information MOSFET Vendors Key MOSFET Type s Applications Contact Information Vishay Siliconix Si4420DY SO 8 package Si4442DY SO 8 package Si3442DV SO 8 package Si7860DP PowerPAK SO 8 Si7892DP PowerPAK SO 8 Si7884DP PowerPAK SO 8 SUB60NO6 18 TO 263 SUB70N04 10 TO 263 International Rectifier IRF7413 SO 8 package IRF7457 SO 8 package IRF7822 SO 8 package IRLBA1304 Super220 FDS6680A SO 8 package FDS6690A SO 8 package Fairchild Semiconductor www siliconix com 203 452 5664 lout lt 10A lout 10A 15A Vcc lt 5V louT 3A Vcc lt 5V lour 12A lout lt 15A Jour lt
42. the output is shut off The GATE will begin ramping one POR timing cycle after the OV pin voltage drops below its threshold An external CRWBR circuit as shown in the typical application diagram provides a time period that an overvoltage condition must exceed in order to trip the circuit breaker When the OV pin exceeds the overvoltage threshold Voy the CRWBR timer begins charg ing the CRWBR capacitor initially with a 45uA current source Once the voltage at CRWBR exceeds its threshold Vcg of 0 47V the CRWBR current immediately increases to 1 5mA andthe circuit breaker is tripped necessitating a device reset by toggling the ON pin LOW to HIGH Power On Reset Start Up and Overcurrent Timer Delays The Power On Reset delay tpog is the time period for the POR pin to go HIGH once the voltage at the FB pin exceeds the power good threshold V A capacitor connected to CPOR sets the interval Loop and tpor is equivalent to the start up delay tstapr see Equation 1 A capacitor connected to CFILTER is used to set the timer which activates the circuit breaker during overcurrent condi tions When the voltage across the sense resistor exceeds the slow trip current limit threshold of 48mV the overcurrent timer begins to charge for a period Let gw determined by Goen If no capacitor is used at CFILTER then Leet ow defaults to bus If toos ow elapses then the circuit breaker is activated and the GATE output is immediately pulled to
43. ut 11 4V POR START UP DELAY 30ms Q2 is TN0201T SOT 23 Additional pins omitted for clarity Figure 6 PCB Connection Sense with ON OFF Control M0235 121903 18 January 2004 MIC2085 2086 Higher UVLO Setting Once a PCB is inserted into a backplane power supply the internal UVLO circuit of the MIC2085 86 holds the GATE output charge pump off until Vu exceeds 2 18V If VCC falls below 2V the UVLO circuit pulls the GATE output to ground and clears the overvoltage and or current limit faults For a higher UVLO threshold the circuit in Figure 7 can be used to delay the output MOSFET from switching on until the desired input voltage is achieved The circuit allows the charge pump MIC2085 3 0 1uF Undervoltage Lockout Rising 11 0V Undervoltage Lockout Falling 10 1V Undervoltage Output 11 4V POR START UP Delay 60ms Additional pins omitted for clarity Micrel R1 to remain off until Vj exceeds 1 el x1 24V The GATE drive output will be shut down when Vj falls below R1 1 el X1 14V Inthe example circuit Figure 7 the rising UVLO threshold is set at approximately 11V and the falling UVLO threshold is established as 10 1V The circuit consists of an external resistor divider at the ON pin that keeps the GATE output charge pump off until the voltage at the ON pin exceeds its threshold Voy and after the start up timer elapses Q1 IRF7822 SO 8 Vour 12V 4A Downstream Signal Figure 7
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