TSC TS3404 handbook
Contents
1. L x Vout Vin AVout Al x ESR Increasing the value of inductance reduces the ripple current and voltage However the large inductance values reduce the converter s response time to a load transient One of the parameters limiting the converter s response to a load transient is the time required to change the inductor current Given a sufficiently fast control loop design the TS3405 will provide either 0 or 100 duty cycle in response to a load transient The response time is the time required to slew the inductor current from an initial current value to the transient current level During this interval the difference between the inductor current and the transient current level must be supplied by the output capacitor to minimizing the response time can minimize the output capacitance required The response time to a transient is different for the application of load and the removal of load The following equations give the approximate response time interval for application and removal of a transient load trise L x lran Vin Vout teat L x IrRan Vout where Itran is the transient load current step trise is the response time to the application of load tract is the response time to the removal of load the worst case response time can be either at the equations at the minimum and maximum output levels for the worst case response time Feedback Compensation Fig 6 highlights the voltage mode control loop for a synchr2. The entire startup sequence typically takes about 5mS Current Sinking The TS3405 incorporates a MOSFET shoot through protection method which allows a converter to sink current as well as source current Care should be exercised when designing a converter with the TS3405 when it is known that the converter may sink current When the converter is sinking current it is behaving as a boost converter that is regulating its input voltage This means that the converter is boosting current into the buck converter input power if the buck converter input power has the same supply source which supplies the bias voltage Vcc to the TS3405 if there is nowhere for this current to go such as to other distributed loads on the Vcc rail through a voltage limiting protection device or other methods the capacitance on the Vcc bus will absorb the current This situation will allow voltage level of the Vcc rail to increase If the voltage level of the rail is boosted to a level that exceeds the maximum voltage rating of the TS3405 then the IC will experience an irreversible failure and the converter will no longer be operational Ensuring that there is a path for the current to follow other than the capacitance on the rail will prevent this failure mode TS3405 6 10 2003 12 rev A Application Guidelines Component Selection Input Capacitor Use a mik of input bypass capacitors to control the voltage overshoot across the MOSFETs Use small ceramic capaci
3. You can use a small single transistor as switch like as JP1 shown in Fig 4 Compensation Break Frequency Equations As in any high frequency switching converter layout is very important Switching current from one power device to another can generate voltage transients across the impedances of the interconnecting bond wires and circuit traces Using wide short printed circuit traces should minimize these interconnecting impedances The critical components should be located as close together as possible using ground plane construction or single point grounding To minimize the voltage overshoot the interconnecting wires indicated by heavy lines should be part of a ground or power plane in a printed circuit board Locate the TS3405 within 3 inches of the MOSFETs Q1 and Q2 The circuit traces for the MOSFETs gate and source connections from the TS3405 must be sized to handle up to 1A peak current Provide local Vcc decoupling between Vcc and Gnd pins Locate the capacitor Cgoot as close as practical to the Boot and Phase pins All components used for feedback compensation should be located as close to the IC a practical 2003 12 rev A TSC SOP 8 Mechanical Drawing SOP 8 DIMENSION i MILLIMETERS INCHES i MIN MAX MIN MAX 4 80 5 00 0 189 0 196 B P 3 80 4 00 0 150 0 157 1 35 1 75 0 054 0 068 1 8 i 0 35 0 49 0 014 0 019 0 40 1 25 0 016 0 049 1 27 typ 0 0
4. protection circuitry to determine when the upper MOSFET can be turned on The sourcing Rds on is 150 and the sink Rds on is 70 Ugate can handle high voltage up to maximum 20V o ot e mekan island plane through the lowest impedance connection available This pin provides the PWM controlled gate drive for the lower MOSFET This pin is also monitored by the adaptive shoot through protection circuitry to determine when the lower MOSFET can be turned on Connect a well decoupled 5V supply to this pin This pin is the inverting input of the internal error amplifier Use this pin in combination with the COMP Pin to compensate the voltage control feedback loop of the converter 7 COMP During soft start and all the time during normal converter operation this pin represents the output of the error amplifier Use this pin in combination with the FB pin to compensate the voltage control feedback loop of the converter Pulling COMP to a level below 0 3V enables soft start process The whole soft start process takes about 5mS This pin is used to monitor the voltage drop across the lower MOSFET for over current protection The OCP threshold is 30mV If Phases is less thean 300mV the upper MOSFET cannot be turned on in the next cycle Block Diagram TS3405 2 10 2003 12 rev A Electrical Characteristics lout OmA and Tj 25 C unless otherwise specified Parameter Symbol _ Test Conditions min Typ Max Unit Vcc supply cur
5. 5 typ 0 10 0 25 0 004 0 009 0 7 0 7 5 80 6 20 0 229 0 244 0 25 0 50 0 010 0 019 A B C D F G K M P R TS3405 10 10 2003 12 rev A WWW ALLDATASHEET COM Copyright Each Manufacturing Company All Datasheets cannot be modified without permission This datasheet has been download from www AllDataSheet com 100 Free DataSheet Search Site Free Download No Register Fast Search System www AllDataSheet com
6. OO TSA 00 TS3404 Single Synchronous Buck PWM Controller Pin assignment Oscillator Frequency up to 300KHz 1 Boot 8 Phase 2 Ugate 7 COMP 0 8V Internal Reference 3 Gnd 6 FB Drives N Channel MOSFETs 4 Lgate 5 Vcc General Description The TS3405 makes simple work out of implementing a complete control and protection scheme for a DC DC step down converter Designed to drive N channel MOSFETs in a synchronous buck topology the TS3405 integrates the control output a adjustment monitoring and protection functions The TS3405 provides simple single feedback loop voltage mode control with fast transient response The output voltage can be precisely regulated to as low as 0 8V with a maximum tolerance of 1 5 over temperature and line voltage variations A fixed frequency oscillator reduces design complexity while balancing typical application cost and efficiency The error amplifier features a 15MHz gain bandwidth product and 8V uS slew rate which enables high converter bandwidth for fast transient performance The resulting PWM duty cycles range from 0 to 100 The protection from over current conditions is provided by monitoring the Rds on of the lower MOSFET to inhibit PWM operation appropriately This approach simplifies the implementation and improves efficiency by eliminating the need for a current sense resistor Features Applications Buck converter Vin operate from 3 3V 14V Vcc operate from 3 75V 6V Buck converter Vi
7. esign factors The power dissipation includes two loss components conduction loss and switching loss The conduction losses are the largest component of power dissipation for both the upper and the lower MOSFETs These losses are distributed between the two MOSFETs according to duty factor The switching losses seen when sourcing current will be different from the switching losses seen when sinking current When sourcing current the upper MOSFET realizes most of the switching losses The lower switch realizes most of the switching losses when the converter is sinking current see the equations below These equations assume linear voltage current transitions and do not adequately model power loss due TS3405 7 10 the reverse recovery of the upper and lower MOSFET s body diode The gate charge losses are dissipated by the TS3405 and do not heat the MOSFETs However large gate charge increases the switching interval tsy which increases the MOSFET switching losses Ensure that both MOSFETs are within their maximum junction temperature at high ambient temperature by calculating tempature rise according to package thermal resistance specifications a separate heatsink may be necessary depending upon MOSFET power package type ambient temperature and air flow Losses while sourcing current Pupper lo x Rds on x D lo x Vin x tsw x Fs PL ower lo x Rds on x 1 D Losses while sinking current Pupper lo x Rds on x D PLoweR
8. lo x Rds on x 1 D lo x Vin x tsw x Fs Where is the duty cycle Vout Vin tsw is the combined switch ON and OFF time Fs is the switching frequency Given the reduced available gate bias voltage 5V logic level or sub logic level transistors should be used for both N MOSFETs Caution should be exercised with devices exhibiting very low Vgs on characteristics The shoot through protection present aboard the TS3405 may be circumvented by there MOSFETs if they have large parasitic impedances and or capacitances that would inhibit the gate of the MOSFET from being discharged below it s threshold level before the complementary MOSFET is turned on 5V Dboot VD FIGURE 5 Upper Gate drive bootstrap Fig 5 shows the upper gate drive Boot pin supplied by a bootstrap circuit from Vcc The boot capacitor Cgoor develops a floating supply voltage referenced to the Phase pin The supply is refreshed to a voltage of Vcc less the boot diode drop Vpp each time the lower MOSFET turns on 2003 12 rev A Application Guidelines continued Output Inductor The output inductor is selected to meet the output voltage ripple requirements and minimize the converter s response time to the load transient The inductor value determines the converter s ripple current and the ripple voltage is a function of the ripple current The ripple voltage and current are approximated by the following equations Al Vin Vout Fs x
9. load as physically possible Be careful not to add inductance in the circuit board wiring that could cancel the usefulness of these low inductance components Consult with the manufacturer of the load on_ specific decoupling requirements Use only specialized low ESR capacitors intended for switching regulator applications for the bulk capacitors The bulk capacitor s ESR will determine the output ripple voltage and the initial voltage drop after a high slew rate transient An aluminum electrolytic capacitor s ESR value is related to the case size with lower ESR available in larger case sizes However the equivalent Series inductance ESL of these capacitors increases with case size and can reduce the usefulness of the capacitor to high slew rate transient loading Unfortunately ESL is not a specified parameter Work with your capacitor supplier and measure the capacitors impedance with frequency to select a suitable component capacitance TS3405 9 10 In most cases multiple electrolytic capacitors of small case size perform better than a single large case capacitor Feedback Divider The reference of TS3405 is 0 8V the output voltage can be set by R and R as shown in Fig 4 The equation is following Vout 0 8 x 1 Ry R4 The R1 should be between 2kQ to 5kQ put the R4 R4 and others compensation component as close to TS3405 as possible Shutdown Pulling low the COMP pin can shutdown the TS3405 PWM controller
10. n can be greater than Vcc Power supplies for microprocessors Subsystem power supplies Cable modems set top box DSL modems 0 8V to Vin output voltage DSP and core communications processor supplies 1 5 over line voltage and temperature Memory supplies Simple single Joop control design Personal computer peripherals Voltage mode PWM control Industrial power supplies gt gt gt gt gt gt gt gt TEREE Loss less programmable over current protection uses lower MOSFET s Rds on Ordering Information Internal soft start Low voltage distributed power supplies Converter can source and sink current Part No Operating Temp Package Fixed oscillator frequency 300KHz TS3405CS 40 85 C SOP 8 vec 45 55 20 20 vec o e O V VooorVewse 20 ia Gnd 0 3V to Vcc 0 3V Ooo uw mns O On ee Lead Temperature 1 6mm 1 16 from case for 10Sec CC CC Ty Ty TS3405 1 10 2003 12 rev A TSC Pin Descriptions nof Pin Descriptions O S SOS 1 Boot This pin provides ground referenced bias voltage to the upper MOSFET driver A bootstrap circuit is used to create a voltage suitable to drive a logic lever N channel MOSFET It can take 20V as the maximum voltage It can be powered by a DC power supply or powered by a boost strap circuit 2 This pin provides the PWM controlled gate driver for the upper MOSFET It is also monitored by the adaptive shoot through
11. onous rectified buck converter The output voltage Vout is regulated to the reference voltage level The error amplifier Error Amp output VE A is compared with the oscillator OSC triangular wave to provide a pulse width modulated PWM wave with a amplitude of Vin at the Phase node The PWM wave is smoothed by the output filter Lo and Co TS3405 8 10 The modulator transfer function is the small signal transfer function of Vout Vea This function is dominated by a DC Gain and the output filter Lo and Co with a double pole break frequency at Fic and a zero at Fesr The DC Gain of the modulator is simply the imput voltage Vin divided by the peak to peak oscillator voltage Vosc Modulator Break Frequency Equations Fic 1 2mxVLoxCo Fesr 1 217 x ESR x Co Compensation Break Frequency Equations Fz 1 21x R2x C Fp 1 2T x R2 x C4 x C2 C1 C2 Fz 1 27 x Ri R3 x C Fp2 1 2T x R x C The compensation network consists of the error amplifier internal to the TS3405 and the impedance networks ZIN and ZFB The goal of the compensation network is to provide a closed loop transfer function with the highest OdB crossing frequency fOdB and adequate phase margin Phase margin is the difference between the closed loop phase at fOdB and 180 degrees Vin Driver i Lout Vout Referenc FIGURE 6 Voltage mode buck converter compensation design 2003 12 rev A Ap
12. plication Guidelines continued The eguations below relate the compensation network s poles zeros and gain to the components R4 R2 R3 C4 Cz and C3 in Fig 7 Use these guidelines for locating the poles and zeros of the compensation network Pick Gain R2 R for desired converter bandwidth Place 1 zero below filter s double pole 75 Fc Place 2 zero at filter s double pole Place 1 pole at the ESR zero Place 2 pole at half the switching frequency Check gain against error amplifier s open loop gain Estimate phase margin repeat if necessary Output Capacitor Selection An output capacitor is required to filter the output and supply the load transient current The filtering requirements are a function of the switching frequency and the ripple current The load transient requirements are a function of the slew rate di dt and the magnitude of the transient load current These requirements are generally met with a mix of capacitors and careful layout Modern components and loads are capable of producing transient load rates above 1A nS High frequency capacitors initially supply the transient and slow the current load rate seen by the bulk capacitors The bulk filter capacitor values are generally determined by the ESR Effective Series Resistance and voltage rating requirements rather than actual requirements High frequency decoupling capacitors should be placed as close to the power pins of the
13. rent Nominalsupply we s m Power on reset threshold Vcc power on reset threshold Hysteresis Rampampitude awos CT Ct v Reference Reference voltage Tolerance Error Amplifier pecan ce ew Gain bandwidth product eswe o Ju m Slew rate sR comes 46 so 92 vis Gate Drivers Vvec SV lLeate 100mA Protection Disable OCP threshold Voce West 5V sweep Phase 30 mw Disable threshold Vosa Sweepcomp os v pa TS3405 za 3 O Single Power 5V Application TS3405 3 10 2003 12 rev A Typical Application continued Vcc_5V Vcc 12V Cbulk CHF ert TS3405 4 10 2003 12 rev A H3 Vin_Gun Boot TS3405 Vgate pi COMP Phase Lgate Maximum load current Component Reference design 5A 10A 15A MOSFET Q10 Q2 Rds on 30MQ Rds on 20mQ Rds on 10mQ Inductor L1 5uF 3uF 1 6uF No of input capacitor C130 C14 1 1 2 C8 c90 C10 1 2 3 No of output capacitor No of decoupling C5A C5B capacitor Reference design capacitor 1500uF ESR 33 Reference design decoupling capacitor 10uF MLCC TS3405 2003 12 rev A Functional Description Start Up The TS3405 automatically initializes upon receipt of power The Power On Reset POR function continually monitors the bia
14. s voltage at the Vcc pin The POR function initiates the Soft Start SS operation after the supply voltage exceeds its POR threshold Over Current Protection OCP The over current function protects the converter from a shorted output by using the lower MOSFET s on resistance Rds on to monitor the current Therefore even the power input voltage is greater than Vcc TS3405 still can support this This method enhances the converter s efficiency and reduces cost by eliminating a current sensing resistor The TS3405 s OCP threshold is a fixed value 300mV when Phase voltage is less 300mV the next on cycle will not be initialized Over Voltage Protection OVP An over voltage protection comparator is monitoring the COMP When COMP voltage is less than 0 3V the Soft Start SS process is initiated Soft Start SS Both POR and OVP initiate the soft start sequence after the over current set point has been sampled Soft Start clamps virtually the error amplifier output COMP pin and reference input non inverting terminal of the error amp to the internally generated Soft Start voltage The oscillator s triangular waveform is compared to the ramping error amplifier voltage This generates Phase pulses of increasing width that charge the output capacitor s when the internally generated Soft Start voltage exceeds the COMP pin voltage the output voltage is in regulation This method provides a rapid and controlled output voltage rise
15. tors for high freguency decoupling and bulk capacitors to supply the current needed each time Q1 turn on Place the small ceramic capacitors physically close to the MOSFETs and between the drain of high side MOSFET Q1 and the source of low side MOSFET Q2 The important parameters for the bulk input capacitor are the voltage rating and the RMS current rating For reliable operation select the bulk capacitor with voltage and current rating above the maximum input voltage and largest RMS current required by the circuit The capacitor voltage rating should be at least 1 25 times greater than the maximum input voltage and a voltage rating of 1 5 times is a conservative guideline The RMS current rating requirement for the input capacitor of a buck regulator is approximately 1 2 the DC load current For a through hole design several electrolytic capacitors may be needed For surface mount designs solid tantalum capacitors can be used but caution must be exercised with regard to the capacitor surge current rating These capacitors must be capable of handling the surge current at power up Some capacitor series available from reputable manufacturers are surge current tested MOSFET The TS3405 requires 2 N channel power MOSFETs These should be selected based upon Rds on gate supply requirements and thermal management requirements In high current applications the MOSFET power dissipation package selection and heatsink are the dominant d
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TSC TS3404 handbook