LINEAR LTC3756 Manual
Contents
1. M2 VISHAY SILICONIX Si2307BDS M3 VISHAY SILICONIX Si2328DS Q1 MMBT5401 100k Figure 2 An 80V buck mode LED driver with PWM dimming for single or double LEDs 50 C discrete component temperature rise far more manageable than the potential heat produced by the 83V string of 1 54 LEDs Even if PWM dimming is not required the PWMOUT dimming MOSFET is useful for LED disconnect during shutdown This prevents cur rent from running through the string of ground connected LEDs possible under certain input conditions If an LED fails open or if the LED string is removed from the high power driver the FB constant voltage loop takes over and regulates the output at 95V until a proper string is attached between LED and LED Without overvoltage protection the LED sense resistor would see zero LED current and the control loop would work hard to increase its output Eventually the output capacitor voltage would go over 100V exceeding the maximum rating of several components While in OVP the OPENLED status flag goes low High Voltage Buck Mode LED Driver with High PWM Dimming Ratio When the input voltage is higher than the LED string voltage the LT3756 can serve equally well as a constant current buck mode converter For ex ample an automotive battery s voltage can present a wildly moving target 38 from drooping voltages to dizzyingly high voltage spikes The buck mode LED driver in Figure 2 is perfect for suc2. boost buck boost mode buck mode SEPIC and flyback topologies Its high voltage rating op timized LED driver architecture high performance PWM dimming host of protection features and accurate high side current sensing make the LT3756 a single IC choice for a variety of high voltage input and high power lighting systems 7 in heat dissipation shown in Figure 4 which shows thermal images of both circuits under equivalent operating conditions The thermal advantages of using synchronous switches are clear Conclusion The 2 phase synchronous boost topology possible with the LT3782A offers several advantages over a non synchronous or a single phase boost topology Its combination of high ef ficiency small footprint heat sink free thermal characteristics and low in put output capacitance requirements make it an easy fit in automotive and industrial applications 7 39
3. 00mA Mi 241 110k 100v x2 0 0250 3906 Vin Vin GATE SENSE LT3756 0 4UF 5 1k 10k T 4700pF M1 VISHAY SILICONIX Si7454DP D1 DIODES INC PDS3100 L1 SUMIDA CDRH127 220 28 7k 400kHz Figure 4 A buck boost mode LED driver with wide ranging V y and Vi zp current the peak inductor current is also equal to the peak switching current higher than either a buck mode or boost topology LED driver with similar specs due to the nature of the hookup The 4A peak switch current and inductor rating reflects the worst case 9V input to 50V LED string at 400mA Below 9V input the CTRL analog dimming input pin is used to scale back LT3782A continued from page 36 other out thus reducing the total output ripple by 50 which in turn reduces output capacitance require ments Theinput current ripple is also halved which reduces the required input capacitance and reduces EMI Finally the power dissipated as heat is spread out over two phases reducing the size of heat sinks or eliminating them altogether 24V at 8A from a 10V 15V Input Figure 1 shows a high power boost application that efficiently produces a 24V 8A output froma 10V 15V input The LTC4440 high side driver is used Linear Technology Magazine January 2009 the LED current to keep the inductor current under control if the battery voltage drops too low The LEDs turn offbelow 6V input due to undervoltage lockout and wi
4. DESIGN IDEAS LY 100V Controller in 3mm x 3mm QFN or MSE Drives High Power LED Strings from Just About Any Input Introduction Strings of high power solid state LEDs are replacing traditional lighting technologies in large area and high lumens light sources because of their high quality light output unmatched durability relatively low lifetime cost constant color dimming and energy ef ficiency The list of applications grows daily including LCD television back lights and projection system bulbs industrial and architectural lighting systems automotive headlamps tail lights and indicator lights computer monitors street lights billboards and even stadium lights As the number of applications ex pands so does the complexity of input requirements for the LED drivers LED drivers must be able to handle wide ranging inputs including the harsh transient voltage environment presented by automotive batteries the wide voltage range of the Li ion cells and wallwart voltages For LED lighting manufacturers and design ers applying a different LED driver for each application means stocking testing and designing with a wide variety of LED controllers This can be an expensive and time consuming proposition It would be far better to use a controller that can be applied to many solutions The LT3756 high voltage LED driver features a unique topological versatil ity that allows it to be used in boost buck boost mode buck mo
5. LED string would result in the output capacitor charging up to the input voltage Although the buck mode components will survive this scenario the LEDs may not survive being plugged back into a potential equal to the input voltage That is a single 3 5V LED might not survive being connected directly to 80V Single Inductor Buck Boost Mode LED Driver One increasingly common LED driver requirement is that the ranges of both the LED string voltage and the input voltage are wide and overlapping In fact some designers prefer to use the same LED driver circuit for several different battery sources and several different LED string types Such a versatile configuration trades some efficiency component cost and board space for design simplicity but the tradeoffs are usually mitigated by the significantly reduced time to market by producing an essentially off the shelf multipurpose LED driver The buck boost mode topology shown in Figure 4 uses a single inductor and can both step up and step down the input voltage to the LED string voltage It accepts inputs from 6V to 36V to drive 10V 50V LED strings at up to 400mA The PWM dimming and OVP are level shifted in a manner similar to the buck mode for optimal performance of these features The inductor current is the sum of the input current and the LED string Linear Technology Magazine January 2009 LED 22uH D1 Vin 9V TO 36V 6V UVLO 10V 50V iene i a eof 4
6. SE package both thermally enhanced For applications with lower input volt age requirements the 40Vyy 75Vour LT3755 LED controller is a similar option to the LT3756 Although it is typically used as an LED driver the LT3756 s voltage FB pin provides a well regulated output voltage if the constant current sense voltage is not used This is a side benefit of the LT3756 s overvoltage protection feature in which the current control loop is superceded by the FB voltage loop in the case of an open LED string thus preventing the controller from a running up the voltage in an effort to maintain current 125W Boost LED Driver for Stadium Lights or Billboards Lighting systems for stadiums spot lights and billboards require huge strings of LEDs running at high power The LT3756 controller can drive up to 100V LED strings with its floating sense resistor inputs ISP and ISN The 125W LED driver in Figure 1 accepts a wide range 40V 60V input taken from the output of a high power transformer The LT3756 s high power GATE driver switches two 100V MOSFETs at 250kHz This switching frequency minimizes the size of the discrete com ponents while maintaining high 97 efficiency thus producing a less than 37 LI DESIGN IDEAS ov 12v FOR pK T OA 1A ILep LT3756 PWMOUT M2 12 4k 51k S 1 OR 2 LEDs 3 5V 7V f 5 OA 1A i W 4 OPTIONAL D1 DIODES INC B2100 L1 SUMIDA CDRH8D38 330 M1 VISHAY SILICONIX Si4484EY
7. de SEPIC flyback and other topologies Its high power capability provides potentially hundreds of watts of steady state LED power over a very wide input voltage range Its 100V floating LED current sense inputs allow the LED string to float above ground as shown in the buck mode and buck boost mode to pologies in this article Excellent PWM dimming architecture produces high dimming ratios up to 3000 1 Linear Technology Magazine January 2009 Vin PVin 40V TO 60V ae LED CURRENT REDUCED WHEN I x2 Vin lt 40V VREF eee SHDN SHDN UVLO by Keith Szolusha PDS5100 113756 0 1uF ey TN L1 2x SERIES SLF12575T 220M4R0 Q1 2x PARALLEL Si7322DN Figure 1 A 125W 83V at 1 5A 97 efficient boost LED driver for stadium lighting A number of features protect the LEDs and surrounding components Shutdown and undervoltage lockout when combined with analog dimming derived from the input provide the standard ON OFF feature as well as a reduced LED current should the battery voltage drop to unacceptably low levels Analog dimming is accu rate and can be combined with PWM dimming for an extremely wide range of brightness control The soft start feature prevents spiking inrush cur rents during start up The OPENLED pin informs of open or missing LEDs and the SYNC LT3756 1 pin can be used to sync switching to an external clock The 16 pin IC is available in a tiny QFN 3mm x 3mm and an M
8. h harsh environments It operates with a wide 10V to 80V input range to drive one or two 3 5V LEDs 7V at 1A In this case both the Vjy pin and ISP and ISN current sense inputs can go as high as 80V PWM dimming requires a level shift from the PWMOUT pin to the high side LED string as shown in Fig ure 2 The maximum PWM dimming ratio increases with higher switch ing frequency lower PWM dimming frequency higher input voltage and lower LED power In this case a 100 1 dimming ratio is possible with a 100Hz dimming frequency a 48V input a 3 5V or 7V LED at 1A and a 150kHz switching frequency Although higher switching frequency is possible with the LT3756 the duty cycle eventually has its limits Generous minimum on time and minimum off time restric tions require a frequency on the lower end of its range 150kHz to meet both the harsh high Vjy to low VL Ep 80Vin to one 3 5V LED and low Vjy dropout requirements 10Vjy to 7V gp of this particular converter The overvoltage protection of the buck mode LED driver has a level shift as well Q1 a pnp transistor helps regulate the maximum allowable Vien 7V VLen 3 5V EFFICIENCY 0 10 20 30 40 50 60 70 80 Vin V Figure 3 Efficiency for the buck mode converter in Figure 2 output capacitor voltage to a level just beyond that ofthe LED string Without the level shifted OVP network tied to FB an open
9. ll not turn back on until the input rises above 7V to prevent flickering In buck boost mode the output voltage is the sum of the input voltage and the LED string voltage The output capacitor the catch diode and to level shift the SGATE signals and drive the synchronous MOSFETs The 250kHz switching frequency optimizes efficiency and component size board area Figure 2 shows the layout Proper routing and filtering of the sense pins placement of the power components and isolation using ground and sup ply planes ensure an almost jitter free operation even at 50 duty cycle Figure 3 shows the efficiency of the circuit in Figure 1 with synchronous MOSFETs measured to 8A and the efficiency of an equivalent non syn chronous circuit using boost diodes measured to 6A The 1 improvement in peak efficiency may not seem signifi cant but take a look at the difference DESIGN IDEAS LY Viep 50V 80 Viep 10V EFFICIENCY 10 15 20 25 30 Vin V Figure 5 Efficiency for the buck boost mode converter in Figure 4 the power MOSFET can see voltages as high as 90V for this design Conclusion The 100V LT3756 controller is osten sibly a high power LED driver but its architecture is so versatile it can be used in any number of high voltage input applications Of course it has all the features required for large and small strings of high power LEDs It can be used in
Download Pdf Manuals
Related Search
LINEAR LTC3756 Manual