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For High Power LEDs, SolarCells, and BatteriesThe best LED drivers accurately regulate LED current for consistent color reproduction and modulate itrapidly for high contrast dimming. They also recognize and survive short and open circuits, monitorand report current levels, guard against overheating, and protect weak power supplies from excessiveload currents. A standard switching converter would require a number of additional expensiveamplifiers, references and passive components to fulfill these responsibilities. In contrast, the LT3763LED driver-controller has these functions built in. Luke Milner, Design Engineer, LinearTechnology Corp., Milpitas, USA

The LT3763 is more than just a highperformance LED driver. Its rich feature setsimplifies the design of other demandingapplications, such as safe charging of asealed lead-acid batteries, or maximumpower point regulation for a solar panel, ora combination of both. The LT3763performs these tasks with high efficiency,even at input voltages reaching 60 V.

Driving LEDsFigure 1 shows the LT3763 configured as ahigh power LED driver. A potentiometer atthe CTRL1 pin permits manual adjustmentof the regulated LED current from 0 to 20A. For thermal regulation of the LEDcurrent, a resistor with a negativetemperature coefficient is mounted nearthe LED and connected from the CTRL2

pin to GND. The resistor network at the EN/UVLO

pin programs the LT3763 to shut down ifthe input voltage falls to less than 10V. Theresistor network at the FB pin defines anopen-circuit condition as when the outputreaches 6V, and should that ever happen,the LT3763 automatically reduces theinductor current to prevent overshoot andpulls down the /FAULT pin to mark theoccasion.The LT3763 is designed to provide

flicker-free LED dimming as shown inFigure 2. This is achieved by pullingPWMOUT low whenever PWM is low andthereby disconnecting the LED, by similarlydisconnecting the compensation networkat VC, and resynchronizing internalswitching clocks to the PWM pulse. These

maneuvers ensure that subsequent pulsesare identical, that the inductor current risesas fast as possible to satisfy the

LEFT Figure 1: Asingle high powerLED (20 A) driverwith analog andPWM dimming

ABOVE Figure 2: PWM dimming performance ofthe circuit in Figure 1

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programmed LED current level, and thatthe LED light never flickers.The LT3763 can be configured as in

Figure 3 to deliver 350 W with 98 %efficiency from a 48 V input. An internalregulator supplies the drivers of the TG andBG pins with enough power for each todrive two of the external NMOS powerswitches. Higher power applications can bebuilt by connecting LT3763s in parallel, sothat current is shared equally between thetwo controllers. This configuration alsoillustrates how the SYNC pin can be usedto synchronize the parallel connectedLT3763s to an external clock.

The high output voltage rating of theLT3763 enables 35 V at the output withthe simplicity of a standard buck converter.The output voltage can be as high as 1.5 Vless than input voltage, and theconfiguration in Figure 4 makes use of thisfeature to charge three sealed lead-acidbatteries in series (up to 45 V) from a 48V supply.

Charging batteriesThe battery charger shown in Figure 4, likeall chargers, must be able to preciselyregulate the batteries’ rated chargingcurrent (constant current mode) until the

battery voltages reach the limit set by theirchemistry. The charger must maintain thatvoltage (constant voltage mode) withoutovershoot until the current drawn by thetrickle-charging batteries becomes verysmall. Once the trickle charge phase iscomplete, the charger should allow thebatteries’ voltages to decay to a relaxedlevel before finally settling at and holdingthat final voltage indefinitely. The combined current and voltage

regulation loops on the LT3763, and itsLED fault handling circuitry, nearly make ita complete battery charger. Only a singleadditional transistor is required to form a

Figure 3: 350 Wwhite LED driver

Figure 4: 3. 3A, six-cell (36 V) SLAbattery charger

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complete battery charging system. The resistor divider at the FB pin has

been designed to program the chargingvoltage to 45 V. As in the case of an open-circuit, when the voltage reaches 45 V, theLT3763 automatically reduces the currentto prevent overshoot as shown in Figure 5. Subsequently, during trickle charging,

the battery draws less current over time.When the charging current reduces to 10% of the regulated current (C/10 batteryspecification), the LT3763’s open-circuitfault condition is triggered. The resultinghigh-to-low transition at the /FAULT pin isused to turn off the gate of the addedtransistor M3 and remove the resistorRFB3 from the feedback network. Theprogrammed output voltage is therebylowered, and the LT3763 stops switchingto allow the batteries to relax on their own. When their combined voltage decays to

the newly programmed value, the LT3763begins switching again and provides a

sustaining current necessary to maintainthe output voltage indefinitely. As anadded benefit, the /FAULT pin transitionserves as a signal that the trickle charginghas begun.

Regulating solar panelsA well-designed solar panel power supplyrequires an intelligent combination ofcurrent and voltage regulation. In anoptimum design, a converter must sensethe voltage on the panel and adjust thecurrent it draws to maintain the inputvoltage at the panel’s maximum powerpoint. If it draws too much current, thevoltage of the high impedance panel willcollapse. If it draws too little current,available light energy is essentiallywasted. In many common solutions, a solar

panel controller designer would use anamplifier to sense the input voltage andadjust the voltage on the current controlpin. The LT3763 includes this function atthe FBIN pin. Simply tie CTRL1 high, tothe 2V reference available at VREF, and adda voltage divider from VIN to FBIN. Whenthe voltage at FBIN falls to nearly 1.205 V,the internal amplifier automaticallyoverrides the CTRL1 voltage and reducesthe load current. This regulates the inputvoltage (the voltage of the solar panel) atthe maximum power point for the panel.The resistor divider on the FBIN pin isshown in Figure 6 and can be customizedto fit the requirements of any solar panel. In the configuration shown in Figure 6,

the converter can generate whatever

inductor current, up to 5 A, is required tohold the panel voltage at 37 V. Inputvoltage feedback is via the voltage dividerat the FBIN pin, which in turn regulatesthe inductor current to what is actuallynecessary to hold the panel at peak powerin any given light condition. As shown in Figure 7, the process of

charging a battery with a solar panel looksvery similar to charging with a lowimpedance supply as before. Thedifference is that the regulated inductorcurrent (charge current) is not preset bythe designer, but is instead adjusted onthe fly via the feedback loop regulatinginput voltage. This effectively minimizescharge time, since input power ismaximized at all times, regardless of panelillumination. Since the LT3763 has the capability of

regulating input voltage and current, aswell as output voltage and current, andprovides a fault flag with C/10, it caneasily be used with a wide variety of solarpanels to charge many different types ofbatteries.

Monitoring current levelsIn each of the applications presented here,the LT3763 provides an additional serviceby monitoring the input and output currentlevels. Voltages across the IVINP and IVINNpins ranging from 0 to 50 mV areamplified with a gain of 20, and theresulting voltage appears at the IVINMONpin. The voltage at the ISMON pin is anidentical amplification of the voltage acrossthe SENSE+ and SENSE– pins, as shown

Figure 5: 36 V SLA battery charging cycle

Figure 6: 70 W solar energy harvester with maximum power point regulation

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in Figure 8. These signals are helpful in systems that

must verify the current provided to LEDs ormeasure the efficiency of voltageconversion. They can also help to estimatethe power provided by a solar panel or tomonitor the current trickling into a chargingbattery as it decays to zero.

Due to the discontinuous input currentof a step-down buck converter, a low-passfilter is typically necessary at the IVINP andIVINN pins as shown in Figure 1 and Figure4. A much smaller filter at the SENSE+ andSENSE– pins may also be useful in filteringhigh frequency noise, but it is not

necessary. Even with these filters, themonitors are fast enough to trackreasonably short PWM pulses as shown inFigure 8. Nevertheless, if a designer is moreconcerned with average current levels thaninstantaneous current levels, thenadditional low-pass filters can be easilyadded to the ISMON and IVINMON pins.

ConclusionThe LT3763 is a versatile step-down buckconverter that integrates many complexfeatures essential for not only LED drivers,but solar harvesters and battery chargers aswell. A PWM driver and current monitorsare included with fault detection, currentlimiting, input and output voltageregulation. Due to its high voltage rating, allof these features can be utilized toilluminate long strings of LEDs or chargestacks of batteries. Available in a 28-leadTSSOP package, the LT3763 is a compact,complete, and efficient power system.

Figure 7: Solar powered SLA battery charging Figure 8: Current monitor outputs in an LEDdriver application with PWM dimming

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