Can’t Find the Right Synchronous Boost LED Driver? Use a Synchronous Buck Converter Instead: Boost Mode Topology Drives 25V, 3A LEDs from 12V Keith Szolusha Synchronous buck converter drivers are commonly used when the current required for high power LEDs, such as 10A–40A projector LEDs, would overstress the components in a nonsynchronous converter. Synchronous rectification limits power losses and thermal rise due to high current in the converter switches. Synchronous rectification can offer the same benefits in high power step-up (boost) LED drivers—even with 1A to 3A LEDs. In contrast to a buck converter, the peak switch current of a boost can be much higher than the LED current, especially when output power is high and the input voltage is low. For instance, take the LT3744 40V synchronous buck LED driver, which is designed to drive high current LEDs for projectors. It features a versatile, floating VEE output that allows it to be used in both high current buck applications and positive-tonegative (buck-boost) topologies where There are a number of situations where a synchronous boost LED driver is not available for a particular application. For some of these cases, a synchronous buck LED driver IC can be used, but instead of operating as a step-down converter, it operates as a step-up, or boost mode* LED driver. Figure 1. Boost mode 9V–16V input to 25V, 3A LED driver with 98% efficiency VIN 9V TO 16V VIN SGND 200k TG EN/UVLO EN/UVLO 1µF 25V M1 220nF D1 INTVCC CTRLT VREF 2V 2.2nF 100k 10k 2V 0V VEE VFNEG ISP ISN L1 4.7µH 10µF 5mΩ VEE VC2 VC3 20k VEE 680pF VEE 102k VEE 22 | April 2015 : LT Journal of Analog Innovation LT3744 BOOST MODE LED DRIVER The LT3744 synchronous boost mode LED driver shown in Figure 1 regulates a 3A, 25V (75W) LED string from an automotive input (9V–16V) at 98% efficiency. Even at this power level, the maximum component temperature rise is 45ºC with a 12V input, as shown in Figure 2. The IC enables easy implementation of both 10:1 analog and 100:1 PWM dimming at 120Hz with ground-referred input signals, even though neither the LED string nor the PWM dimming MOSFET are connected to GND. Although the 5mΩ sense resistor sets a 10A peak switch current in this application, the solution can be altered to operate with a 6V input and a 15A peak switch current; with an appropriately valued inductor and a lowered undervoltage lockout. 1M 43.2k VEE RT 4× 10µF 50V M2 PWM1 FB PWM2 PWM3 SYNC PWM_OUT2 PWM_OUT3 PWM_OUT1 SS LED_ISP VC1 10nF VEE BG FAULT CTRL1 CTRL2 CTRL3 30.1k VEE LT3744 SW 25V 3A LED 2× 10µF 25V BOOST 30.9k the anode of the high current LED is connected to ground to satisfy heat-sinking requirements. It is the floating VEE feature that allows us to effectively use this part, originally designed for buck applications, as a synchronous boost mode LED driver. M3 20mΩ LED_ISN M1, M2: INFINEON BSC026N04LS M3: INFINEON BSC010N04LS D1: PMEG4002EB D2: PMEG4010 L1: WÜRTH 7443551470 4.7µH VEE D2 The negative VEE rail on the LT3744 can reach −21V. The LT3744 simplifies design by handling the level-shifting of ground referenced input control signals. design ideas Synchronous rectification is a feature commonly called on to limit power losses in high power LED drivers. As shown, a synchronous buck LED driver can be used as a boost mode converter, capitalizing on the wide availability of this feature in high performance buck regulator ICs. A simple ground referenced PWM input signal is level shifted to PWMOUT, so no additional level shifting circuitry is required to control the PWM MOSFET. Likewise, the LED current-setting sense resistor can be tied directly to the negative VEE rail, all the way down to −21V. LEVEL-SHIFTED GND AND V EE The level-shifting, positive-to-negative conversion, feature of the LT3744 LED driver is designed to support high current grounded-anode LED drivers. Nevertheless, the same feature can be used for boost mode applications where the LED string is connected between VIN and a negative VEE potential. Because the Figure 3. Boost mode current ripple, duty cycle and voltage stress are the same as those of traditional boost regulators. VIN = 12V VLED = 23.45V ILED = 3A NO FORCED AIR Figure 2. Boost mode LED thermal scan shows cool operation. LED sense resistor and PWM dimming MOSFET are both located at the bottom of the LED string, the level-shifted no more complicated than a traditional S2 L1 L1 VOUT VIN CIN PWMOUT signal from the ground-referred PWM input yields a topology that looks S1 VIN = LED+ VIN COUT + CIN COUT S2 S1 VOUT = VLED – VEE (NEGATIVE) = LED– CONTINUOUS MODE CURRENT VOLTAGE VOUT S1 VIN 0 VOUT D1 L1 0 VIN 0 VIN – VOUT VOUT COUT VIN CONTINUOUS MODE IIN 0 VOUT S1 0 IOUT 0 D1 IOUT 0 L1 0 IOUT 0 CURRENT VOLTAGE IOUT 0 VIN 0 VOUT VIN 0 IOUT 0 0 VIN 0 VIN – VOUT VOUT COUT IIN IOUT 0 0 IOUT 0 0 IOUT 0 April 2015 : LT Journal of Analog Innovation | 23 The LT3744 synchronous buck LED driver can be used as a high efficiency 9V–16V input, 25V LED 3A boost mode LED driver, yielding 98% efficiency for a 75W converter. The unique ability of this IC to level shift control signals as needed from SGND to –VEE makes it possible to produce a floating boost mode topology with no more components than needed in a traditional boost. boost PWM dimming setup. The input side looks like a straightforward LED driver, with CTRL analog dimming, SYNC input and enable inputs all referred to signal GND, no matter where negative VEE lies. The LT3744’s VEE can go all the way down to −21V. Open LED overvoltage protection is set at about 26.5V for the 25V VLED application. With some open LED overshoot in mind, this limits the VIN minimum to about 6V for a 25V VLED boost mode application before negative VEE goes beyond the −21V limit. To operate at the minimum 6V input, the solution in Figure 1 requires a lower undervoltage lockout and a sense resistor and inductor that can accommodate 15A peak switch current limit. With lower VLED strings (at any current level), the minimum input voltage can be dropped to 4V VIN with some simple adjustments. MORE ABOUT BOOST MODE CONCLUSION A boost mode converter has many of the same properties of traditional boost regulators. As shown in Figure 3, with the exception of the unusual topological hookup and the main control of high side switch S1 instead of a low side switch, this boost mode converter features the same duty cycle, ripple current and voltage stress that a tradition boost converter would. If synchronous rectification is not required, a nonsynchronous buck regulator can be used as a boost mode driver, with S2 replaced by a typical catch diode D1, as in a traditional boost regulator. Synchronous rectification is a feature commonly called on to limit power losses in high power LED drivers. As shown here, a synchronous buck LED driver can be used as boost mode converter, capitalizing on the wide availability of this feature in high performance buck regulator ICs. Specifically, the LT3744 synchronous buck LED driver can be used as a high efficiency 9V–16V input, 25V LED 3A boost mode LED driver, yielding 98% efficiency for a 75W converter. The unique ability of this IC to level shift control signals as needed from SGND to –VEE makes it possible to produce a floating boost mode topology with no more components than needed in a traditional boost. n The gain-phase Bode plot in Figure 4 shows that even the control loop of boost mode converter acts like a traditional boost regulator. With a 10kHz crossover frequency, 60° of phase margin, and −15dB of gain margin, the LED driver shown in Figure 1 is stable and reliable. 60 180 50 150 40 120 30 90 20 60 10 30 0 0 –10 –30 –20 –60 –30 –90 –40 –120 –50 –150 –60 24 | April 2015 : LT Journal of Analog Innovation 1 10 FREQUENCY (kHz) –180 100 PHASE (DEGREES) GAIN (dB) Figure 4. Boost mode control loop gain and phase shows typical bandwidth. Notes: * Boost mode is a patent pending technology.