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.
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.
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-to-negative (buck-boost) topologies where 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.
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.
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.
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 VEE
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 LED sense resistor and PWM dimming MOSFET are both located at the bottom of the LED string, the level-shifted PWMOUT signal from the ground-referred PWM input yields a topology that looks no more complicated than a traditional 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
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.
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.
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.
* Boost mode is a patent pending technology