Detailed explanation of new dimming solution based on LED driver

Power drivers that generate high modulated current pulses are used in many lighting applications, from high current LEDs in DLP projectors to high power laser diodes. For example, in high-end video projectors, high power LEDs are used to generate colored lighting. The RGB LEDs in these projectors require precise dimming control for accurate color mixing – where more control is available than simple PWM dimming. Typically, to achieve the wide dynamic range required in color mixing, the LED driver must be able to quickly switch between two completely different modulated peak current states and superimpose PWM dimming without causing any damage. The LT3743 is able to meet these demanding accuracy and speed requirements.

The LT3743 is a synchronous step-down DC/DC controller that uses fixed frequency, average current mode control to accurately regulate the inductor current through a sense resistor in series with the inductor. The LT3743 can regulate the current in any load with ±6% accuracy over an output voltage range of 0V to 2V below the input rail.

Precise, wide-range LED current control is achieved by combining accurate analog dimming (high and low light states) with PWM dimming. Analog dimming is controlled via the CTRL_L, CTRL_H and CTRL_T pins; PWM dimming is controlled via the PWM and CTRL_SEL pins. The LT3743 achieves fast transitions between high and low analog states by using a unique approach of externally switched load capacitors, allowing the regulated LED current level to be changed in a few μs. The switching frequency can be set from 200kHz to 1MHz (via an external resistor) and synchronized to an external clock from 300kHz to 1MHz.

Switched Output Capacitor Topology

In a traditional current regulator, the voltage across the load is stored in the output capacitor. If the load current changes suddenly, the voltage in the output capacitor must be charged or discharged to match the new regulated current. During the transition, the current in the load is not well controlled, resulting in a slow load current response time.

The LT3743 solves this problem by using a unique switched output capacitor topology that achieves ultra-fast load current rise and fall times. The basic concept behind this topology is that the LT3743 acts as a regulated current source, providing drive current to the load. For a given current, the voltage drop across the load is stored in the first switched output capacitor. When a different regulated current state is required, the first output capacitor is turned off and the second capacitor is turned on. This allows each capacitor to store the load voltage drop corresponding to the desired regulated current.

Figure 1 shows the basic topology with the various control pins. The PWM and CTRL_SEL pins are digital control pins that determine the state of the regulated current. The CTRL_H and CTRL_L pins are analog inputs with a 0V to 1.5V full-scale range that produce a regulated voltage of 0mV to 50mV across the current sense resistor.

Figure 1: Basic switched capacitor topology

The timing waveforms corresponding to various states of the PWM and CTRL_SEL pins are shown in Figure 2. When PWM is low, all switching operations are terminated and both output capacitors are disconnected from the load.

Figure 2: LED current PWM and CTRL_SEL dimming

Although the LT3743 can be configured with switched output capacitors, it can easily adapt to any conventional analog and/or PWM dimming scheme.

Switching cycle synchronization

The LT3743 synchronizes all switching pulse edges to the PWM and CTRL_SEL rising edges. Synchronization gives the system designer the freedom to use any periodic or non-periodic PWM dimming pulse width and duty cycle. This is an essential feature for high current LED drivers in recovering from a zero or low current state to a high current state. By restarting the clock when the CTRL_SEL or PWM signal goes high, the inductor current will immediately begin to ramp up without waiting for a clock rising edge. Without synchronization, the phase relationship between the clock pulse edge and the PWM pulse edge will be uncontrolled, which may cause significant jitter in the LED light output. When an external clock with a SYNC pin is used, the switching period will be resynchronized to the external clock within 8 switching cycles.

A 24V, 20A LED driver with switched output capacitors for high-end DLP projectors. High-end DLP projectors require extremely high quality image and color reproduction. To achieve high color accuracy, color deviations in individual LEDs are corrected by mixing in the colors of the other two color LEDs. For example: when the red LED is conducting at full current, the blue and green LEDs are turned on at low current levels so that they can be mixed in to produce accurate red light. This approach requires the ability to quickly switch between lower (about 2A) and higher (about 20A) LED currents to maintain the PWM dimming pulse edges. Figure 3 shows a 24V/20A LED driver designed for high-end DLP projectors.

Figure 3: 24V/20A LED driver using switched output capacitors

The lower switching frequency of 450kHz allows the use of a very small 1.0μH inductor. The transition time between the high and low current states is approximately 2μs at 25% ripple current. The large 1mF output capacitor stores the voltage drop across the LEDs at the two different current states and provides the instantaneous current when the MOSFET dimming switch turns on. Using several low ESR capacitors in parallel is critical to achieve fast LED current transitions.