In many lighting applications, power drivers that generate modulated current pulses, from high-current LEDs in DLP projectors to high-power laser diodes, etc., are used. For example, in high-end video projectors, high-power LEDs are used to produce colored illumination. The RGB LEDs in these projectors require precise dimming control for accurate color mixing—in this case, in addition to simple PWM dimming, more control is available. In general, to achieve the wide dynamic range required for color mixing, the LED driver must be able to quickly switch between two completely different modulated peak current states and superimpose PWM dimming without any damage. The LT3743 meets 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 inductor current through a sense resistor in series with the inductor. The LT3743 is capable of regulating the current in any load with an accuracy of ±6% over an output voltage range of 0V to "2V below the input rail."
Accurate, wide-range LED current control is achieved by combining accurate analog dimming (high-luminosity and low-luminance states) with PWM dimming. Analog dimming is controlled by the CTRL_L, CTRL_H, and CTRL_T pins; PWM dimming is controlled by the PWM and CTRL_SEL pins. By using a unique approach to externally switching capacitors, the LT3743 enables fast transitions between high and low analog states to change the regulated LED current level in a few μs. The switching frequency can be set from 200kHz to 1MHz (by using an external resistor) and synchronized to an external clock with a frequency range of 300kHz to 1MHz.
Switching output capacitor topology
In a conventional current regulator, the voltage across the load is stored in an output capacitor. If the load current changes abruptly, the voltage in the output capacitor must be charged or discharged to match the new regulated current. During the conversion, 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 that is responsible for supplying the 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 enables each capacitor to store a load voltage drop corresponding to the desired regulated current.
Figure 1 shows the basic topology with 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 full scale range of 0V to 1.5V, producing a regulated voltage of 0mV to 50mV across the current sense resistor.
Figure 1: Basic Switched Capacitor Topology
Figure 2 shows the timing waveforms for the various states of the PWM and CTRL_SEL pins. 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 a switched output capacitor, it can easily accommodate any traditional analog and/or PWM dimming scheme.
Switch 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 cycle or non-periodic PWM dimming pulse width and duty cycle. For high current LED drivers, this is an essential feature in the recovery from a zero current or low current state to a high current state. By restarting the clock when CTRL_SEL or the PWM signal goes high, the inductor current will immediately begin to ramp up without waiting for a rising edge of the clock. When synchronization is not used, the phase relationship between the clock edge and the PWM pulse edge will be uncontrolled, which may cause significant jitter in the LED light output. When using an external clock with a SYNC pin, the switching cycle is resynchronized to the external clock within 8 switching cycles.
A 24V, 20A LED driver for high-end DLP projectors with switched output capacitors. High-end DLP projectors require extremely high quality images and color reproduction. In order to achieve high color accuracy, the color deviation in each LED is corrected by mixing the colors of the other two colored LEDs. For example, when the red LED is in full current conduction, the blue and green LEDs will be turned on at a low current level 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 edge. Figure 3 shows a 24V/20A LED driver for high-end DLP projectors.
Figure 3: 24V/20A LED Driver with Switched Output Capacitor
The lower switching frequency of 450kHz allows the use of a very small 1.0μH inductor. At 25% ripple current, the transition time between the high current state and the low current state is approximately 2 μs. The large 1mF output capacitor stores the voltage drop across the LED in two different current states and provides the instantaneous current when the MOSFET dimmer switch is turned on. For fast LED current conversion, it is critical to use several low-ESR capacitors in parallel.
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