Single FET controls LED array
Len Sherman, Maxim Integrated Products, Sunnyvale, CA -- EDN, April 12, 2001
White-LED backlights are gaining acceptance because they offer higher reliability and simpler drive circuitry than backlights based on CCFL (cold-cathode-fluorescent-lamp) and EL (electroluminescent) technology. As a result, white-LED backlights are increasingly common in PDAs (personal digital assistants), cell phones, digital cameras, and other portable devices. A design in which the display requires backlighting for extended periods needs an efficient circuit that drives the LEDs with a controlled current and eliminates the wasted power associated with current-limiting resistors. Figure 1 shows a switch-mode boost design that regulates current instead of voltage. Because all the LEDs are connected in series, they all receive the same current without the need for ballasting resistors. Identical currents help achieve uniform intensity. And, because the output current is low (20 mA in this case), the output-filter capacitance, C2, can be smaller than for a load consisting of parallel-connected LEDs.
The circuit's 90% conversion efficiency offers a distinct power-saving advantage over resistor-limited and linearly regulated designs. It might appear that a series-LED connection is unsuitable for applications in which some (but not all) LEDs must be off. A cell phone, for example, sometimes needs that capability for occasions when the display is off but the keypad remains lit. Or, a PDA might need to play a sound file while maintaining illumination in the buttons but not the display. In the circuit of Figure 1, switching off individual LEDs or groups of LEDs is not a problem, even with series drive. Applying a logic-high level to the gate of a simple MOSFET switch, Q2, turns off a subset of LEDs by shunting their current. The remaining LEDs (for the keypad, for example) remain on, and their intensity remains constant because IC1 regulates their current, by sensing the voltage across R2 (300 mV at full brightness). When the circuit turns the LEDs on and off, the R2-C4 network at the gate of Q1 slows the load changes sufficiently to prevent transients in the LED drive current. Other features include adjustable intensity via the ADJ pin and full shutdown via the 
pin.
