White-LED driver touts high efficiency

White LEDs, the most recent addition to the LCD backlight, find common use in providing backlight for color LCDs. Thanks to their size and white-light output, they appear in small, portable devices with color displays, such as PDAs and cellular phones. Like other LEDs, a white LED needs a constant-current source—typically, on the order of 15 to 20 mA. The forward voltage of a white LED is approximately 3.5V. Most products use multiple LEDs to provide adequate backlight for a display. Because the LED's brightness depends on its forward current, these multiple diodes commonly connect in series to ensure that the same current flows through each of them. You need approximately 14V to forward-bias four series-connected LEDs, starting from the nominal operating voltage, 2.7 to 4.2V, of a single-cell lithium-ion battery. Boost regulators usually provide this operating voltage. A current-sense resistor, which you insert in series with the LEDs, closes the feedback loop. However, it is important to minimize the voltage drop across this resistor to increase efficiency. Currently available integrated boost regulators commonly use a 1.24V bandgap voltage as the feedback reference, which results in 1.24V loss across the current-sense resistor, a loss that represents approximately 7% loss in efficiency. Figure 1 shows an interesting LED-drive circuit.

You use the SP6682, a standard, regulated charge-pump circuit, in an unusual manner to control the external switch, Q₁. This IC incorporates an internal 500-kHz oscillator, which would normally drive charge-pump capacitors to double the input voltage. The circuit in Figure 1 uses no charge-pump capacitors. Instead, the oscillator output appears on Pin 7 and drives Q₁ on and off. Q₁, L₁, D₁, and C₁ function as a conventional boost regulator, which builds up voltage across C₁. When this voltage exceeds the sum of the diodes' forward drop, current starts to flow. The circuit senses current across R₁ and compares it with a 0.3V reference voltage inside the chip. This circuit provides efficiencies as high as 87%, a figure that exceeds that of any integrated boost regulator. Several factors are responsible for the increased efficiency. First, the chip integrates the 0.3V reference voltage, which is significantly lower than the typical 1.24V. This reference voltage appears in series with the LEDs and therefore constitutes an efficiency loss proportional to the value of the reference. Second, a discrete MOSFET provides low on-resistance and high switching speed, parameters superior to those of any integrated switch.

Q₁ is a low-cost device that comes in a tiny SOT-23 package. Also, the excellent drive capability of the charge-pump IC ensures low switching losses. By changing the type of the MOSFET you use, you can make a trade-off between desired efficiency and cost. The breakdown voltage of the MOSFET limits the maximum output voltage; you can adjust the voltage to drive a system with as many LEDs as you need. (Larger displays use eight to 12 LEDs.) For dimming purposes, applying a PWM signal to the Enable pin causes the regulator to shut down and restart. This function allows you to precisely control LED brightness.