带渐进调光功能的白光LED驱动器.doc

White Light LED Driver With Gradual Dimming S.K. Loo, Michael Day PMP/Portable Power DC-DC Applications ABSTRACT Many consumer products using white light LEDs for illumination and backlighting provide gradual LED illumination at turnon and turnoff. This gradual illumination is usually achieved with the aid of a microprocessor that provides PWM dimming control. This circuit uses the Texas Instruments TPS61040 white light LED driver to provide gradual illumination at turnon and turnoff without the use of a microprocessor. The following circuit generates a 101mA constant current to drive up to 6 white LEDs for portable applications such PDAs and digital cameras. The input operating range is 1.8 V to 6 V, which covers two-cell alkaline and NiMh inputs (1.8 V to ~3 V), three-cell alkaline and NiMh inputs (2.7 V to ~4.8 V), and single-cell Li-Ion inputs (3 V~4.2 V). This boost converter uses R1 to set a constant current through the LEDs. A detailed description of this circuit can be found in the TPS61040 data sheet (SLVS413). The circuit shown in Figure 1 instantly produces the maximum programmed load current at turnon and instantly goes to zero current at turnoff. The circuit shown in Figure 2 slowly (1–2 seconds) increases and decreased the LED brightness at turnon and turnoff. Only a slight modification of the original circuit is required to produce this effect. On application of the enable signal, R3 slowly charges C3, which slowly turns on Q1. The TPS61040 is immediately enabled through D2. During turnon, the initial resistance from FB to ground is extremely high, limiting the LED current to 0 mA. As Q1 slowly turns on, the effective resistance gradually drops. The drop in resistance directly correlates to a rise in LED current. The LEDs transition from 0% brightness to 100% brightness in 1–2 seconds, depending on the time constants chosen. Figure 3 shows the relevant waveforms during turnon. When ENA goes low during turnoff, D2 prevents C4 from disc