Ajmal Godil
Electronic Design
Consumer-based LED applications have really taken off. You now find them in home lighting, airplane cabin lights, automobile lights, MP3 players, and elsewhere. In the past, most LED drivers were based on some sort of charge pump, where the input voltage was multiplied by two and the LED voltage was post-regulated by an internal low-dropout regulator. But some high-power LEDs require much higher current before they start emitting light. Therefore, most of today's industry-standard LED drivers use a boost topology, since the current can be up to a few amps.
For battery-powered handheld applications, such as cell phones and PDAs, system manufacturers require that the LED drivers provide some sort of dimming function. That's because battery life is inversely proportional to light intensity, since light intensity is directly proportional to LED current. The simplest technique adopted by chip vendors is to provide an EN pin that turns on the boost regulator only when the voltage is above a certain threshold.
Because most handheld applications use a microprocessor or a microcontroller, it's very easy to generate a rectangular pulse of specified frequency, amplitude, and duty cycle. If you apply this pulse-width modulation (PWM) signal to the EN pin, you can increase or decrease the LED current – and thus the brightness – by varying the duty cycle.
This simple approach works very well for dimming the LEDs, but it injects high ripple current on the input supply. In some systems, this becomes unacceptable because it drags down the input supply voltage. The peak-to-peak input ripple current can reach to around 3 A, which is too high, when the LED current is reduced from 700 mA to 350 mA. In the testing for this article, these results were obtained by using a 3-V, 10-kHz, 50% duty-cycle PWM signal on the EN pin.
One way to address this issue is to use a boost LED driver such as the Intersil EL7801, which has an EN pin and a LEVEL pin. A dc voltage on this pin controls the LED current. This circuit also uses a 10 kHz PWM signal. But rather than feed it to the EN pin, you can minimize the input-supply, peak-to-peak ripple current by passing the PWM signal through a low-pass RC filter (R4 and C3) with a time constant of 2 ms (Fig. 1).
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| Figure 1. | Using a boost LED driver with a LEVEL pin in addition to an EN pin, and then adding a low-pass filter to the driver circuit (R4-C3), reduces the ripple current. |
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If a time constant is much bigger than 1/fPWM, the low-pass filter produces an average voltage that can be connected directly to the LEVEL pin to control the LED current. Therefore, a 5-V, 50% duty-cycle waveform would produce a 2.5-V waveform. After internal level shifting, that would correspond to 500 mV on the LEVEL pin.
The average voltage for the LEVEL pin (VAVG) can be calculated by the below formula:
where VPWM = PWM amplitude, D = duty cycle.
Then the average current for the LED (IAVG) is
In the example in the figure, R11 is 0.2 Ω.
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| Figure 2. | With the improved driver circuit in Figure 1, the input-supply ripple current drops to a negligible level, approximately 200 mA. |
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Under the same test conditions as before, the improved circuit exhibited a greatly reduced ripple current when dropping the LED current from 700 mA to 350 mA (Fig. 2). The input ripple current has been lowered to a negligible level.
