Posted on 01 April 2019

Simple Technique for LED Driver Applications

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Minimizing input supply ripple current

LED applications in the consumer market have really taken off like a wild mushroom. In the past they were very popular in cell phone applications, but now you find them in home lighting applications, airplane cabin lights, automotive headlights, MP3 players, etc.

By Ajmal Godil, Intersil


Most of the LED drivers in the past were based on some sort of charge pump, where the input voltage was multiplied by 2x and the LED voltage was post regulated by same sort of internal low dropout regulator.

Some high-power LEDs require much higher current before they start emitting light. Therefore, most of the current industry standard LED drivers are based on boost topology since the current can be up to a few amps. For handheld applications such as cell phones and PDAs, which are powered by battery, system manufacturers require the LED drivers to provide some sort of dimming function. The reason for this is that battery life is inversely proportional to light intensity, and light intensity is directly proportional to LED current. The simplest technique that chip vendors have adopted is to provide an EN pin that turns-on the boost regulator when the voltage is above a certain threshold; otherwise, it shuts-off. Since most of these handheld applications use a microprocessor or a microcontroller, it is very easy to generate a rectangular pulse of a certain frequency, amplitude and duty-cycle. If you apply this PWM signal to the EN pin, one can increase or decrease the LED current by varying the duty cycle. So by increasing the duty-cycle, you force more current into the LED and it looks brighter compared to decreasing the duty-cycle, where you force less current into the LED, and it looks dimmer. This simple approach works very well for dimming the LEDs, but it injects high ripple current on the input supply, and in some systems this is not acceptable since it drags the input supply voltage down. See Figure 1, which shows the input supply current when the LED current is reduced from 700mA to 350mA by using a 3V, 10 KHz, 50% duty cycle PWM signal on the EN pin. This figure shows that the peak-to-peak input ripple current is approximately 3A, which is too high.

Top is an input supply ripple current of approximately 3A pk-pk - Bottom is a PWM signal used for reducing LED current from 700mA to 350mA.

The circuit in Figure 2 addresses this issue by using the Intersil’s EL7801 boost LED driver that not only offers the “EN” pin but also provides 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 feeding it to the EN pin, one can minimize the input supply peak-to-peak ripple current by passing this PWM signal through a low-pass RC filter (R4 and C3) with a time constant of 2mS. This low-pass filter with a time constant >> 1/Freq._PWM would produce an average voltage that can be connected directly to the “Level” pin to control the LED current. So a 5V, 50% duty-cycle waveform would produce 2.5V waveform, which after internal level shifting would correspond to 500mV on the Level pin. The average voltage for the level pin can be calculated by the formula below:

EL7801 schematic for reducing input supply ripple current for dimming

Vavg_Level_Pin = (PWM_Amplitude *Duty Cycle)*0.2

Iavg_LED = Vavg_Level_Pin / R11 (R11 in Figure 2 is 0.2 ohms)

Figure 3 shows the input supply ripple current under the same test conditions by reducing the LED current from 700mA to 350mA. It can be seen that the input ripple current has been reduced to a negligible level.

Input supply ripple current for schematic in Figure 2 reduced to approximately 50µV




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