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 is unacceptable
because it drags down the input supply
voltage. shows the input supply
current when the LED current is
reduced from 700 mA to 350 mA by
using a 3-V, 10-kHz, 50% duty-cycle
PWM signal on the EN pin. The peak-to-peak input ripple current is approximately 3 A, which is too high.
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 10kHz 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 ()
.
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
can be calculated by the below formula:
VAVG Level pin = (PWM amplitude *
duty cycle) * 0.2
Then,
IAVG LED = VAVG Level pin/R11
In the example in the figure, R11 is 0.2 Ω.
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 ( ). The input ripple current has
been lowered to a negligible level.