LED applications are booming: backlighting for LCD TVs,
projectors, and computer displays, camera flashes, and
vehicle brake lights and taillights are just a few. All require
constant-current LED drivers that offer a high dimming ratio,
high efficiency, precise adjustable current, and low cost.
To save space, some LED drivers can handle multiple strings
of LEDs. For instance, the LT347drives four LED strings, totaling
100 W. If all of the driver channels aren’t needed for LEDs,
leftover channels can be used to produce regulated power supplies,
eliminating additional circuitry required for a separate
supply. This article shows how
to do just that—use a four-channel
LED driver to drive three
constant-current RGB LED
strings and employ the fourth
channel as a constant-voltage
source.
The example system is an
LCD projector that requires
three color LED strings: red,
blue, and green. Each string uses
1-A LEDs and is driven in a buck
mode topology (see the figure).
The LCD panel also requires a
bias voltage, which is supplied
by the fourth LT3476 channel.
To use the fourth channel as
a voltage source, the channel’s
CAP and LED pins must be
properly tied to the top voltagesense
resistor (R1). This configuration
creates a boost circuit
that generates a 15.5-V output at
700 mA from a 9-V supply, with
90% efficiency.
In a classical voltage-regulator
design, you can assume the
voltage-feedback error amplifier
draws no current (actually, in the
10-nA range) from the output.
This isn’t true in the example
circuit. The driver’s CAP pin
and LED pin bias currents are on the order of 70 µA. This level is necessary for the high-side
current sensing required to properly drive the LEDs. As a result,
R1 has to be small enough to limit the error caused by the bias
current. The recommendation is less than 15 O. Given VOUT
and R2, R1 can be calculated using the equation:
R1 = R2/(VOUT/0.105 V – 1)
where 0.105 V is the regulated sense voltage between CAP and
LED when the VADJ pin is tied to the Ref pin.
