Typical uses today include
backlighting for LCD
TVs, projectors, computer
displays, camera flashes, and
vehicle brake lights. All need
constant-current LED drivers
that are economical and offer
high dimming ratio, high efficiency,
and precise adjustable
current. To save space, some
LED drivers are designed to
light multiple strings of LEDs.
For instance, Linear Technology’s
LT3476 can drive
four LED strings, totaling
100 W. The chip is basically a
constant-frequency, currentmode
regulator. In applications
where there are leftover
driver channels, the extra
channels can be applied to
the task of producing regulated
electrical power. This
eliminates the need for extra
circuitry otherwise used with
a separate power supply.
In the accompanying diagram,
an LT3476 drives three
constant-current RGB LED
strings. Its fourth channel
serves as a constant-voltage
source. A typical application
might be in an LCD projector
which needs three color LED
strings for red, blue, and green.
There are six 1-A LEDs in each
string, and each string is driven in
a buck-mode topology. The LCD
panel also needs a bias voltage
source, and the fourth channel
can provide this bias.
To use one of the LT3476 channels
as a voltage source, the CAP
and the LED pin of the channel
must be tied to the top voltagesense
resistor R1. Channel four
is a boost circuit that generates
a 15.5-V output at 700 mA from a
9-V supply with 90% efficiency.
A point to note is that the CAP
and LED pins are the inputs to
a voltage-feedback-error amplifier
in the chip. This amplifier
regulates the current through
the sense resistor RSNS and hence
through the LED string. In a classical
voltage-regulator design,
it is safe to assume the voltagefeedback-
error amplifier draws
no current (10-nA range) from the
output. This assumption is not
valid in the circuit shown here.
The CAP pin and the LED pin bias
current of the LT3476 are on the
order of 70 μA. This current level
is necessary for the high-side current
sensing to properly drive the
LEDs. As a result, the value of the
sense resistor R1 must be low
enough to limit the error caused
by the bias current. The recommendation
is to set R1 at less than
15 Ω. Given VOUT and R2, R1 can
be calculated using the equation:
R1 = R2/(VOUT/0.105 V–1)
In this equation, 0.105 V is the
regulated sense voltage between
CAP and LED when the VADJ pin is
tied to the Ref pin directly.
— Hua (Walker) Bai, Applications
Engineer, Linear Technology Corp.