Boost-then‑Buck LED Drivers Enable Wide PWM Dimming
Range with Wide-Ranging Input Voltages
Keith Szolusha and Taffy Wong Multichannel LED drivers are primarily designed to power
multiple LEDs or multiple LED strings, sometimes of different
colors or lengths, from a single IC. These drivers, however,
include a number of features that allow for other compelling
uses. The LT3797 3-channel LED driver, for instance, can
be configured to produce boost-then-buck capability with
one channel as a boost voltage preregulator while the other
two channels are configured as buck mode LED drivers.
When the input voltage source is wideranging and can be both above and below
the LED string rating, a buck-boost or
SEPIC topology is commonly used. These compared to buck-only converters, and
lower efficiency and higher conducted
EMI compared to boost-only regulators.
One way to avoid these problems is to
boost a wide-ranging input with a voltage preregulator and use that as the input
to a buck-only LED driver. This has the
advantages of step-up and step-down,
high PWM dimming bandwidth, and
lower conducted EMI. Since the LT3797
has three channels that can be used
for either voltage regulation or driving …