L DESIGN FEATURES Dual Step-Up Converter Drives White
LEDs with 1000:1 PWM Dimming
by Keith Szolusha Introduction
Notebook computers, large-screen
handheld PDAs, dashboard displays,
and automotive and avionic in-cabin
entertainment LCD panels are illuminated with strings of high power white
LEDs. White LEDs are the preferred
over other backlight technologies because they provide true white light at
a high enough intensity for daylight
viewing, and enough dimming capability for nighttime use. LEDs also offer
relatively long life spans and a lack
of hazardous materials. LED strings
lining the edges of these LCD panels provide uniform brightness when
driven with a constant current.
The maximum switch voltage of an
LED driver limits the number of LEDs
that it can drive in series. It may seem
that paralleling LEDs is a good way
to increase the capacity of a driver
IC, but parallel LEDs must be wellmatched in forward voltage; otherwise
un-matched LED strings cause uneven
currents and thus uneven brightness. …