Versatile TFT LCD Bias Supply and White LED Driver
in a 4mm × 4mm QFN – Design Note 440
Eddy Wells
Introduction
The makers of handheld medical, industrial and consumer
devices use a wide variety of high resolution, small to
medium sized color TFT LCD displays. The power supply
designers for these displays must contend with shrinking board area, tight schedules, and variations in display
types and feature requirements. The LTC ®3524 simplifies
the designer’s job by combining a versatile, easily programmed, TFT LCD bias supply and white LED backlight
driver in a low profile 4mm × 4mm QFN package. A second nonsynchronous boost converter generates the
voltage required to regulate one or two LED strings at
up to 25mA each. LED current can be adjusted by either
analog or digital means, optimizing the TFT display for
varying ambient light conditions. Each string is independently enabled and can contain 1 to 5 LEDs in series.
Internal circuitry maintains equal current in the strings,
even when the forward voltage drops of the LEDs do not
match. Open LED protection is provided to prevent the
output from exceeding 24V. The LTC3524’s 2.5V to 6V input supply range is ideally
suited for portable devices powered from Li-Ion or
multiple alkaline or nickel cells. Both the LCD and LED
drivers operate at 1.5MHz, allowing the use of tiny, low
cost, inductors and capacitors. 3-Output TFT Supply with Digitally Dimmed LED …