DC688A: 5V @ 40mA, ±10V @ 2mA, 2x 20mA (@ 21V Max) LED Drivers

Demonstration Circuit DC688 is a quad output power supply intended for use in small to medium size TFT panels. The circuit features the LT1942 quad output switching regulator and generates an LED driver along with the triple output supply required for the TFT panels.

With an input voltage range of 3V to 4.2V, two step-up regulators and one inverting regulator provide AVDD, VON and VOFF bias voltages for the LCD panels. Another high power boost is used to drive 12 LEDs (2 strings of 6 each). For 3V minimum input voltage, up to 20mA current is available for each LED string. A built-in LED ballast circuit is included to precisely match the LED currents in the two strings. The LED driver has a precise dimming control. A voltage between 0.25V to 2.5V can be applied to CTRL4 pin to control the dimming level.

An internal high side PNP provides delayed turn on of the VON signal for panel protection. A P-channel MOSFET driven by the output of PGOOD pin is added at the output of AVDD to provide output disconnect in shutdown.

All four switchers are synchronized to the internal 1MHz clock, allowing the use of low profile inductors and ceramic capacitors. Both the AVDD boost converter and the LED driver have soft-start to limit inrush current.