APPLICATION INFORMATION

APPLICATION OVERVIEW

OPERATING PRINCIPLE

TPS60250

TPS60252

Most of the current handsets fall into one of three categories. First is the clamshell design, with a main display on

the inside, a secondary display on the outside and a keypad backlight. Second is the bar design, with a main

display and a keypad backlight. Third is the slide type (slide-up and slide-down) design, with a main display and

two keypad banks (inside and outside). The TPS60250/2 are well suited for use in these three major phone

designs because the devices have 7 individually regulated white LED current paths that drive up to five white

LEDs in the main display and up to two white LEDs in sub display with regulated constant current for uniform

intensity. The main and sub display LED channels drive up to 25 mA and the auxiliary LED output (DM5) that

drives up to 80 mA can be assigned to keypad backlight, torch light, or low cost/weak camera flash application

The TPS60250/2 circuits use only 4 external components: the input/output capacitors and 2 charge pump flying

capacitors. The few external components combined with the small 3mm

× 3mm QFN package provide for a small

total solution size. By combining independent control of three separate banks of backlight LEDs with low cost and

weak flash capability, the TPS60250/2 help designers minimize power consumption especially in the case of a

light load condition while reducing component count and package size.

Charge pumps are becoming increasingly attractive in battery-operated applications where board space and

maximum height of the converter are critical constraints. The major advantage of a charge pump is the use of

only capacitors as storage elements. The TPS60250/2 charge pumps provide regulated LED current from a 3 V

to 6 V input source. The devices operate in two modes. The 1

× mode, where the input is connected to the output

through a pass element, and a high efficiency 1.5

× charge pump mode. The IC maximizes power efficiency by

operating in 1

× and 1.5× modes as input voltage and LED current conditions require. The mode of operation is

automatically selected by comparing the forward voltage of the WLED plus the voltage of current sink for each

LED with the input voltage. The IC starts up in 1

× mode, and automatically transitions to 1.5× if the voltage at

any current sink input (DM_or DS_) falls below the 100 mV transition voltage. The IC returns to 1

× mode as the

input rises. Figure 13 provides a visual explanation of the 1

× to 1.5× transition.

In 1.5

× mode, the internal oscillator determines the charge/discharge cycles for the flying capacitors. During a

charge cycle, the flying capacitors are connected in series and charged up to the input voltage. After the on-time

of the internal oscillator expires, the flying capacitors are reconfigured to be in parallel and then connected in

series to the input voltage. This provides an output of 1.5

× the input voltage. After the off-time of the internal

oscillator expires, another charge cycle initiates and the process repeats.

Copyright © 2007–2008, Texas Instruments Incorporated

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Product Folder Link(s): TPS60250 TPS60252