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APPLICATION INFORMATION

APPLICATION OVERVIEW

OPERATING PRINCIPLE

TPS60250

SLVS769 – APRIL 2007

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 is well suited for use in these three major phone designs

because it has 7 individually regulated white LED current paths and that drive up to five white LEDs in 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 25mA and an auxiliary LED output (DM5) drives up to 80mA that can

The TPS60250 circuit uses only 4 external components: the input/output capacitors and 2 chargepump 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 helps designers minimize power consumption especially in case of

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 chargepump provides regulated LED current from a 3-V to

6-V input source. It operates 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.

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