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APPLICATION INFORMATION

APPLICATION OVERVIEW

OPERATING PRINCIPLES

TPS60255

SLVS825 – MARCH 2008

Most of the current mobile telephone handsets fall into one of these designs:

• Clam Shell (Figure 17)—a main display on the inside, a secondary display on the outside, and a keypad

backlight.

• Slide type (Figure 18)—slide-up and slide-down design, with a main display and two keypads (inside and

outside).

• Bar (Figure 19)—a main display and a keypad backlight.

Charge pumps are becoming increasingly attractive for driving LEDs for the display backlight and keypad

backlight in handsets, where board space and maximum converter height are critical constraints. Its major

advantage is use of only capacitors as storage elements. TPS60255 is well suited for use in all three major

phone designs.

The device provides six 25 mA current regulators and one 80 mA current regulator. The current regulators are

divided into three groups (A, B, and C), with each group having its own independent current program register and

ON/OFF control. This promotes dividing and combining the groups for various LED driving configurations

including LCD backlight, keypad backlight, and camera flash light. See APPLICATIONS CIRCUITS for example

cell phone application circuits.

The TPS60255 adopts 1

× and 1.5× charge pump configuration. The device monitors the voltage of the current

feedback pins (Gx pins) and automatically switches between 1

× and 1.5× mode to ensure current regulation

regardless the variations of input voltage and LED forward voltage.

The

TPS60255

uses

only

four

external

components,

the

input/output

capacitors

and

two

charge-pump-flying-capacitors. This combined with the 16-pin, 3 mm

× 3 mm QFN package (0.8 mm height),

provides for a small, low-profile total solution.

The TPS60255 charge-pump provides regulated LED current from a 2.7 V to 6.0 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× mode if the voltage at any current sink input

(GAx, GBx, or GC) falls below the 100-mV transition voltage. The IC returns to 1

× mode as the input voltage

rises. 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

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

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

8

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Product Folder Link(s): TPS60255