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EN1 + EN2

0 V

0 A

0 V

20 ms/div

3 LED’s Connected,

DETAILED DESCRIPTION

OPERATION

LED CURRENT ADJUSTMENT (ISET)

R

ISET +

V

ISET

I

LED

K

TPS60231

SLVS544 – OCTOBER 2004

STARTUP TIMING

Figure 11.

The TPS60231 uses a fractional conversion charge pump to generate a supply voltage for the integrated current

sinks. These current sinks are used to ensure a constant current for each LED. Depending on the input voltage

and programmed LED current, the charge pump either operates in the 1x mode or in the 1.5x mode. By

switching automatically between these two modes, the circuit optimizes power conversion efficiency as well as

extends operating time by allowing the discharge of the battery completely.

The charge pump can generate 75 mA of output current, so each of the 3 LED outputs can be powered with up

to 25 mA of current. The maximum LED current is set by a resistor connected to the ISET pin. This resistor

programs a reference current, which is current mirrored to set the LED current.

Applying a PWM signal to the EN1 pin and/or the EN2 pin controls the LED brightness. See a detailed

description in the section Analog Dimming Using ISET Pin.

A resistor programs a reference current, which is current mirrored to set the LED current. The voltage at the

ISET pin depends on the status of EN1 and EN2. The current in each LED is typically 260 times the current

through the resistor at ISET.

The LED brightness can however also be controlled by an analog control signal that is fed into the ISET pin.

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