TPS60230
www.ti.com
SLVS516A – MAY 2004 – REVISED JUNE 2004
STARTUP TIMING
5 V/div
EN1 + EN2
0V
VOUT
1 V/div
V
I
= 3.6 V, I
LED
= 20 mA,
5 LEDs Connected,
T
A
= 255C
0V
I
I
100 mA/div
0A
20
µs/div
Figure 11.
DETAILED DESCRIPTION
OPERATION
The TPS60230 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 125 mA of output current, so each of the 5 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.
LED CURRENT ADJUSTMENT (ISET)
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.
V
R
+
ISET
k
ISET
I
LED
V
ISET
— Voltage from ISET pin (0.2 V, 0.4 V or 0.6 V) to GND, see Table 1
I
LED
— Current per LED from Dx pin to GND
k — I
SET
to Dx current ratio
The LED current varies linearly from 0 mA to I
LED(max)
mA by applying a PMW signal with 0% to 100% duty cycle.
The LED brightness can however also be controlled by an analog control signal that is fed into the ISET pin.
9
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