TPS60230_14 Datasheet, PDF (9/21 Pages) Texas Instruments – WHITE LED CHARGE PUMP CURRENT SOURCE WITH PWM BRIGHTNESS CONTROL

TPS60230
www.ti.com
SLVS516A – MAY 2004 – REVISED JUNE 2004
DETAILED DESCRIPTION (continued)
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.
RISET
+
VISET
ILED
k
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.
SOFT START
The TPS60230 has an internal soft start circuit to limit the inrush current during startup. This prevents possible
voltage drops of the input voltage if a high impedance power source is connected to the input of the TPS60230.
When the device starts up with an output voltage that is below the input voltage, the output capacitor is charged
directly from the input with a current source. The output current increases linearly until the output reaches within
300 mV of the input voltage. When the programmed output current can be reached with the 1x mode, the
TPS60230 terminates the soft start and begins normal operation. When the desired output current cannot be
reached, the charge pump begins operation in 1.5x mode and pumps the output voltage up to the needed level
to reach the programmed output current.
ENABLE (EN1, EN2)
The enable pins EN1 and EN2 are used to enable the device or set it into shutdown. The TPS60230 is enabled if
one of the enable pins is pulled higher than the enable trip point of 1.3 V. The device starts up by going through
the soft start routine as described in the section Soft Start. Pulling both pins to GND, after a delay, programs the
device to shutdown. In shutdown, the charge pump, current sources, voltage reference, oscillator, and all other
functions are turned off and the supply current is reduced to 0.1 µA.
EN1 and EN2 can also be used for dimming. The logic levels at EN1 and EN2 set the minimum voltage at the
current mirrors and the voltage at the ISET pin to GND. This sets the current at the LEDs to be either the full
current or a fraction of the full current. See Table 1 for further details. The maximum current through the LEDs is
set by a resistor connected between ISET and GND.
EN1 and EN2 can also be used for PWM dimming. The PWM signal can either be applied to EN1 or EN2, or
both inputs can be tied together and the PWM signal can be applied to both pins. Depending on the
configuration, the current during PWM dimming is switched between 0 mA and its maximum (EN1 and EN2
connected to the PWM signal) or between 0 mA and 1/3 of the full LED current if EN2 = 0 and EN1 is toggled.
When EN1 = 0 and EN2 is toggled, the output current can be changed between 0 mA and 2/3 of the full range.
Table 1. Enable Levels
ENABLE LEVEL
EN2
EN1
0
0
0
1
1
0
1
1
MODE
SHUTDOWN
VISET = 200 mV
VISET = 400 mV
VISET = 600 mV
LED CURRENT
0
1/3
2/3
Full
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