TL4242_09 Datasheet, PDF (6/14 Pages) Texas Instruments – ADJUSTABLE LED DRIVER

TL4242
SLVS641A – APRIL 2008 – REVISED MAY 2009 .............................................................................................................................................................. www.ti.com
APPLICATION INFORMATION
Figure 5 shows a typical application with the TL4242 LED driver. The three LEDs are driven by a supply current
that is adjusted by the resistor, R_REF, preventing brightness variations due to forward voltage spread of the LEDs.
The luminosity spread arising from the LED production process can be compensated via software by an
appropriate duty cycle applied to the PWM pin. Therefore, it is not necessary to select LEDs for forward voltage
or luminosity classes. The minimum supply voltage calculates as the sum of the LED forward voltages, the
TL4242 drop voltage (maximum 0.7 V at a LED current of 300 mA) and the maximum voltage drop at the shunt
resistor R_REF of 185 mV (max).
V_BAT
I
RADJ GND
RO
SI
Q
D
100 nF
Microcontroller
10 µF 10 kΩ
ST
PWM
I
Q
TL4242
REF
GND D
LED
Dragon
47 nF
0.47 Ω
0.25 W
R_REF
Figure 5. Application Circuit
The status output of the LED driver (ST) detects an open-load condition, enabling supervision of correct LED
operation. An LED failure is detected as a voltage drop at the shunt resistor (R_REF) below 25 mV (typ). In this
case, the status output pin (ST) is set low after a delay time adjustable by an optional capacitor connected to
pin D.
The functionality and timing of ST and PWM are shown in Figure 6. The status delay can be adjusted via the
capacitor connected to pin D. Delay time scales linearly with capacitance, C_D:
6
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