TL4242QKTTRQ1 Datasheet, PDF (10/21 Pages) Texas Instruments – ADJUSTABLE LED DRIVER

TL4242-Q1
SLVS732E – JULY 2010 – REVISED JULY 2013
STOP
TAIL
TLC555-Q1
VDD
OUT
TL4242-Q1
Q
PWM
REF
Vin
www.ti.com
LED
Dragon
R_REF
Figure 9. Stoplight and Taillight Application Circuit
Figure 9 shows the application circuit of the stoplight and taillight including an automotive-qualified timer,
TLC555-Q1, the duty cycle of which is programmable by two external resistors. One can see that driving the
STOP signal high pulls the PWM pin constantly high, creating 100% duty cycle. Thus the LEDs operate at full
brightness. When the TAIL signal is high, the LEDs operate at 25% brightness because the TLC555-Q1 timer is
programmed at a fixed duty cycle of 25%.
Thermal Information
This device operates a thermal shutdown (TSD) circuit as a protection from overheating. For continuous normal
operation, the junction temperature should not exceed the thermal-shutdown trip point. If the junction temperature
exceeds the thermal-shutdown trip point, the output turns off. When the junction temperature falls below the
thermal-shutdown trip point, the output turns on again.
Calculate the power dissipated by the device according to the following formula:
P = (V_I - V_O ) ´ I_O + V_I ´ I_Q
In the formula, V_I represents the input voltage of the device, V_O stands for the output voltage, and I_O means the
output current of LED and I_Q is the quiescent current dissipated by the device. The very small value of I_Q
sometimes allows one to neglect it.
After determining the power dissipated by the device, calculate the junction temperature from the ambient
temperature and the device thermal impedance.
T_J = T_A + q_JA ´ P
PCB Design Guideline
In order to prevent thermal shutdown, T_J must be less than 150ºC. If the input voltage is very high, the power
dissipation might be large. Currently there is the KTT (DDPAK) package which has good thermal impedance, but
at the same time, the PCB layout is also very important. Good PCB design can optimize heat transfer, which is
absolutely essential for the long-term reliability of the device.
• Maximize the copper coverage on the PCB to increase the thermal conductivity of the board, because the
major heat-flow path from the package to the ambient is through the copper on the PCB. Maximjum copper is
extremely important when there are not any heat sinks attached to the PCB on the other side of the package.
• Add as many thermal vias as possible directly under the package ground pad to optimize the thermal
conductivity of the board.
• All thermal vias should be either plated shut or plugged and capped on both sides of the board to prevent
solder voids. To ensure reliability and performance, the solder coverage should be at least 85 percent.
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