J

A

JA

T

T

P

=

+ q

´

I

O

O

I

Q

P

(V

V ) I

V

I

=

-

´

+

´

TL4242-Q1

PWM

Q

REF

Vin

VDD

OUT

TLC555-Q1

STOP

TAIL

R

REF

LED

Dragon

TL4242-Q1

SLVS732E – JULY 2010 – REVISED JULY 2013

www.ti.com

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:

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.

PCB Design Guideline

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.

10

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