REF

Q max

REF

V

I

R

=

TL4242-Q1

PWM

Q

REF

Vin

VDD

OUT

TLC555-Q1

STOP

TAIL

R

REF

LED

Dragon

12

TL4242-Q1

SLVS732G – JULY 2010 – REVISED APRIL 2017

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Product Folder Links: TL4242-Q1

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8.2.1.3 Application Curves

Figure 8. Start-Up Waveform

Figure 9. 50% Duty Cycle Waveform

8.2.2 Stoplight and Taillight Application

For many automobiles, the same set of LEDs illuminates both taillights and stop lights. Thus, the LEDs must

operate at two different brightness levels, full brightness for the stoplight and 10% to 25% brightness for the

taillight. The easiest way to achieve the different brightness is dimming by pulse-width modulation (PWM), which

holds the color spectrum of the LED over its whole brightness range. The maximum current that passes through

the LED is programmable by sense resistor RREF.

Figure 10. Stoplight and Taillight Application Circuit

8.2.2.1 Design Requirements

DESIGN PARAMETER

EXAMPLE VALUE

RREF

1 Ω

8.2.2.2 Detailed Design Procedure

Obtain the maximum current, IQmax, that passes through the LEDs by Equation 4:

(4)

For example, if RREF equals 1 Ω, as VREF is a fixed value range from 168 mV to 185 mV, IQmax should be 168 mA

to 185 mA.

Figure 10 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%.