PWM

Via to GND Plane

ST

GND

REF

D

NC

I

To µC

To pullup

source

Q

To power supply

To µC

RREF

TL4242

SLVS641B – APRIL 2008 – REVISED MARCH 2015

www.ti.com

9 Power Supply Recommendations

The input voltage at the I pin must be greater than the sum of the LED forward voltages, the voltage drop across

the TL4242 (from I to Q), and the voltage drop across the shunt resistor RREF. For an example of this calculation,

refer to Typical Application. The input voltage should not exceed the recommended maximum operation voltage

of 42 V.

10 Layout

10.1 Layout Guidelines

The REF pin should be routed directly to the shunt resistor, RREF. If there is a long PCB trace between the LED

string and the shunt resistor, the REF pin should connect close to the shunt resistor (rather than close to the LED

string) to allow for accurate sensing across the shunt resistor.

The traces for I and Q will carry the full LED current. These traces should be the appropriate width to carry the

LED current.

The exposed thermal pad on the bottom of the TL4242 should be connected to the PCB. The thermal pad helps

to dissipate heat in the case of high power dissipation in the device. To further enhance the thermal performance

of the device, the thermal pad can be connected by vias to the ground layer in the PCB.

10.2 Layout Example

Figure 8. PCB Layout

12

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