I have a customer (Raytheon) using the TL4242 LED driver. He is driving a single LED at 20mA, using a 9.1 ohm resistor for the current sense resistor. The LED forward voltage varies from 2.2 to 3.8 and the power supply is 5V. The PWM frequency is 200 Hz.
He wants to see if he could reduce the turn-on transient during Pulse Width Modulation. He has a number of these devices and has staggered the PWM inputs to reduce the size of the transient, but if he could reduce the edge rate it would limit the extent of the harmonics. He observed that the TL4242 looks to be a feedback amplifier, so he thought that adding a small capacitor (0.1uf) in parallel with the LED would produce an integrator effect that would slow the current rise during turn on. He is also looking at paralleling the capacitor with 5k ohms to discharge the capacitor while the LED is off.
Any comments or potential gotchas?
Thanks, David
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David Hildebrand
Analog Field Applications
South Region
Texas Instruments, Inc.
Office: 214-479-0917
Cell: 972-841-6499
david.hildebrand@ti.com
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