TPS92691: Boost LED driver output spikes

Roman Simonyan

Part Number: TPS92691

I designed a boost LED driver based on TPS92691. It drives 56V 5A COB LED from 22-50V input. It works ok.

Oscilloscope shows small spikes on input (100mV), but large spikes on output (3V). The spikes are from switching (400 kHz).

I started with 4.7uF X7R output capacitor. Then added one more, spikes were reduced. Then added one more, which reduced spikes little more, but adding more capacitance does not change anything anymore.

I mesure with probe with wire coiled around it.

When the MOSFET conducts, the first bigger spike (ringing) occurs, then the MOSFET opens and the second spike occurs. Why this may happen?

Can the spikes be reduced somehow? Also, is it bad, will it harm anything?

over 7 years ago

0 Tuan Tran74 over 7 years ago

TI__Mastermind 18890 points

Hi Roman,

Is this scope capture of the voltage across the output capacitors? If it is then there are ESL in the caps and you might want to use a smaller cap 0.01uF ceramic X7R across the output caps to reduce the ESL.

If it's voltage across your LED then it's most likely pickup on your layout and scope.

Thanks Tuan.

0 Roman Simonyan over 7 years ago

Intellectual 440 points

During the redesign process, I noticed that the resistor on the current sense RC LPF was shorted, so, there was no LP filtering. I am not sure exactly how it was affecting the circuit, but now it looks much cleaner. The same waveform, but without spikes, just the ripple.

Here is the waveform, probed exactly as in the post above, on output capacitor:

So, even in not so good layed out PCB, the driver works very well. Thank you TI!

0 Tuan Tran74 over 7 years ago in reply to Roman Simonyan

TI__Mastermind 18890 points

Hi Roman,

It's good to hear that you have resolved the issue...Thanks Tuan

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TPS92691: Boost LED driver output spikes