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TPS61196: LED light off when the dim duty rise quickly up to 99.8% from 20%

Part Number: TPS61196

Hi Team,

We got a question from our customer regarding abnormal phenomenon.

The phenomenon is that the anode voltage rise up to OVP point when the dimming duty is changed from 20% to 99.8% quickly. (then LED is light off because of OVP = 58.3V.)

The condition is ;

Vin=12V, 4 LED strings (Unused 2 strings are connected to ground.)

The phenomenon appears;

1.) when the dimming duty is changed from 20% to 99.8% quickly.

The phenomenon DOES NOT appear;

1.) when the dimming duty is changed slowly.(20% to 99.8%)

2.) when the LED strings are reduced to 3. (4strings to 3strings)

3.) when Vin is over 12.5V.

4.) when the dimming duty is fixed 99.8%.

At the beginning of question, we suspected not enough the performance of load transient. So, we asked them to change the compensation circuit at COMP pin or to add Cff.

However those were no effected.

I am at a standstill on this issue. What should I check any other point ?

(Would you mind if you check the schematics on private message? The schematic of their board is confidential information.)

Best Regards,

Koji Hamamoto