LP8863-Q1: wrongly alarm open string fault

Miranda Gu

Part Number: LP8863-Q1

Hi team,

My customer is using LP8863-Q1 to drive 4-string LED. Unused led strings were tied to ground.

However, the device still alarmed open string fault. And this problem only happened when output capacitor didn't discharge completely. Please refer to the start-up waveform below. Could you give some suggestions to solve it?

Green: Vboost_out Pink: INT Blue: EN Yellow: Vin

↑ Output capacitor didn't discharge completely, if restarted again, open string fault would alarm as Vboost_out > Vboost_ovp_high = 46.8V

↑ Output capacitor discharged completely, if restarted again, open string fault wouldn't alarm as Vboost_out < Vboost_ovp_high = 46.8V

Appreciate your kind support!

Miranda

over 4 years ago

0 Shirley He over 4 years ago

TI__Expert 5475 points

Hi Miranda,

Which 4 channels did you use? From LED0 to LED3 or not? Do you mean the other two unused channels were reported as open fault?

0 Miranda Gu over 4 years ago in reply to Shirley He

TI__Genius 14827 points

Yes, unused LED4&5 strings were reported as open fault.

0 Shirley He over 4 years ago in reply to Miranda Gu

TI__Expert 5475 points

Hi Miranda,

Did you also see the invalid LED string configuration fault, besides the LED open fault?

The device uses the LEDx pin voltage to auto-detect the LED string configuration. Since the boost output is not fully discharged, the LEDx pin voltage is not fully discharged. The LED string configuration detection may not work properly. It may take the LED0 and LED3 as connected together, which is not any one of 8 configurations that the device supports. So the invalid LED string configuration fault will be reported, and it will turn to 6 channel/60 degree mode. In that case, the channel 4&5 will be reported as open.

0 Shirley He over 4 years ago in reply to Shirley He

TI__Expert 5475 points

Hi Miranda,

Will this behavior impact your system? Do you have any concern if the device reports the open fault during this kind of start-up condition? Does the working scenario that starting-up before the boost is not discharged really exists in actual system?

0 Miranda Gu over 4 years ago in reply to Shirley He

TI__Genius 14827 points

Hello Shirley,

No other faults.

Yes, it affects system fast ON/OFF tests, so customer cannot accept it.

0 Shirley He over 4 years ago in reply to Miranda Gu

TI__Expert 5475 points

Hi Miranda,

I'll close this thread since we have already communicated by email.

0 Miranda Gu over 4 years ago in reply to Shirley He

TI__Genius 14827 points

Sure, many thanks for your kind help.

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LP8863-Q1: wrongly alarm open string fault