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LP55231: Unintentional Output Off

Shinichi Yokota
Genius 9715 points
Part Number: LP55231

Hello,

Not all but some LP55231 units show behavior that unintentionally turns off outputs of D1 to D6 after the following procedure. I suspect it has something to do with its charge pump function because the behavior won't appear in the forced-to-bypass mode (1×).

What do you think is the cause of this behavior?

Procedure

  1. "Automatic mode selection" (11b) is selected in the CP_MODE (bits [4:3]) of the MISC register.
  2. Set a value in the range from 0xFA to 0xFF to the D4 PWM register.
  3. After about 100 ms, set a value of 0xF8 or less to that D4 PWM register, and then, outputs of D1 to D6 are turned off.

Output Loads

No LEDs are connected to the outputs.

  • D1 to D5
  • D6
  • D7 to D9 - NC

Best regards,
Shinichi Yokota

  • 0
    TI__Expert 4160 points

    Hi Yokota-san, 

    I have two questions from my side: 

       1. Are there multiple LP55231 on one PCB which share a common power supply?

       2. Would you clarify the purposes of this load condition of D1 to D6 (No driven LEDs)? 

    If it works with 1x bypass mode, I will try to recur the same problem on LP55231 EVM board following the procedures you provided and examine the waveform of charge-pump output. 

    Best Regards,

    Aaron

  • 0 in reply to Aaron Bing
    TI__Genius 9715 points

    Aaron-san,

    Aaron Bing said:
    Are there multiple LP55231 on one PCB which share a common power supply?

    No, there is no LP55231 on this PCB that shares the same supply voltage with this one.

    Aaron Bing said:
    Would you clarify the purposes of this load condition of D1 to D6 (No driven LEDs)?

    External MOSFETs are placed at D1 to D5 to drive an LED array. The LP55231 doesn't drive the LED array directly.

    A logic buffer is placed at D6, and the buffer drives the PWM signal for an LCD backlight. Only D6 output's PWM signal is used.

    Have you ever seen this behavior? I'm looking forward to your feedback.

    Best regards,
    Shinichi Yokota

  • 0 in reply to Shinichi Yokota
    TI__Expert 4160 points

    Hi Yokota-san, 

    I've never seen this happening to LP55231 due to the specific load condition with external MOSFETs to drive LED arrays. 

    Shinichi Yokota said:
    After about 100 ms, set a value of 0xF8 or less to that D4 PWM register, and then, outputs of D1 to D6 are turned off.
    • After the step #3, are D1~D6 outputs automatically recovered or they need reset the register values?
    Shinichi Yokota said:
    Not all but some LP55231 units show behavior that unintentionally turns off outputs of D1 to D6 after the following procedure
    • So that some LP55231 on other PCBs can work fine with CP 1.5x function. Would you examine the output waveforms and monitor the register read data when unintentional OFF condition occurs? If it's individual LP55231 issue, I assume a ABBA swap test is also considerable. 

    Best Regards, 

    Aaron