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LP87524-Q1: The output is abnormal when EN signal is low

Zirui Su
Expert 5340 points
Part Number: LP87524-Q1

Hi team

Here we have encountered a issue that when LP87524 outputs abnormally. When the EN is high (green waveform), the LP87524Q1 output is 1.8V but when EN from high goes low, the LP87524Q1 output is abnormal and the voltage value is around 1V, here is the waveform:

Green one is EN signal and the yellow one is LP87524Q1 output, could you please help to analyze it and what potential factors will cause this issue? Here we also dump the LP87524Q1 register, thank you.

87524dump.txt 
/ # i2cdump -f -y 7 0x60
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f    0123456789abcdef
00: 52 3b 80 2c c4 14 80 2c e4 24 61 00 c5 00 b1 00    R;?,???,?$a.?.?.
10: b1 00 13 17 15 10 19 0d 00 42 10 00 00 00 10 cc    ?.??????.B?...??
20: cc 81 01 55 55 00 00 00 55 a6 00 81 50 77 00 06    ???UU...U?.?Pw.?
30: dc 59 4d 9a 53 40 18 03 00 01 51 00 00 00 00 00    ?YM?S@??.?Q.....
40: 0b 35 09 4e 61 04 00 00 00 00 00 00 00 00 00 00    ?5?Na?..........
50: 00 00 06 00 00 25 00 00 00 00 00 00 00 00 00 00    ..?..%..........
60: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
70: 00 00 00 00 00 00 00 00 00 00 00 02 00 00 00 00    ...........?....
80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
90: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
c0: 20 1c 03 1d c6 30 00 00 00 00 00 00 00 00 00 00     ????0..........
d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................

  • 0
    TI__Genius 11020 points

    Hello Zirui,

    It's difficult to decipher the register dump.

    Questions to help with the investigation:

    - What is the full part number for the device?

    - What settings they are writing to the device and how they are controlling the bucks? 

    - Can they measure the supply voltage, ENx pins and I2C when they see this behavior?

    - What buck output this is? Are other bucks ok?

    One reason I could think is the device is reset and returned back to the default OTP settings where the voltage is set as 1V.

    Thanks.

    Regards,

    Tomi Koskela

  • 0 in reply to Tomi Koskela
    TI__Expert 5340 points

    HI Tomi,

    Here I update the information for you to check:

    The full part number is LP87524PRNFRQ1

    Here is the schematic for you to review:

    Since other lanes of BUCK are normal just the BUCK 3 is abnormal, and we checked that the BUCK 3 control mode is from EN pin combine with register. But when EN3 is low, the LP87524 buck 3 will output a roughly 1V voltage which is quite strange. Please help to double check the control mode and the schematic design why the BUCK 3 will output an abnormal 1V in addition to the suspicion of reset to default OTP, customer does not use OTP.

  • 0 in reply to Zirui Su
    TI__Genius 11020 points

    Hello Zirui,

    So the control mode is EN3 combined with the register? 

    What is connected to the BUCK3? Can they disconnect the output from the load to see that the voltage is not leaking from somewhere else? In shutdown mode there is 230ohm pull-down resistor that discharges the output and it is possible that something else is overdriving the output.

    Is the buck3 switching in this state they see the wrong output voltage? 

    Did they check the other EN pins, I2C and supply voltage like I mentioned in earlier message?

    About the schematic in general, do they have more capacitance close to the load on these rails? It is recommended to have at least 2x22uF on each buck output to make sure stability is good. See more details in this app note: https://www.ti.com/lit/an/snva881/snva881.pdf

    Thanks.

    Regards,

    Tomi Koskela