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LP8725: PMIC 'BUCK2_FLAG MASK' bit issue

Part Number: LP8725

LP8725 PMIC has a 'BUCK2_FLAG MASK' bit (bit7) in BUCKCONTROL (0x0C) register

 

We are finding that if this bit is changed from 1 (mask the BUCK OK flag) to 0 (no masking of BUCK OK flag)

this causes the PMIC fault detection functionality to assert its RESET_N output, resetting our MCU - which it shouldn't if BUCK2 is OK.

BUCK2 is our always on MCU VDD_1V8 supply

'BUCK1_FLAG MASK' can be in either state without causing a reset.
BUCK1 is unused and disabled by our MCU firmware

However if BUCK1 regulator is changed from disabled to enabled, then the 'BUCK2_FLAG MASK' bit can be unmasked without causing a reset.
Again 'BUCK1_FLAG MASK' can be in either state without causing a reset

  • Aivaras,

    I have assigned this question to the expert on this device, and I expect you will receive a response by end of day (US Central time).

  • Hi Aivaras,

    Can you please provide a scope shot of this RESET_N behavior after disabling BK2_FLAG MASK? Please show at least the following signals:

    1. RESET_N
    2. EN
    3. VIN2
    4. Buck 2 output

    Thanks,

    Gerard

  • Attached is a trace of I2C write setting BK2_FLAG MASK bit to 0 and subsequent assertion of RESET_N 30ms later

     

    PMIC_nRESET_OUT is LP8725 RESET_N [digital trace]

     

    PMIC_EN is LP8725 EN/PWR_ON [digital trace – always high]

     

    VDD_1V8 is LP8725 BUCK2 output voltage

     

    PCIe_+3V3 is LP8725 VIN
    The test jig used for this measurement is showing more noise than actually present on this voltage ..

     

    Channel 5 and Channel 6 are LP8725 I2C interface

     

     

    After a further 30ms there is a glitch on BUCK2 output voltage – is this PMIC restarting ?
    [see second zoomed in waveform]

     

    and after 30ms more RESET_N de-asserts ..

  • Hi Aivaras,

    Thank you for providing additional details.

    Please allow me some time to investigate whether we have any hardware to reproduce this behavior. I will get back to you with an update tomorrow. 

    Thanks,

    Gerard

  • Hi Aivaras,

    Unfortunately we do not currently have hardware at our Dallas site. I am checking with our Finland team and will provide more information early next week. 

    Thanks,

    Gerard

  • Hi Aivaras,

    I apologize for the delay. Our Finland team also did not have any hardware available, so we have scheduled a build of some EVMs to investigate this issue. 

    Thanks,

    Gerard