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LMZ21701: LMZ21701 1.8v failure

Michael Hershkovich
Prodigy 230 points
Part Number: LMZ21701

Hi,

Im using on my board 3 implamentations of the LMZ21701 component for power rails: 1.8v, 2.5v and 3.3v.

This is not my first time using this switcher however first time using it for 1.8v power rail.

The problem is that I'm getting about 2.6v at the output instead the 1.8v.

I've checked the design, calculated again the passives

values, measured them and everything is OK according

to the datasheet.

I've noticed one thing which may explain this: the voltage on the FB pin is constantly 1.15v and it

supposed to be 0.8v comparing the other circuits

Implementations (3.3v and 2.5v)

All the other things seems to be ok.

The Power good is ok

The voltage output is climbing lineary to 2.6v

The resistors values is exacly as proposed by the datasheet.

The voltage input is 5v

The enable input is high to 3.3v and powersupply

responds to low input to off state.

Before I'm replacing the component with a new one

maybe the resistors values which proposed by the

datasheet are wrong?

Thanks,

Miki

  • 0 in reply to Michael Hershkovich
    TI__Guru**** 191445 points

    I do not support LMZ21701 directly, but perhaps I can offer some advice.  It does appear the FB resistor divider values are correct.  Even with 2.6 V output, the power good pin may still indicate power is good as the power good function for LMZ21701 only monitors for under voltage conditions, not over voltage.  So it appears you have a system with multiple output voltages including 3.3 V.  What type of loads do you have on your board?  Any processors or FPGA with 3.3 V I/O?  A common cause of this type of overvoltage is leakage of the 3.3 V power rail onto the 1.8 V rail thru an FPGA or processor.  You can check it by disconnecting the 1.8 V output from any external loads and trace back to find the source of the leakage.

    Let me know if this helps.

  • 0 in reply to JohnTucker
    TI__Intellectual 1155 points
    Hi Miki,

    while you are testing John's suggestions, it could also help us, if you provide us the topmark of the device.

    Is the device switching at all? Can you probe this and provide also a picture?

    Best regards,
    Michael
  • 0 in reply to JohnTucker
    Prodigy 230 points

    Hi John, you were right! the VDD_VIO_TP is connected to MAX10 Altera's FPGA (used for monitor) and for some reason even before the FPGA is burned it generates 2.6v on the analog input pin... Anyway I've removed the R790 series resistor and now the power supply is working good with 1.8v at its output.

    Thanks!

    Miki