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WEBENCH® Tools/LMZ22003: Output Voltage Latched to 6.3V (Max Output Voltage)

Danson Evan Garcia
Prodigy 120 points
Part Number: LMZ22003

Tool/software: WEBENCH® Design Tools

I have been trying to debug this circuit for some time now. 

The LMZ22003 seems to be latching to max output voltage and even if I change the RFBB resistor, I can't seem to get the latched output voltage to go to the desired voltage level. The 5V rail for the enable pin is activated much later but that should not affect the output voltage at all. Is it possible that the chip itself is not functioning any longer? Also, the voltage at FB pin is greater than 1.6V.

Below is the schematic diagram of the part. 

-Dan

  • 0
    TI__Mastermind 18940 points
    Hello Danson,

    1.6V on FB is definitely not right.
    Did the regulator work for some time or did you get this behavior at time zero?

    If possible, please also share your board layout.

    Regards,
    Denislav
  • 0 in reply to Denislav Petkov
    Prodigy 120 points

    Hi Denislav,

    I got the behavior at time zero. At time zero, the EN pin is set to low and then sent high at a later even then the value never changes. What voltage should I expect on the FB pin? At the current configuration, I am getting 2.2V which should be correct based on voltage divider equation when Vout is 6.6V. But after doing further calculation, VFB should be 0.796V as stated on the datasheet. (Please verify this).

    Board Layout should be fairly straight forward. I took out the TOP and BOTTOM Ground pours so that it is easier to visualize the traces.

  • 0 in reply to Danson Evan Garcia
    Prodigy 120 points
    I removed R3 from the circuit to see if the VFB = Vout = 0.8V. But VOUT stays at 6.4V output and so VFB = 6.4V as well.
  • 0 in reply to Danson Evan Garcia
    TI__Intellectual 1155 points

    Hi Danson,

    it might be the case that your FB pin is damaged. The absolute maximum ratings for this pin are -0.3V to 2.5V.

    According to your latest post, you have measured now +6.4V at the FB pin. I would not trust the device any more...

    Did you try to reproduce the problem with another device or is this the only device, where this failure happened?

    Best regards,

    Michael