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TPS796: Output Voltage drops from 2.5V to 2.2V

Nanjunda M
Expert 2485 points
Part Number: TPS796

Hi,

We are using TPS79625DCQR in one of our designs.

We are converting from 3.3V to 2.5V in our case. Our total continuous current requirement is about 800mA. Attached the circuit schematics below.

On the board we are seeing the voltage drop from 2.5V to about 2.2V. (Ref image 5V_3.3V(FPGAbank2con).jpg)

Suspecting on the devices connected we tried lifting the pin4(output pin) of the device and we observed the output. We see that the Power is about 2.5V swinging(Ref: 5V_2.5V(red)Pinlifted.jpg)

After this to clear the board side, we provided an external supply of 2.5V. Observed that the circuits were driving just 360mA. & when probed we did not observe any swings as in previous case.

Now what could be the reason for this dip in the voltage.

Can anyone please help me with this.

Thanks & Regards,
Nanjunda M

  • 0
    TI__Mastermind 23361 points

    Hi Nanjunda,

    Thanks for providing the scope-shots and schematic. The device TPS79625DCQR  you used is a fixed version device. There are no external resistors needed to set the output voltage. Please consider removing the extra C138, R91, R92 and recheck the output. Please also not to lift output pin when you check the output voltage, as the output pin needs to have a minimum 1uF capacitor for the LDO to be stable.  

    Regards,
    Jason Song

  • 0 in reply to Jason Song
    Expert 2485 points

    Hi Jason,

    As you can see the R91 & R92 are DNP(not mounted) on the board but we have mounted the C138 as per the datasheet suggestions

    Please let me know whether this can cause issue for the dip in the voltage?

    Please note that the lifting of the pin was done just for experimental purpose not for the usage, you can ignore this if the behaviour shown with waveform is expected.

    Thanks & Regards,

    Nanjunda M

  • 0 in reply to Nanjunda M
    TI__Mastermind 23361 points

    Hi Nanjunda,

    The other side of the C138 capacitor needs to be grounded instead of connecting to Vout. You can try to remove this cap to see if the output voltage issue still exists.

    Regards,

    Jason Song

  • 0 in reply to Jason Song
    Expert 2485 points

    Hi Jason,

    Thanks for the inputs.

    When we removed the capacitor (placed across the NR pin and output) we observed the proper output.

    Regards,

    Nanjunda M