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TPS82085: POWER GOOD is active but the Output Voltage is wrong

Steffen Jahr
Prodigy 220 points
Part Number: TPS82085

I use the Part to generate 1.8V from 3.3V.

The Voltage on the TPS82085 Output is wrong, around 0.45V. I think this comes from the load ICs that have several power supplies.

I think, that the part is not correctly soldered.

On the filter on top of the part I measure around 2.5V. (So the voltage is higher than the Vout nominal)

Althougth tte output Voltage is wrong, the power good pin is active.

Is this behaviour correct?

  • 0
    TI__Expert 3611 points

    Hi Steffen,

    Can you send the schematic of your design?
    What are you using for supplying the input of the TPS82085 and what is the load of your application (Iout)?

    When measuring the output voltage, can you please measure it over the output capacitor.
    Also send the measurements of VIN, EN, VOUT and PG when the device is starting up, so I can review the start-up waveforms.

    Regards,
    Davor

  • 0 in reply to Davor Pavlovski
    Prodigy 220 points

    Input 3.3V com from a 15W DC/DC with a filter and a 50mOhm Resistor in series.

    The Inpit Voltage is switched on with a TPS22025, that is activated 

    The Enable(PWRGOOD_VCCINT) is delivered by another TPS82085 that starts before, so the Input Voltage had enougth time to stabilize.

    Output Current in this state is low, around 25mA

    I measured the voltage over C831. 0.45V

    I measured the Voltage in top of the TPS82085: around 2.45V (I'm not sure about the correct value)

    In >90% of my boards all is fine, I think this is an individual Error of the solder process.

    BUT the PG pin becomes active (LOW), and enables the start of further power stabilizers (what is the initial problem for my question in this forum)

    The failing Boards are in a repair loop, so I can't measure any more on them.

    Greetings,

    Steffen

  • 0 in reply to Steffen Jahr
    Prodigy 220 points

    (sorry, some clearings)

    The input voltage is switched on with a TPS22025, that is activated some ten milliseconds after the DC/DC converter voltage is on.

    ...

    BUT the PG pin becomes active (OPEN), and enables the start of further power stabilizers (what is the initial problem for my question in this forum)

    ...

    In summary I think the part works as it is described:

    When the output pin is open than the output of the part goes to a voltage that is higher than 95% of the voltage on the foldback pin.

    Output is in my case around 500% of FB pin  voltage.

    PG pin is than active. Not optimal, but according the written rules.

    Greetings,

    Steffen

  • 0 in reply to Steffen Jahr
    TI__Expert 3611 points

    Hi Steffen,

    Yes the PG pin behaves as it should according to the datasheet.
    But I was still wondering why you would have the issue of Vout being larger than nominal for some of your boards.
    Please check the soldering and check with your board manufacturer.

    Regards,
    Davor

  • 0 in reply to Davor Pavlovski
    Prodigy 220 points

    Hi Davor,

    thanks for the message.

    As I already mentioned in my first post, the part is not correctly soldered.

    When the Output has no connection, the voltage divider delivers a very small voltage to the FB pin and the output of the Part goes to the maximum.

    Unfortunately is than the PG pin in ok state.

    This meets the description "more than 95% of VOUT nominal" but in summary this is a functional deficit of the part.

    In my opinion, adding an upper border for the PG output would be a good idea.

    At the end I can live with this issue.

    Greetings, Steffen