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TPS7A90: TPS7A9001DSK, in 5.25V to 4.5V and 5.25V to 5V circuit, output voltage will not ramp to ideal value quickly.

Max Han
Intellectual 2260 points
Part Number: TPS7A90

Hi,

TPS7A9001DSK is used by customer.

5.25V to 4.5V/0.5A

5.25V to 5V/0.5A

schematic is as below.

at startup test, customer see that output voltage will ramp up to a value which near ideal value, then going up slowly to ideal value.

for example. 5.25V to 4.5V/0.5A

voltage ramp up to 4.278V, then going up slowly, after about 40ms, output voltage equals to 4.5V.

customer wants to konw what causes this phenomenon?

it there method to solve it?

thanks.

  • 0
    TI__Intellectual 2260 points

    5.25V to 4.5V/0A

    5.25V to 4.5V/0.18A

    5.25V to 4.5V/0.5A

    5.25V to 5V/0A

    5.25V to 5V/0.18A

    5.25V to 5V/0.5A

    all above test, output waveforms are similar.

  • 0 in reply to Max Han
    TI__Expert 6825 points

    Hi Max,

    This is the correct behavior of the TPS7A90. What is happening here is that the soft-start circuit is on until Vout reaches 97% of its nominal value. Once Vout gets this high the NR pin (reference to the error amplifier) is charged through the internal RNR resistor until it reaches the proper reference value.

    One thing you could try is reducing the size of the NR/SS capacitor and then changing the startup from the fast setting to the slower setting to get the same startup time you would have using the other SS_CTRL setting.

    Let me know if this helps you out.

    Regards,

    Mark