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TPS84250 Output loading/dropping.

dharmendra gowra
Prodigy 30 points
Other Parts Discussed in Thread: TPS84250

Hi,

I have designed a small piggyback card using TPS84250 for generating the 12.5V@1A from 16-36V DC input. The board was assembled and tested and able to see the 12.5V coming. But when I connect the o/p to a load which is a oscillator (OCXO@500mA), I am seeing the voltage getting dropped to 8.5V, some times I see the voltage fluctuation b/w 8.5V to 7V at very low frequency 10-16Hz . Please let me know what could be reason for problem.

I am attaching the schematic and layout for reference and below are the experiments which I had done already and no improvements found.

1) changed the RT resistor to 200 KOhms.

2) changed the soft start cap to 100nf. 

3) added 20 ohms series resistor on the positive lines and found 9.5 volts at the IC end and 8.5 volts at the load end (OCXO). The OCXO is Expected   to take < 500mA current 

8311.SBCU_PSDB_PIGY_BACK_CARD.brd

4338.PSDB_PIGY_BACK_CARD.pdf

Regards

Dharmendra G

  • 0
    TI__Intellectual 1710 points

    Hi Dharmendra,

    Thanks for your post.  Can you please verify the followings before we can further assist you:

    1.  What was the input voltage you tested with?  Was it a voltage changing between 16V to 36V?  What was the frequency?  Can you post unload the waveform?  Moreover, was there any current limit being set at the input?

    2.  The R4 was shown as 549K in the schematic.  Were you running at 600kHz switching frequency?

    3.  What was the output waveform look like?  Can you also include the waveform?

    Thanks again, look forward to hearing from you soon.

    Best regards,

    CY Hung

     

  • 0 in reply to CY Hung
    Prodigy 30 points

    Hi Hung,

    Please see my answers.

    1) The input voltage was 28V DC  feed from the external power supply. No current limit was set at the power supply and capable to provide the current up to 10A.

    2) R4 was replaced with the 212K to set the switching freq to 900khz.

    3) I tried to take the waveform, but this time i did not see the variation but the o/p voltage is constant 8.5V

    Regards

    Dharmendra G

  • 0 in reply to dharmendra gowra
    TI__Intellectual 1710 points

    Hi Dharmendra,

    Can you please confirm if the Pin 25 has been connected to Pin 21, 22, 23, 24 (PH)?  If so, the unit will not function properly.  The Pin 25 is DNC (Do Not Connect), and should be left open.

    Please let me know should you have any further question.

    Thanks,

    Hung 

  • 0 in reply to CY Hung
    Prodigy 30 points

    Hi Hung,

    Yes Pin 25 is connected to PH in layout, which is mistake. I have initiated the correction in the layout. I will update you the result by tomorrow.

    Regards

    Dharmendra G 

  • 0 in reply to dharmendra gowra
    TI__Intellectual 1710 points

    Thanks.  Please let me know if we can be of further assistance.

    Hung