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TPSM82822: Output voltage deviates from the design value

Hung-liang Chen
Prodigy 80 points
Part Number: TPSM82822

I used TPSM82822 to generate 3.3V auxiliary power for CPU AM3354BZCZA80.
When I run the circuit alone, 3.3V is provided correctly.
But when I supply this power to the CPU AM3354BZCZA80, the 3.3V will become to about 4.2V and cause damage to the CPU.
I would like to know, what are the possible reasons for this circuit to make the voltage deviation from the value I designed?
thanks!

  • 0
    TI__Genius 13160 points

    Hi Hung-liang,

    Do you have more caps close to the CPU? More than 47uF on the output can cause it to be unstable. Is the output oscillating or stable at 4.2V? Would be great if you can get some scope shots of VOUT and SW node when the CPU is connected. SW node would be one terminal of the inductor on top of the IC.

    Best regards,

    Varun

  • 0 in reply to Varun John
    Prodigy 80 points

    Hi Varun,

    Thanks for your reply. but i need your more support.

    Yes, I have more caps close to the CPU and eMMC. The total cap is 46.8uF.
    Although the value is less than requirement, I still remove 30uF cap.

    The action still do not work! following is the D2D output at start
    before change.

    after change

    And i probe the node at following fig. there is no switching signal. it is a 4.2V of a flat signal.

  • 0 in reply to Hung-liang Chen
    TI__Genius 13160 points

    Hi Hung-liang,

    Is there anything else connected to the 3.3V rail that can inject a voltage into the rail? You can also mail me (v-john@ti.com) your complete schematics.

    Is the feedback voltage higher than the target of 600mV? If so, can you try adding an extra load to the output to see if it discharges and regulates back to the 3.3V?

    Best regards,

    Varun

  • 0 in reply to Varun John
    Prodigy 80 points

    It is glad to discuss with you! The following is the test result from your suggestion.

    Since the feedback is 800mV, and there is no switching it looks like the device is in PFM mode.

    After I added a dummy load 150ohm, everything is all right. the Vfb is 600mV and the output is back to 3.3V.