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TPSM63606: Using a digital potentiometer with an RFBT causes oscillation.

Shinichi Inoue
Guru 22281 points

Part Number: TPSM63606

Dear Specialists,

My customer is considering TPSM63606 for variable power supply with digital poteitiometer.

However, they encountered oscillation.

I would be grateful if you could advise.

---

I created a variable-output power supply by using digital poteitiometer for RFBT.

Increase output voltage causes oscillation.

Reducing RFBB increases the stable voltage range.

When RFBB=2.4kohm, output 7V stable and RFBB=1kohm, output 11V stable.

I suspect the oscillation is caused by FB loop due to the parastic capacitance of digital potentiometer.

Could you please advise three questions listed below?

(1) When add CFF between Vout and FB pin, result any improvement?

How much pF should be connected?

The digital potentiometer is used AD5291 between A pin(85pF) and W pin(65pF).  

(2) Would it be advantage to put  the digital potentiometer to RFBB, instead of RFBT?

(3) Would there any othe measures?

---

I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    TI__Genius 15750 points

    Hi Shinichi,

    85 pF and 65 pF seem high enough to affect the loop response significantly. It might be possible to reduce the FB resistors and increase Cout to minimize this impact.

    What is the input and output voltage range and load current? Could you also share your schematic?

    Regards,

    Niranjan

  • 0 in reply to Niranjan Nair
    Guru 22281 points

    Hi Nirainjan,

    Thank you for your reply.

    I confirmed the customer regarding Input Cap, Output Cap and Output Voltage Range.

    Input Cap: 10uF x 2

    Output Cap 47uF x 3

    Output Voltage Range 1V~12V

    Niranjan Nair said:
    85 pF and 65 pF seem high enough to affect the loop response significantly.

    Does this mean 85pF, 65pF or 150pF?

    Niranjan Nair said:
    It might be possible to reduce the FB resistors and increase Cout to minimize this impact.

    I mentioned before

    Shinichi Inoue said:
    When RFBB=2.4kohm, output 7V stable and RFBB=1kohm, output 11V stable.

    The customer have confirmed that smaller RFBB, the more effective.

    Would it be better to make more smaller?

    How many uF should the output capacitor be?

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi 

  • 0 in reply to Shinichi Inoue
    TI__Genius 15750 points

    Hi Shinichi,

    Shinichi Inoue said:
    Does this mean 85pF, 65pF or 150pF?

    The usual range of values we use for feedforward capacitances are in few 10s of pF to 100s of pF. So any of these values, being in this range, can affect the loop response.

    Shinichi Inoue said:

    Would it be better to make more smaller?

    How many uF should the output capacitor be?

    Let me check on these. Can you tell me their frequency setting and the load current too?

    Regards,

    Niranjan

  • 0 in reply to Niranjan Nair
    Guru 22281 points

    Hi Niranjan,

    Thank you for your reply.

    I'll confirm switching frequency and load current with the customer.

    When I obtain, I'll share with you.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    Guru 22281 points

    Hi Niranjan,

    I'm sorry for late reply. I was on holidays.

    I could obtain feedback from the customer.

    The switching frequency is 1.14MHz(RT=11.3kohm)

    The load current is 3.5A(start up) and 0.7A(steady state)

    The output voltage is 1~12V(adjustable by digital potentiometer)

    Could you please advise the suitable CF value?

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi 

  • 0 in reply to Shinichi Inoue
    TI__Genius 15750 points

    Hi Shinichi,

    No issues.

    I will check and should be able to get back to you early next week.

    Regards,

    Niranjan

  • 0 in reply to Niranjan Nair
    TI__Genius 15750 points

    Hi Shinichi,

    Could you ask them to try with the 2.4k RFBB and with 150 uF effective (derated) output capacitance?

    These show okay results in simulation.

    Regards,

    Niranjan

  • 0 in reply to Niranjan Nair
    Guru 22281 points

    Hi Niranjan,

    Thank you for your reply.

    I'll confirm with the customer.

    Could you please wait for a couple of time.

    When I obtain, I will send to you.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    TI__Genius 15750 points

    Hi Shinichi,

    Sure, let me know how it goes.

    I will close the thread for now, you can reopen when you hear back from customer.

    Regards,

    Niranjan