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-12V Generation Discontinuance issue of TPS63700

JongRyung Park43
Prodigy 90 points
Other Parts Discussed in Thread: TPS63700

Hi everyone.

I'm using TPS63700DRC DC/DC invertor to convert 5Vdc to -12Vdc.

Today we performed power interruption testing which is to interrupt power for 120~130msec during EUT operation.

We performed the testing with measuring -12V and SW waveform.


When we cycled (5Vdc off --> on) 5Vdc with 130msec duration, sometimes (1 or 2 times per 20 power cycle) -12V was not properly generated.

DC/DC invertor circuit we are using is exactly same as the circuit in the datasheet of TPS63700.

Whenever we met -12V fail, SW pulse stopped. The voltags of FB, Vref and COMP were 1.1V, 1.16V and 2.7V respectively when we met -12V fail.

The load of the DC/DC invertor is resistive load (fourteen 10kohms resistors are connected in parallel).


For troubleshooting, we changed 4.7nF capacitor of COMP pin with 1nF, 10nF and 1uF. For three capacitor change cases, -12V failure did not occurred

for 100 times 5Vdc cycle. We also removed 4.7nF capacitor then performed test and -12V came out well.


So, I have two questions as below:

1. What is the function of COMP pin? Why the performance of TPS63700DRC varies per the capacitance change of COMP pin?

2. I intend to remove or change the capacitor with 1uF. Is the removal or change the capacitor acceptable solution?


Thanks,

K. H. Han

  • 0
    TI__Guru**** 150820 points
    Is such a power cycle likely to occur in your end application?

    You should look at waveforms of Vin, Vout, and inductor current to see what is different about the times when the IC does not start.

    The COMP pin is part of the control loop compensation. It affects the IC's speed of response to transients. You can certainly change its value, but I don't think a value 200x greater than the standard value is a good idea. More like 10nF or 22nF seems reasonable. You should do detailed system testing of start-up, transients, etc. to validate the new value.
  • 0 in reply to Chris Glaser
    Prodigy 30 points

    Thank you for your answer Mr. Glaser.
    I'm a colleague of JongRyung.
    We already measured the waveform of Vin, Vout and SW.
    Due to the SW pulse discontinuance, Vout went to 0V.
    Vin waveform has no suspect things. Actually we are using same DC/DC invertor at the same board for another -12V generation.
    We compared Vin waveform with the other DC/DC invertor. Although Vin waveform of both invertors were exactly same, one of them only has this issue.
    The only different thing is load. One invertor has capacitive load but the other has resistive load.
    I knew that TPS63700 is very sensitive about load, so we tried to adjust COMP capacitor.
    Because detail information about COMP pin is not included in the TI datasheet we don't know about the detail function about COM P pin.
    But, we know that if we increase or decrease the COMP cap. -12V issue can be improved. 
     
    We chnaged the COMP cap. to 22nF based on your answer then performed power intrruption testing.

    -12Vref still have fail but the frequency of failure decreased so we intend to change 22nF to 47nF.

    Would this change be OK?
    Thanks,

  • 0 in reply to Seong Jong Lee
    Prodigy 90 points
    We performed the Power Interruption test with several COMP Cs.
    - COMP C = 47nF, fail at 430 times
    - COMP C = 330nF, OK util 1000 times

    considering inside COMP block of TPS63700, using 330nF in COMP CAP have any problem ?
    (as you know we don't know inside TPS63700, so we ask you..)
    Next week we will do detailed system testing of start-up, transients, etc. with 330nF.

    Best regards,
  • 0 in reply to JongRyung Park43
    TI__Guru**** 150820 points
    Is it the capacitively loaded rail that does not start sometimes? How much capacitance is on this rail? What level is Vout at when it tries to restart? Does adding a small load to discharge Vout affect the startup success?
  • 0 in reply to Chris Glaser
    Prodigy 90 points
    Is it the capacitively loaded rail that does not start sometimes?
    How much capacitance is on this rail?
    -> No, Load cap is not. only 4.7uf x 4 is in our circuit(same to circuit in datasheet).
    What level is Vout at when it tries to restart?
    -> -13.4V
    Does adding a small load to discharge Vout affect the startup success?
    -> output resistance is totally 714ohm(Vout to GND) in the circuit.
    I didn't add more a small resistor.do you think a small resistor is need ?
  • 0 in reply to JongRyung Park43
    TI__Guru**** 150820 points
    I think we discussed this in your other thread last month. I do not recommend attempting to start into a pre-biased output voltage. You need to discharge the output voltage before attempting to restart.

    As well, the larger COMP cap may create a longer SS time which reduces the peak currents seen at startup.
  • 0 in reply to Chris Glaser
    Prodigy 20 points

    I will using a 330nF(COMP Capacitor).

    What do you think?

  • 0 in reply to user4348285
    TI__Guru**** 150820 points
    It sounds rather large. Be sure and validate your circuit's operation with that value.
  • 0 in reply to Chris Glaser
    Prodigy 20 points
    I have validated my circuit.
    Yesterday I tested in a low temperuture (-40deg).
    --> result: No problem.
    Today I test in a High temperuture(71deg).

    If COMP Capacitor has 330nF.
    I want an answer to whether there is any problem with the internal blocks of the TPS63700.
  • 0 in reply to user4348285
    TI__Guru**** 150820 points
    For sure, we haven't validated a value that high but I don't see a reason why it would not work. You have done well by validating the circuit at temperature extremes.