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TPS63700: Does not start if 4.10V < VIN < 4.25V

user4867287
Intellectual 435 points
Part Number: TPS63700


Hi,

The TPS63700 does not start if Vin>4.10V AND Vin<4.25V. It works fine while Vin<4.10 OR Vin>4.25.

The circuit is the same as figure 2 in the TPS63700 data sheet (however, R3=680k).

It always start switching 600us after EN is set. It switches for 500us while the output ramps down to -5.5V and then stops switching.

When ok it thereafter rests for 100us while the output ramps up to -5V. It then continues to switch and regulates the output to -5V.

When not ok (4.10V<Vin<4.25V) it does not start switching again, and the output ramps up to 0V. 

Any idea what to do?

Reagrds

C-H

  • 0
    TI__Expert 7925 points
    Hi,

    Could you please show us the scope plots during the startup?

    Best regards,
    Milos
  • 0 in reply to Milos
    Intellectual 435 points

    TPS63700.pdfHi,

    Plots enclosed.

    Best Regards

    C-H

  • 0 in reply to user4867287
    TI__Expert 7925 points
    Hi,

    That is strange, I will check your configuration tomorrow on the EVM. Is the only difference R3=680 kΩ? Is your PCB layout as close as possible to the one shown in the datasheet?

    Best regards,
    Milos
  • 0 in reply to Milos
    Intellectual 435 points
    Hi,
    The only schematic changes are R3 680k and D1 PMEG3015EH.
    I have also checked the layout in the old CAD software where this old board originally was designed.
    I can see that several layout improvements can be done. There are also some bad component selections. Input and output decoupling capacitors are of 0603 size.
    After checking the current capacitor supplier I can see that the 10uF expected is not more than 1 uF when DC-biased by 5V.
    I suspect this causes high switching noise that ocassionally triggers internal protection (over current?) and stops the TPS63700 switching.
    Do you have any other idea?

    I increased the capacitance and the problem dissappeard. But I do not know how much "functional margin" I have.
    Is there a way that I can measure/check the margin I have from triggering internal protection?

    Somehow I need to decide if increasing decoupling capacitance is enough or if I also must improve the board layout.


    Best Regards
    C-H
  • 0 in reply to user4867287
    TI__Expert 7925 points
    Hi C-H,

    I just checked the startup behavior on the EVM, with component values as on Figure 2 from the datasheet, and R3 changed to 681 kΩ. There were no startup issues at any input voltage.

    Please follow the recommendations from the datasheet regarding the required capacitance, including the voltage/temperature/ageing derating. Too little capacitance will make the device unstable.

    And also try to follow the recommended PCB layout as close as possible. We can also check the layout. If it is too sensitive to put it here publicly, you can send it to lbs_request@list.ti.com.

    Best regards,
    Milos
  • 0 in reply to user4867287
    TI__Expert 7925 points
    Hi C-H

    We haven’t heard back so we're assuming this resolves your question. If so, please click the "This resolves my issue" button. If not, just post a reply below, or create a new thread if the thread has locked due to time-out.

    Thanks and regards,
    Milos