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Are there a situation which will cause TPS23751 switch in VFO/PWM frequently?

Lonsn
Prodigy 180 points
Other Parts Discussed in Thread: TPS23751, TPS23751EVM-104

I meet with a strange situation when using TPS23751. The 5V output load is about 7W and will change 1-2W according to load change. I observed the SRD pin of TPS23751 will output a square waveform with 125KHz frequency sometimes. It seems the TPS23751 switch the VFO/PWM frequently. It caused that sometimes the 5V output is 4.7V and sometimes it is 5V. Is it a normal situation when work in such load? Should I fix the TPS23751 in a VFO or PWM mode? How to fix the work mode?

Thanks!

  • 0
    TI__Mastermind 41760 points
    Lonsn,
    This is currently being looking into, an answer will be posted when available.
  • 0 in reply to Darwin Fernandez
    Prodigy 180 points

    Add some more debug info:

    We found that we are actually in the transition mode.  The schematic is same as TPS23751 EVM.  When the load is about 4.xWatt, it's VFO mode, and 5.xW in PWM mode.  In VFO mode, the output is stable 5V. We use the electronic heavy load to let TPS23751 EVM work in PWM mode, the output is also stable 5V.  Since the application is a movie player, the load will change from 4W-7W according to the audio volume.

    Our questions are belowing;

    1. Is it normal for VFO/PWM switch so quickly?(about sometimes 100KHz)

    2. Is it normal for voltage output drop to 4.6V when working in little-medium load(5-6Watt) PWM mode. SRT is set to 1.25V. 

    3. Is it suitable for us to change the SRT voltage to change the transition mode?

    4. When we connect SRT to ARTN and fix it to PWM mode, the voltage output is stable but only have 4.6V. How can we improve it?

    5. Are there any power limitations or other limitations for us to fix it in VFO mode? How to set TPS23751 to work in VFO mode?

    Regards,

  • 0 in reply to Lonsn
    Prodigy 180 points

    Add schematic.

    PoE.pdf

    We found that the CTL pin voltage is not stable.(2.1v - 2.3v). Maybe it is caused by the load change.

    We change the default SRT resistor 33.2K to 41.2K, now the SRT voltage change from 1.25V to 1.4V. It seems to work in VFO mode now and stable. But we are not sure if this change is OK. Please help to analyze.

  • 0 in reply to Lonsn
    TI__Mastermind 41760 points
    Hi Lonsn,
    I see no issues with this change. 1.4V is still within the recommended operating range of SRT.
  • 0 in reply to Darwin Fernandez
    Prodigy 180 points

    We think this is only a walkaround. When the load goes up around the new transition point, we will meet this issue again. How to adjust the parameters to let voltage output not drop when in PWM mode with little-medum load.

  • 0 in reply to Lonsn
    TI__Mastermind 42795 points

    TPS23751EVM-104 was designed with an extra circuit to reduce the natural hysteresis occurring between PWM and VFO (and VFO-PWM) modes using the Q8/R40 components below. This was done for efficiency "dead spot" reasons. If you want to tweak this hysteresis, you can adjust R40 or remove the circuit altogether.

    As a bit more background on the EVM design, please refer to the attached test report with focus on section 7 and 14. Section 13 shows the feedback loop response (PWM full load mode). PWR104E1_TestReport.pdf