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TPS23751 can not get stable 5V output when work in PWM mode

Lonsn
Prodigy 180 points
Other Parts Discussed in Thread: TPS23751, TLV431

Hi,

We have a custom design using TPS23751 to implement 5V poe output. Attachment is the schematic. Almost same as TPS23751-EVM except some MOSFET.

For light load VFO mode, 5V output is fine. But we meet with 2 questions for PWM mode.

1: When the load is medium, for example 3ohm-3.5ohm, the SRD output waveform switch from 0 - 12V periodically, it's not stable 0 or stable 12V. We can force it work in PWM mode by setting SRT to zero, but we get the following second issue.

2: The output can't get 5V in PWM mode, only 4.6V for medium load, as the load increase, the output voltage goes up.  It can get 4.9V when load is 4A. The pin1 of TLV431 can't fix in 1.25V, it only has 1V. It seems the total TLV431 feedback system can work normally.

Please advise how to debug this issue, thanks!

5v_poe.pdf

  • 0
    TI__Mastermind 40100 points

    Hi Lonsn,

    When you say unstable, does the output still recover? The EVM's user's guide shows the output power is in PWM mode at higher currents and works okay. It seems that you are hitting current limit sooner so the ouput voltage is dropping.

    Your schematic is nearly the same as the EVM. Did you verify your stability issue under the same test conditions on the EVM? If it is working okay then best thing to do is replace the small changes made your schematic to the original EVM schematic. You may also check impedances between the EVM and your board to make sure that some of the components were not misassembled.

    Regards,

    Darwin 

  • 0 in reply to Darwin Fernandez
    Prodigy 180 points

    Darwin,

    The output voltage will not change if the load resistor doesn't change.  The strange issue is that output voltage will rise as the load resistor decrease in PWM mode. So we don't think it hits the current limit.  

    We have made the schematic same as EVM but issue still exist.  Maybe some components are misassembled but we are still checking. We want to know if there is some debug methods to fix the problem quickly. For example, why the reference voltage of TLV431 can't  reach 1.25V. It seems the total feedback loop(TLV431, TPS23751, NA5730)  has some problems in PWM mode.

    Regards,