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TPS23758: The SR MOS and TPS23755 comparison

Part Number: TPS23758
Other Parts Discussed in Thread: TPS23755, PMP7355

Hi Expert, 

My customer is using TPS23758 in their IP CAM product. They need the 5V/ single output. Based on their request, we have proposed the TPS23758 to them. 

Recently they are doing the test on our EVM board, and want to make some modification. They ask some question regarding to TPS23755 and Diode Rectify. I have checked other discussion thread in E2E and see there are some discussion regarding to above questions. However still have some item need your help. 

1. Customer replace the SR MOS by Diode on TPS23758EVM. They found output voltage change from 5V to 10V. From my understating the Diode mainly affect the efficiency, why the output voltage also change?

2. Do TPS23758 has internal algorithm for the SR control? Because the parameter of 755 and 758 looks like the same, customer want to know the detail of the difference. 

Thank you!

Pin

  • Hey Pin, 

    To answer both questions, the TPS23758 is made to operate in PSR with a synchronous (MOSFET) flyback ONLY. And the TPS23755 can support a diode flyback PSR ONLY. The TPS23758 cannot support a diode flyback with good regulation. The reason why is proprietary, so we can't share with customers, but the information they need to know is the TPS23758 will not have good regulation in a diode flyback.

    We'd strongly suggest switching either the design (to optocoupler feedback, or using a MSOFET) or changing the IC to the TPS23755. 

    They could recommend the PMP7355, but this design is not very efficient so that is why we do not recommend 5V diode flyback.

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • Hi Michael, 

    Thank you for the information. I will definitely highlight the the compatibility of SR MOS and Diode. 

    I have one follow up question, if customer use TPS23755 in the 5V system, is there any concern?

    Or PMP7355 will be the better option for customer?

    Thank you!

    Pin

  • Hey Pin, 

    The TPS23755 has the same design constraint that it needs to be a diode flyback for PSR. 

    When we go to 5V in the TPS23755, the issue is the diode flyback. with a 0.7V diode drop, this is more significant than in a 12V design. This is a major factor in the efficiency, so the diode flyback at 5V is very low. 

    So overall, we typically don't recommend it, unless the customer can accept it?

    We also have a 5V buck (non-isolated) design with the TPS23755 that i can share?

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments