LM25122-Q1: How do we implement the BY-PASS function

April,

Thanks for reaching out with your questions.

Does the customer have the LM25122 configured in FPWM or DEM operation?

Thanks,

Garrett

Hi Garrett,

   They configured in FPWM, below is SCH, VCC: 7.6-7.8V . Any concern from your side? mode.

April,

What is the input voltage?

Also when the customer indicates that the bypass mode is not working is the MOSFET completely off or is the MOSFET pulsing on and off? Can you check the voltage between the BST and SW to ensure that it is above the voltage is above the BST to SW undervoltage?

Thanks,

Garrett

Hi Garrett,

       Input voltage : 14.1V , VIN,

       The waveform shows in bypass mode the MOSFET completely off.  Below is the voltage we measured, 

      MOSFET Q3 is off, the body diode covers ~ 0.6v voltage drop.  The voltage on SW and OT are both 14.1V, no voltage gap 0.7V, SCH is as attached, Do you have any idea?

      We also measure the below pin to GND voltage,

   1. BST to GND,   24.5V

    2. VIN: 14.1V,

    3. HO to GND ; 14.1V

    4. SW to GND : 14.1V,

    5. BOOST on output point ； boost_output : 13.4V ,  BOOST to SW = 0.6V, body diode on MOSFET conduct the voltage.

      

     _DCDC converet 400W_A1_0510_1.pdf

  Not sure why the circuit does not work on by-pass mode.

Hi Garrett,
We have tried changing the R4 to 100kohm, but issue is still exist. Please help to check if anything else will cause the issue.

Hi Garrett,
Anu update? Thanks, It is an urgent case, so need your strong support here. Thanks,