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TPS40304: Vout is abnormal

Moon Wang
Intellectual 1285 points
Part Number: TPS40304

Hi Team,

My customer designed a power with Vin=4V, Vout= 3.5V, Iout= 15A. Below is their schematic.

The R21, R28 are empty. The R18 is 2 Ohm.

Now they found the Vout is only1.3V without load. 

When they connected a 2Ohm load Resistor, there is no Vout.

How to solve this problem?

BR,

Moon

  • 0
    TI__Guru 55080 points

     

    It is likely an issue that the boot-strap is not getting enough voltage to drive the high-side MOSFET and it is tripping high-side OC.  As the output voltage comes up to 2V, the inductor ripple current increases to almost 17A peak to peak.  That combined with a VERY fast soft-start of 200μs may be triggering repeated OC shut-downs.

    You can confirm that by looking at the EN/SS pin and SW pins for repeated soft-start attempts.

    1) try increasing C26 from 3.3nF to 33-100nF for a longer soft-start.

    2) Verify that R18 us 2Ω and not the 2kΩ showing in the schematic

    3) If increasing C26 is not sufficient, you can try adding a Schottky diode from BP to the junction between R18 and C19 to improve boot-strap recharge

    4) If B+ (input voltage) is never above 6.5V, you can connect BP directly to VIN to bypass the internal LDO and increase available gate drive.

    5) Since the high-side current limit is based on the drop from VIN to DW at the TPS40304, check to see how VIN is connected to the input voltage.  If VIN at the high-side MOSFET is dropping more than the VIN at the IC, it can reduce the available peak current and trigger high-side OC.