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LM5122: Bypass Operation

George Engel
Prodigy 10 points
Part Number: LM5122

Hello,

 

We have an 8-phase design we are trying to operate Forced-PWM mode and switch to Bypass operation when the input voltage exceeds the output voltage and Forced-PWM mode when the input voltage is below the output voltage.

 

Typically, the output voltage set point is ~27.4VDC and the input voltage can range from ~24VDC to ~36VDC dependent on load power requirements. Once the converter is in Bypass operation, we are not seeing the converter switch back to Forced-PWM mode. See the attached scope capture (Ch1-Output Voltage, Ch2-Anode D6, Ch3-Input Voltage, Ch4-Input Voltage).

 

What input/output conditions are required for the controller to return to Forced-PWM mode?

 

 

  • 0
    TI__Guru* 76745 points

    Hi George,

    Thank you for designing with the LM5122.  I am afraid your circuit did not run in bypass mode. Bypass and FPWM mode is entered with SS completes soft start.  You SS capacitor is very large, and the it seems the high side FET is never turned on, and it just use D6 (equivalent to high side FET's the body diode) to conduct. This is why your Vout has about a diode drop below the regulation point when VIN is high.  Then, when Vin drops, your output voltage stays high (seemingly you did not apply any load), and it is higher than the 27.3V setting point, no wonder it does not switch. 

    Can you modify your circuit SS to the following, and turns on the high side FET in bypass mode when VIN is high, then Vout will follow VIN when VIN is dropping, and once Vout gets below the regulation setting point (27.4V), the circuit will run in FPWM.

    Improved SS.docx

    Thanks,

    Youhao Xi, Applications Engineering