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LM5121: LM5121 in a DCDC 24V to 60V@3A boost converter

Part Number: LM5121

Dear Team,

Can you please support the following customer question below ?

The design is now in bring up tests validations.

The design is set: to: swFREQ =400KHz, Mode= Diode Emulation , External limiting for the Inductor current MAX pk  = 12A (Rsense=5mOhm + Ext voltage offset)

The problem is: AFTER powerup, I am changing the load from Zero to 1A and the controller enters to a latch mode.

Watching after the Various controls Inputs of the controller,  NO Fault condition was noticed (as you can see in the attached pictures to this mail)

No current limit was reached,

No UVLO was crossed down,

No SS_pin was pulled down.

 

I would appreciate if you / TI team can put some light solving it.

In addition Two Questions:

1) how I can configure the design to work  in “Bypass Operation” mode

2) how can I control (AT POWER UP) the time between the DG raise to VGS Threshold detection time ?  (how the controller detects it?)

Please see below waveforms and schematics.

   Scanned file.pdf

Best regards,

Nir.

  • Hi Nir,

    Based on the waveforms it looks like the control loop might not be stable. There are few things I would like you to check

    1. Please ensure that the RT/SYNC resistor is 22.5kOhm.
    Based on the schematic you attached the resistor value was not changed to make the switching frequency 400kHz.

    2. change the compensation values
    Based on a quick calculation of the control loop it looks like it might be marginally stable for some output power cases. Please try the following values.
    -R25 = ~45kOhm
    -C20 = 47nF
    -C25 = 100pF
    These should provide a stable output voltage.

    3. Were all the ceramic capacitors on the output removed?
    I suggest adding some in to help reduce the high frequency ripple.

    Please let me know if you have any questions.

    Thanks,

    Garrett