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TPS54331: input voltage lower than output voltage when ORing

Søren Larsen
Prodigy 70 points
Part Number: TPS54331
Other Parts Discussed in Thread: LM66100

Hi all,

I have a setup with a TPS54331 configured to 5V output and want to OR between a primary(24V) and secondary (5V USB) power sources.
 

The secondary 5V-usb source in only used during maintenance to update the firmware on the board. Usually, the primary power source is disconnected during the flashing process, however the design should be capable of having both sources present at the same time.
When the secondary supply is connected and the primary is disconnected, the TPS54331 has 5V-Vd2 on its input.
What happens when the input is lower than the desired output(5V)?
The input would be over the UVLO so the IC should still be on. It is correctly understood that the output then would be input*0.9(min duty cycle)?

Do you have any advice on other ways of doing the ORing circuit?

Thank you.

  • +1
    TI__Mastermind 26296 points

    Hi Larsen

    Yes if the Vin is lower than the Vo_set, the converter operates under maximum duty cycle and the vout is calculated in the equation(32)in the datasheet. This is ok for the buck converter. The thing we need to considerate is that the load of TPS54331, does if need accurate 5V all time? Is the around 4.5V output of TPS54331 acceptable for it's load? If it is, this ORing configuration should be ok.

    Thanks and best regards.

  • 0 in reply to Gui He
    Prodigy 70 points

    Hi He,

    Thank you for your answer.

    The 4.5V might be too much of a voltage drop, but I’m considering exchanging D2 with an "ideal diode" IC to get a lower voltage dro. For example, the LM66100.

    Best regards,

  • 0 in reply to Søren Larsen
    TI__Mastermind 26296 points

    Hi Søren

    Replacing D2 to a ideal diode will help reduce the voltage drop on the D2, but please notice the output will still be 5V*Dmax,  Dmax is about 0.9 as you said.

    What' s the vin range spec for the load in the block diagram?

    Thanks and best regards.

  • 0 in reply to Gui He
    Prodigy 70 points

    Hi He,

    Thank you for the answer.

    The load Vin range is actually 4.75V to 5.25V(RS485 IC) as the buck converter drives a LDO and RS485 IC.
    So it will not be sufficient to have the USB connection on the input side of the buck as 5V*Dmax will be lower than 4.75V.

    Could a solution be to place the USB and diode on the output side of the buck as shown below?

    Then replace D2 with a ideal diode of a lower voltage drop.

    Would there be any downside to this setup?
    I'm a bit unsure if a diode in series with the Buck output is strictly needed?

    Thank you.

    Best regards,

  • 0 in reply to Søren Larsen
    TI__Mastermind 26296 points

    Hi Søren

    A diode should be necessary to block the inverse energy. Anyone of the D1/2/3 will do:

    You can place the diode in series with output, which is D1, but the Vout set by feedback resistors should be higher considering the voltage drop on D1;

    If you place diode in series with inductor, which is D1, no need to consider the voltage drop on diode, but both D1 and D2 will bring power loss and cause lower efficiency;

    If place diode on the Vin side, D3, the power loss will be lower since the Iin is smaller than Iout.  But please configure the Vin-EN voltage divider to set Vin UVLO >5V,- to make sure the device won't switching because of the reverse energy of Vout.

    Thanks and best regards.

  • 0 in reply to Gui He
    Prodigy 70 points

    Hi Gui,

    Thank you for sharing your knowledge. I believe inserting D3 and disabling the converter UVLO >5V with Ren1,Ren2 and equation 1 and 2(from datasheet) is the best solution for me as it solution with the lowest power loss. 

    The D2 will then again be replaced with a Ideal diode.

    Could you please elaborate on what happens if UVLO is not set above 5V? How is the devices switching from the reverse energy? 
    I cant seem to find any literature describing this.

    Again, thank you.

    Best regards,

  • 0 in reply to Søren Larsen
    TI__Mastermind 26296 points

    Hi

    I will check and get back to you later.

    BR.

  • +1 in reply to Søren Larsen
    TI__Mastermind 26296 points

    Hi Søren

    When the Vout of TPS54331 is 5V, it will charge Vin up through inductor and the body diode of high-side switch, if the EN is higher than UVLO, the buck will switching, which is not desired.

    BR.

  • 0 in reply to Gui He
    Prodigy 70 points

    Hi Gui,

    Thank you for the explanation and help.
    I appreciate it!

    Best regards,