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TPS54521: Problem with 100% duty cycle in TPS54521

Jay Vaghasiya
Prodigy 20 points
Part Number: TPS54521

Hi,

I have designed a circuit using the TPS54521, Vin=5v to 12v, Vout=5.1v. I understand that if Vin drops below 5.1v, Vout should track Vin because ic having 100% duty cycle.

But in my circuit if input voltage drops below 5.3V, Output voltage is going to completly turn off. After hiccup cycle period its againg turn on. This cycle is continuously happen. Its not follow Vin. Iout is 1A.

IC is not getting hot.

Is there any mistack in my circuit desing?

In scope Yellow channel is Output voltage and Blue channel is voltage accros low side mosfet(PH to GND pin).

  • 0
    TI__Genius 9550 points

    hi,

    Will give feedback on Monday after study your description.

  • 0
    TI__Expert 4720 points

    Hi,

    The reason is that the EN divide resistor is not properly designed.  if input voltage drops below 5.3V, Output voltage is going to completly turn off, because EN voltage drops bellow threshold and IC turns off. 

  • 0 in reply to Neal Zhang
    Prodigy 20 points

    I have make EN divider resistor based on below formula given in datasheet. And for R1=150k, R2=39k, Vstart = 5.69V and Vstop=4.98V  comes.

  • 0 in reply to Jay Vaghasiya
    TI__Expert 4720 points

    That‘s right. So if you want the device to work at Vin<5V, you need to adjust the EN resistors accordingly.

  • 0 in reply to Neal Zhang
    Prodigy 10 points

    I know that, but my question is why its not work properly upto Vin 5.2V when turn off threshold is set to 4.9V. I have also checked with set Vstart@ 5v and Vstop @4.6v, still its not works fine below 5.3v.

  • 0 in reply to Jay Vaghasiya1
    TI__Expert 4720 points

    Hi,

    Here are some notes to debug:

    1. there is current flowing on Vin wire, so if you set it as 5.3V, the actual Vin that IC see is smaller. The EN voltage IC see is also smaller. To avoid it, you can set Vstop to far smaller than 5V. For example, set it to 2V.

    2. The IC can work at 100% duty cycle when Vin is connected to split power, rather than tie to PVIN. When Vin is tied to PVIN, IC may not work well at Vin<4.8V. I suggest you measure the C21 voltage to see the real Vin now.

    3. What's your spec when using this part? Vin, Iout? If you need the IC work at low Vin, suggest connect PVIN and VIN to different power source