TPS54521: Problem with 100% duty cycle in TPS54521

Jay Vaghasiya

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).

over 5 years ago

0 Jason Wang (BSR-MV) over 5 years ago

TI__Genius 9550 points

hi,

Will give feedback on Monday after study your description.

0 Neal Zhang over 5 years ago

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 Jay Vaghasiya over 5 years ago 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 Neal Zhang over 5 years ago 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 Jay Vaghasiya1 over 5 years ago 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 Neal Zhang over 5 years ago 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

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TPS54521: Problem with 100% duty cycle in TPS54521