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In general, will a buck converter allow current to pass through if Vin is less than the configured output voltage?

Specifically, I'm reading the TPS54308 datasheet and I'm not seeing an answer to my question.

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    \$\begingroup\$ Some bucks do, but some may have an under-voltage lockout circuit (UVLO). \$\endgroup\$
    – user57037
    Commented Mar 25, 2019 at 5:13
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    \$\begingroup\$ Yes , UVLO is the designer’s responsibility \$\endgroup\$ Commented Mar 25, 2019 at 5:14
  • \$\begingroup\$ Thanks, I didn't realize that Under Voltage Lock Out (UVLO) was the term I was concerned with. \$\endgroup\$ Commented Mar 25, 2019 at 5:25
  • \$\begingroup\$ A "portable" two-way radio system I once worked with had a battery pack that could be charged from the cigarette lighter of a vehicle. It could also be charged from a powersupply. Users complained that they would charge the radios overnight, get in the truck with a radio, and drive to a work site - radio battery is suddenly empty. They were plugging the radio in to charge in the truck, then starting the truck. The charger for the battery pack was a switching power supply the had no UVLO. The radio battery pack was accidentally being used as a booster to start the truck. \$\endgroup\$
    – JRE
    Commented Mar 25, 2019 at 10:29

1 Answer 1

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Yes, many Buck regulators (including the TPS54308) will do this. As long as the input voltage doesn't trip a UVLO protection (either internal to the chip or part of your circuit), it will pass through at 100% duty cycle trying to reach the desired output voltage. If you want a higher cutoff voltage than the chip provides, you'll need an external circuit to measure the input and drive the EN pin accordingly.

From your datasheet:

UVLO specs

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  • \$\begingroup\$ Based on the excerpt in your answer and section 7.3.5 of the same document, it seems like I can configure a higher UVLO value using the EN pin, but I cannot go below the internally configured for UVLO value for the Vin pin, correct? \$\endgroup\$ Commented Mar 25, 2019 at 5:24
  • \$\begingroup\$ Right. If your VIN drops to say, 3.0V, it's going to shut off no matter what. If you wanted a cutoff of 5V you could set up a comparator with the output going to the EN pin. The frequent hurdle there is having some other supply rail to compare to. \$\endgroup\$
    – Joe Baker
    Commented Mar 25, 2019 at 5:30
  • \$\begingroup\$ It looks like the en has a specific cutoff voltage so you don't need a comparator. Just put a voltage divider on Vin and feed the output to en. Data sheet explains it. \$\endgroup\$
    – user57037
    Commented Mar 25, 2019 at 7:34
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    \$\begingroup\$ Please, specify that this answer is specific to the TPS54308. There is no general answer for each buck converter, some have 100% duty, some have not. \$\endgroup\$ Commented Mar 25, 2019 at 9:30

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