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LMZ31710: Bypass of the Buck DC-DC converter when the input voltage is less than the output voltage specified

Islam Alaa
Prodigy 10 points
Part Number: LMZ31710
Other Parts Discussed in Thread: , LMZM23601

Hello, 

I have a question related to the usage of the LMZ31710 Buck DC-DC converter. In my case, I have an input voltage of 7.4 volts and tuned the output voltage to be 5.25v. In this case, the device should operate normally and should be able to supply the needed current to the system (up to the limit of the device which is 10A).

I am mainly asking about what would happen if the input voltage dropped below the level of the output voltage. i.e. If the input voltage is 4.7 and the output voltage is tuned to be 5.25v, how would the device behave at this point. 

I used the LMZ31710EVM-001 to test this condition, and found out that the output voltage will match the input voltage with a small drop given that the input voltage doesn't pass the UVLO limit, but what about the current capabilities of the device and would it be able to supply the system with the needed current or not?

Looking forward to your reply and thanks in advance. 

  • 0
    TI__Guru 50120 points

    Hi Islam,

    What you are seeing is the effect of running the LMZ31710 in voltage dropout. As you mentioned the output voltage will "trail" the input voltage with a diode drop. The device will still be able to provide power to load your downstream circuits. Depending on the output current, the device may enter voltage dropout earlier. For reference you can look at Figure 2-5 of a recently released power module (LMZM23601). In these graphs you will noticed that lowering the input voltage while at full load of 1A will result in the device entering voltage dropout first. The output will scale linearly to the input voltage with a diode drop.

    Regards,

    Jimmy