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LM3478: LM3478 boost converter with a Vin higher than Vout

Diogo Condeco
Prodigy 10 points
Part Number: LM3478

Hi All,

My name is Diogo Condeço and I work as an Electronic Engineer for Glory Global Solutions.

We are currently using the LM3478 in one of our boards and we would like to know if there is an issue with using it with a Vin higher than Vout.

In more detail:

Our Vin can be from a battery (10 to 14.5V) - in this configuration the DC/DC controller will be working as a normal boost converter. The circuit is working no issue there.

Alternatively, when there is external power available Vin can be from 21.6V to 26.4V.

I have configured the DC/DC to output 21V. This means when Vin is present, from the external power supply, at the resistor divider responsible for getting the VFB to the LM3478 there will be >21V, thus VFB > 1.26V. This means Mosfet is not switching.

So my output voltage will be something like 21V in battery mode, and Vout = Vin (minus the drop in the diode) in power supply mode. My only constrain is to get 21V at the output, but I can get 22, 23 or up to 26.4V with no problem. So Vout can follow the Vin voltage from the power supply.

Off course the Diode and the inductor of the boost converter will have to handle the amount of current required by the circuit load. I am also complying with the electrical constrains: VFB < +7V and Vin < +45V.

I know this is not the common use of a boost converter, but I am wondering if there is any reason not to use it like this.

I have tested it, and it is working fine, but I need to understand if it can be used in this conditions in a reliable matter.

Thanks in advance for you attention,

Best regards,

Diogo

  • 0
    TI__Mastermind 21720 points

    Hi Diogo,

    Thanks for reaching out with you question.

    There should be no issue to have the input voltage above the target output voltage. As you mention the output voltage will follow the input voltage in this case. The only thing that needs to be assured is that the diode and inductor can handle the current when operating in this mode.

    Please let me know if you have any questions.

    Thanks,

    Garrett