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LTM4625 pre-biased to 5V - startup. Is that possible

Our device must be powered by either the USB-C port (Vusb=5V) or an auxillary supply (Vaux=20V).

We plan to use the LTM4625 to step down the auxillary voltage to the Vlocal=5V supply. The Vaux and Vusb may come on and off in any order.

The diagram is shown below.

The USB path is isolated with an ideal diode (see bottom). But the question for here is:

Do I need an isolating diode in the output of LTM4625 also?

Page 14 of the datasheet seems to say so:

Do not pre-bias LTM4625 with an output voltage higher than INTVCC (3.3V) or a voltage higher than the output voltage set by feedback resistor (RFB).

Vlocal = Vusb=5Volt if Vaux is comming after Vusb - this will violate first half of this requirement (but not the second half) and render the circuit malfunctioning in this case. 

So an isolation diode is needed.Right?

In the datasheet page 14 it continues by mentioning:The LTM4625 accomplishes this by forcing discontinuous mode (DCM) operation until the TRACK/SS pin voltage reaches 0.6V reference voltage. This will prevent the BG from turning on during the pre-biased output start-up which would discharge the output.

What is BG? This is not defined in the datasheet.

Thanks for any input you may have.

....Just in case someone need an ideal diode.

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  • Hi larsen,

    Sorry for the delay. Yes in this kind of setup the diode would also be needed on the output of the LTM4625 to prevent the pre-bias limitation of 3.3V max. Also you want to avoid the case Vaux is low condition (ie low impedance off) where LTM4625 would be off, which can forward bias the LTM4625 top mosfet's body diode conducting current back to Vaux potentially causing damage if not current limited.