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By mooreaa

I am trying to drive a high side mosfet switch using a logic level signal from a USB power line... when the USB power is present, I want to shut off the other power source. When the usb is not present, I need to have the mosfet conducting to allow power from this other source.

The other power source has a max input voltage of 60V, so I need some kind of driver to control the mosfet.

I'm looking at the Linear LTC4440-5, but I just not quite sure how to wire it up, maybe I'm looking at the wrong kind of driver?
By UofMEShop
How much current do you need to switch ?
An optically coupled mosfet ("OptoMOS", "PhotoMOS", etc) will provide the high side isolation. Your USB power drives its internal led at say 10 mA which then shines onto and turns on an isolated mosfet. The AQV252G will do 60 V at 2.5 A
By lyndon
I think a depletion mode FET will do what you need. But they can be hard to find in high-current models and you will need a negative voltage supply.

Look at fig. 10 here: ... =191900470

Alternately, can you use a NC relay? If you're only switching a few amps, the coil currents can be quite low.
By Philba
You should tell us what the other power source is as that will have some impact on what you should do. What feeds the power supply and what kind of power supply is it? Do you want to switch the input to the power supply or the output from it.

Also, are you trying to power some electronics from one of 2 sources? One being USB and the other the undefined power source?

Finally, what kind of current draw are you looking at? USB can only supply up to 500 mA and notebooks often times are limited to 100 mA.

I'm not saying that the two suggestions are wrong but with out more information, it's hard to know either way.
By mooreaa
I have two power sources to power my mcu, USB and VIN.

VIN is from 5 to 50V (motor controller board so big batteries can be used).

The VIN path goes through a switching regulator to give me about 800mA at 3.3V.

But, when there is USB power I want to disable the VIN so that the device is USB powered and there is no drain on the battery. The USB is sufficient to drive the circuit so the power there is not an issue.

What I am trying to do is switch off the battery when the USB input is present...

Since VIN is high voltage, I am trying to employ some kind of a mosfet based switch. The mosfet would help to minimize the power loss across it as well as allow me to handle the high voltage input.

I tried something like this that works in simulation but I know that my VBoost-TS voltage goes beyond that of the allowed values...

By Philba
Does your switcher have a control input? Most do. Wouldn't it make more sense to control the switcher rather than control the input to the switcher? You should be able to find one for the first stage that has low standby current.
By mooreaa
Heres another approach I found:

Linear Tech's LT4356-1

The document describes a simple application using a high side n channel fet which almost works... The input in this case would be the SHDN pin...

the one problem with this method is that the shdn pin is inverted!!! argh. So I need to invert the signal here... however.... I need to drive the inverter from the high voltage line, since its logic needs to be high when the usb power is disconnected and low when the usb is plugged in... in this manner.

So what can I do for an inverter which is powered from 50-60v... but produces an output of 5v?
By Philba
Use an NPN transistor with a pull up to V+. R1 should be set to limit Ib to 1 Ma or so, R2 in the the range 4.7K to 10K and R3 in the 100K+ range. Any NPN with the right voltage max will work. The 2N3904 WONT work, too low Vce Max. You might want to add a divider stage on the output to ensure you get 5V.
One other possibility is an emitter follower - might be simpler by a couple of resistors.