Why use an FET on battery charger output?

Question

Hi,I'm working on a battery charger for a Palm Pilot.  This fits inside the modem docking case, where space is limited.  It basically takes 12v, regulates it down to 4.1v with a LM317L, and feeds it to the docked palm pda.There's a circuit on the web that does this:  The circuit has an FET as a switch on the output, to prevent the palm from discharging into the LM317 when 12v is not applied.  Can this FET simply be replaced by a diode?  Since the circuit has a variable regulator, it can be adjusted up to compensate for the voltage drop across the diode.If this is right, then a 3 lead part gets replaced with a smaller 2 lead part.  And the diode actually takes up zero space, as it could be used as the output wire with some spaghetti tubing or heat shrink.Am I missing something here?

Rheilly Phoull · Accepted Answer

One day  got dressed and committed to textSeems feasible to me, dont forget the diode will drop more volts than theFET.--Regards ..... Rheilly Phoull

Paul Jones · Answer

You could do a LM431 and a few PNP's to pull down the 12vdc to 4.1 . But the LM431 needs to draw thru a 300 ohm R to start a PNP or the 431 will turn on prematurely at 2.0 vdc. and there are hundreds of circuits .... I don't like the 317 .

It's easy to add a coil and a diode to make the LM431 circuit a "switcher" or "buck" regulator ....

Lee · Answer

A larger voltage drop is the only difference, right?  The drop can be adjusted out on the LM317.Is there any other benefit in using an FET?Rheilly Phoull wrote:

kell · Answer

The mosfet works as a kind of variable resistor. At higher output currents the gate voltage drops and its resistance increases, limiting current. A series diode won't do that. As a current-limiting scheme, the mosfet is about as simple as you can hope for. Also, he says exact voltage is critical with lithium batteries; you can get the 317 output dialed in very precisely. With a diode on the output, you will have a varying voltage drop across the diode according to the current.

Why use an FET on battery charger output?

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