P-Channel MOSFET Load Switch w/ Soft-Start

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I am making a board that will have 3 high-side P-Channel MOSFETs used
a load switches to 3 different loads. They will switch on/off 12V,
24V, and 36V battery packs. They need to have soft-start on-switching
(and fast off-switching). Load currents will be 1-10Amps. They
will be controlled by PIC outputs. I don't know much about using
MOSFETs. Is there such a thing as a P-Channel MOSFET driver with soft-
start that will work in the 12-36V range? I have seen a couple parts
that would be perfect but they're for low voltage supplies like 5V.
Or if I made my own driver with discrete parts, I'm not sure how to do
it. It needs to be small, I was hoping to do it with an SOT-23-5
sized part. Can anyone help? Thanks.

 

[[email protected]]

Soft-start ON rise time would be nice to have it be a few
milliseconds, or adjustable by a 1206 capacitor. As for switch-OFF
time, I don't really have an answer. From reading datasheets of other
MOSFET drivers, I have seen fast OFF times as being a feature of the
drivers, so I assumed that meant fast OFF switching time was good
thing to have. This design will be for a generic battery pack
controller board that will potentially be powering several different
types of electronic loads.
An example of a part I'm looking for would be similar to the Fairchild
Semi FDG901D. This part would be perfect but it only works to 6V.

 

[Rene Tschaggelar]

I am making a board that will have 3 high-side P-Channel MOSFETs used
a load switches to 3 different loads. They will switch on/off 12V,
24V, and 36V battery packs. They need to have soft-start on-switching
(and fast off-switching). Load currents will be 1-10Amps. They
will be controlled by PIC outputs. I don't know much about using
MOSFETs. Is there such a thing as a P-Channel MOSFET driver with soft-
start that will work in the 12-36V range? I have seen a couple parts
that would be perfect but they're for low voltage supplies like 5V.
Or if I made my own driver with discrete parts, I'm not sure how to do
it. It needs to be small, I was hoping to do it with an SOT-23-5
sized part. Can anyone help? Thanks.

P Channel FETs in the supply are controller by an NPN on the GND.
Meaning have a pullup on the gate of 20k or so, and have an NPN with
an emitter resistor on GND forming a current sink. The base can be
connected to the logic.

Choose the emitter resistor such that sufficient current flows while
the voltage on the gate doesn't exceed 10V or so.

Speed is an issue for the resistor values.

Rene

 

[[email protected]]

What is the advantage of using an NPN as the P MOSFET controller
instead of using a small signal logic-level N MOSFET to control it??