Re: Logic Level MOSFET High Side Switch (almost dead horse)


PeteS wrote:
jecottrell65@xxxxxxxxx wrote:
Hello All,

I've read many of the related posts and still have some questions and
look forward to all the sage advice that is available here.

Application:

Batt(15V) ---> MOSFET ---> LDO (switched) -----> Application (also
switched)

The LDO requires some large caps (220uF +) and to reduce cost I'd like
to use cheapies (more leakage). Since I need to conserve battery life,
I'd like to isolate them to eliminate the battery drain due to the
leakage.

(Note: I know it would make more sense to probably use a switcher, but
the LDO, I think, makes more sense for the short duty cycles and the
frequency that the application is going to be used...)

I need to switch the MOSFET (high side) with 3.3V or 5V logic levels.

Application will idle at 70mA, rise to 250mA for 5mS, pulse to 1.5A for
10mS, then return to idle.

Don't you need to maintain a stable voltage for your controller anyway?
A switching controller would not draw much power without loads.

(15V-5V) * 1.5A = 15W, that's a lots of energy wasted, although for a
short time.


Driving Factors:

Single part solution
Availability
Cost
Size

Questions:

Is there a single part that will switch a 15V(Vdrain) supply at
3.3V-5V? I can figure out how to do it by tying the gate to 15V and
then switching that w/ a transistor. But I'd like to not use all the
extra parts.

Part of the problem is my understanding of MOSFETs and deciphering
datasheets.....

Any help would be much appreciated,

John


The followings might work, but does not satisfy the requirements of low
cost and part count.

For the app you need, I would use two FETs.

A P-channel for the pass transistor and a N-channel for the control.

I don't have an ascii schematic tool here (and I don't remember where
to download it), so a description will have to do.

Your control would connect to the gate of a typical N-channel small
signal device (the VN2222LL comes to mind as a through hole part, but
there are lots of other alternatives - you could just as easily use a
NPN small signal BJT here), source to ground, drain connected to the
gate of the pass transistor _through_ a resistor (about 1k). The gate
of the N channel should be pulled to ground through either a resistor
or the control circuitry.

The source of the pass transistor would be connected to the battery,
drain to the LDO. (That keeps the body diode reverse biased under
normal conditions).

You should have a resistor (perhaps 100k) from source to gate of the
pass transistor. From the gate of the pass transistor to ground you
should have a cap [perhaps 1nF] - this will slow down the turn-on of
the device so there isn't quite so huge an inrush current when you turn
it on. It will, of course, also slow down the turn off ;)

Note as I don't know how fast you need the supply after turn on, I
can't be precise about components.

As to the pass transistor, you'll need something with very low Rds[on]
as you want to pass 1.5A.

I suugest something like the Philips PMN50XP (although that's in a
surface mount package).

Rds[on] 60 milliohm (Vgs = -4.5V, Ta = 25C), Iddc - 3A to 100C.

In your application, something like this would probably be fine.

Cheers

PeteS

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