Logic Level MOSFET High Side Switch (almost dead horse)

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

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.

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

Pete,

That is essentially what I had from another engineer. I guess that is the tried and true way to skin the cat. A sincere thanks for taking the time to pound out the explanation on the keyboard.

linnix,

MCU uses a few uA 99% of the time, a few mA 0.9% of the time... I figured for simplicities sake the power overhead in the LDO was worth the savings in parts and assembly.

Thanks to all,

John

So, you need a separate stable PS for the MCU anyway. Furthermore, to ensure a stable current draw of 1.5A, you need enough time (probably more than the few msec load time) to ram the LDO up and down. Power is still wasted during ram-up and ram-down.

Go to

for one example from the Profet line of logic level high side switches. One of them should do what you want. We use six or eight to drive various

24 VDC solenoid coils and little halogen light bulbs in one product and almost never have a failure - quite rugged and reliable.

-- Regards, Carl Ijames carl.ijames at verizon.net

Carl,

That's what I was looking for! I found some other parts similar but were out of production. Do you know of anyone that carries small quantities for prototyping and limited production?

Thanks,

John

Sorry, no. We get that board as a complete assembly by an outside vendor. I think the two or three times we had to replace one, we just took a good one from a board with other problems :-). I'll ask tomorrow at work, though.

-- Regards, Carl Ijames carl.ijames at verizon.net

Carl,

I found a similar part, BTS410 that looks like it will do the job. It's available from Digikey.

Do you use the STATUS pin? If not do you just let it float?

Thanks again,

John