Logic Level MOSFET High Side Switch (almost dead horse)

[[email protected]]

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.

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

 

[PeteS]

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.

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

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

 

[linnix]

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.

 

[[email protected]]

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

 

[linnix]

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.

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.

 

[Carl Ijames]

Go to http://www.infineon.com/upload/Document/ITS410E2_20060328.pdf 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.

 

[[email protected]]

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

 

[Carl Ijames]

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.

 

[[email protected]]

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