3.3V Logic-Level MOSFET

T

Ted Lechman

Guest
Has anyone already found what I'm looking for?

I'm looking for a high side MOSFET to control power to a low power ( <
100ma ) device. The voltage to be switched is 3.3V and the control
(gate) voltage to the MOSFET is also 3.3V (not 5V). the RDSON is not a
major factor. Do you know of a MOSFET part that will work at these
gate and input voltages?

Thanks
Ted
 
Ted Lechman wrote:

Has anyone already found what I'm looking for?

I'm looking for a high side MOSFET to control power to a low power (
100ma ) device. The voltage to be switched is 3.3V and the control
(gate) voltage to the MOSFET is also 3.3V (not 5V). the RDSON is not a
major factor. Do you know of a MOSFET part that will work at these
gate and input voltages?

Thanks
Ted
Click to expand...
Vishay/Siliconix has a number of them. Just look for "logic level" on
their selector guide (then read the data sheet with care).

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
 
On 20 May 2004 16:01:13 -0700, the renowned eastwood132@yahoo.com (Ted
Lechman) wrote:

Has anyone already found what I'm looking for?

I'm looking for a high side MOSFET to control power to a low power (
100ma ) device. The voltage to be switched is 3.3V and the control
(gate) voltage to the MOSFET is also 3.3V (not 5V). the RDSON is not a
major factor. Do you know of a MOSFET part that will work at these
gate and input voltages?
Click to expand...
Lots of them (assuming you're looking for a p-channel MOSFET). For
example, the IRF7425 which has 0.013 ohm Rds(on) at -2.5V. About 85
cents US in 100's.

Look for "logic" parts with a very low Vds (something like -20V) and
many of them will fit your specifications if you want to search for an
optimal part.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
Tim Wescott wrote...
Ted Lechman wrote:

I'm looking for a high side MOSFET to control power to a low power
( < 100mA ) device. The voltage to be switched is 3.3V and the control
(gate) voltage to the MOSFET is also 3.3V (not 5V). the RDSON is not
a major factor. Do you know of a MOSFET part that will work at these
gate and input voltages?

Vishay/Siliconix has a number of them. Just look for "logic level"
on their selector guide (then read the data sheet with care).
Click to expand...
Many small power MOSFETs (which can switch 1A or more with the usual
5V gate drive) can work quite well with 3.3V drive (which is a TTL
"HI" level). For example, the common 2n7000 and 2n7002 will switch
up to 50mA just fine with 3.3V gate voltage. Perhaps not offically,
according to the data sheet, but in fact they do, as can be shown
using a mathematical analysis of the data-sheet numbers. Switching
100 - 200mA with 3.3V is a bit tougher for legacy parts, but as Tim
says, there are lots of new "logic-level" parts that work well. For
example the Supertex TN0702, available in the TO-92 package, and the
TN0604, available in TO-92 and SMT. But sadly most of the attractive
new logic-level parts are *only* available in SMT packages, like the
Fairchild nds331 and nds335, the Philips pmv30, pmv31 and pmv56, and
the Vishay Si2302, which are all in SOT-23 packages. These parts can
switch several amps.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)
 
Winfield Hill wrote:
Tim Wescott wrote...

Ted Lechman wrote:

I'm looking for a high side MOSFET to control power to a low power
( < 100mA ) device. The voltage to be switched is 3.3V and the control
(gate) voltage to the MOSFET is also 3.3V (not 5V). the RDSON is not
a major factor. Do you know of a MOSFET part that will work at these
gate and input voltages?

Vishay/Siliconix has a number of them. Just look for "logic level"
on their selector guide (then read the data sheet with care).


Many small power MOSFETs (which can switch 1A or more with the usual
5V gate drive) can work quite well with 3.3V drive (which is a TTL
"HI" level). For example, the common 2n7000 and 2n7002 will switch
up to 50mA just fine with 3.3V gate voltage. Perhaps not offically,
according to the data sheet, but in fact they do, as can be shown
using a mathematical analysis of the data-sheet numbers. Switching
100 - 200mA with 3.3V is a bit tougher for legacy parts, but as Tim
says, there are lots of new "logic-level" parts that work well. For
example the Supertex TN0702, available in the TO-92 package, and the
TN0604, available in TO-92 and SMT. But sadly most of the attractive
new logic-level parts are *only* available in SMT packages, like the
Fairchild nds331 and nds335, the Philips pmv30, pmv31 and pmv56, and
the Vishay Si2302, which are all in SOT-23 packages. These parts can
switch several amps.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)
Click to expand...
So, has Supertex been around for years and me too dim to notice, or did
they have a different name last year?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
 
Tim Wescott wrote:
So, has Supertex been around for years and me too dim to notice, or did
they have a different name last year?
Click to expand...
1976.
--
Tim Hubberstey, P.Eng. . . . . . Hardware/Software Consulting Engineer
Marmot Engineering . . . . . . . VHDL, ASICs, FPGAs, embedded systems
Vancouver, BC, Canada . . . . . . . . . . . http://www.marmot-eng.com
 
Tim Wescott wrote...
Winfield Hill wrote:
Tim Wescott wrote...
Ted Lechman wrote:

I'm looking for a high side MOSFET to control power to a low power
( < 100mA ) device. The voltage to be switched is 3.3V and the control
(gate) voltage to the MOSFET is also 3.3V (not 5V). the RDSON is not
a major factor. Do you know of a MOSFET part that will work at these
gate and input voltages?

Vishay/Siliconix has a number of them. Just look for "logic level"
on their selector guide (then read the data sheet with care).

Many small power MOSFETs (which can switch 1A or more with the usual
5V gate drive) can work quite well with 3.3V drive (which is a TTL
"HI" level). For example, the common 2n7000 and 2n7002 will switch
up to 50mA just fine with 3.3V gate voltage. Perhaps not offically,
according to the data sheet, but in fact they do, as can be shown
using a mathematical analysis of the data-sheet numbers. Switching
100 - 200mA with 3.3V is a bit tougher for legacy parts, but as Tim
says, there are lots of new "logic-level" parts that work well. For
example the Supertex TN0702, available in the TO-92 package, and the
TN0604, available in TO-92 and SMT. But sadly most of the attractive
new logic-level parts are *only* available in SMT packages, like the
Fairchild nds331 and nds335, the Philips pmv30, pmv31 and pmv56, and
the Vishay Si2302, which are all in SOT-23 packages. These parts can
switch several amps.

So, has Supertex been around for years and me too dim to notice, or
did they have a different name last year?
Click to expand...
Supertex has long been a favorite of mine, initially because they
habitually offered each die in an array of different packages. But
they're not as well known as other companies, and now most of their
interesting parts (e.g. low-capacitance high-voltage FETs in TO-220
power packages) have been discontinued. <sigh>

BTW, I misread the O.P.'s question, and all my suggested parts are
N-channel types. Spef suggested a P-channel part, the IRF7425,
causing me to carefully re-read the question, which asked for a
"high side MOSFET" switch. Oops! Sorry about that!

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)
 
Hi Winfield,

Most of the time I found it easier to use PNP transistors in cases like this
where it is just about switching on power to a device. In a Darlington setup if
there wan't enough drive power. At least then you are guaranteed that only a
few hundred millivolts get dropped. FETs are often too fickle when operated so
close to their steep region. Suddenly you receive a shipment that is on the
high side of Ugs and things get toasty.

Regards, Joerg

http://www.analogconsultants.com
 
Joerg wrote...
Hi Winfield,

Most of the time I found it easier to use PNP transistors in cases like this
where it is just about switching on power to a device. In a Darlington setup
if there wan't enough drive power. At least then you are guaranteed that only
a few hundred millivolts get dropped. FETs are often too fickle when operated
so close to their steep region. Suddenly you receive a shipment that is on
the high side of Ugs and things get toasty.
Click to expand...
This is eminently sensible. These days we love power MOSFETs. But we
may seek to use them even if they're less than ideal for an application.
That's not good. Driving a PNP transistor with 4 to 10mA in order to
switch up to 100mA makes good sense. The PNP is cheaper and easier to
get. And it's very well understood. Low Vgs FETs are becoming widely
available, and are dependable batch to batch. But they're expensive,
and may go out of stock or even be discontinued. Not a happy scene.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)
 
Hi Winfield,

With "cheaper" you actually chose a really important word here. Bipolars beat FETs
almost all the time under a few hundred mA. Higher up it can be different.

Academia is really a bit behind on this topic. They do not teach cost thinking well
enough, in my times not at all. There should be seminars where students a presented
with a huge list of parts or for that matter with the URL for Digikey. Then they'd
have to create a certain circuit and get graded on what it costs. Of course it also
has to work.

The part suggestions in Art of Electronics are great, especially for folks getting
started. Maybe a new edition could contain rough budgetary pricing. That would
teach engineers a lesson or two on the penalties of going an easy route. We almost
always can get that panacea chip or dream transistor that does most of the grunt
work but at the end of the day that may blow the budget.

Regards, Joerg

http://www.analogconsultants.com
 
Joerg wrote...
Hi Winfield,

With "cheaper" you actually chose a really important word here.
Bipolars beat FETs almost all the time under a few hundred mA.
Higher up it can be different.

Academia is really a bit behind on this topic. They do not teach
cost thinking well enough, in my times not at all. There should
be seminars where students a presented with a huge list of parts
or for that matter with the URL for Digikey. Then they'd have to
create a certain circuit and get graded on what it costs. Of
course it also has to work.

The part suggestions in Art of Electronics are great, especially
for folks getting started. Maybe a new edition could contain rough
budgetary pricing. That would teach engineers a lesson or two on
the penalties of going an easy route. We almost always can get
that panacea chip or dream transistor that does most of the grunt
work but at the end of the day that may blow the budget.
Click to expand...
OK, thanks for the thought, good idea. And it comes at a good
time in our writing.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)
 
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