P-Channel MOSFET for logic level drive

A

Andrew Holme

Guest
Can anyone recommend a small P-channel MOSFET suitable for 5V logic level
gate drive?

This is for an RS232-port-powered RS232 data output signal from a PIC.
Source will go to +5V, gate to a PIC output, and the drain is resistively
pulled-down to a negative rail. I did this years ago, but I can't remember
what MOSFET I used. Farnell don't seem to specify any of their P-channel
MOSFETs as being for logic level drive.

TIA
Andrew.
 
Andrew Holme wrote...
Can anyone recommend a small P-channel MOSFET suitable for 5V logic
level gate drive?

This is for an RS232-port-powered RS232 data output signal from a
PIC. Source will go to +5V, gate to a PIC output, and the drain is
resistively pulled-down to a negative rail. I did this years ago,
but I can't remember what MOSFET I used. Farnell don't seem to
specify any of their P-channel MOSFETs as being for logic level
drive.
Click to expand...
There aren't many logic-level p-types to choose from. Supertex
has a few. Spehro Pefhany suggested the IRF7425, which has 13
mill-ohms Rds(on) (max) at -2.5V, 8.5 mill-ohms at 4.5 volts.
It's in an SO-8 package, about 85 cents US in 100's.

What are you looking for?


--
Thanks,
- Win
 
Andrew Holme wrote:
Can anyone recommend a small P-channel MOSFET suitable for 5V logic level
gate drive?

This is for an RS232-port-powered RS232 data output signal from a PIC.
Source will go to +5V, gate to a PIC output, and the drain is resistively
pulled-down to a negative rail. I did this years ago, but I can't remember
what MOSFET I used. Farnell don't seem to specify any of their P-channel
MOSFETs as being for logic level drive.

TIA
Andrew.
Click to expand...

Fairchild FDV304P comes to mind.


--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
crcarleRemoveThis@BOGUSsandia.gov
NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to reply.
 
On Tue, 24 May 2005 18:52:39 +0100, the renowned "Andrew Holme"
<andrew@nospam.com> wrote:

Can anyone recommend a small P-channel MOSFET suitable for 5V logic level
gate drive?

This is for an RS232-port-powered RS232 data output signal from a PIC.
Source will go to +5V, gate to a PIC output, and the drain is resistively
pulled-down to a negative rail. I did this years ago, but I can't remember
what MOSFET I used. Farnell don't seem to specify any of their P-channel
MOSFETs as being for logic level drive.

TIA
Andrew.
Click to expand...
Why don't you want to use a PNP BJT?

The BSS84 is one p-channel part spec'd at 5V.


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
 
Hello Spehro,

Why don't you want to use a PNP BJT?
Click to expand...
Yes, less cost.

The BSS84 is one p-channel part spec'd at 5V.
Click to expand...
One of my favorite parts but only if it really has to be a FET.

Regards, Joerg

http://www.analogconsultants.com
 
Winfield Hill wrote:
Andrew Holme wrote...

Can anyone recommend a small P-channel MOSFET suitable for 5V logic
level gate drive?

This is for an RS232-port-powered RS232 data output signal from a
PIC. Source will go to +5V, gate to a PIC output, and the drain is
resistively pulled-down to a negative rail. I did this years ago,
but I can't remember what MOSFET I used. Farnell don't seem to
specify any of their P-channel MOSFETs as being for logic level
drive.

There aren't many logic-level p-types to choose from. Supertex
has a few. Spehro Pefhany suggested the IRF7425, which has 13
mill-ohms Rds(on) (max) at -2.5V, 8.5 mill-ohms at 4.5 volts.
It's in an SO-8 package, about 85 cents US in 100's.

What are you looking for?
Click to expand...
It's a one-off hobby project (GPIB-->RS232 using PIC16F84). Up to a few
ohms would not be a problem for Rds(on), since the pull-down will be at
least 1k. Package wise, I would prefer not to go surface mount, but I can
handle them if necessary. I'll look at the IRF7425. Thanks.
 
Spehro Pefhany wrote:
On Tue, 24 May 2005 18:52:39 +0100, the renowned "Andrew Holme"
andrew@nospam.com> wrote:

Can anyone recommend a small P-channel MOSFET suitable for 5V logic
level gate drive?

This is for an RS232-port-powered RS232 data output signal from a
PIC. Source will go to +5V, gate to a PIC output, and the drain is
resistively pulled-down to a negative rail. I did this years ago,
but I can't remember what MOSFET I used. Farnell don't seem to
specify any of their P-channel MOSFETs as being for logic level
drive.

TIA
Andrew.

Why don't you want to use a PNP BJT?
Click to expand...
Yes, I could do. I was thinking of saving the base current, but then I
suppose it'll only be micro-amps.

The BSS84 is one p-channel part spec'd at 5V.
Click to expand...
Thanks, I'll look at that part.
 
Hello Andrew,

It's a one-off hobby project (GPIB-->RS232 using PIC16F84). Up to a few
ohms would not be a problem for Rds(on), since the pull-down will be at
least 1k. Package wise, I would prefer not to go surface mount, but I can
handle them if necessary. I'll look at the IRF7425. Thanks.
Click to expand...
Then Spehro's suggestion (BSS84) should be the ticket. It's cheap.

Regards, Joerg

http://www.analogconsultants.com
 
Andrew Holme wrote:

Can anyone recommend a small P-channel MOSFET suitable for 5V logic level
gate drive?

This is for an RS232-port-powered RS232 data output signal from a PIC.
Source will go to +5V, gate to a PIC output, and the drain is resistively
pulled-down to a negative rail. I did this years ago, but I can't remember
what MOSFET I used. Farnell don't seem to specify any of their P-channel
MOSFETs as being for logic level drive.

TIA
Andrew.
Click to expand...
Screw the MOS Fet, use a 2n3906 pnp transistor...

http://members.cox.net/berniekm/spydar.gif

(See rs232 circiut in lower left)


--
Luhan Monat: luhanis(at)yahoo(dot)com
http://members.cox.net/berniekm
"Any sufficiently advanced magick is
indistinguishable from technology."
 
Andrew Holme wrote:
Winfield Hill wrote:
Andrew Holme wrote...
Can anyone recommend a small P-channel MOSFET suitable for 5V logic
level gate drive?

This is for an RS232-port-powered RS232 data output signal from a
PIC. Source will go to +5V, gate to a PIC output, and the drain is
resistively pulled-down to a negative rail. I did this years ago,
but I can't remember what MOSFET I used. Farnell don't seem to
specify any of their P-channel MOSFETs as being for logic level
drive.
There aren't many logic-level p-types to choose from. Supertex
has a few. Spehro Pefhany suggested the IRF7425, which has 13
mill-ohms Rds(on) (max) at -2.5V, 8.5 mill-ohms at 4.5 volts.
It's in an SO-8 package, about 85 cents US in 100's.

What are you looking for?

It's a one-off hobby project (GPIB-->RS232 using PIC16F84). Up to a few
ohms would not be a problem for Rds(on), since the pull-down will be at
least 1k. Package wise, I would prefer not to go surface mount, but I can
handle them if necessary. I'll look at the IRF7425. Thanks.
Click to expand...

Zetex also makes some PMOS in TO92 with <5V thresholds.


Good day!



--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
crcarleRemoveThis@BOGUSsandia.gov
NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to reply.
 
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