Low drop-out current sink with switch?

[William P.N. Smith]

I'm looking for a constant current sink that'll control somewhere
around one half to one amp or so with a low dropout voltage and a
control pin.

I'm currently awaiting some LM2941 LDO regulators, which (configured
for current mode) look fairly ideal except the minimum voltage drop is
going to be something like a couple of volts (1.275V for the reference
voltage and a quarter to a half a volt for the device).

Is there a way to do something like this using a MOSFET with (say) a
constant gate voltage? My initial look at the IRLD024 makes it look
like the current will vary considerably with temperature, and a bias
resistor on the source pin gets me back up to the 1.5 volt range for a
dropout, and then I still need an on/off control...

Any thoughts?

Thanks!

--
William Smith wpns@compusmiths.com N1JBJ@amsat.org
ComputerSmiths Consulting, Inc. www.compusmiths.com

 

[mike]

William P.N. Smith wrote:

I'm looking for a constant current sink that'll control somewhere
around one half to one amp or so with a low dropout voltage and a
control pin.

I'm currently awaiting some LM2941 LDO regulators, which (configured
for current mode) look fairly ideal except the minimum voltage drop is
going to be something like a couple of volts (1.275V for the reference
voltage and a quarter to a half a volt for the device).

Is there a way to do something like this using a MOSFET with (say) a
constant gate voltage? My initial look at the IRLD024 makes it look
like the current will vary considerably with temperature, and a bias
resistor on the source pin gets me back up to the 1.5 volt range for a
dropout, and then I still need an on/off control...

Any thoughts?

Thanks!

There used to be a thing called a sense fet. Brought out one fet cell
on a separate source pin. You still need a negative power supply for
all the biasing and sensing stuff, but it only has to carry a small
fraction of the total current.
mike

--
Bunch of stuff For Sale and Wanted at the link below.
laptops and parts
4in/400Wout ham linear amp.
Honda CB-125S
400cc Dirt Bike 2003 miles $450
Police Scanner, Color LCD overhead projector
Tek 2465 $800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/

 

*** post for FREE via your newsreader at post.newsfeed.com ***


"William P.N. Smith" wrote:

I'm looking for a constant current sink that'll control somewhere
around one half to one amp or so with a low dropout voltage and a
control pin.

I'm currently awaiting some LM2941 LDO regulators, which (configured
for current mode) look fairly ideal except the minimum voltage drop is
going to be something like a couple of volts (1.275V for the reference
voltage and a quarter to a half a volt for the device).

Is there a way to do something like this using a MOSFET with (say) a
constant gate voltage? My initial look at the IRLD024 makes it look
like the current will vary considerably with temperature, and a bias
resistor on the source pin gets me back up to the 1.5 volt range for a
dropout, and then I still need an on/off control...

Any thoughts?

Thanks!

Use a low voltage rail-to-rail Op Amp with beyond-the-rail inputs to
drive a NPN switching transistor. Sense the current with a tiny resistor
in the emitter feed and configure this combination as a constant current
source. A suitable Op Amp would be KM4101 (runs on a single 2.7V supply,
60mA output capacity). Use the enable pin of the Op Amp to switch the
current source on and off (KM4101 quiescent current 0.1mA).

The 260MHz bandwidth of the KM4100/KM4101 is overkill, of course.

Just an idea, I didn't try it!

Martin.


-----= Posted via Newsfeed.Com, Uncensored Usenet News =-----
http://www.newsfeed.com - The #1 Newsgroup Service in the World!
-----== 100,000 Groups! - 19 Servers! - Unlimited Download! =-----