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Op amp brownout circuit

S

Steve Morgan

Jan 1, 1970
0
I am designing an Atmel-based circuit with an associated single supply
op amp. I do not want the op amp Iq to deplete batteries any further
than down to about 1V after the programmed micro brownout.

Note that the batteries are not to be manually switched off.

What is the simplest way of doing this?

Thank you,

Steven
 
F

Fred Bloggs

Jan 1, 1970
0
I am designing an Atmel-based circuit with an associated single supply
op amp. I do not want the op amp Iq to deplete batteries any further
than down to about 1V after the programmed micro brownout.

Note that the batteries are not to be manually switched off.

What is the simplest way of doing this?

Use an OA with "shutdown":
http://cache.national.com/ds/LM/LMV341.pdf
 
J

Joerg

Jan 1, 1970
0
Hello Steve,

If Fred's solution is too expensive you could use a pnp transistor in
the supply that has a Schottky plus series resistor connected between
base and ground. That'll let go when the voltage drops too low.

However, this cheap trick will make the 1V cutoff a bit "mushy" and will
continue to draw a little base current until you are further down the
Vbe curve.

Best might be to let the micro monitor the voltage and turn off
everything if below 1V. You'll also need a pnp to switch the opamp
supply as FETs won't cut it at these low Vgs levels. Well, a pnp is
cheaper anyway. Whatever you do make sure that the opamp inputs and
outputs are not driven into unhealthy regions when its power is turned off.

Regards, Joerg
 
T

Tim Shoppa

Jan 1, 1970
0
Best might be to let the micro monitor the voltage
and turn off everything if below 1V.

Do the new Atmel microcontrollers go that low? Last I checked their
brownout stuff kicks in around 1.8V. (That is pretty good compared to
some others... but I'd still love to hear of something at 1V.)

Tim.
 
F

Fred Bloggs

Jan 1, 1970
0
Joerg said:
Hello Steve,

If Fred's solution is too expensive you could use a pnp transistor in
the supply that has a Schottky plus series resistor connected between
base and ground. That'll let go when the voltage drops too low.

However, this cheap trick will make the 1V cutoff a bit "mushy" and will
continue to draw a little base current until you are further down the
Vbe curve.

Best might be to let the micro monitor the voltage and turn off
everything if below 1V. You'll also need a pnp to switch the opamp
supply as FETs won't cut it at these low Vgs levels. Well, a pnp is
cheaper anyway. Whatever you do make sure that the opamp inputs and
outputs are not driven into unhealthy regions when its power is turned off.

Regards, Joerg

I think an even cheaper trick would be to power the OA through an ATMEL
I/O pin or use a TinyLogic gate to power it.
 
K

Ken Smith

Jan 1, 1970
0
Fred Bloggs said:
I think an even cheaper trick would be to power the OA through an ATMEL
I/O pin or use a TinyLogic gate to power it.
Yes, IIRC, the ATMEL chip can easily sink enough current to switch the
ground side of the op-amp power connections.


If you don't need much performance from the op-amp, there are some that
draw very little. The have a gain-bandwidth of only several KHz but the
DC characteristics aren't too bad.
 
J

Joerg

Jan 1, 1970
0
Hello Tim,
Do the new Atmel microcontrollers go that low? Last I checked their
brownout stuff kicks in around 1.8V. (That is pretty good compared to
some others... but I'd still love to hear of something at 1V.)

Probably not. You are right, 1V is too low for the usual micro. But
Fred's suggestion of powering the opamp through an IO port is a good
idea. As long as the code will turn power off to the opamp when the
micro detects that the voltage gets too low for its own sake. Kind of
like how a laptop goes from standby to hibernate when the battery begins
to fade.

Regards, Joerg
 
J

Joerg

Jan 1, 1970
0
Hello Fred,
I think an even cheaper trick would be to power the OA through an
ATMEL I/O pin or use a TinyLogic gate to power it.


That is an excellent way to do it. However, the bypass cap across the
opamp and maybe across other associated circuitry needs to be taken into
account. The current peak caused by charging and discharging it might
exceed abs max limits. Possibly a resistor in series with the I/O can help.

Regards, Joerg
 
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