OpAmp differential voltage range ?

[Hammy]

I thought I found an ideal micro power OpAmp for another low power LDO
I'm building in the LMV641. Small offset, RR output, 10MHz GBW, 2.6
V/us and still pretty good drive capability +/- 20mA.

http://www.national.com/ds/LM/LMV641.pdf

There's one thing I'm concerned about it specs a max differential
input of +/- 0.3V. Under normal operation this shouldn't be a problem
but at start up and shutdown this may be exceeded. Is this likely to
hurt the reliability of OpAmp? Should I place current limiting
resistors on both opamp's inputs?

Schematic

http://i38.tinypic.com/108643d.png

 

[John Larkin]

On Tue, 14 Oct 2008 13:58:52 -0400, Hammy <spamme@hotmail.com> wrote:

I thought I found an ideal micro power OpAmp for another low power LDO
I'm building in the LMV641. Small offset, RR output, 10MHz GBW, 2.6
V/us and still pretty good drive capability +/- 20mA.

http://www.national.com/ds/LM/LMV641.pdf

There's one thing I'm concerned about it specs a max differential
input of +/- 0.3V. Under normal operation this shouldn't be a problem
but at start up and shutdown this may be exceeded. Is this likely to
hurt the reliability of OpAmp? Should I place current limiting
resistors on both opamp's inputs?

Schematic

http://i38.tinypic.com/108643d.png

Many opamps have clamp diodes between their inputs. A few mA of
transient current won't do any harm. Your circuit looks OK as-is.

Incidentally, your circuit is backwards. Convention is input on the
left, output on the right.


John

 

[Hammy]

On Tue, 14 Oct 2008 12:32:33 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

Many opamps have clamp diodes between their inputs. A few mA of
transient current won't do any harm. Your circuit looks OK as-is.

Thanks John.

This is the first LDO I've built that actual performs identically to
the spice simulation.

Incidentally, your circuit is backwards. Convention is input on the
left, output on the right.

Guilty as charged. I don't know why I do that sometimes. If you look
in the bottom right corner you'll see I stuck with convention for my
1/Beta schematic. Input to the left.

John