LMV321: LMV321 Input voltage range when unpowered

Andy Whyte

Part Number: LMV321
Other Parts Discussed in Thread: LM321LV, TLV9001

I would like to use a small single opamp as a buffer monitoring battery input for a signal, the power is supplied by uC output pin, so is 3.3V, the device will be powered off most of the time to reduce energy.

When this happens the input will float (through large resistor) to the battery voltage, which can be 4.2V or 12V. This is within the acceptable range of the opamp input, but maybe not when not powered up nor if greater than Vcc.

Is there any way to tell what the behavior of this or an alternative opamp is in this situation? Is there a rail-rail op amp that would do this? Apologies if the datasheet states this.

over 1 year ago

0 Andy Whyte over 1 year ago

Prodigy 25 points

added a simplified schematic for clarity.

0 Clemens Ladisch over 1 year ago

Guru 280000 points

The absolute maximum ratings forbid voltages above 5.7 V.

The LMV321A and LM321LV have clamping diodes to V+, so larger voltages are allowed as long as the current is limited. This current flows into the V+ supply and might (partially) power up any devices connected to it.

You should use an opamp with shutdown like the TLV9001‍S.

0 Jerry Madalvanos over 1 year ago in reply to Clemens Ladisch

TI__Mastermind 28476 points

Hey Andy,

If the resistor divider is already being disconnected via FET to save power, why not just connect a fet above the resistor divider and avoid overvolting the op amp altogether? During power on, the current consumption increases slightly, but this will keep the op amp from being damaged.

Best,
Jerry

0 Andy Whyte over 1 year ago in reply to Jerry Madalvanos

Prodigy 25 points

Thanks for the suggestion, this is the way I'll go - actually no need for a buffer at this point just reduce the resistor values and use a PMOS.

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LMV321: LMV321 Input voltage range when unpowered