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TLV2772A: Rail-to-rail output?

ArTzy
Genius 14959 points
Part Number: TLV2772A
Other Parts Discussed in Thread: TLV9052, LM358

Hi Experts,

We would like to ask assistance on this query about TLV2772A output behavior when input signal is outside recommended level.
From the datasheet, recommended Vdd is 2.5 to 6V. Cx would like to operate it with 3.3Vdd.

Question:
[1] What would happen to the output if the input signal is greater than (Vdd - 1.3)volts? Will it reduce the output by 1.3V?

[2] Datasheet shows it's a rail-to-rail but why does maximum input signal is limited to (Vdd-1.3V)?

Please enlighten us. Thank you.

Regards.
Archie A.

  • 0
    Guru 266610 points

    1. The behaviour is undefined. When both inputs are in the invalid region, their difference can no longer be determined correctly.

    2. The datasheet say "rail-to-rail output". The input is not rail-to-rail.

    There are opamps with rail-to-rail inputs, e.g., TLV9052.

  • 0 in reply to Clemens Ladisch
    TI__Mastermind 26441 points

    Hey Archie,

    To add on to Clemens comments. There are a couple different things the output can do when it's no longer in the linear operating range. The below data is for LM358, which similarly has an input common-mode range of in to V- (like the TLV2772A). When operating at 3.3V the output will indeterminately rail positive or level off at the upper end of the common mode range. However, this is just for unity gain. There are numerous factors that determine the output, and when you exceed the common-mode voltage range (CMVR) many of these factors vary over process (read: random). 

    Therefore, if you want determinate operation I would remain within the CMVR of the device. If you want your CMVR to extend to both rails, I would look at a rail-to-rail input op amp like the TLV9052 that Clemens mentioned.