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LM7332: Why the LM7332 datasheet does not mention the protection diode between inputs

Part Number: LM7332
Other Parts Discussed in Thread: OPA2990

Hi,

 Waste weeks to fig out the LM7332 issue. why not mention the protection ( a pair of back-to-back diodes  across the amplifier inputs) in the datasheet?

  • Former Member
    0 Former Member

    Hello Chuanxiao,

    I am sorry to hear about your displeasure.

    Can you please describe your problem?  Do you need any assistance?  Perhaps we can avoid similar issues in the future.

    Regards,

    Daniel

  • Hi Chuanxiao,

    the absolute maximum rating of differential input voltage is +/-10V. So, there shouldn't be back-to-back diodes sitting across the amplifier inputs. But even if there were some, they would have current limit resistors in series and you won't "see" them during normal operation.

    Kai

  • Hi,

    You can find out there  are  back-to-back diodes sitting across the amplifier inputs by the multimeter., but not mentioned on the datasheet that makes the debug time consume 

  • thanks for the reply, we cannot remove the diodes inside the OPAMP, just replace with ISL28218FBZ

  • Hi Chuanxiao,

    take care, the most OPAmps are not intended to be used as comparators!

    Kai

  • Former Member
    0 Former Member in reply to Chuanxiao Li1

    Hello Chuanxiao,

    I'm still not sure why the input diodes being there would be an issue because, as the datasheet specifies, you can have up to +/-10V of differential input voltage.  Are you concerned about there being 10V across the inputs of channel A when Q2 is off?

    I agree with Kai.  If you would like to use the parts as comparators, which is what is shown in the above schematic, then I would recommend using a true comparator part.  The Renesas part you have mentioned is not a comparator, but an op amp.

    If you have some specifications you would need your part to meet, I would recommend you open a new thread asking for a comparator suggestion.  Someone from that team will reach out to you and help you find a part that works better for your application.

    Regards,

    Daniel

  • Hi Daniel,

    This is not our design (contracter design), I do not think you can apply 10V across the input (you only can apply 1V across the input b/c the protection diodes). the comparator cannot drive the BJT (20mA) . anyway, not our design, just making current circuit working.

    Thanks

    C. Li

  • Actually, there is more to Chuanxiao comment about switching to the ISL28218 dualop amp - 

    I was not aware of this, but apparently that design team decided using an op amp as a comparator comes up so often (while most op amp folks hate that) they designed this part to support that function -quite different than the very high input dfif voltage in the LM7332 absmax table - which looks like an error that should be fixed perhaps. 

  • Former Member
    0 Former Member in reply to Michael Steffes

    Hi Chuanxiao,

    From this thread and the LM7332 datasheet, the inputs of the op amp can go up to +/-10V.  See below. 

    Again, it is not advisable to use an op amp as a comparator, for speed reasons if for nothing else.  We have a couple of resources on this that I think you would find helpful.

    1.  https://e2e.ti.com/blogs_/archives/b/thesignal/archive/2012/03/14/op-amps-used-as-comparators-is-it-okay

    2.  https://www.allaboutcircuits.com/industry-articles/using-operational-amplifiers-as-comparators/

    Thanks for the input Michael!  While it does seem that the Renesas part does not need back-to-back input diodes, I am still not sure that the part would be fast enough to have similar performance to a comparator.  But, this would be up to Chuanxiao.

    Regards,

    Daniel

  • Former Member
    0 Former Member in reply to Former Member

    Hi Chuanxiao,

    Something else you might want to take a look at is our new part, the OPA2990.  This new part has an interesting input protection scheme, shown on page 24.  It should satisfy your current needs while also having a faster slew rate and better price structure than the competitor's part.

    Regards,

    Daniel

  • thanks for this matter