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LM2901: Input common mode range

Naser Pour Aryan
Prodigy 20 points
Part Number: LM2901
Other Parts Discussed in Thread: LM393, TLV1704

Hello,

I am a development electronics engineer in a company in Nuremberg, Germany.
We are at a development stage when we cannot change the circuit without extensive tests anymore.
We have an issue regarding the common mode input range of LM2901. We are exceeding the prescribed range of 0 to Vcc-2V at inputs.
In our circuit the supply of the comparator is connected to 3.3V.
Please consider this scenario:
Comparator minus input is in common mode range.
Plus input connected to a square wave (Ton~=Toff) between 162mV and 2.6V and a period of 100µs, so we exceed the common mode by 1.3V.
It is most important for us that the output toggles between low and high states with every input period, not necessarily looking square shaped.
Will the ouptput toggle in the temperature range -40°C to 105°C? How much delay will we have at the edges?

As in our case one input exceeds the common mode range as much as 1.3V, how much will the deviation from the datasheet be? (values from datasheet: Input bias current (Iib): Max -500nA, Input Offset Voltage (Vio): Max 15mV, Response time: 1,3µs)

Thanks much and yours sincerely

  • 0
    TI__Guru 63896 points

    Hello Nasser,

    Please see the LM393 family Application Note. Pretty much all your questions are answered in there.

     https://www.ti.com/lit/an/snoaa35a/snoaa35a.pdf

    You will be relying in the LM2901's "feature" where as long as one input is within the valid range, the output will be correct - but at a cost.

    The cost is there can be an additional delay created as the input goes in/out of the valid input range and the internal circuitry recovers. We have not fully characterized this - as it is not within specifications - but the delay can be an additional microsecond or two.

    Your toggle frequency is fairly low, and your input signal is large, so I would not expect it to "miss" any pulses.

    If you want to be sure, then you should use a rail-to-rail input device, such as the TLV1704. The TLV1704 can drop-in replace the LM2901 in the TSSOP package. Or decrease the input signal amplitude to keep it below 1.3V or increase the comparator supply voltage to >4.6V.

    Probably the easiest is to decrease the input amplitude with a simple voltage divider on the input.


  • 0 in reply to Paul Grohe
    Prodigy 20 points

    Hello Paul,

    Thanks a lot for your helpful answer but I want to address another problem we have concerning the common mode of LM2901:

    Actually the other problem we have is that the above mentioned LM2901 has its other input set at a reference voltage which can be in worst case 1.352V. As said above the supply is at 3.3V. So the common mode range is between 0V and 1.3V. So we are 50mV above the allowed boundary and thus according to the application note may have problems at cold temperature. 

    We asked another producer (our second source: STMicroelectronics) also about this problem and they said that the component will function according to the datasheet down to a temperature of -38°C.

    We would like to know your comment on this as you are the main producer on our list. Down to which temperature there'll be a deviation from the datasheet?

    Or you think our system will function down to -40°C anyways because it is slow? (please see my first message on this, we have a toggling signal at 10kHz at the other input)

    As we said we can't just change this reference voltage on the circuit at this point easily because we have finished our tests in the production process and a new change will cause a lot of test costs.

    Yours sincerely,

    Naser Pour Aryan

  • 0 in reply to Naser Pour Aryan
    TI__Mastermind 26570 points

    Naser

    We would not expect the LM2901 to fail under that condition.

    Chuck