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LM7341: used as comparator

Kolbein Blakar
Prodigy 20 points
Part Number: LM7341
Other Parts Discussed in Thread: TLV1811, TLV1841, TLV1851

Tool/software:

Will the input bias current increase when the LM7341 is used as comparator? The differential voltage between the inputs is 1-4V and well within the supply voltages.

Best regards

Kolbein Blakar

  • 0
    TI__Guru 60596 points

    Hello Kolbein

    See page 17 of the LM7341 datasheet, section "LM7341 AS A COMPARATOR"

    Note that this is a bipolar device and the bias current will change direction as you approach within a volt of the rail. See figures 26-28

    Is there a particular reason you want to use an op-amp? The op-amp would make a very poor comparator compared (ha!) to a real comparator.

    12us prop delay is on par with our micro-power comparators that draw <1uA of supply current (vs. 1mA).

    I'm assuming you need high voltage (>12V) and a push-pull output.

    Have a look at the following "real" comparators - all have the same pinout as the LM7341:

    TLV1811 - 40V, 0.45us at 10uA

    TLV1841 - 40V, 65ns at 75uA

    TLV1851 - 40V, 14us, similar speed to the LM7341, but at only 440nA supply current

    All the above are CMOS input devices, so the input currents are in the sub nA's.

  • 0 in reply to Paul Grohe
    Prodigy 20 points

    Hello Paul

    Thank you very much.

    I need to elaborate the question. The IC is used in a "perfect diode circuit" to clamp an analogue signal to a reference, to limit the maximum level. The analogue signal is variable and most of the time lower than the reference. In this time there will be a voltage between the inputs of the opamp, and IC will behave more like an comparator. When the analogue signal increases to the reference  or higher, the output of the IC will go low, pull current from the analogue signal circuit and limit the signal, due to the output impedance of the analogue signal circuit. This becomes a precise clamp to limit an analogue signal. In this situation it is linear and needs to be an opamp.

    The reference (Iout Ref) is set by an DA. The supply voltage are -3V and +5V.

    In the test of the PCBAs we set the DA to minimum output level (=0), and the analogue signal -1,5V. This sets the inputs of the opamp to 0V and -1,5V. In this test the results indicate there is a current into the input pin of the opamp (LM 7341).

    My reading of the figure 26-28 is: they are marked Ibias ver Vcm. I read VCM as common mode voltage, that is: both inputs at same voltage. 

    My question is still: when there is a voltage like 1,5V between the input, will this change the current into the inputs of the opamp.

    With regards

    Kolbein

  • +1 in reply to Kolbein Blakar
    TI__Guru* 77421 points

    Hi Kolbein, 

    Kolbein Blakar said:
    My question is still: when there is a voltage like 1,5V between the input, will this change the current into the inputs of the opamp.

    Paul mentioned that LM7341 is bipolar op amp, but the datasheet did not say that there is a back-to-back diode protection between input. Based on the Figure 26-28 Ib plot, the LM7341 may be bipolar in design, though the datasheet did not indicate it clearly (may be there is a typo in LM7341 AS A COMPARATOR section). It is possible that the input back-to-back diodes in LM7341 are designed to be rugged, and it can withstand higher differential input voltage, as long as the input current is limited to +/-10mA as shown in the datasheet. 

    Typical bipolar op amps have the following input configuration, where the differential input voltage can not exceed +/-0.7Vdc. If you have current limiting resistor at the input (limiting current below +/-10mA), then differential input of +/-1.5Vdc will be ok, providing that the configuration did not violate the abs. max. ratings in this part. In any case, this is a good practice to limit the input current to the LM7431 op amp.  

    You can use LM7341 op amp as comparator, but the rising and fall timings are limited by the slew rate, which is approx. 1.9V/usec. If a design requires fast step response, use comparators are better option.  

    If you have other questions, please let me know. 

    Best,

    Raymond