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TLV3544-Q1: Maximum Input Voltage

Part Number: TLV3544-Q1

At the risk of asking a Really Stupid Question (TM) :

One reason for using CMOS / JFET amplifiers is the very low input bias current - they can work well when you need to present a high input impedance to the signal source, but there appears to be a paradox : The Absolute Maximum Input Voltage is listed as supply plus or minus 0.5 volts. High impedance precludes the use of Schottky diodes for clamping to ensure that the maximum input voltages are not exceeded - the reverse leakage current of a Schottky will induce a significant voltage drop across a high input impedance, generating errors. The temperature dependency of the reverse leakage is cruel - whilst it may be almost acceptable at 25 degrees, it certainly is not at 105 degrees.

I saw a glimmer of hope in the datasheet's Note 2 on the Absolute Maximum Input Voltage : "Input pins are diode-clamped to the power supply rails. Input signals that can swing more than 0.5V beyond the rails must be current limited to 10mA". This *suggests* that the rating is stated not because there will be gate oxide damage by exceeding the rails, or any sort of latch-up, but rather that the issue is with the power dissipation in these diodes. If that is true, then the use of a high input impedance may in and of itself be adequate protection (although that is a little bit of a stretch of reasoning), but at worst one could add external low leakage silicon clamping diodes and not necessarily cause damage when the signal source exceeds the Maximum Voltage, since the diodes will limit the input pins to sensible, slightly-beyond-the-rails, voltages and the high impedance means that only tiny amounts of current are possible.

My question is whether this interpretation of the Absolute Maximum Voltage data is correct, and that the limiting element is the thermal limit of the input protection diodes, not a gate oxide or latch-up issue ?

Many thanks,


  • Hi,

    On the TLV3544-Q1 if the inputs exceed the internal ESD diode clamps, the diodes will turn on and clamp the voltage. However, if an unlimited amount of current is allowed to flow into the diodes, it will damage the device. It is possible to safely allow the inputs to go beyond the diodes clamping voltage as long as the input current is limited to a maximum of 10mA under all conditions. Any currents exceeding 10mA may damage the device.

  • Hiya Jacob!

    Many thanks for answering my question and confirming my suspicion that the damage would happen to the ESD diodes. Assuming I do limit the possible current to 10mA or less, would it be safe to say that external clamping diodes are NOT required ? What I mean by this is : are the internal ESD diodes going to get upset at conducting (albeit at less than 10mA) for a significant percentage of the time, and/or are the reverse recovery characteristics of the ESD diodes "inferior" to what could be achieved externally hence making the provisioning of external diodes beneficial in a signal rather than a protection sense ?

    Many thanks again,


  • Hi Pat,

    As long as you keep the current under the 10mA limit the diodes should be okay for reliability. However, I do believe these devices use an edge triggered ESD diode device on the positive side, so I would caution that they may not clamp as desired on that side. In terms of recovery, the ESD diodes are going to be inferior from an external clamp. If you plan to be overdriving the input frequently I would advise to use external diodes since you then do not have to worry about current limits and should have quicker recovery. The one thing you do need to be aware of is the added capacitance of the external clamping diodes, and should try to choose ones with low capacitance.

    Another option would be to use external clamps and a small series resistor before the inputs. This will help to further protect the inputs and isolate some of the capacitance of the added diodes.

  • Hiya Jacob!

    Many thanks again for taking the time to respond and clarifying the nature of the ESD diodes. I shall use external ones as you advise.

    It is interesting that you mention the capacitance. I have been "doing battle" with that - high input impedance is a real pain in that regard - as is leakage current in semiconductors connected to the input signal path!

    Best regards,