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SN74AUP2G17: Over Voltage at Input and Short Circuit at Output side for the Overall system

Srinadh Moka
Prodigy 205 points
Part Number: SN74AUP2G17
Other Parts Discussed in Thread: THS3491, TLV1871

Tool/software:

Hi,

We are developing a ground based receiver with different inputs and the outputs. Can you suggest the protection circuit/IC which is support for the over voltage protection at the input side and short circuit at the output side.

Input Signal: 1PPS signal

Output Signal: 1PPS signal, IRIG B output each output should be driving capability with 50E impedance.

Suggest the Over voltage protection at input side and Short circuit protection in the output side without degrading the main signal.

And somewhere in the system i want to divide the 1PPS signal into 2 for two different systems, can i use SN74AUP2G17 by providing the same 1PPS signal to the input of channel 1 and 2 then getting 2 output from the buffer. or suggest the best scheme to divide the 1PPS signal.

  • 0
    Guru 317770 points

    1. What is the signal voltage?

    2. Is the output line terminated with a 50 Ω resistor at the receiver? If yes, what is the voltage that the receiver expects at its input?

    3. What voltages do you want to protect against? ESD, transient voltages, or shorts to another voltage? Which one?

  • 0
    TI__Guru* 97066 points

    Hi Srinadh,

    The SN74AUP2G17 will support voltages on the input up to 4.6V without suffering any damage, but we do recommend keeping the inputs below 3.6V for the best reliability. There are many ways to prevent over-voltage, and they will depend on the type of over-voltage you expect to see (see Clemens's question #3). ESD protection is specialized and depends on the type/level of protection required. For normal over-voltage protection, for example, possibly being shorted to a 12V supply, then a series resistor + shunt diode connected to VCC should suffice to protect the input.

    For the output, a series resistor is sufficient to protect against a short circuit. The AUP family of devices can support up to 20mA safely at the output, so the best resistance value to allow a short circuit safely will be R = VCC / 0.02. Assuming a 3.3V supply, then this would be a 165Ω resistor (67Ω should be sufficient due to the internal resistance of the device, which is about 100 Ω under short circuit conditions at 3.3V -- internal resistance is larger at lower voltages). I would go with 67Ω if this is intended as a protection for only failure modes, and 100Ω if you think shorting will be a common event at the output port.

    This does mean that there will be an impedance mismatch at the output for your 50 Ω connection, but typically having a higher resistance will not cause any major issues for such a slow signal.

    If you supply additional details for the system and expected special conditions we can probably help with a more specific solution.

  • 0 in reply to Clemens Ladisch
    Prodigy 205 points

    Hi,

    1. Signal Voltage we considering is 3.3V 

    2. We are not terminating the signal with 50E but we are maintaining 50E impedance for 1PPS signal.

    3. I'm not considering the situations like surge and transient voltages what if the input voltage is exceed the required. Required/which is considering in the input is 3.3V if suppose the input voltage is 5/10V then my system will effect. In that condition suggest the overvoltage protection for 1PPS without effecting the rise and fall time of the 1pps signal basically it is 5ns and pulse width is 20us.

  • 0 in reply to Emrys Maier
    Prodigy 205 points

    Hi Maier,

    Here my consideration is i'm receiving the 1PPS signal from the external source to my system. Then i'm receiving the signal with series resistor and diode as you mentioned for over voltage protection followed by THS3491 which is also for protection purpose we have selected. Does it really suits here? and the output from the THS3491 is not degrade the rise and fall time 5ns of the 1PPS signal and also pulse width 20us. And the output is divided into 2 to provide the signal into 2 independent IC's like one is for FPGA and then another one is for TDC-GPX2 in the system. Here to divide the signal i'm using SN74AUP2G17.

    Is this configuration works without signal degradation or not. If not please suggest the best approach.

  • 0 in reply to Srinadh Moka
    Guru 317770 points

    AUP devices can work with 3.3 V signals, and have a typical output impedance of about 50 Ω. Adding a short-circuit protection resistor at the output will slow down the signals a little bit, but with the delays being symmetrical for rising and falling edges, will not degrade the signal.

    I do not understand what you are using the THS3491 for. If the input signal is clamped with diodes/resistors, the AUP buffer is enough. If you want to use a high-voltage device, better use a high-speed comparator like the TLV1871.