This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

MUX36D04: Signal Input current when no power is given

Part Number: MUX36D04

Hi,

We are using MUX36D04 in our circuit and we observed that, when power supply to this switch goes off, the current drawn by the signal input reaches 7mA.
Currrently we are using 240E as current limiter. But for how long can this current can be tolerated by the switch ?

  • Hi Kushal,

    The spec that states the amount of current drawn by a switch when VCC = 0V  is called powered off protection, or INC PWROFF. Here is an example from a datasheet of  a different device for this spec.

    Unfortunately the MUX36 device does not have this powered off protection feature. We may be able to help you figure out a solution for you system but can you first clarify what the "240E as a current limiter" is? Is this a resistor of 240ohms? 

    One app note that we have for system level protection of the MUX36 is linked here. 

  • Hi Dakotah,

    Thanks for yor response, it really helped us to look into our problem closely.
    240E is 240 ohms resistor.
    Our maximum input is 10V and our Vdd = 17V and Vss = -17V.

  • Hi Kushal,

    This case that you are mentioning is not with the specs of this device. The MUX36D04 does not have powered off protection feature that I mentioned early.

    The issue with having VDD power removed is that the device can actually backpower VDD through the ESD protection diode. You mentioned having 7mA flowing through your 240ohm resistor and into the device when VDD = 0V. What is the voltage on the resistor node near the sensor? Is the 7mA at the max input voltage of 10V?

    Can you take the following measurements:
    1) What is the sensor voltage when drawing 7mA?
    2) What is the voltage across your resistor?
    3) What is the voltage at VDD with the signal present and current flowing?

    If the sensor voltage is 10V with a 7mA current then the drop across your 240ohm resistor should be around 1.7V. If this is the case then VDD could be potentially backpowered to around 7.5V.

    Please let me know what results you get.

  • Hi Dakotah,

    Thanks for your reply

    1.)Sensor voltage is 6.5 V, there are 8 such channels having 6.5V at all the channels.
    The current being drawn from the input supply is 100mA for all the channels

    2.)across 240 ohms, 7.6mA current is there and the voltage across that was1.837V.

    3.)When the power comes again, the VDD is only 2V instead of 17V and huge current is being drawn from the power supply (not the input section, at input section the current drawn is still 100mA)

  • Hi Kushal,

    Thanks for collecting that data. 

    It seems that the device is being operated outside of the ABS max in this case and leading to device failure. This is evident by your statement of trying to power the device back up and VDD is held at 2V while drawing a large current.

    Here is the spec listed in the datahseet.

    This state the max current at 30mA for only one channel at a time. if this was split into the 8 channels that would be around 3.75mA max per channel. You have a 240ohm resistor in your circuit, can you clarify the purpose of the resistor? Can the value be changed? The app note that I referenced early talks about using a current limiting resistor like this resistor but of a large value. If we can increase the resistance we may be able to limit the total current to within a safe operating condition.

    Also, what is the state of you enable pin in this VCC=0V case? While you are in this powered down state signals may still be present on the output due to this backpowering of VDD. This should be evaluated for safe operation of the system.

  • Hi Dakotah,

    1.) If the datasheet says, 30mA per channel, then how it is in ABS max (in our case around 7mA current flowing) .
    2.) The enable pin voltage was 0.515V. This is because 17V was loading the main source from where the 3.3V was powered.
    3.) We have increased the current limiting resistor to higher value, and the issue is solved.

  • Kushal,

    1) I highlighted note (2) from the ABS max table in the last post. This states "Only one pin at a time". This is saying that the max of 30mA can flow through a single channel at a time. This spec is saying the the 30mA is the max the device can support regardless of how many channels are on. (e.g. 30mA is the max if 1 channel is on, 3 channels, or all of the channels). This is why the 7mA is in the ABS max range.

    2)Enable votlage of 0.515V is still a logic low according to VIH and VIL specs. While the part can be within ABS max values make sure that the system is still functional and you are not passing through any signals during power off operation that will damage entire signal chain.

    3)Glad to hear the the higher resistance is doing what it is supposed to and the issue is solved.