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SN7407: Turns On When Powered Down

Matt Lang
Intellectual 540 points
Part Number: SN7407
Other Parts Discussed in Thread: SN74LS07

We have run into an issue with the SN7407DR chips. We found that when power is removed from the product, as the 5V rail ramps down, the buffer output will turn on when Vcc reaches around 2.88V.

Experiment Setup:

Vcc of +5V

Gnd = 0V

Buffer input driven to 3.3V throughout the whole experiment, even when 5V power is removed. To insure the input is not the cause of the power down pulse.

The 5V rail must have some bulk capacitance to simulate a 5V rail with bulk capacitance anywhere from 22uF to 100uF.

Put a pull up resistor on the buffer output to another power rail, either the 3.3V or another rail that does not get powered off with the buffer Vcc. To measure output with scope



Experiment Procedure:

Remove the power source for the buffer Vcc 5V rail, but make sure the bulk capacitance remains connected to the buffer chip after power is removed.
When Vcc gets to around 2.88V, the buffer output will turn fully on and bring the output low. The duration the output stays low depends on how large the Vcc bulk capacitance is.

Notes:
1) We are not operating the IC below it's rated Vcc. At some point our PCB has to be powered off and we can't have our IO turning on when they are not supposed to. 

2) Our buffer chip we tested on the bench has these markings: 71A4C5M 7407 G4

3) The specific input and output pins tested are: Input = Pin 11 (5A) Output = Pin 10 (5Y)

Questions:
1) Is this a known issue with these ICs?

2) What is a suggested solution to this?

3) Do you have a newer revision of this part or an alternate that does not have this issue?

  • 0
    Intellectual 540 points

    Thought I would post my scope screen shots showing the issue.

    The image shows the Vcc 5V rail (blue) ramping down. And the buffer output (yellow) with a pull up resistor to a separate 5V rail that stays on. Also the buffer input is driven to 3.3V the whole time. Vcc had a 100uF cap.

    You can see that around 2.88V the buffer turns on, bringing the output low. Until Vcc reaches around 700mV.

  • 0 in reply to Matt Lang
    Guru 269850 points

    The IC is operated below its rating; see the minimum VCC in the recommended operating conditions table.

    The output transistor switches on when its base current is not shunted to ground by the other transistors (see the schematic below). This happens when VCC  is not high enough for that chain of transistors.

    To ensure that the outputs are high impedance during power up/down, you need some buffer with an OE input, e.g., 74xx244. What electrical characteristics do you need?

  • 0 in reply to Clemens Ladisch
    Intellectual 540 points

    When you say "The IC is being operated below its rating". Yes, the Vcc is well below what it's rated to when the issue happens, but the buffer is not being "operated". It's just sitting there idle. All ICs must be able to handle being turned off without toggling their outputs. Just seems like a big issue and could cause lots of problems.

    I would need something completely drop in replacement. Something that has an output default of "off" when Vcc is low so the buffers don't turn on.

    What about the SN74LS07DR parts? Would these turn on when Vcc is low?


  • 0 in reply to Matt Lang
    Guru 269850 points

    Only logic gates with OE are designed to be able to disable the outputs when the supply voltage is too low.

    The SN74LS07 has three diode drops for both the output transistor and the shunt transistor. It might work, but this is not something that is guaranteed or tested, so the only way to find out is to test it yourself.

  • 0
    Intellectual 540 points

    For completeness, I purchased some of the SN74LS07DR parts. They do not have the same issue with my setup. I do not see any "power down pulse". I will continue to test and they will most likely be a drop in replacement.