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?