• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 18 subscribers
  • Views 235 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

SN74HCS367: SN74HCS367 self biasing issue

Norman Oliva
Prodigy 210 points
Part Number: SN74HCS367
Other Parts Discussed in Thread: SN74AHCT244-Q1, SN74AHCT244Q, SN74AHCT367

Hello, we are using SN74HCS367QPWRQ1 as our buffer driver fed from our MCU PWM outputs ( 3.3V ), we noticed that due to the internal clamp diode from input to Vcc, said input coming from our MCU PWM is enough to self bias the chip since Vcc can operate at as low as 2V. In our application we just shorted the enable pin to gnd and we are relying on the VCC to turn the device off to meet our safety interlock requirement but it was  defeated because the internal clamp is self biased by our MCU via our input pulses. Is this expected?

Below is our implementation:  Once the MCU starts to pulse, you can take off 5VS and the buffer will still work due to self biasing from internal clamp.

If we add reverse diode in series and add pull-up resistor at the input pin, will that cause any issue? 

thanks,

Norman

  • +1
    Guru 261330 points

    HC(S) family devices have clamping diodes to VCC and can be accidentally powered through them. You should use a logic family with overvoltage-tolerant inputs like AHC, LV-A, or LVC.

    Please note that 3.3 V signals are not compatible with plain CMOS inputs at 5 V. You need a device with TTL-compatible inputs, e.g., the SN74AHCT244Q or SN74AHCT244-Q1.

  • 0 in reply to Clemens Ladisch
    Prodigy 210 points

    Hello, we finally got pin to pin compatible replacement with TTL logic p/n is SN74AHCT367PW. In our application, we will intentionally cut the 5Vdd supply to comply with our S2 safety requirement. In this case, the input will be present for 25mS while the 5Vdd is already at 0V level. Please see illustration. 

  • +1 in reply to Norman Oliva
    Guru 261330 points

    The SN74AHCT367 will work. (I did not mention it because it is not AEC-Q100 qualified.)

    The inputs allow 5 V for any amount of time, independently of supply voltage. (Diode clamps would be needed only if you wanted to protect against voltages above the absolute maximum rating of 7 V.)