Kjeldgaard
- Joined Apr 7, 2016
- 476
After a little searching, I managed to find an old data sheet of a simple non buffered inverter, wherein there is shown equivalent circuit: 74HCU04. Each of the six inverters looks like the following:
There are diodes from the inputs and outputs to Vcc and Gnd, and only when the voltage at the input rises above Vcc or goes below Gnd, the leak current (besides Input leakage current, which is typically a small uA value) in the protection diodes. Data on the CMOS logic is usually between 10 and 20 mA as max-current clamp.
When we're talking about, not used, CMOS logic inputs must be tethered to Vcc or Gnd, then it's to keep the cross currents down. The following graph shows the relationship between the input voltage, cross current and output voltage:
It can be seen that with about half an Vcc input voltage, then current is typically over 20 mA, and input should be within a half volts from Gnd or Vcc before this cross-current is minimized.
There are diodes from the inputs and outputs to Vcc and Gnd, and only when the voltage at the input rises above Vcc or goes below Gnd, the leak current (besides Input leakage current, which is typically a small uA value) in the protection diodes. Data on the CMOS logic is usually between 10 and 20 mA as max-current clamp.
When we're talking about, not used, CMOS logic inputs must be tethered to Vcc or Gnd, then it's to keep the cross currents down. The following graph shows the relationship between the input voltage, cross current and output voltage:
It can be seen that with about half an Vcc input voltage, then current is typically over 20 mA, and input should be within a half volts from Gnd or Vcc before this cross-current is minimized.