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SN74AHC14Q-Q1: Input Current (Ii) flow direction

Cedrick
Expert 8270 points
Part Number: SN74AHC14Q-Q1

One of our customer has a inquiry about SN74AHC14Q-Q1, please see the inquiry below:

The SN74AHC14QPWRQ1 is being used in multiple ECU's for automotive application, where the input voltages may be present when the supply rail is off (0 V). Our question is in regards to input current "Ii" specification which shows < 1 uA worst-case when Vi = 5.5 V or GND, and VCC = 0 V to 5.5V.

1) Where does the input current flow? Into the positive rail, ground, or outputs?

Based on the datasheet, it looks as if there is no input diode clamp to the positive rail which leads me to believe any current input would be directed to ground. I need to verify this though because if it does go to the positive rail, it will backfeed our 3.3V supply and turn on critical circuitry which must remain in a known-off state.

Thank you.

Regards,

Cedrick

  • 0
    Guru 303560 points

    The AHC/AHCT Designer's Guide (SCLA013) says:

    Figure 12 shows protective circuits used for advanced high-speed CMOS devices. To meet the requirements outlined previously, the protective circuit is constructed in two stages. The input is first protected by a thyristor consisting of transistors Q2 and Q3. This provides coarse protection. If the input voltage rises above about 15 V, transistor Q1 breaks down and fires the thyristor. The latter then short circuits the high currents. Resistors R1 and R2 have values of only a few ohms. Therefore, the holding current of the thyristor is several tens of mA. When the current is reduced again at the end of the discharge, the thyristor is extinguished. Transistors Q4, Q5, and Q6 operate as fine protection and are intended principally to protect the input from excessive voltages. When there are overvoltages at the input, these transistors are driven into breakdown and limit the voltage, while resistor R3 limits the current.

    A two-stage protection circuit also is to be found at the output of the circuit. The previously mentioned thyristor (Q11, Q12, and Q13) provides coarse protection. Diodes D1 and D2 limit the voltages at the output to tolerable levels.

    The typical leakage currents will be much smaller than 1 µA (except when under extreme temperatures and other stresses).

    It looks as if leakage to ground is most likely., but there might also be a little bit through Q4/Q5 to VCC. Leakage through all the MOSFET gates to the output is basically nonexistent.