Posted on Jan 4, 2013

Since standard CMOS logic operates down to 3-V supply voltages and is specified as low as 30 p.A maximum current sinking/sourcing capability, it is necessary to use a buffer transistor to provide the required current to the IRED if CMOS is to drive the optocoupler. As in the case of the low output TTL families, the H74A output can drive a multiplicity of CMOS gate inputs or a standard TTL input given the proper bias of the IRED. A onelogic stage drives the IRED on.

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This circuit will provide worst-case drive criteria to the IRED for logic supply voltages from 3 to 10 V, although lower power dissipation can be obtained by using higher value resistors for high supply voltages. If this is desired, the worst-case drive must be supplied to the IRED with minimum supply voltage, minimum temperature and maximum resistor tolerances, gate saturation resistance, and transistor saturation voltages applied. For the H74 devices, minimum IRED current at worst-case conditions, zero logic state output of the driving gate, is 6. 5 mA and the HllLl is 1.6 mA.

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