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SN74HCT08: Difference with SN74ACT08

Emma Wang
Expert 4770 points
Part Number: SN74HCT08
Other Parts Discussed in Thread: UCC23313, , SN74ACT08

Hi expert,

My customer use two SN74HCT08 driver 4 UCC23313, but since the current drive is not enough, the output voltage will drop to 4.2V with 5V VCC.

if they use one SN74HCT08 driver 4 UCC23313, the voltage will drop to 3.8V.

UCC23313 need 16mA at max, and customer test showed it need 8mA on each UCC23313.

So I suggest customer to use a high current AND gate,SN74ACT08, this is better and output voltage is 4.5V.

Question 1: And customer hope to understand why these two devices have difference on IOL and IOH.

Besides,  they found the SN74ACT08 will eat some duty cycle.

When input is 5% and output is only 2% during soft start phase. Question2 :and they want to know what cause this phenomenon?

BR

Emma

  • 0
    Guru 277110 points

    Different logic families are designed for different applications. AC is faster than HC, and has more drive strength, so it is a better solution to drive the UCC23313's input.

    Please show the schematic. If you're using a resistor to limit the current, then you can simply reduce the resistor's value to compensate for the AND gate's output impedance.

    All devices (HC, AC, and UCC) are designed to have symmetrical propagation delays, so they should not change the duty cycle. Please show an oscilloscope trace of both the input and the output signal.