Problem with SN74HCT373 IC while starup of equipment

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swapnil0511

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At first we were using ic HD74LS373 but since its low availability in local market we shifted to SN74HCT373 but the problem is when we start the equipment at that time relay attach to this ic are turned on and off for short duration but we did not had this problem in HD74LS373
can anyone suggest me any changes in design or any other alternative for HD74LS373
 

If you connect the output-enable (NOT OE) line to earth via a capacitor and to Vcc via a resistor, the outputs will be in their high impedance state while the capacitor charges, so fiddle with the time constant to get a delay long enough so it ignores the settling time of the power supply unit.
Frank
 

74LS573 is an alternative. It is functionally equivalent to 74LS373 , but has modified pin layout which helps PCB designing.
 

Hi
thanks for ur reply chukey and ahsan

and chuckey i connected 220ohms register between Vcc and Latch enable pin and 0.01uf capacitor between latch enable and ground this solves the problem while during startup but when the equipment is turn off again there is chattering of relays.

Can anyone help me with this
 

You appear to have many irregular ripples when your V+ is not stable going up or down. Each logic interface can react differently.

Therefore you must decide what state your logic should be in or have an uninterrupted power supply.

If V+ is interrupted or AC is lost or brownout , you must define the sequence of events in a controlled manner or even if multiple supplies exist.

I would suggest, without knowing these details a "Power Good" detector is required for !OE.

How you define this, depends on previous details.
One example may be if V+< (Nom-10%) then OE= high.(off)
This could be done with Programmable zener chip. You may need to add an appropriate time delay for ON and instant for OFF.
 
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