SN74LVC244A: Slew rate consult for 1OE and 2OE pin

Stanley Dai

Part Number: SN74LVC244A
Other Parts Discussed in Thread: SN74LVC125A, SN74LVC126A,

Hi Team,

Customer is consulting whether we have slew rate requirements for received signal on 1OE and 2OE pins? Our D/S have not shown this spec.

Best,

Stanley

over 3 years ago

0 Clemens Ladisch over 3 years ago

Guru 316690 points

All CMOS devices without Schmitt-trigger inputs have such a requirement. (And it's the same for the control and data inputs.)

If the datasheet does not specify it, look at other device in the same logic family. The SN74LVC125A datasheet specifies a limit of at most 8 ns/V; the SN74LVC126A datasheet has 10 ns/V.

0 Stanley Dai over 3 years ago in reply to Clemens Ladisch

TI__Expert 5810 points

So which one is for SN74LVC244A? Pls help us confirm it. The D/S of SN74LVC244A doesn't contain it. Thanks.

0 Clemens Ladisch over 3 years ago in reply to Stanley Dai

Guru 316690 points

This is not known. To be safe, use the most restrictive one.

0 Stanley Dai over 3 years ago in reply to Clemens Ladisch

TI__Expert 5810 points

Which one is recommended by your side? Pls confirm it.

0 Clemens Ladisch over 3 years ago in reply to Stanley Dai

Guru 316690 points

These limits are specified as maximum, so the most restrictive one is the smaller number.

0 Stanley Dai over 3 years ago in reply to Clemens Ladisch

TI__Expert 5810 points

But the D/S of SN74LVC244A doesn't have this info. If these limits are necessary, customer needs the confirmed info from BU side. Just simply tell me which one you recommend to customer. 8ns/V or smaller?

0 Clemens Ladisch over 3 years ago in reply to Stanley Dai

Guru 316690 points

As far as I can see, 8 ns/V should work for any LVC device. But I am not a TI employee.

+1 Emrys Maier over 3 years ago in reply to Stanley Dai

TI__Guru* 97046 points

Hi Stanley,

Please use 8 ns/V as the maximum transition rate for this device.

Also note that this is always in the "Recommended Operating Conditions" section of the datasheet -- it is not a measured specification, but a recommendation for operating the device without any issues.

Operating a device with standard CMOS inputs using slow inputs can cause issues as described here: How does a slow or floating input affect a CMOS device?

0 Stanley Dai over 3 years ago in reply to Emrys Maier

TI__Expert 5810 points

Hi Emrys,

Customer just feedbacks other vendors like Nexperia 74LVC244APW, which has no slew rate requirements for 1OE and 2OE pins. So they want to double check with us that can1OE and 2OE pins be free of above 8ns/V transition rate requirements?

Best,

Stanley

+1 Clemens Ladisch over 3 years ago in reply to Stanley Dai

Guru 316690 points

That information is wrong; Nexperia's 74LVC244A has slew rate requirements on all inputs, including 1/OE and 2/OE.

The Nexperia datasheet says that "Schmitt-trigger action at all inputs makes the circuit tolerant of slower input rise and fall times". However, "slower" in this case means only up to the specified 10 ns/V, and not any slower than that. "Schmitt-trigger action" is not the same as real Schmitt-trigger inputs. The application note Understanding Schmitt Triggers says:

There are parts that have Schmitt trigger action. This means that they will have a small amount of hysteresis that will help with noise rejection but will still have an input rise and fall time limit. These will usually not have any VT specs in the datasheet and will have rise and fall time limitations specified for the inputs in the recommended operating conditions.

Please note that TI's LVC inputs also have Schmitt trigger action, but the TI datasheet does not mention it because the Δt/ΔV specification (if the datasheet does not forget to include it) is enough to characterize the inputs for the circuit designer.

0 Stanley Dai over 3 years ago in reply to Clemens Ladisch

TI__Expert 5810 points

Thanks Clemens, very clear.

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SN74LVC244A: Slew rate consult for 1OE and 2OE pin