SN74HCT74: Switching characteristics vs. temperature

Mike Pascucci

Part Number: SN74HCT74
Other Parts Discussed in Thread: SN54HCT74

In the datasheet for the SN74HCT74, page 4 shows a table for "over recommended operating free-air temperature range." There are three columns including "TA = 25°C", "SN54HCT74," and "SN74HCT74."

I understand that each of the "SNxxxxxxx" columns are showing the maximum switching times for the overall temperature ranges (-55 ~ 125°C and -40 ~ 85°C), and "TA" is giving typical and max for room temperature (25°C), but is there a plot to show how changes in temperature change the switching times? Or at least something basic like that switching time increases or decreases with heat?

Here's the datasheet I'm looking at: http://www.ti.com/lit/ds/symlink/sn74hct74.pdf

Thanks,

Mike

over 5 years ago

0 Mike Pascucci over 5 years ago

Prodigy 200 points

*Copy and paste mistake: I meant to refer to the table as "switching characteristics over recommended operating free-air temperature range."

0 Emrys Maier over 5 years ago in reply to Mike Pascucci

TI__Guru* 96591 points

Hey Mike,
There's generally not going to be anything that detailed on a device this old. I can take a look at the char data when I get back in to work tomorrow and see if we have something.

0 Emrys Maier over 5 years ago in reply to Emrys Maier

TI__Guru* 96591 points

Hey Mike,
I didn't find anything like a plot of delay over temperature, however I did find some detailed characterization data for a very similar device. It looks like there isn't a huge change over temperature for the HC(T) family of devices. I saw delays increasing by 1 to 2ns as temp increases from -40 to +85C.

Please note that the range on the datasheet is the only limit we can guarantee -- if you're counting on the delay always being the same that it is today, that may not be the case. Changes in manufacturing can shift those numbers as long as they still remain within the datasheet spec.

0 Mike Pascucci over 5 years ago in reply to Emrys Maier

Prodigy 200 points

Hey Emrys,

Thanks - just knowing that delays increase with temperature is helpful. So is it correct to assume that the max shown in the "switching characteristics" table (in the datasheet I linked) is for the upper temperature limit (+85C)? For example, for 5.5V VCC at +25C, max CLK to Q/Q' is 25ns, but the max becomes 31ns for +85C?

-Mike

0 Emrys Maier over 5 years ago in reply to Mike Pascucci

TI__Guru* 96591 points

No, the maximum is listed for the absolute worst case - ie a device with the worse shift in process for delay + temperature change. Like I said, I only saw about 1ns of change over temperature, so a device with a typical value of 15ns should shift from about 14ns to 16ns over temp (typical).

0 kai klaas69 over 5 years ago in reply to Mike Pascucci

Guru 140200 points

Hi Mike,

see figure 3-22 of this link:

www.next.gr/.../Digital-CMOS-Circuits-Tutorial-page-4.php

Kai

0 Mike Pascucci over 5 years ago in reply to Emrys Maier

Prodigy 200 points

Okay, thanks. Also, I'm not sure if I should ask this here or if I should start a new thread, but do you know if there's any information on the resistance between the Q and GND pins when Q is low?

0 kai klaas69 over 5 years ago in reply to Mike Pascucci

Guru 140200 points

Hi Mike,

this can be estimated from the specifications of datasheet:

An example:

At IOL = 4mA and 25°C the typical output voltage is 0.17V. So, the on-resistance of the NMOS-FET of output stage is about 0.17V / 4mA = 42.5R.

Kai

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SN74HCT74: Switching characteristics vs. temperature