SN74AHC541：undershoot for about 5ns

Part Number: SN74AHC541

Hi. Team.

I’m using SN74AHC8541N , SN74AHC541N , SN74HC273N and SN74AHC273PWR.

I have some questions to undershoot of VI and VO.

The undershoot may be exceeded -0.5V in the application. which exceeding abs max rating of VI and VO.

Each devices datasheet say “The input and output voltage ratings may be exceeded if the input and output current ratings are observed.” .

So I believe each device can exceed -0.5V as long as IIK and IOK are lower than -20mA and ±20mA respectively.

It is my understanding. correct?

If so , could you advise damping resistor value to suppress the current for each devices?

Thanks.

Regards,

Tsuyoshi.

Part Number: SN74AHC541

Hi. Team.

I’m using SN74AHC8541N , SN74AHC541N , SN74HC273N and SN74AHC273PWR.

I have some questions to undershoot of VI and VO.

The undershoot may be exceeded -0.5V in the application. which exceeding abs max rating of VI and VO.

Each devices datasheet say “The input and output voltage ratings may be exceeded if the input and output current ratings are observed.” .

So I believe each device can exceed -0.5V as long as IIK and IOK are lower than -20mA and ±20mA respectively.

It is my understanding. correct?

If so , could you advise damping resistor value to suppress the current for each devices?

Thanks.

Regards,

Tsuyoshi.

Hi Tsuyoshi,

Tsuyoshi Makino said:

So I believe each device can exceed -0.5V as long as IIK and IOK are lower than -20mA and ±20mA respectively.

This is correct. Keep in mind the SN74HC273N has positive input/output clamp diodes and the AHC devices have positive output clamp diodes.  Please see this [FAQ] Can the input voltage (Vi) to my logic device be higher than the supply voltage (Vcc)?

The below image shows the input/output clamp diode structure of the SN74HC273. If the input voltage is higher than Vcc the positive clamp diode will become forward biased and allow current to flow to Vcc. 

A schematic would be helpful here, I don't know how these devices are connected. I'd need to know the input and output voltages of each device to help recommend damping resistors.

Regards,

Sebastian 

Hi. Sebastian.

Sorry for the lack of application information for you to advice damping resistor value to suppress the current for each devices.

I give how these devices are connected, and input and output voltage.

could you advise damping resistor value to suppress the current for each devices, again?

• Devices connection：

• Input and output voltage：

1. CPU : output   VOL=  0 ~ 0.4V   , VOH = 4.35 ~ 5.25V.  (Cin max = 10pF)

• Observed undershoot :

1. For the DATA, Vpeak undershoot = -0.87V. Time period below max rating(-0.5V) is about 3ns.
2. For the /EN0, Vpeak undershoot = -0.95V. Time period below max rating(-0.5V) is about 3ns.
3. For the /EN1, Vpeak undershoot = -0.69V. Time period below max rating(-0.5V) is about 0.9ns.
4. For the /EN2, Vpeak undershoot = -1.04V. Time period below max rating(-0.5V) is about 2ns.
5. For the /EN3, Vpeak undershoot = -0.84V. Time period below max rating(-0.5V) is about 3.1ns.

Regards,

Tsuyoshi

Hi Tsuyoshi, 

The observed undershoot should not be a concern as long as the current at the I/Os are limited to 20mA. I'd recommend using series resistors of 1kOhm to keep the current well below 20mA.

Regards,

Sebastian 

Hi. Sebastian

I'm wondering if I can estimate IIK and IOK from IBIS models.

However, there are no hspice data for SN74AHC273, SN74AHC8541 and SN74HC273 on the device home page.

If there is any measure that can estimate IIK and IOK ,could you kindly help?

or any idea that I can check IIK and IOK?　 

Regards,

Tsuyoshi

Hi. Sebastian

Thank you for the help.

Tsuyoshi