SN74AXC4T774-Q1: SN74AXC4T774QPWRQ1

Ananthu Mohan

Part Number: SN74AXC4T774-Q1

Hi,

In one of our projects, we used the level translator IC SN74AXC4T774QPWRQ1 as an replacement for SN74AVC4T774PWR in our production boards. We've observed failures with the level translator in our production boards after this alternate part assembly.

We examined the IC for functionality and found the problem, a signal connected with one of the four channels is the source of the problem. This signal is connected to an IC with internal weak pullup and the level translator input pin (B4). The voltage on the input pin was low even though the pin is driven by a weak pullup. So we have isolated the input pins by removing the connected resistors, and measured the resistance of the IC input pins. All of the IC inputs had a resistance of around 50K - 60K Ohm (measured at power ON condition, VCCA=1.8V & VCCB=3.3V), and the leakage measured through series resistors was higher than 25uA (This is when we added an external 100KOhms pullup to drive the pin. The voltage on the pin was around 1.5V).

The other three signals are well-driven signals, therefore the functionality was passed. The one that failed had a low driven strength from the other IC, but it was definitely capable of driving SN74AXC4T774QPWRQ1 since SN74AVC4T774PWR was working fine earlier.

Could you please help us to find as to why this low resistance and high leakage is observed and kindly help us to resolve this as soon as possible?

Thanks,

Ananthu Mohan M

www.e-consystems.com

over 3 years ago

0 Clemens Ladisch over 3 years ago

Guru 316780 points

The guaranteed maximum input leakage current is 8 µA at 125 °C.

Apparently, your device is damaged. This is typcially caused by out-of-range voltages. Can you show an oscilloscope trace of that input while working?

0 Ananthu Mohan over 3 years ago in reply to Clemens Ladisch

Prodigy 60 points

Attached the oscilloscope image as referred above.

Input to B4 seems to be only 0.4V when working. Whereas If we open the series resistor, 3.3V is coming on the other side.

0 Ananthu Mohan over 3 years ago in reply to Ananthu Mohan

Prodigy 60 points

Note: When we isolated the series resistors and checked the resistance in the input pins, below are the observations.

(i) In OFF Condition, all the input pins had a resistance of 0.9M Ohm.

(ii) In Power ON Condition, the resistance on the input pins is only around 50-60KOhm.

0 Clemens Ladisch over 3 years ago in reply to Ananthu Mohan

Guru 316780 points

Please show the input signals that actually occur in practice. I suspect you have spikes or ringing above 4.2 V.

0 Ananthu Mohan over 3 years ago in reply to Clemens Ladisch

Prodigy 60 points

Could you please restate the below term.

"Input signals that actually occur in practice"

0 Clemens Ladisch over 3 years ago in reply to Ananthu Mohan

Guru 316780 points

Sorry; I'd like to see the input signal with an undamaged device. Because that might give a clue what caused the damage.

0 Ananthu Mohan over 3 years ago in reply to Clemens Ladisch

Prodigy 60 points

We have faced issue in all the boards mounted with SN74AXC4T774QPWRQ1, whereas it is working fine with SN74AVC4T774PWR.

We have also tested this alternate part in our other project boards, the input resistance is same as around 50-60K Ohms (series resistor isolated) in power ON condition.

Is it okay, if we probe the input signal with SN74AVC4T774PWR and send the same to you?

0 Clemens Ladisch over 3 years ago in reply to Ananthu Mohan

Guru 316780 points

It is unlikely that the input signals behave the same with the AVC device, so that would probably be not as helpful.

Do all AXC devices end up with the same ~ 50 kΩ input resistance?

0 Ananthu Mohan over 3 years ago in reply to Clemens Ladisch

Prodigy 60 points

Yes, All the AXC devices have resistance between 50-60KΩ when measured in power ON condition (series resistors isolated).

0 Ananthu Mohan over 3 years ago in reply to Ananthu Mohan

Prodigy 60 points

As we stated earlier, we have used this AXC device in other projects which are working fine. We have checked the input resistance in Power ON condition by isolating the series resistors but still the input resistance is around 50-60KΩ.

Whereas in AVC device, the input resistance is in the range of Mega Ω (tested in the same way as above mentioned).

0 Emrys Maier over 3 years ago in reply to Ananthu Mohan

TI__Guru* 97046 points

Hi Ananthu,

Currently the product expert for this device is not available - he will be back next week.

It sounds to me like these devices are violating the datasheet specification here:

I'd like to put off starting a return until we can get the product expert to weigh in on this issue. Is this something that we can discuss with him next week?

0 Ananthu Mohan over 3 years ago in reply to Emrys Maier

Prodigy 60 points

Hi Emrys,

Ok, we could discuss about this next week.

Once your product expert is available, please update us at the earliest.

0 Emrys Maier over 3 years ago in reply to Ananthu Mohan

TI__Guru* 97046 points

Thanks Ananthu,

I will send him a request to post here upon his return.

0 Dylan Hubbard over 3 years ago in reply to Emrys Maier

TI__Mastermind 45815 points

Hi Ananthu,

I'm looking into this now, thanks for allowing time for me to get back in office. Give me another 24 hours to continue ramping up on the material already provided and look into the issue. I have a feeling this behavior could be a result of the glitch suppression circuitry of the AXC family.

0 Dylan Hubbard over 3 years ago in reply to Dylan Hubbard

TI__Mastermind 45815 points

Hi Ananthu,

In the Power on condition, what is the state of the OE pin?

0 Ananthu Mohan over 3 years ago in reply to Dylan Hubbard

Prodigy 60 points

Hi Dylan,

In our design, the OE pin is tied to GND through a Zero Ohm resistor in series.

0 Dylan Hubbard over 3 years ago in reply to Ananthu Mohan

TI__Mastermind 45815 points

Hey Ananthu,

Thanks for that info, one last confirmation I will need:

Please replicate the scope shot above showing the issue, but instead of monitoring the supply could you have a probe on the OE pin?

I just want to make sure there isn't something weird going on with that input that could lead to the leakage you are seeing.

0 Ananthu Mohan over 3 years ago in reply to Dylan Hubbard

Prodigy 60 points

Hi Dylan,

Please find the oscilloscope image for the same.

0 Dylan Hubbard over 3 years ago in reply to Ananthu Mohan

TI__Mastermind 45815 points

Hey Ananthu,

Thanks for the added channel. This is definitely unusual behavior. I'm going to check with my designers to see if they have any more ideas before I have you initiate a FA. Would you be able to share a schematic on e2e or would you be willing to send it via email?