SN74LVC8T245: SN74LVC8T245 Output Enable input & voltage problem

Yoav Nadler

Prodigy 110 points

Part Number: SN74LVC8T245

Hi,

i have this component in my design.

Direction input is configured from A to B.

Output enable input is configured to be enabled (using a 4.75kohm resistor to GND).

By mistake the OE pin was shorted to 3.3V (resistor pull down to GND was still connected) and it seems to cause a voltage problem.

In general, i won't connect an input to a voltage without a resistor but, i want to understand why it cause a problem.

The input, OE, should have an input resistance that should protect it, i've tried to search DS to find it without luck...

Thanks in advance for your help,

Best regards,

Yoav.

over 6 years ago

0 Emrys Maier over 6 years ago

TI__Guru* 96591 points

Hi Yoav,
We will get back to you when we are in the office next week.

0 Yoav Nadler over 6 years ago in reply to Emrys Maier

Prodigy 110 points

Thanks for your reply,

i performed more tests and it seems that the input resistance of the OE pin is low,

What info can you add about that?

Thanks, Yoav.

0 Clemens Ladisch over 6 years ago in reply to Yoav Nadler

Guru 243790 points

The OE pin is a CMOS input, so it should have a very high resistance. (This is implied by the I_I specification in the datasheet (even if it forgets to mention OE)). A pull-up or pull-down resistor is needed only if you want to have a default value that can be overridden; a series resistor is needed only if you expect out-of range voltages and want to limit the current through the protection diode.

3.3 V is inside the absolute maximum ratings, so that should not have been a problem.

Is the low resistance to GND or to V_CC? Anyway, this sounds as if the input transistors are fried.

0 Yoav Nadler over 6 years ago in reply to Clemens Ladisch

Prodigy 110 points

Hi,

in my design i have an external 4.75K ohm, but when i measure the resistance (power is down) i measure 105 ohm.

so, i can assume the component is damaged?

thanks.

0 Clemens Ladisch over 6 years ago in reply to Yoav Nadler

Guru 243790 points

Between what two points are you measuring the resistance?

Does the chip work, or not?

0 ShreyasRao over 6 years ago

TI__Guru 56010 points

Yoav,

As Clemens rightly mentioned, the OE is CMOS high input impedance pin with very large resistance in the range of Mohm (as can be calc by the V/I from the input leakage current Ii)
If you are seeing a low resistance on the OE, then the device is most likely damaged.
Is this on all the devices in the lot or on one / few devices?

Logic

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SN74LVC8T245: SN74LVC8T245 Output Enable input & voltage problem