SN74AHC1G09: Open Drain Output characteristic

Mike Greenwood

Part Number: SN74AHC1G09
Other Parts Discussed in Thread: SN74LS09,

In an existing application, I’m trying to replace an SN74LS09 which currently interfaces with 5V logic to a part that will interface with new 3.3V logic. Ideally, I would like to do it with a drop-in replacement, but if that is not possible then to do something else with as few parts as possible.

The one option that I’m looking at is to use an open-drain SN74AHC1G09 operated from a supply of 3.3V, but I have question on the rating of the maximum output voltage. I’m trying to figure out if the output is diode clamped to Vcc or not. It seems to indicate that it is, but I want to make sure. Also, it seems odd to me to have an open-drain output clamped to Vcc, but maybe that is normal.

If I have a resistor pullup of 1.5 kΩ to 15V on the output of the SN74AHC1G09;

1) What will the output voltage be when it is a high output.

15V? OR,
4V because it is diode clamped to the supply of 3.3V?

i. Current is limited to about 7.3 mA which is less than the absolute maximum of 20 mA.

2) Was the output of the SN74LS09 also clamped to Vcc or not?

From the data sheet:

Absolute Ratings

(2) The input and output voltage ratings may be exceeded if the input and output current ratings are observed.

Thanks for any help you can provide.

over 6 years ago

0 Emrys Maier over 6 years ago

TI__Guru* 96782 points

Hi Mike,
The output voltage maximum (Vcc + 0.7V) can be exceeded, however the Supply Voltage Range rating cannot (7V max), so the absolute maximum for the output voltage would be Vcc + 0.7V @ Vcc = 7V --> Vo(max) = 7.7V.

If you want to do 15V, I would recommend looking at the CD4k line of parts, as they are the only logic devices rated from 3V to 18V -- or just use an external FET rated to 20V with a normal push-pull output CMOS AND gate.

I'm going to have to double check on the output clamp diode since it seems odd to me that a device would be open drain but still have a clamp diode on the output, and I don't have any on hand to verify. The datasheet definitely seems to indicate that this is the case, however.

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

TI__Guru* 96782 points

Hi Mike,
I have verified that the SN74AHC1G09 does _not_ have a positive clamp diode on the output.

I will add this to our errata list so we can update the datasheet.

0 Mike Greenwood over 6 years ago in reply to Emrys Maier

Prodigy 50 points

Thanks for your help. That answers my questions.

Logic

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SN74AHC1G09: Open Drain Output characteristic