SN74LV125A-Q1: LVTTL Compliance

Matt Child

Part Number: SN74LV125A-Q1
Other Parts Discussed in Thread: SN74LVC125A-Q1, SN74LVC1G125-Q1

We are looking at using this device to output an LVTTL signal. We will be supplying the device with a nominal voltage of 3.3V (Max: 3.4 V).

From the datasheet the V_OL Max is 0.44 at 3V Vcc (and presumably slightly higher for 3.3V), which is just slightly above the 0.4 V VOL Max required for LVTTL. Can we use an external pulldown resistor from the outputs of the buffers to bring this voltage down to meet LVTTL spec?

over 4 years ago

0 Clemens Ladisch over 4 years ago

Guru 317370 points

The output voltage depends on the current (see [FAQ] What is the output voltage (VOH or VOL) when the output current is X or the supply voltage is Y?). When connected to a CMOS input, the current is approximately zero, so V_OL is approximately zero.

What is the actual required drive strength?

0 Matt Child over 4 years ago in reply to Clemens Ladisch

Prodigy 100 points

Hi Clemens,

For now we are trying to be compatible with four devices, which we estimate will draw between 3 mA and 15 mA. I have done some of those calculations you have linked me to and I am still unsure of the answer I'm afraid!

0 Clemens Ladisch over 4 years ago in reply to Matt Child

Guru 317370 points

The LV family is designed for 8 mA at 3.3 V. For 15 mA, consider the SN74LVC125A-Q1 or SN74LVC1G125-Q1.

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SN74LV125A-Q1: LVTTL Compliance