DS26LV32AQML: Inquiry on Selecting Alternate-Failsafe Termination Resistor Values in AN-903

hm lee

Part Number: DS26LV32AQML

Tool/software:

Dear TI Support,

I am reviewing Application Note AN-903 regarding Alternate-Failsafe Termination design.

In the document, resistor values are presented for a 5 V supply. However, I would like to know how to properly select the resistor values when VCC = 3.3 V.

Could you please provide:

The recommended resistor value ranges (RA, RB, RC, RD, RT) for 3.3 V operation.
The calculation formulas or design guidelines used to determine the appropriate resistor values.

This will help me ensure that the receiver input differential voltage (VTH ≥ 200 mV) is satisfied under open/short/fault conditions at 3.3 V supply.

Thank you for your support.

Best regards,

3 days ago

0 Clemens Ladisch 3 days ago

Guru 317280 points

The resistor values shown in the application note are just examples; they should be adjusted for the expected peak voltages and the driver's drive strength.

These values will also work with a 3.3 V supply; they result in a voltage of about 300 mV between the receiver's inputs when the line is idle.

0 BOBBY 3 days ago

TI__Guru 73761 points

https://www.ti.com/lit/an/slyt324/slyt324.pdf?ts=1758917145109&ref_url=https%253A%252F%252Fwww.google.com%252F

-Bobby

0 hm lee 16 hours ago in reply to Clemens Ladisch

Prodigy 10 points

Thank you for your reply.
May I know the formula used to determine Vth?
Specifically, I would like to understand how the 300 mV threshold is derived under a 3.3 V supply.

0 BOBBY 6 hours ago in reply to hm lee

TI__Guru 73761 points

Hm,

I'm not sure where the 300mV threshold you are referring to is from.

The RS485 standard which this device was designed around states the max VTH+ is 200mV. So most people who design an external fail safe bias will aim for 200mV.

-Bobby

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DS26LV32AQML: Inquiry on Selecting Alternate-Failsafe Termination Resistor Values in AN-903