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SN65HVD78 A / B Input Equivalent Circuit

I am trying to solve a problem the this transceiver causes when in the powered down state. There is a sneak path to the (unpowered) 3.3 V supply through the A/B inputs of the transceiver from an active bus. The data sheet shows an equivalent input circuit - Figure 18 - see below. There are three resistors in this circuit (R1, R2, R3), but the values are not given in a table like other TI transceiver data sheets. Can you please tell me the values.

For now I am trying to design with the values from the SN65NVD10 data sheet R1 = 45K;   R2,R3 = 9K. Are these the right values?

- Jim Thommes

  • Hi Jim,

    The equivalent circuit diagram in the data sheet is just a representation of the application circuit and does not show the actual circuit implementation. R1 is a passive resistor that has a value of 50k ohms. R2 has a value of 7.8kohms but there is a diode in parallel with it and when the bus voltages rise above Vcc this body diode starts to conduct and what you see on Vcc is the voltage drop across R1 minus a diode drop. R3 is a passive resistor of 10.6k ohms in value. Please let me know if you have any more questions?

  • Thanks Michael. This helps explain a lot that I am seeing. With this information, I can complete a compatible design with other parts.

    For your information, we are having trouble with a SPI EEPROM 256Kb. This EEPROM part has a Power On Reset that latches and does not release until the Vcc goes below approx 0.0 to 0.2 volts. It is on the same 3.3 V bus as several of these transceivers that remain on active buses when 3.3 V is powered down. The result is the 3.3 V never reaches 0.0 and the EEPROM does not work properly on the next power up cycle - reads / writes erratically.

    - Jim Thommes