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SN65HVD1781: Icc current consumption details

Pradeepkumar Selvaraj1
Prodigy 190 points
Part Number: SN65HVD1781
Other Parts Discussed in Thread: THVD2450

Dear Team,

I am using SN65HVD1781DR chip for my RS485 interface.

I have queries wrt supply current values of different values provided at different places.

1) From electrical characteristics table 7.6, Icc is 5mA as highlighted below.

In the same table, there is dynamic characteristics is taking me to different place having higher supply currents of 16mA or 33mA as shown in two different characteristics diagram provided in below snip,

So, what is the significant difference between 5mA provided in the first table, and the 16 or 33mA provided in the dynamic characteristics graphs.?

As per my design, I am using Vcc 3.3V & 120 ohm termination. So, for 1V differential output voltage, the current required into the 120-ohm resistor should be 1V/120 = 8.3mA

So, the calculation tells that it should take minimum of 8.3mA. but how it is mentioned as 5mA in the datasheet.?? I am confused. Please help me to understand.

Regards,

Pradeep. S

  • 0
    Guru 284000 points

    The "no load" specifications are measured without any terminations.

    A typical RS-485 bus is terminated at both ends, so the load is 60 Ω (54 Ω worst case). And the differential output voltage is much larger than 1 V.

  • 0 in reply to Clemens Ladisch
    Prodigy 190 points

    Thank you Clemens.

    It is mentioned 1V typical as VOD in below snip taken from the datasheet. Since I am using 3V3 as supply, so i considered 1V as differential output voltage. So, could you please tell how maximum it can go up to for 3v3 supply?

    And regarding power dissipation, please clarify how to calculate it. Since in the datasheet, power dissipation is provided only for the case of 5.5V supply as shown below.

    Please clarify.

    Regards

    Pradeep. S

  • 0 in reply to Pradeepkumar Selvaraj1
    Guru 284000 points

    At 3.3 V, it is indeed 1 V typical.

    There is no information about the maximum.

    I guess there is no power dissipation specification for 3.3 V because the SN65HVD1781 is not strictly RS-485 compatible at 3.3 V. I would recommend to use a more modern device like the THVD2450.

  • 0
    TI__Guru 60540 points
    Pradeepkumar Selvaraj1 said:

    1) From electrical characteristics table 7.6, Icc is 5mA as highlighted below.

    This one is related to the device not actively driving/switching and not receiving an active switching input. (generally no load)

    Pradeepkumar Selvaraj1 said:
    In the same table, there is dynamic characteristics

    Dynamic refers to actively driving a signal into a load. 

    Pradeepkumar Selvaraj1 said:
    different place having higher supply currents of 16mA or 33mA as shown in two different characteristics diagram provided in below snip,

    Figure 1 shows the current draw expected from driving B-A against a 54ohm resistance. This is related to the output drive strength of the A-B drivers. At higher Vcc, the Ron of the FETs are lower so it can drive more current.

    Figure 2 shows an RMS current value which you can kind of think of it as a way to turn an AC measurement into an average DC measurement. Figure 2 shows the additional supply current as frequency increases. I believe this is showcasing the additional loses due to shoot through current of the receiver channel.

    Pradeepkumar Selvaraj1 said:
    So, what is the significant difference between 5mA provided in the first table, and the 16 or 33mA provided in the dynamic characteristics graphs.?

    First table is basically the device is turned on but no doing anything. 

    The -16mA is the amount of current a single differential driver is sourcing.

    33mA is the Supply current when the device is actively driving (it is also sourcing current through A-B which is included in this number). You can use this value to estimate the power consumption of the device itself. You would multiply by the 3.3V to approximate the power consumption in this example.

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 190 points

    Thank you so much Bobby & Clemens.