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THVD1410: About PD of THVD1410 when 32 node decvice connected

Jimmy WANG
Genius 3265 points
Part Number: THVD1410

Tool/software:

Hi team,

As I know RS-485 can support 32 devices up. 

So Ro of one device defined 12KR and 12K/32 then got 375R. if VCC=5V 

5V/375R=13mA 

PD=5V*0.013A=65mW  Is correct? 

  • 0
    TI__Guru 59170 points
    Jimmy WANG said:
    As I know RS-485 can support 32 devices up. 

    Not exactly. RS 485 defines a unit load as 1mA at 12V and it can support up to 32 unit loads. Modern RS485 devices don't have that much leakage current in each device. The device you chose (THVD1410) has a leakage current of 125uA max or 96k ohm load (12V/125uA = 96k ohms). If one unit load is 12k ohms then THVD1410 has a unit load of 1/8th. So if your entire RS485 bus supports 32 unit loads and all of your devices are THVD1410s then your total amount of THVD1410s allowed would be 256 (32x8) not 32.

    Jimmy WANG said:

    5V/375R=13mA 

    PD=5V*0.013A=65mW  Is correct? 

    I think this is correct when you look at the bus when it's idle. (RS485 driver is inactive)

    So you would normally have two termination resistors in parallel with the 375 ohm load when it's actively driving. Normally the termination resistance is 120 ohms. So 120//120//375 this is about 51 ohms. 

    I think the total power of the system at max load would be 5^2/51 (P=V^2/R) = 490mW would be the absolute worst case I think. This would be the total power of the system, the device's power dissipation would be lower than this since most of the load would be on the resistors of the system (voltage drop across the device's NFET/PFET wouldn't be 5V).

    -Bobby

  • 0 in reply to BOBBY
    Genius 3265 points

    Hi team,

    So if the max load is 256 devices. The THVD1410 PD is 490mW?

    Because I consider LDO ability  can support or not

  • 0 in reply to Jimmy WANG
    TI__Guru 59170 points
    Jimmy WANG said:
    So if the max load is 256 devices.

    It depends on the RS485 device. I Used THVD1410 as an example but you would need to check the unit load of each unique device.

    Jimmy WANG said:
    The THVD1410 PD is 490mW?

    This is a DC measurement and a worst case scenario of a max loaded bus. If we assume the device can see a short circuit and pull 250mA of current then the worst case would be 250mAx5V = 1.25W. During that situation you wouldn't want your power supply to collapse so you would probably need it to be able to handle more than 250mA since it will likely need to source other systems. 

    So max DC worst loading would be 490mW but worst case scenario of a short would probably be 1.25W. 

    In actual use, it would probably be much less than this.

    -Bobby