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THVD1454: Total Current Draw when termination and driver are enabled

Matt Lang
Intellectual 560 points
Part Number: THVD1454
Other Parts Discussed in Thread: THVD1500

Tool/software:

I have a question on the THVD1454DRCR part. I am designing an RS485 bus that is powered via the ethernet cable with a 24V bus. I need to keep the current draw of the RS485 nodes as small as possible because of this. 

I want to be able to en/dis the termination resistor in firmware. I was looking into use a PhotoMOS SSR to do this. Like the AQY282SZ or TLP175A. But I do see TI makes RS485 transceivers with integrated termination switches. I would prefer not to have to power the transceiver via the 24V bus. But if the THVD1454DRCR + termination switch draw little current, I might be able to use it.

Specs:

  1. 250kbps max. Probably 115,200
  2. We are designing to have a total round trip cable resistance of around 120Ohm (60Ohm for ground wire) 1000s of feet of cable. So we need to keep the total current draw down so that we do not exceed the +12V,-7V common mode specs of RS485 when we see ground potential differences.
  3. Half Duplex
  4. Non-Isolated

Questions:

  1. Is using a SSR like the ones mentioned common and can be be made robust?
  2. How much current does the termination resistor switch in the THVD1454 consume when it's enabled? The datasheet says that the typical supply current when driver is enabled and termination enabled is around 39mA (48mA max). Which is quite large compared compared to the recommended forward LED current of the AQY282SZ  of 5mA. At 5mA, the AQY282SZ has a very stable 1Ohm Rdson for the full temperature range.
  3. I plan on using the THVD1500DR combined with the AQY282SZ. How much capacitance can we add to the RS485 bus with the combination of AQY282SZ and THVD1500DR? How much capacitance can the THVD1500DR  handle with it's 24mA driver before we start to see issues. I am planning to have a max of 50 nodes on the bus with a around 2000ft of cable max.

Any tips, tricks, guidance and concerns would be appreciated. 

  • +1
    Guru 317320 points

    2. The power consumption of the switch itself is negligible. The specified supply current includes the current through both termination resistors.

    3. The THVD1500 typically drives 33 mA on a normal bus, and can drive up to 95 mA into a short. Long cables can have quite a high capacitance, so the additional capacitance of the SSR should not matter.

    How often do you actually transmit over the bus? If it is idle most of the time, you can reduce power consumption by using AC termination (see RS-485 Basics: When Termination Is Necessary, and How to Do It Properly). At very low data rates, you might be able to work without termination.