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ISO1176T: Is the supply current for VCC2 can be generated by one transformer only when two ISO1176T are used?

Part Number: ISO1176T

 Hello guys,

 One of my customers is considering using ISO1176T for their new products. They'd like to use two ISO1176T per one product. But they need to reduce the product cost. So they asked me whether the supply current for VCC2 of both ISO1176T can be generated by one transformer connected to either ISO1176T. (Then another ISO1176T D1/D2 terminals are opened.) Do you have any answer for the question? 

 Could you please tell me your answer or comment?

 Your reply would be much appreciated.

 Best regards,

 Kazuya Nakai.

  • Hello Kazuya-san,

    Thank you for using our E2E forum.

    I understand the overall problem that you/customer are trying to solve here. Technically this can be done, but we need to confirm some details.  I will also give you a backup solution in case you need it :)

    Solution 1:

    • Two ISO1176T devices; only one device's D1, D2  pins (transformer driver pins) are used and the other is open. Having only one set  of D1, D2 pins connected is good because it avoids transformer errors.
    • Vcc2 = 5V ( = isolated power supply generated from transformer + optional LDO)  is shared across two devices
    • Now this will work  only if Vcc1 is = 5V . So your config will be 5V in, 5V output. You can use a 1: 1.5 turns ratio transformer (you need more than 1:1 ratio to compensate for diode drop and LDO drop in the secondary side). This will ensure enough power is available in the secondary side.
    • EVM guide for this is here . the DA2303AL transformer will give you the 1:1.5 ratio.  You can pick similar transformer designs from your preferred vendor.
    • Now, RS485 needs about 70mA for each chip (driver). Add the Icc2 quiescent current for each chip and that is around 18mA. Let us approximate this , add margin and call it 100mA per chip.  So you'll need approx 200mA in total for normal operation for two devices on the secondary side.  Due to transformer ratio, this will be = 200 * 1.5 = 300mA in primary side and one ISO1176T should be able to provide this with a 5V supply (Vcc1 = 5V)
    • this is good for normal operation.
    • In RS485, if a short circuit happens, then the current consumption can go higher 135mA (DC) and 250mA (transient). In this case, the power supply will current limit before the transceiver. But this is okay in most causes, because it is reacting to a fault on the bus (short circuit).

    Solution 2 (backup):

    • You can use one SN6505 device (standalone transformer driver). It can provide 1A at 5V on the primary side, along with other features.
    • You can then use this with just  one transformer + rectifier + (optional) LDO to power two ISO1176 (non-T) devices.
    • This can be used in 5V to 5V or even 3.3V (micro side) to 5V (RS485 side) configuration. Extra power to spare and can handle more devices also!
    • Pretty reasonable in size and cost. 

    You/your customer can pick whichever works best.

    I hope this helps!

    Best regards,

    Abhi

       

  • In reply to Abhi Aarey:

    Hello Abhi,

    Thank you very much for your detail answer.
    I could understand well because of the answer.

    Thank you again and best regards,
    Kazuya Nakai.