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Pull-Up Resistor at Output of ISO7240C

James Liang
Prodigy 70 points
Other Parts Discussed in Thread: TPIC6C596, ISO7240C

Currently we have the Pull-Up Resistor at Pin-8 (/G) of TPIC6c596, and Pin-8 (/G) is also connected to CPU.

Thus during power-up, Pin-8 (/G) is pulled to Vcc to disable all 8 Outputs.

After CPU initialization, this Pin-8 (/G) is set to GND by CPU control to enable 8 Output signals.

 

Now we like to add ISO7240C between TPIC6c596 and CPU for isolation.

(However, this will create more timing issue between two isolated power supplies during power up.)

Will this Pull-Up Resistor still work with ISO7240C?

Do we really need this Pull-Up Resistor or not ?

 

Best Regards,

  • 0
    TI__Genius 14015 points

    Hi James,

    I'm not very familiar with the TPIC6C596 or your requirements, but I believe you can just tie pin 8 of the TPIC6C596 to GND so it will always be enabled, and have the CPU control the ISO7240C by toggling the ENABLE pin. You will need a pull-down resistor on the ENABLE pin so that during power up, the device doesn't become enabled. 

    Does this sound like it would work for you?

    Thanks,
    Jason Blackman 

  • 0 in reply to Jason Blackman
    Prodigy 70 points

    Hi, Jason:

     

    Thanks for your prompt reply.

     

    We use several TPIC6C596s to control relays and LEDs.

    The reason why we didn't tie Pin 8 (/G) directly to GND is that relays will behave randomly during power-up until CPU is ready.

    So we need the Pull-Up Resistor to force all  outputs of TPIC6C596 being "OFF" during power-up period.

    (Indeed TPIC6C596 does not have "ENABLE" pin.)

     

    Based on datasheet of ISO7240, it seems that the outputs will be "HIGH" during power-up of VCC2.

    If true, then we could use one output of ISO7240 to control Pin 8 (/G) without the need of the Pull-Up resistor.

    If we still keep the need of the Pull-Up resistor, is there any side effect on ISO7240?

     

    Best Regards,

     

    James Liang

     

  • 0 in reply to James Liang
    TI__Genius 14015 points

    Hi James,

    Thank you for the added information. I don't think the outputs of the ISO7240 will be guaranteed to be "HIGH" during power-up of Vcc2, so I recommend using the pull-up resistor on the /G pin. This shouldn't pose a problem for the ISO7240 as long as the resistor value is large enough that the +/-4mA output driver can pull the pin all the way to a "LOW." 

    Thanks,
    Jason Blackman 

  • 0 in reply to Jason Blackman
    Prodigy 70 points

    Hi, Jason:

    Thanks a lot.

     

    James Liang