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SN75468 connected to STM32F427ZGT6 uC

Brian Cavanagh
Intellectual 285 points
Other Parts Discussed in Thread: SN75468, ULN2003A

Hi,

As you can see in my attachment, the SN75468 COM pin is tied to VCC (12VDC) and E is tied to GND1.

The outputs of the SN75468 are connected to a relay, also seen in the attachment.

The inputs on the SN75468 receive a signal from an STM32F427ZGT6 uC.  The uC is powered from 3V3 and GND1.

I am using an output pin on the STM32F to drive the input on the SN75468.  I'm not clear if this will work or not since I never used this circuit in the past.

I did go through the SN754X datasheet and read the Electrical Characteristics (also attached) and Vi(on) has a max input of 3 volts (@ IC = 300mA).  This is where I get confused is about the current, Ic.  

My questions are:

1.  Is there any unforeseen issues, that is not apparent, that I should not connect my uC output to the SN75468?

2.  The uC's I/O's are CMOS and TTL compliant.  Likewise, the SN75468 is CMOS and TTL compliant.  Will the SN75X require a lot of current from the STM32F?

3.  Will the STM32F even work if connected to the SN75468?

Thanks,SN75468.docx

Brian

  • 0
    TI__Mastermind 24975 points
    Brian,

    If you are not using a Vcc > 50V, we have another device that is more fitting and less expensive that you would be able to use in this application. I would recommend looking at the ULN2003A device for this application. Ultimately these two devices are functionally equivalent but the ULN2003A has a lower voltage tolerance (50V) but is good enough for your application where Vcc <50V. The following responses apply to your questions both for the SN75468 and the ULN2003A

    1. There should be no problem with driving the inputs directly from the uC, as long as the microcontroller can source 1-2 mA in order to bias the internal structure and turn on the darlington pair.

    2. For Input voltage of 5V, the maximum current into the device is specified at 0.5mA, and it is less for a lower voltage. Look at the input current specification for this.

    3. I don't foresee any other issues with this application. This is a very typical relay driving application. A microcontroller can drive the inputs of the device in order to allow a much higher current to flow through the relay to ground.

    I do have an additional comment:
    You have flyback diodes connected to the relay. While this is fine, one advantage of the SN75468 or the ULN2003A is that it has these diodes integrated and connected back to the COM pin. As long as you connect the COM pin to the same voltage as the relay, then this integrated flyback
    diode should take care of things without needing the BAT54 diode.

    I hope this helps, please let me know if you have additional questions and I would be happy to help!

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Intellectual 285 points

    Hello Michael, thank you for you thorough answer, this helped me tremendously.

    Yes, to your question, I will have less than 50 on VCC, only 12VDC. Thank you again.

    Brian

  • 0 in reply to Brian Cavanagh
    TI__Mastermind 24975 points
    Brian,

    Absolutely, very happy to help!

    Best,
    Michael