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MAX3243: VCC power check

Shu-Cheng LIN
Guru 11785 points
Part Number: MAX3243
Other Parts Discussed in Thread: MAX3232

Hi Sirs,

Sorry to bother you.

1.As schematic, have any risk if the D1 connection +3.3V? The pressure drop of D1 is 0.35V~0.4V

2.What is the VCC current of the MAX3243? 0.3mA?

Schematic:

  • 0
    TI__Mastermind 25705 points
    Shu-Cheng,

    If D1 is supplied with 3.3V, you would have the risk of IC not working normally. 3.3-0.4=2.9V, which is below the operation region of MAX3243. Another point is, if you change the supply to 3.3V, the capacitor values need to be changed to follow datasheet guideline as well. MAX3243 typical Icc is 0.3mA with no load. If any channels (Dout) turns on, the current could be much higher.

    Regards,
    Hao
  • 0 in reply to Hao L
    Guru 11785 points

    Hi Sirs,

    Could you help provide MAX3243 reference schematic for us check?

    We have a question for MAX3243 +VCC power pin.

    Does TI have suggest +VCC install diode or other protection component?

    Thanks!

  • 0 in reply to Shu-Cheng LIN
    Guru 11785 points
    Hi Sirs,
    Sorry for pushed, any update on this case?
    Thanks!!
  • 0 in reply to Shu-Cheng LIN
    TI__Mastermind 25705 points
    Shu-Cheng,

    I'm sorry for the late reply. It was a US holiday yesterday.

    You can refer to the application session on the datasheet for the reference schematic. In general it's not recommended to add diodes or other protection on VCC. The side effect of such components, like voltage loss or current limit, could create issues in applications.

    Regards,
    Hao
  • 0 in reply to Hao L
    Guru 11785 points

    Hi Sirs,

    Thanks for your reply.

    Double check again. UART signal is +3P3V power rail from EC. MAX3243 VCC connect to +5V power.

    Does function can work normally? Is this design no problem?

    Please kindly to check. Thanks!

  • 0 in reply to Shu-Cheng LIN
    TI__Mastermind 25705 points

    Shu-Cheng,

    Is there any special reason why you want to use different supplies? MAX3243 works with 3.3V supply. You just need to change all C1-C4 to 0.1uF cap. Please choose voltage rating of the cap higher than 10V.

    Regards,

    Hao

  • 0 in reply to Hao L
    Guru 11785 points

    Hi Sirs,

    Thanks for your reply

    Due to this serial optional card is old project using. The optional card can’t adjust any RLC value. It’s already production on old project.
    The old project SIO and UART are 5V power. MAX3243 VCC also connect to 5V.
    Customer expects use this old project optional card on New project. But this new project changes to EC and UART are 3.3V power.
    So we need double check MAX3243 VCC connect to 5V and UART signal is 3.3V power.

    Could you help check our design could be work? Have any risk? Thanks!

  • 0 in reply to Shu-Cheng LIN
    TI__Mastermind 25705 points

    Shu-Cheng,

    Since the input is TTL, DIN pins should be OK. However you need to check if MCU can take 5V signal from ROUT pins. If MCU cannot, you may need to use resistor divider to protect its pins. You can refer to this app note for some detail.

    0044.3V MCU to 5V XCVR Interface.pdf

    Please let me know if you have more questions.

    Regards,

    Hao

  • 0 in reply to Hao L
    Guru 11785 points

    Hi Sirs,

    Thanks for your reply.

    Based on previously feedback, Looks VCC pin is not recommended to add diodes or other protection. Just like diodes D1 as following screenshot.

    This diode purpose is protect leakage power from outside device.

    Does MAX3242 VCC pin can prevent power leakage?

    Please help to check. Thanks!

  • 0 in reply to Shu-Cheng LIN
    TI__Mastermind 25705 points

    Shu-Cheng,

    Can you explain the case when the power leakage happens? Do you mean when the external power supply is below ground? If it's the case, MAX3243 cannot prevent the current flowing out of the device.

    Regards,

    Hao

  • 0 in reply to Hao L
    Guru 11785 points

    Hi Sirs,

    Thanks for your reply

    If system power off. RS-232 has connected to serial card. I’m worry about the outside RS-232 will via UART signal leakage to MAX3243 and then MAX3243 VCC leakage to MB side. So VCC pin install the diode to prevent power leakage.

    By this case. MAX3243 VCC cannot prevent the current flowing out of the device. It may must add diode on VCC pin. Is that right?

  • 0 in reply to Shu-Cheng LIN
    TI__Mastermind 25705 points

    Shu-Cheng,

    It's a good thinking. But if the power is off, MAX3243 I/O pins should be in high impedance mode. I'm not sure the MB side could be powered up by this leakage path. Even there exists some kind of leakage. The path should be high resistive and the current would be very limited. Please let me know if you have more questions.

    Regards,

    Hao

  • 0 in reply to Hao L
    Guru 11785 points

    Hi Sirs,

    Thanks for your reply

    Do you have MAX3243 power consumption for reference?

    Thanks!

     

  • 0 in reply to Shu-Cheng LIN
    TI__Mastermind 25705 points

    Shu-Cheng,

    You can refer to this blog for the detail and some data (example 2).

    e2e.ti.com/.../active-rs232-power-consumption-why-isnt-it-in-the-data-sheet

    I checked that MAX3243 and MAX3232 have similar driving behavior. With the on channel number, data rate and load capacitance, you can get power dissipation estimate by scaling.

    Regards,

    Hao