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Inquiry regarding power supply configuration on VCC2 pin of ISO35T

Atsushi Okui
Genius 4015 points
Other Parts Discussed in Thread: ISO3086T, ISO35T

Hello, all

Now we have one inquiry from our customer who is considering to use ISO3086T or ISO35T.

They have fixed the VCC1 input voltage to 3.3V, however has not fixed the VCC2 input voltage on either of these devices.

Also, they would like to use DA2304 (turns ratio 1:2.2) than DA2303-AL(1:1.5) which seems to be flexible usage on both VCC2=3.3V or VCC2=5V.

In case VCC1=3.3V and VCC2=3.3V for ISO35T, please let us confirm whether the configuration below could be suggested.

VCC1=3.3V => DA2304 => 6V => TPS76333 => 3.3V => VCC2

We thank you in advance for your information.

Best regards,

  • 0
    TI__Mastermind 21769 points

    Okui-san,

    I apologize for the delay in getting back to you.

    The recommended 1:1.5 turns ratio, is used to boost the voltage up enough so the LDO has ample head room for drop out. The TPS763xx family of LDOs are all able to take up to 10V in on the input, so having around 6 volts will not be an issue.

    A possible issue that could arise is with thermal dissipation, although this is only for specific cases. Depending on the package, and the ambient temperature, you could run into an issue. For example with the DBV package, with poor thermal relief in the board layout (low K), the device can only dissipate 154mW of power. At a dropout of 2.7 volts, it can source a maximum of 57mA of current continuously. Under normal operation (no short bus faults), the device will draw much less than 57mA of current, so this should not be an issue.

    From the isolated RS485 devices perspective, as long as the LDO is dropping the voltage down to the recommended range, there should be no issues.

    Thanks,

    John

  • 0 in reply to John Griffith
    Genius 4015 points

    Griffith-san,

    Thank you for your reply.

    I found that TPS763xx family LDO could not drive ceramic capacitor.

    Instead of this device, TPS709xx (for example TPS70933) could drive this.

    Please let us clarify whether we could use TPS709xx family device could be used on this case.

    We thank you once again for your information.

    Best regards,

  • 0 in reply to Atsushi Okui
    TI__Mastermind 21769 points

    Okui-san,

    Again, depending on the operating temperature and package variant you are using there could be issues. If you have specific questions about the TPS70933, I would recommend posting a question in the Linear Regulators forum.

    http://e2e.ti.com/support/power_management/linear_regulators/default.aspx

    The worst case short circuit current is limited to 60mA for the driver. The rest of the transceiver will draw a max od 16mA, therefore the total is 76mA for a fault. During normal operation, this should be much less.

    Thanks,

    John

  • 0 in reply to John Griffith
    Genius 4015 points

    Griffith-san,

    Thank you for your continuous support towards the inquiries from our customer.

    With regard to short circuit current on ISO3086T, I have additional inquiry from them.

    Please refer to the item below, and feedback us with your comment.

    You mentioned above as "The worst case short circuit current is limited to 60mA for the driver", meanwhile, on page 6 of the application report below says that "Driver output short-circuit limit 250 mA for VSHORT –7 V to 12 V".

    http://www.ti.com/lit/an/slla036d/slla036d.pdf

    Please let us clarify whether 250 mA short-circuit limit should be taken in consideration for ISO3086T or not.

    This term will be important to select transformer and LDO for VCC2 supply.

    We thank you once again for your information.

    Best regards,

  • 0 in reply to Atsushi Okui
    TI__Mastermind 21769 points

    Okui-san,

    I apologize, what I meant to say was during normal operation (without a fault), the driver should consume a maximum of 60mA of current which is on page 2 of the datasheet. If you take into account the receiver and quiescent current which is roughly 15mA more, then during normal operation the device should draw at most 75mA of current.

    The short-circuit current of the device is 200mA which is found on page 3 of the datasheet on the driver electrical characteristics table. I apologize for any confusion.

    By the way the 250mA number in the applications note that you attached comes from the RS485 standard. In the standard it states that “the peak current magnitude shall not exceed 250mA.” This is not a requirement stating the driver needs to be that strong, it is actually a limitation put into place to limit the current that the low side of the driver will sink if multiple nodes are sourcing, while one node is sinking current.

    Please let me know if you have any other questions, again sorry for the confusion.  

    Thanks,

    John