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ISOW1412: Treatment for EN/FLT pin

Part Number: ISOW1412

Hello expert,

I have question about treatment for EN/FLT pin.
Accoridng to datasheet of "pin functions", TI explain as shown below.

* If EN is floating, DC-DC converter is enabled (internal pull-up resistor)

On the other hand, according to "Figure 11-3. Application circuit for ISOW14x2", this pin is pulled up 3.3V via 4.7k.

According to pin functions, it seems user can leave this pin as floating, however do user have better to pull up for this pin ?

BR,

  • Hi Machida-san,

    Thanks for reaching out.

    * If EN is floating, DC-DC converter is enabled (internal pull-up resistor)

    Your understanding is correct, the device has internal pull-up which is going to enabled the device when EN is left floating.

    According to pin functions, it seems user can leave this pin as floating, however do user have better to pull up for this pin ?

    The internal pull-up resistors are generally high in resistance values to keep power consumption to minimal. Hence, it is generally advised to pull-up the pins externally as well. Hence, in this case as well, we recommend that you pull-up the EN pin externally through a pull-up resistor. Typical value that we recommend is 4.7kΩ but it can be anywhere between 1kΩ to 10kΩ. Thanks.


    Regards,
    Koteshwar Rao

  • Hello Rao-san,

    Thank you for your reply.
    I have following two questions.

    Q1. Can you show approximate internal pull up resistor for EN/FLT pin ?
    Q2. Why do you recommned external pull up ? (Is it from viewpoint of external noise ?)  

    BR,

  • Hi Machida-san,

    Please find my inputs below.

    1. Please refer to the section 10.8 titled Device I/O Schematics in ISOW1412 datasheet. This section lists schematics for all input and output pins including the EN/FLT pin. I am copying it below for your reference. As shown in the schematic, the internal pull-up is 550kΩ.
    2. Like I mentioned earlier, the internal pull-up resistor is high in value. A high-value pull-up is a weak pull-up and hence, it can be easier for external noise to affect the pin. Thus it is recommended to use a stronger external pull-up to drive the pin stronger. a 4.7kΩ resistor strong enough to hold the pin HIGH sufficiently against noise.

    Let me know if you have any further questions, thanks.


    Regards,
    Koteshwar Rao