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The TUSB320HA IC has a pin named EN that needs to be set high to enable the chip. According to the datasheet:

  • VIH_EN, the voltage needed to set EN high, should be between 1.05V and 3.65V
  • IIH, the high-level input current, should be between -20uA and 20uA
  • REN, the internal pulldown resistance for EN, is 500k ohms

By Ohm's law, applying 3.3V directly to EN should send 6.6uA of current, comfortably within the IC's tolerances.

My understanding is that a pull-up resistor is needed in circuits where EN may be set low. The pull-up resistor would prevent 3.3V from shorting to ground.

However, in my circuit EN is routed only to 3.3V and always set high. Is it correct to say a pull-up resistor between 3.3V and EN is unnecessary in this context?

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3 Answers 3

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Datasheet has examples.

Example design description says, if external control is not needed, EN can be tied to 3.3V.

Look at the datasheet for further info.

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You do not need a pullup resistor.

However there are circumstances where even if the intention is to always have the device enabled, it's still worth using a resistor.

  • For trouble-shooting. When you're debugging a problem, it can be useful to disable a particular IC, whether you're investigating its power consumption, or software enumerating it.
  • Some automatic test programs need an IC to have all its options available if you are going to use their standard board test library. It can be cheaper to include the resistor, than have to manually alter the test program.
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My recommendation to always have pull-up for EN may be ill-advised or even misleading, but, still, understanding the danger of receiving more downvotes, I am taking a risk to ask the question in the body of my answer. It is done not because of the lack of respect for SE policy that discourages asking questions in answers, but only to keep the track of the OP question context.

What confuses me is the persistence (for a few years) of incorrect (?) description of EN/EN_N pin functions in the TI datasheet for the case of external control operation: If controlled externally, must be held low at least for 50 ms after VDD has reached its valid voltage level, for both EN and EN_N pins. Can someone confirm (or disprove) that this is in error and that for TUSB320HA (EN pin) it must read must be held HIGH at least for 50 ms?

I understand that the OP question is about the enable signal controlled internally, but the seeming "asymmetry" of device initialization signals makes arguing via example circuits for TUSB320LA (as only these are given in the datasheet) at least insufficient.

My original (most probably, incorrect) answer, edited into the conditional mood:

See the table in the datasheet, page 3, section 5 Pin Configuration and Functions. The descriptions for rows EN_N/EN read:

EN_N Enable signal; active low. Pulled up to VDD internally to disable the TUSB320L device. If controlled externally, must be held low at least for 50 ms after VDD has reached its valid voltage level.

EN Enable signal; active high. Pulled down to GND internally to disable the TUSB320H device. If controlled externally, must be held low at least for 50 ms after VDD has reached its valid voltage level.

One can infer from these descriptions that the devices, when controlled internally, carry out the same initialization operations, and in the active high device certain "internal" enable must be held low for at least 50 ms. If this is the case, and if the EN signal is not buffered, shorting the EN pin to 1.8/3.3V may prevent the device from performing the initialization operation.

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  • \$\begingroup\$ This is wrong. See datasheet examples. Also note the wording, "if" it is controlled. And it is not controlled. \$\endgroup\$
    – Justme
    Commented Oct 5, 2022 at 5:43
  • \$\begingroup\$ The datasheet shows only schematic drawings of typical applications with TUSB320LA devices and no TUSB320HA examples. It is true that EN_N in these drawings is tied to GND, not pulled. As for specifically TUSB320HA I almost sure you are right: although without supporting drawings, the subsection 8.2.1.2 Detailed Design Procedure states In this case, external control of the EN pin is not implemented and therefore the EN pin is tied to 1.8 V or 3.3 V. \$\endgroup\$
    – V.V.T
    Commented Oct 5, 2022 at 7:10
  • \$\begingroup\$ A bit confusing is the row with the EN_N description in Pin Functions: ...must be held low at least for 50 ms ... which is probably a typo (must be held high?). Have you had a hands-on experience with a design for TUSB320HA application, where pin EN is tied to 1.8/3.3V? \$\endgroup\$
    – V.V.T
    Commented Oct 5, 2022 at 15:51
  • \$\begingroup\$ Thank you for your answer. I think you are right that TI's datasheet has some errors. This TI forum thread says EN should be held high (not low) for 50ms and the purpose is to enable I2C. Another answer pointed out that part of the datasheet says, "...external control of the EN pin is not implemented and therefore the EN pin is tied to 1.8 V or 3.3 V," the key word being "tied", so it seems OK to omit a pull-up resistor. \$\endgroup\$
    – ide
    Commented Oct 5, 2022 at 17:46
  • \$\begingroup\$ It's hard to believe that TI's datasheet has some errors, even after customers had exposed possible flaws and TI engineers discussed these on the e2e forum. I am just curious if someone has a hands-on experience with this EN-tied-to-I2C-supply-voltage configuration for TUSB320HA and what is their opinion about the issue. It would be great if you drop a message to this post after you implement and test your design. \$\endgroup\$
    – V.V.T
    Commented Oct 6, 2022 at 1:05

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