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sn74alvc164245-ep power sequence

Calum Mackinnon
Genius 4155 points

Hi,

In the datasheet there is a power up consideration section see below in bold -

My customer had the following questions -

We connected the VCCB to 5V and VCCA to 3.3V.  Our power sequence has the 5V coming up before the 3.3V.  Does item#2 “Power up the control side of the device (VCCA for all four of these devices) implied that there is a power sequence involve that requires VCCA up before VCCB?

Furthermore, it recommends the OE pins be tied to the VCCA, whereas we have it connected to GND - I assume this is OK?

"TI level-translation devices offer an opportunity for successful mixed-voltage signal design. A proper power-up
sequence always should be followed to avoid excessive supply current, bus contention, oscillations, or other
anomalies caused by improperly biased device pins. To guard against such power-up problems, take these
precautions:
1. Connect ground before any supply voltage is applied.
2. Power up the control side of the device (VCCA for all four of these devices).
3. Tie OE to VCCA with a pullup resistor so that it ramps with VCCA.
4. Depending on the direction of the data path, DIR can be high"

Cheers

Calum

  • 0
    TI__Guru 71526 points

    Hi Calum,

      First, apologies for the delay in answering this question, I wanted to be sure there were no alternative "well, you could do this" answers.  To be sure, the datasheet is stated correctly and very direct.  In addition, a supporting App Note, SCEA021A is even more emphatic about the power-on sequence, also explaining that damage could occur if it is not followed:  www.ti.com/lit/an/scea021a/scea021a.pdf .   Refer to pages 7 > 12, and in particular, the statement on pg 12 about " . .  strict power sequencing requirements. . ." : 

    And for their question about OE tied to GND, no, this is not acceptable.  It needs to be tied to VCCA, as you don't want the output driver to be active while the power-up is occurring.

    Yes, they will probably have to add some supervisory or sequencing circuitry, and that is the overhead required when using these translators.

    -Leonard