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Original question:

SN74LVC2T45: SN74LVC2T45 power up sequencing

SN74LVC2T45: Problem if "Power-up Consideration" isn't followed

Taito Takemura
Expert 3200 points
Part Number: SN74LVC2T45
Other Parts Discussed in Thread: SN74LXC2T45

Hi experts,

Could you please answer the question below?

[Backgorund]

Customers want to:
   Level translation from 1.8V output to 3.3V and level translation from 3.3V output to 1.8V
Requirement:
   3.3V rises several tens of ms after 1.8V rises.

[Questions]

(1)The data sheet has the following description for the power-on sequence, but what kind of problems can be expected if this is not followed? Could you please tell me it? They are considering using SN74LVC2T45, but they may not be able to satisfy the blue and yellow parts at the same time, so please let us know.

(2)　Could you tell me if there is a readily available others that can convert 1.8V to 3.3V & 3.3V to 1.8V?

Thank you for your help.

Best Regards,

Taito Takemura

  • 0
    Guru 316780 points

    2. Use the SN74LXC2T45 instead.

  • 0 in reply to Clemens Ladisch
    TI__Expert 3200 points

    Hi Clemens,

    Thank you for your answer.

    Could you please give me any comments about (1)?

    Best Regards,

    Taito Takemura

  • 0 in reply to Taito Takemura
    TI__Guru 50916 points

    Hi Taito-San,

    Typically, the supply that controls the device is recommended to be ramped up first so that the internal circuitry is all active and there's no internal floating nodes while the other supply ramps. However, ramping both together works just fine as well.

    Hence, if VCCA is the control (with A-port as an input) and you ramp VCCB (B-port as an output) first; you can have invalid logic levels / glitches since it will be floating / unknown. But if 3.3V is the control, there should be no concerns.

    For the LXC recommendation, please see [FAQ] What should be done with unused I/O pins of the level translator devices? as they are no concerns for floating pins (compared to the LVC device), thanks.

    Best Regards,

    Michael.

  • 0 in reply to Michael Ikwuyum
    TI__Expert 3200 points

    Hi Michael,

    Thank you for your answer.

    Could you please answer my additional questions?

    (1) "Control"

    When A port is an input port and B port is an output port, VCCA is the control. When B port is an input port and A port is an output port, VCCB is the control.

    Is this correct? I do not understand the meaning of "control" well. Could you please provide a supplementary explanation?

    (2) If A is control.  You ramp VCCB (B-port as an output) first; you can have invalid logic levels / glitches => After you ramp up VCCA, you don't have them. Is this correct?

    (3) "But if 3.3V is the control, there should be no concerns." Could you please give me the reason?

    (4) Should I reverse the startup order of A and B when B is control? (If B is control, you should ramp up Vccb first, correct?)

    (5)"Datasheet"

    Datasheet seems to say that you should ramp up Vcca first absolutely  (Parts: A proper power-up ... ... 3.Vccb can be ramped up with or after Vcca ).

    However I think that you should ramp up the voltage of input side from your explanation. Which is correct?

    Thank you fro your help.

    Best Regards,

    Taito Takemura

  • 0 in reply to Taito Takemura
    Guru 316780 points

    1. The control pin (DIR) is powered by VCCA.

    2. When VCCB is powered up first, the B side does not know what the direction should be.

  • 0 in reply to Taito Takemura
    TI__Guru 50916 points

    Hi Taito-San,

    In addition to Clemens' feedback, please see the below, thanks.

    Taito Takemura said:

    When A port is an input port and B port is an output port, VCCA is the control. When B port is an input port and A port is an output port, VCCB is the control.

    Is this correct? I do not understand the meaning of "control" well. Could you please provide a supplementary explanation?

    Yes A side is the control when A to B translation and B side is the control for B to A translation.

    Taito Takemura said:
    (2) If A is control.  You ramp VCCB (B-port as an output) first; you can have invalid logic levels / glitches => After you ramp up VCCA, you don't have them. Is this correct?

    Yes, once fully ramped up there should be no concerns.

    Taito Takemura said:

    (3) "But if 3.3V is the control, there should be no concerns." Could you please give me the reason?

    (4) Should I reverse the startup order of A and B when B is control? (If B is control, you should ramp up Vccb first, correct?)

    If B to A translation and B is ramped up, A can be ramped up just fine. However, ensure that no pins are left floating. If so, LXC would be the recommendation.

    Taito Takemura said:
    However I think that you should ramp up the voltage of input side from your explanation. Which is correct?

    Yes, the datasheet powerup sequence is a recommendation depending on the control circuitry.

    Best Regards,

    Michael.