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SN74ABT244A: 3.3V Operation and Replace to SN74LVC244A

Kenichiro
Mastermind 9165 points
Part Number: SN74ABT244A
Other Parts Discussed in Thread: SN74LVC244A

[ SN74ABT244A ] 3.3V Operation and Replace to SN74LVC244A

Hi,

I know this is not "smart" discussion, however can you help me to support our customer?

My customer mistakenly has designed the power supply circuit for SN74ABT244A with 3.3V, while datasheet specify typical 5V supply.
They realized this mistake after start shipping, unfortunately, some products have been shipped to their customer.
According to customer, ABT244A works as expected even with 3.3V power supply, means as specified with function table, however they are going to replace this board with fixed board.

Can you comment whether ABT244A can work with 3.3V supply?
(Of course, no guarantee required)


And as a replacement of ABT244A, customer has found SN74LVC244A.
This device can be powered with 3.3V, but their schematics has pull-up resistor to 5V.

I don't think this can be acceptable, because Vo (high/low state) of absolute max is specified as Vcc + 0.5V.



Do you have any other ideas of repalcement?

Thank you for your help in advance.
Regards,
Ken

  • 0
    Guru 279850 points

    Running a device outside the recommended operating conditions is not something that TI would actually test. Your own (your customer's) observations are a better indication than anything that TI could say.

    All logic devices have the restriction that an active output must not be forced above VCC. However, the pull-up resistor does not necessarily do this.

    When you connect an output in the low state to a pull-up resistor, a current will flow through that resistor into GND, and the voltage level at the pin actually is (near) zero.

    When you connect an output in the high state to a pull-up resistor, a current will flow through that resistor into VCC, and the voltage level at the pin actually is (near) VCC. However, for VCC to keep stable, this requires that the power supply is able to sink all that current; if it's not, then the VCC voltage will rise. To ensure that this does not happen, add a bodge resistor at the device between VCC and GND that is able to steer all that current to ground (Rshunt ≤ 3.3 V / (5 V − 3.3 V) × Rpullup). (This will increase the power consumption when the output is not in the high state.)

  • 0 in reply to Clemens Ladisch
    Mastermind 9165 points

    Hi Clemens,

    Thank a lot for sharing your thought with me!
    It makes my understanding more clear.

    I will consult with my customer!
    Regards,
    Ken