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SN74LVC8T245: Recommend a suitable level shift for customer

Sulyn ZHANG
Expert 8700 points
Part Number: SN74LVC8T245
Other Parts Discussed in Thread: SN74CBTK6800

Hi Team,

My customer is looking for level shift, can you help recommend one, thanks!

1. VCCA=1.8V, VCCB=5V;

2. >=8 bits;

3. Support Direction control;

4. Support hot insertion or live insertion function;

Best regards,

Sulyn 

  • 0
    TI__Guru* 96591 points
    Hi Sulyn,
    The SN74LVC8T245 is probably the best device for their application.
  • 0 in reply to Emrys Maier
    TI__Expert 8700 points

    Hi Maier,

    Thanks for the support! So does SN74LVC8T245 support hot or live insertion? I just want to confirm because I can't find more detailed description in datasheet, thanks a lot for your support!

    Best regards,

    Sulyn

  • 0 in reply to Sulyn ZHANG
    TI__Guru* 96591 points
    Hot/live insertion is a very vague request - unless you are referring to the TI app report on live insertion. Can you describe exactly what they plan to do? Will the device be disabled during connection? Will the internal bus be communicating? Will the voltage be held at a valid level during the off-time? What is the tolerance of the bus to changes?
  • 0 in reply to Emrys Maier
    TI__Expert 8700 points

    Hi Maier,

    From customer request, I draw a simplified diagram for better understand, as below:

    Pls note that before plugged in, the Plugged in board don’t have power supply until it was plugged into CPU board. And for the plugged in board, the GND and

    power supply will be connected to CPU Board seperately, GND first and Power Supply after(Like USB plug in sequence). Customer request:

    1. During plug in, firstly level shift IC won't be damaged;

    2. During plug in, The plugged in board Level shift IC won't affect the communication on the bus, thus keep high impedance during Plugged in(Customer can control the level shift IC on the plugged board or this level shift IC can handle this situation itself by hardware configuration, pull down, pull up .etc);

    Either 8 channels or 16 channels are ok for customer to use. So do we have recommended level shift IC to satisfy here?

    Best regards,

    Sulyn

  • 0 in reply to Sulyn ZHANG
    TI__Guru* 96591 points
    Thank you Sulyn,
    That is very clear and helps me greatly!

    The SN74LVC8T245 should work well for their application. I can't tell from the above drawing if the bus connection is shared or not -- if it is, they might consider adding a precharge enabled devices as described below.

    The SN74LVC8T245 has Ioff protection, so when there is no power applied, the device's I/O pins are in the high impedance state. To ensure that the I/O pins remain in a high impedance state during the hot-plug operation, the OE\ pin must be connected to Vcc through a pull-up resistor. Applying any voltage within the device's operating range to the I/O pins while in the high-impedance state will not cause damage to it regardless of the Vcc state.

    If the bus is shared and there is active communication on the bus while a new device is attached, there is a chance for data corruption because the SN74LVC8T245 does not have the "output precharge" feature -- ie the outputs will likely have zero charge when plugged in, and could potentially pull the line low if it is being driven high by another device on the line. The only devices I know of with the output precharge feature are bus switches specifically intended for hot-swapping activities, such as the SN74CBTK6800. There is an application report that describes this feature in detail here: www.ti.com/.../scda009.pdf -- I can't say from the above drawing if they need this for certain or not, but they should at least know about it.

    I would highly recommend adding ESD protection to each line separately to prevent damage to the level shifting IC -- this device is not intended as primary ESD protection for a system, and any external plug has a high likelihood of receiving an ESD event.