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SN74ACT244: The output of High Z input

Victoria Qin
Expert 3450 points

Part Number: SN74ACT244
Other Parts Discussed in Thread: SN74LXC8T245, SN74LXCH8T245

Dear experts,

My customer XIZI is using SN74ACT244 in AC inverter as buffer. The signal chain is: DSP ->first stage SN74ACT244 -> second stage SN74CT244 ->ACPL W341 ->IGBT. Customer is using 2 SN74ACT244 cascade, as below figure.

Customer is doing functional safety certification test so they are considering some abnormal situation as below:

Let's look at 2Y1 ->2A1, 2Y0->2A0. The output of first stage IC is high-Z, the pull-up pull-down resistors R57 and R63 are broken which means the second stage IC has 'floating' input. 

What is the output status of the second stage IC? How is the driving ability of this status, I mean if the second stage IC's output could drive ACPL W341?

Regards,

Victoria

  • 0
    Guru 325670 points

    When an input is floating, it might be low, or high, or oscillate; see [FAQ] How does a slow or floating input affect a CMOS device?

    Devices with bus-hold inputs would keep their last value, but should not be used with pull-up/-down resistors; see section 8.3.10 in the SN74LXC8T245 datasheet.

  • 0 in reply to Clemens Ladisch
    TI__Expert 3450 points

    Hi Clements,

    1. In SN74LXC8T245 datasheet, is this table same as SN74ACT244? When the input of SN74ACT244 is Hi-Z, the output keep last value?

    2. Do you suggest to delete the pull-up/down resistors? I didn't see 8.3.10 in SN74LXC8T245 datasheet.

    Regards,

    Victoria

  • 0 in reply to Victoria Qin
    Guru 325670 points

    1. CMOS inputs always have a high impedance. The question is whether they have a valid voltage or are floating. A bus-hold input never floats. The '245 is a transceiver, so the table shows whether the port is an input or an output.

    2. Sorry, I meant the SN74LXCH8T245. Only the "H" device has bus-hold inputs. Using a bus-hold device prevents you from using pull-up/-down resistors; whether your circuit works with this is your own decision.

  • 0 in reply to Clemens Ladisch
    TI__Expert 3450 points

    Hi Clemens,

    Thanks for your reply.

    The socket customer using is SN74ACT244, no "H" in the PN, maybe it doesn't have a bus-hold input. 

    Clemens Ladisch said:
    When an input is floating, it might be low, or high, or oscillate;

    It's clear because the output depends on the input voltage pick up level.

    Could you explain customer's application more directly: The output of first stage IC is high-Z. Does the voltage level of previous stage high-Z have influence on next stage's output?

    Regards,

    Victoria

  • +1 in reply to Victoria Qin
    Guru 325670 points

    When the output of the previous stage is disabled, and when the pull-up/-down resistors are broken, then the input voltages at the next stage are floating, and the output state is undefined (high or low or oscillating).