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SN74AVC4T245: High Shutdown Current

Part Number: SN74AVC4T245
Other Parts Discussed in Thread: SN74AXC4T245, SN74AXC8T245


I'm trying to use two SN74AVC4T245 chips as simple level translators between a MCU (1V8, always on) and a peripheral (3V3 when on but usually powered down). The arangement is shown in attached picture. 4 I/O signals going out from the MCU and 3 coming back in the opposite direction.

I'm finding that when the 3V3 supply on port B is powered down, the lower of the two devices shown is causing a high sleep current to be drawn, presumably from the 1V8 supply (VSUP_MCU). In this state, pins 1 and 16 of both devices are set high by an MCU I/O line, to disable the outputs.

On the upper device (which seems to have no problems) the 4 inputs on the left are driven low by the MCU. The 3 signals on the lower device go to MCU inputs which are disabled, so effectively all signals on the A port see Hi-Z. The B port is powered down and close to 0V, with the B inputs also close to 0V.

Having found this issue I've looked again at the datasheet and note this on the front page:

"The input circuitry on both A and B ports is always active and must have a logic HIGH or LOW to prevent excess ICC and ICCZ."

I guess this means I need some kind of pull down to prevent them floating, so questions are:

1 - should I have pull-downs on the B inputs, even though this side is powered down? If not essential I'd rather not as this board is extremely tight.

2 - for the unused signal on 1A2/1B2, is it safe to connect 1A2 direct to ground given that this is an output, or is it necessary to add something like 10k to ground on both sides? Again, I'd rather not as I'm very short of space.

3 - when B port is powered down and output enable is high to disable outputs, is it possible to minimise current on the other port if the signals on the A port are properly pulled down?



  • You can get cross currents on floating pins only if they are powered. So you need to add pull-ups or pull-downs only on floating pins on the powered side. As far as I can see, this would be the A pins of U702. (I guess the A pins of U701 are still driven.)

    Unused pins that are not outputs must be grounded. When /OE is high, neither A nor B pins are outputs. A resistor is not needed as long as the A and B pins always have the same value.

  • Hey Gordon,

    To add to Clemens response, I would recommend taking a look at the SN74AXC8T245 device for this application. It will be able to accommodate all of the signals and their directionality in one IC. You will have to sacrifice one of the OE pins (I saw two different control signals for each). If that isn't something you can do then I would recommend using the SN74AXC4T245 which is a drop in for the current device being used. From this FAQ , you can see that you are able to leave the I/O floating for this device. 

    These suggestions may help with your board space concerns.

  • Hi Clemens and Dylan,

    thanks both of you for the replies. It looks like the simplest option is to use the existing devices, but on the 3 channels going to MCU inputs, configure the MCU to have pull-downs on its pins when in shutdown mode. For the unused channel I'll update so that the pins on the unused channel (1A1 and 1B2) are both directly connected to ground.

    I like the suggestion of using the AXC8T245 but for now I'll probably not change it. This looks to be the only 8-bit translator that can be split into two groups (which would be ideal) but it does look to have higher current consumption than the AVC4T45 that I'm using just now so it's probably safer to stay with the existing device but to make sure the A port pins are all pulled to GND.

    Thanks for your help - appreiated.