SN74LVCH16240A: Bus Hold With Optocoupler

engiz

Part Number: SN74LVCH16240A

Tool/software:

Hi,

We have a design that basically converts 24V inputs via optoisolators (ELD207) then they are fed to a microcontroller over inverting buffers, namely SN74LVCH16240A.

The circuit is essentially as below, the problem is, once the state changes (i.e., V-EXT HIGH, buffer input low, hence inverted buffer output high) it stays that way. Is this due to the 10k pullup at the buffer input? Maybe we can reduce it to 2k2. Apart from that, we'd be glad to hear about any other recommendations. Thanks.

7 months ago

0 Clemens Ladisch 7 months ago

Guru 316440 points

The datasheet specifies that you might need 500 µA to switch the inputs.

Consider using inverters without bus-hold inputs.

0 engiz 7 months ago in reply to Clemens Ladisch

Prodigy 35 points

Thanks, that seems to be the most logical thing to do (I hope we can find a pin compatible one). Apart from that, isn't there a way to make this work or the topology is entirely incompatible (since we already have the board we are unlikely to do anything dramatic :()

+1 Clemens Ladisch 7 months ago in reply to engiz

Guru 316440 points

2.2 kΩ should work (3.3 V / 2 / 2.2 kΩ = 750 µA).

8-bit buffers are more common and would have a larger selection.

0 engiz 7 months ago in reply to Clemens Ladisch

Prodigy 35 points

It did indeed, thanks. In fact we ordered some that were stocked in Digikey but since the issue is resolved we may continue with 2k2's for now.

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SN74LVCH16240A: Bus Hold With Optocoupler