CD74HC595: Output state when Vcc not present

Ian Southgate

Prodigy 40 points

Part Number: CD74HC595
Other Parts Discussed in Thread: SN74LV595A

Tool/software:

Can I expect the outputs QA - QH to be high-Z when the device is not powered?

Thought I'd check this before I make any assumptions.

Thanks

Ian

4 months ago

0 Clemens Ladisch 4 months ago

Guru 281490 points

In theory, yes. In practice, the ESD clamping diodes to V_CC prevent you from applying a voltage above 0 V to the outputs.

0 Malcolm Lyn 4 months ago

TI__Mastermind 19620 points

Hi Ian,

As Clemens said since this device has positive ESD diodes on the output the device is not guaranteed to be Hi-Z when VCC = 0. For this feature you need a device that specifies Ioff such as the SN74LV595A.

Best,

Malcolm

0 Ian Southgate 4 months ago in reply to Clemens Ladisch

Prodigy 40 points

Cheers thanks for the info.

0 Ian Southgate 4 months ago in reply to Malcolm Lyn

Prodigy 40 points

Thanks Malcolm, I'll take a look at that device.

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CD74HC595: Output state when Vcc not present