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SN74HCS139: Voltage output level when not supplied

Giorgio Valente

Prodigy 130 points

Part Number: SN74HCS139
Other Parts Discussed in Thread: SN74AHC139, SN74LVC139A, SN74LV139A

Tool/software:

Dear Support,

i am using SN74HCS139PWR: i am not supplying Vcc on pin 16 and channel enable signal is high (in my case i have 3.3V on pin 1). In a "normal" case i would expect high level voltage on output pins but, since i am not supplying Vcc, i think i should measure about 0 V or something like that.

Actually i am measuring 2.6V on Vcc pin and on all output pins: how it that possibile? What am i missing?

Thank you in advance.

Giorgio

0 Arodi Lopez 9 months ago

TI__Prodigy 290 points

Hello, the device does not support a Vin higher than Vcc, see the recommended operation conditions table

The device contains diodes in the inputs and outputs which is causing a voltage at the output, see this FAQ for further information

Can the input voltage (Vi) to my logic device be higher than the supply voltage (Vcc)?

0 Giorgio Valente 9 months ago in reply to Arodi Lopez

Prodigy 130 points

Hi Arodi,

thank you for the answer.

I really need to simulate an off state scenario: can i put Vcc=0 V and all input low logic level?

Thank you

0 Clemens Ladisch 9 months ago in reply to Giorgio Valente

Guru 295730 points

When powered off, 0 V is the only allowed input voltage.

Consider the SN74AHC139, SN74LV139A, or SN74LVC139A, which have overvoltage-tolerant inputs.

0 Giorgio Valente 9 months ago in reply to Clemens Ladisch

Prodigy 130 points

Hi Clemens,

thank you for the answer: i will try to put Vcc= 0V and all input to 0 V, i can't replace the component in this phase of the project.

In this condition can i expect 0V on output pins?

Thank you

+1 Clemens Ladisch 9 months ago in reply to Giorgio Valente

Guru 295730 points

When powered off, the outputs are not actively driven. But they also have clamping diodes, so if any external voltage is applied, it must be 0 V.

Devices with the I_off feature would allow other voltages at the outputs when powered off.

0 Giorgio Valente 9 months ago in reply to Clemens Ladisch

Prodigy 130 points

Hi Clemens,

the condition Vcc= 0V and low input voltage is ok for my case.

I will take in to account Ioff feature for future projects.

Thankl you

BR