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[FAQ] What's the difference between logic output types (push-pull, open-drain, 3-state)?

Emrys Maier
Guru* 96842 points

FAQ: Logic and Voltage Translation > Output Parameters >> Current FAQ

Push-Pull Output

A push-pull output can source current in the high state or sink current in the low state. In modern CMOS devices, the most common configuration for a push-pull output is shown here:

Output State

Positive Driver ( pFET )

Negative Driver ( nFET )
HIGH ON OFF
LOW OFF ON

When the output is in the high state:

The p-channel MOSFET is on and sources current from VCC to the output.

The n-channel MOSFET is off and does not allow current to flow to GND.

When the output is in the low state:

The p-channel MOSFET is off and does not allow current to flow from VCC.

The n-channel MOSFET is on and sinks current from the output to GND.

Open-Drain Output

An open-drain output can only sink current in the low state. In modern CMOS devices, the most common configuration for an open-drain output is shown here:

Output State

Negative Driver ( nFET )
Hi-Z OFF
LOW ON

When the output is in the high-impedance state, the n-channel MOSFET is off and does not allow any current to flow.

When the output is in the low state, the n-channel MOSFET is on and sinks current from the output to GND.


3-State Output

A 3-state output can be in one of three states: driving high, driving low, or not driving (high impedance). The most common configuration for a 3-state output is shown here:

Output State

Positive Driver ( pFET )

Negative Driver ( nFET )
HIGH ON OFF
LOW OFF ON
Hi-Z OFF OFF

When the output is in the high state:

The p-channel MOSFET is on and sources current from VCC to the output.

The n-channel MOSFET is off and does not allow current to flow to GND.

When the output is in the low state:

The p-channel MOSFET is off and does not allow current to flow from VCC.

The n-channel MOSFET is on and sinks current from the output to GND.

When the output is in the high-impedance state:

The p-channel MOSFET is off and does not allow current to flow from VCC.

The n-channel MOSFET is off and does not allow current to flow to GND.