What is Push-Pull Circuit Topology?

A push-pull circuit uses two active components, most common is transistors (FET, or BJT) to “push” or “pull” current through a load. You might also hear the phrase “totem pole” as a reference to push-pull architecture.

Below is a circuit example of a push-pull circuit.

A push-pull circuit

This case uses some n-type silicon and some p-type silicon. Either a BJT or a MOSFET can be used here. You can think of this circuit as a switch that can pull high or low depending on which transistor is fully ON.

The p-type transistor sources current out of the pin. IOH is a common spec that signifies current being sourced from a device or “leaving” the device.

The n-type transistor sinks current into the pin. IOL is a common spec that signifies current being sunk into the device or “entering” the device.

Both IOL and IOH are common terms and electrical specs in most datasheets. See the output current specs for TXE8116.

Electrical Specifications table for TXE8116 page 7/37

Note that for every VOH spec there is a corresponding IOH spec.

For every VOL spec there is a corresponding IOL spec.

With every output low or high current there is an output low or high voltage. These are the driver characteristics and show electrical “strength” at various load currents.

You will note that the “SDO” output (orange) is the serial data output which is one of the SPI signals. This is the data output back to the microcontroller or processor. We can determine that the “SDO” pin is a push-pull output because it has both VOH and VOL specs. It can output a high logic voltage VOH = VCC - 0.4 V when sourcing an IOH current of 3 mA. Likewise, the device can output a low logic voltage VOL = 0.4 V when sinking an output low current IOL = 3 mA.

The IOL spec (blue) shows the low level output current for the interrupt pin /INT. You can tell that this pin is NOT a push-pull driver because it does not have a VOH/IOH spec. This pin is open-drain, and can only drive a LOW logic signal. Thus only an IOL spec is given and a corresponding output low voltage. At a current sink of 4 mA, you can expect the output low voltage from the /INT pin to be VOL = 0.4 V.

See more on open-drain circuit topology here.