Open collector and push-pull configurations are IC pin driving techniques (particularly output pin). Nowadays ICs such as microcontrollers, digital signal controllers and processors are already configurable. The programmer can set the output pin to either open collector or push-pull implementations. Open collector is an implementation wherein the IC will not provide the supply by itself the external circuit by floating its collector and let the external pull up to provide bias on the external circuit. Push-pull on the other hand is an implementation wherein the IC is set to supply the external circuitry. Figure 1 is a representation of the driving strategy of the output pins of an MCU, DSC or DSP (not really exact but good enough to demonstrate open drain and push-pull). Q2 and Q3 (enclosed in green) is the circuit for push-pull while Q4 (enclosed in yellow) is the circuit for open collector configuration.
If the output pin configuration is open collector, Q4 will be used. By the time that Q1 is needed to be biased, the base of Q4 is driven low as a result it is operating at cut-off and the collector of Q4 is typically open. Since Q4 is operating at cut-off and there is no pull-up internally, Q1 will not be biased. That’s why R2 is present. R2 pulls up to Vcc and then current will flow from Vcc to R2, R1 and then VBE of Q1. By the time that Q1 is needed to be turned off, the base voltage of Q4 is set to high so that it will be saturated. The base of Q1 is pulled down to ground. The current will flow from Vcc to R2 and then Q4. This current is called as the sink current.
When push-pull is used, Q2 and Q3 will be selected. When Q1 is needed to be biased, the base of Q2 and Q3 are driven by a high signal and because of this Q2 will saturate and Q3 is at cut-off. The current is then flow from the Vcc of the IC to Q2, output pin, R1 and finally to the VBE of Q1. Since the IC is able to bias Q1, the resistor R2 will not be needed anymore. The current that is flowing during this time in the pin of the IC is called as the source current. By the time that Q1 is needed to turn off, the base of Q2 and Q3 are driven low so that Q3 will saturate and pull down the base of Q1. The current (sink current) will then flow from Vcc to R2, IC pin then finally to Q3.
Open Collector with Internal Pull up
In modern ICs, internal pull up is can be set as well. The R3 in figure 2 is the internal pull up. The designer can use an open collector configuration without an external pull up by utilizing this. However this is not a strong pull up. The usual value for internal pull up is high and maybe not able to bias Q1 properly when the required base current is high. Another thing to take note is enabling the internal pull up with the external pull up R2. As you can see, R3 and R2 are just parallel so the effective pull up resistance is the parallel of the two. The current may increase and you have to consider if there is no adverse effect to the circuit.
Advantages and Disadvantages
The advantage of an open collector configuration is that the MCU, DSC or DSP will not be stressed. In actual circuit design and application, Q4 will turn on only during turn-off or start up (initialization) thus most of the time it is at cut-off. The drawback is that you need an additional resistor for external pull up.
For push pull configuration, the advantage is lower component count. You don’t need the additional external pull up anymore. The drawback is possible stress on the IC. Imagine if all the output pins are configured push-pull, this will be a huge dissipation that might degrade the performance of the IC or even shorten its life.
In actual product designs, some output pins are configured open collector while other is push-pull. This is really up to you. Be sure only to consider device limits such as source and sink currents to avoid damage.