Open-collector outputs are useful for transmitting buffered digital/pulse signals to the external world when high drive is needed. In harsh operating environments like automotive systems, however, these signals need to be protected against accidental shorts to the positive supply that would instantly kill the drive transistor.
The popular approach of inserting a series resistor in the open-collector output has drawbacks. On the one hand, using a high-value, low-wattage resistor may create enough voltage drop in the line to prevent the signal from reaching a logic-low voltage level at the far end. On the other hand, using a smaller-value resistor may require it to be sized for several watts to withstand a continuous short circuit without damage. In an automotive system, which can have a battery supply voltage as high as 16 V, a 100-Ω series resistor would need to dissipate more than 2.5 W.
The simple addition of a transistor and resistor to the open-circuit drive overcomes this dilemma by limiting current through the drive transistor. Transistor Q2 provides the open-collector drive for a system logic signal (see the figure). Resistor R2 monitors the drive current through Q2. When the voltage across R2 becomes high enough (around 700 mV), Q1 starts conducting and diverts the base drive of Q2. This diversion of the base drive restricts the CE current through Q2, preventing damage even under a dead short circuit to the positive rail. The value of R2 determines the current limit.
Resistor R2 is of low enough value that it does not inhibit the drive’s ability to reach logic low. A 75-Ω resistor, for instance, would restrict current through Q2 to about 10 mA. This means R2 would only have to dissipate 7.5 mW, allowing a common 1/8-W resistor to easily withstand permanent short circuits.