Carl Spearow
EDN
High-side drivers find common use in driving grounded solenoid coils and other loads. Short-circuit protection for such drivers is essential for avoiding damage from wiring faults and other causes. Polymer fuses are generally too slow, and discrete current-limiting circuits are large and cumbersome.
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| Figure 1. | This simple high-side driver provides current limiting as well as transient protection. |
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The circuit in Figure 1 uses a small, low-dropout linear regulator as a high-side switch and provides inherent current limiting and thermal shutdown. The regulator comes in an SO-8 package. The zener diode provides transient protection, and the output capacitor ensures stability of the circuit. The circuit can drive a 24 V load at 100 mA. These are adequate specs for many solenoid valves, relay coils, and other moderate loads.
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| Figure 2. | In the bottom trace, the output current limits itself to 160 mA during a short circuit. |
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During a short circuit, the regulator limits the current to 160 mA. This current causes the die to overheat and enter a thermal-shutdown state. Upon removal of the short circuit, the device cools down and resumes normal operation. The top trace in Figure 2 is the output voltage during a 1.3-sec short circuit. The bottom trace is the short-circuit current, which limits itself at less than 200 mA. Note that the regulator goes into thermal shutdown after 500 msec, and the IC then toggles on and off until removal of the short circuit.
