Inverted regulator increases choice and reduces complexity

Most circuits are referenced to ground, where relatively low-voltage components can monitor and control the low side of a load but not the high side. For example, nearly any low-voltage rail-to-rail-input op amp can detect a voltage increase indicating overcurrent through a resistor that connects between the load and ground. To do the same thing on the high side, you typically select a differential amplifier that tolerates high common-mode voltage. This approach limits the component choices for the input amplifier and brings up the question of how to respond to an overcurrent. The differential amp produces a low ground-referenced signal from a high-side event, but you can prevent a high-side overcurrent resulting from a short to ground only by turning off the high-side power. In effect, the differential amp translates the high-side signal into the low-side domain in which you must then translate the response back into the high-side domain.

A simpler approach for any high-side overcurrent-protection circuit references the entire circuit to the high-side rail. Such circuits typically consume little power, which a small, three-terminal linear regulator can easily supply. However, this approach requires an unusual configuration employing a negative regulator whose ground pin connects to the high-side rail and whose input connects to system ground. There are no other connections to system ground. All “ground” points of the overcurrent-protection circuit connect to the regulator’s out pin.

Figure 1 shows a two-phase-stepper-motor, fast-acting, self-resetting high-side circuit breaker with a 24V power supply to the motor and a 12V power supply to the circuit breaker that is referenced to 24V. The circuit breaker sees the 24V motor’s power rail as 12V referenced to its local ground, which the regulator’s output provides. Like all negative linear regulators, the circuit requires a 6.8-µF tantalum capacitor.


FIGURE 1

R10 and R12, both 0.33Ω, 1W resistors, provide current sensing for the two phases. High-side power flows through a sense resistor and a P-channel MOSFET to the high-side input of an H bridge (not shown), which drives one motor winding. Current in either phase can cause the sense voltage to increase to 0.5V, triggering the breaker. The circuit responds by turning off both MOSFETs. It then waits 20 msec and turns them back on, automatically clearing momentary shorts.

Caption
Figure 1: Current-sense resistors turn off MOSFETs when current through them exceeds a limit.