Greg Sutterlin
EDN
The simplicity of low-side current monitoring can mask the advantages of a high-side approach. You can monitor load currents in a power supply, a motor driver, or another power circuit on either the high or the low side (ground). However, don't let the ease of low-side monitoring cause you to overlook its dangers or the advantages of a high-side approach. Various fault conditions can bypass the low-side monitor, thereby subjecting the load to dangerous and undetected stresses. On the other hand, a high-side monitor connected directly to the power source can detect any downstream failure and trigger the appropriate corrective action. Traditionally, such monitors required a precision op amp, a boost power supply to accommodate the op amp's limited common-mode range, and a handful of precision resistors. Now, the MAX4172 IC can sense high-side currents in the presence of common-mode voltages as high as 32 V (Figure 1). IC1 provides a ground-referenced current-source output proportional to the high-side current of interest. This output current, equal to the voltage across an external sense resistor divided by 100, produces a voltage output across a load resistor.
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| Figure 1. | A current-sense amplifier and a few transistors form a low-cost circuit breaker. | |
IC1 and a few external parts form a low-cost circuit breaker. RSENSE senses load currents, and Q1 controls the currents. The design accepts inputs of 10 to 32 V; you can easily modify it to operate from voltages as low as 6.5 V. The initial application of VIN and VCC places the breaker in its trip state. Pressing S1 resets the breaker and connects power to the load, thereby activating Q1, Q3, and Q4B. Q3 powers IC1, and Q4B establishes the overcurrent threshold, VTHRESH = VCC – VBE(4B). Because VCC (2.7 to 5.5 V typical) equals 5 V and the base-emitter voltage of Q4B is approximately 0.7 V, VTHRESH is typically 4.4 V. The circuit trips at a nominal load current of 1 A. The values for RSENSE, RTHRESH, and ROUT are functions of the system's accuracy and power-dissipation requirements. First, select RSENSE=50 mΩ and RTHRESH=10 kΩ. Then, calculate
where ILOAD is the trip point (1 A) and Gm (IC1's typical transconductance) equals 0.01 A/V. Thus, ROUT = 10 kΩ.
Applying power to Q3 and Q4B causes Q4B to conduct, which establishes VTHRESH and activates Q3 to power IC1. A fraction of the load current through RSENSE mirrors to the IC1 output and appears as a voltage, VOUT, across VOUT. Q4B turns off when VOUT increases above (VTHRESH + VBE(4BA)), turning off Q3 and causing a drop in V+ (IC1, pin 8). When V+ reaches 2.67 V (typical), PG goes high, thereby tripping the breaker by turning off Q1. Q2 adds feedback to ensure a clean turn-off at the trip level. Current draw in the tripped state is minuscule and equals the VCC load current, 0.5 mA typical. Press S1 to reset the breaker. The design is intended for low-cost applications in which the absolute accuracy of the trip current is not critical. The accuracy, which depends on variations in VCC and the base-emitter voltages of Q4A and Q4B and on the error current through R4, is approximately ±15% at a trip current of 1 A.
