Build a 32V Circuit Breaker with the MAX4172

In a power supply, motor drive, or other power circuit, the load current can be detected on the high voltage side (power supply side) or on the low voltage side (ground side). In comparison, it is easier to perform on the low voltage side than on the high voltage side, but It also inevitably abandoned the advantages of high-pressure side detection. Some faults may not be detected by the low-voltage side detector, thus putting the load in a dangerous and unmonitored state.

On the other hand, the high-voltage side detector is directly connected to the power supply and can detect any downstream faults and trigger appropriate corrective actions. Previously, forming such a detector required a precision operational amplifier, a high voltage power supply (to accommodate the op amp's limited common-mode range), and a large number of precision resistors. Now, with the MAX4172 monolithic IC, high-side currents can be detected at up to 32V common-mode voltage. The MAX4172 in a tiny μMAX package provides a ground-referenced current source with an output current proportional to the high-side current to be sensed. This output current is numerically equal to the voltage across the external sense resistor divided by 100, allowing it to flow through a load resistor, resulting in a very simple voltage output.

Using the MAX4172 with several inexpensive external components can make a low-cost circuit breaker. The load current is detected by RSENSE and controlled by Q1. The design accepts an input voltage of 10V to 32V and can be operated at a voltage of 6.5V after a simple adjustment.

At the beginning of the addition of VIN and VCC, the circuit breaker is in the triggered state. Press S1 to reset the circuit breaker and switch on the power supply and load. Q1, Q3 and Q4b are also activated. Q3 powers IC1 and Q4b establishes an overcurrent threshold.

VTHRESH = Vcc - Vbe(Q4b) (1)

Since VCC (typically 2.7V to 5.5V) is equal to 5.0V and Q4b has an emitter voltage Vbe (Q4b) of approximately 0.7V, VTHRESH is typically 4.3V. The normal load current threshold is 1A. The size of RSENSE, RTHRESH, and ROUT is determined by the system accuracy and power consumption. First select RSENSE=50mΩ and RTHRESH=10.0kΩ.

ROUT = Vcc/ILOAD×RSENSE×Gm (2)

Trigger current in the formula ILOAD = 1A, RSENSE = 50mΩ, IC1 transconductance Gm is equal to 0.010A / V, so that ROUT = 10.0kΩ. Applying power to Q3 and Q4b turns on Q4b, which establishes VTHRESH, and Q3 activates IC1. The load current flowing through RSENSE mirrors the output of IC1 in a small proportion and flows through ROUT to provide an output voltage VOUT. When VOUT increases above VTHRESH+Vbe(Q4a), Q4b turns off and also turns off Q3, causing the voltage on the V+ pin (IC1 pin 8) to drop. When V+ reaches a typical value of 2.67V, the PG outputs a high level, which in turn turns off Q1 and puts the circuit breaker into the trigger state. The feedback introduced by Q2 guarantees a clean shutdown at the trigger point. The current consumption in the trigger state is very small, equal to VCC load current, typically 0.5mA.

Press S1 to reset the breaker. To reset automatically, replace S1 with a transistor or comparator with open-drain output. The design shown in the figure is suitable for low-cost applications where there is no strict requirement on the absolute accuracy of the trigger current. The accuracy of this design depends on VCC, Vbe, and the error current flowing through R4. The accuracy is approximately ±15% at a trigger current of 1.0A.