Introduction of 28V Output Circuit Composed of MAX4373 Current Amplifier
The MAX4373 is a high-side current-sense amplifier with a comparator and reference. The comparator's latching output (COUT1) allows the device to form a circuit breaker, but the data sheet gives the circuit only for +2.7V to + 5.5V supply voltage range. By adding a general-purpose NPN transistor, the voltage range can be extended to 28V as shown.
In the circuit breaker, the NPN transistor Q1 expands the swing of the comparator output COUT1.
Under normal operating conditions, the open-drain comparator output is low, allowing 430μA of nominal current to flow from the emitter of the transistor through R2 to COUT1. The same (approximately the same) current flows through the collector and R1, producing slightly higher -6.4V VGS voltage to drive p-channel MOSFET.
In this design, when a current of 900 mA flows through the current-sense resistor R6 of 100 mΩ, a voltage drop of 90 mV is generated, which is multiplied by 50 times the gain, and a voltage of 4.5 V is obtained at the OUT terminal. Comparator CIN1 input threshold is 600mV, select the 6.5:1 resistor divider network to get the required set value. R4 selects 68kΩ and R5 is 10kΩ, which gives an actual load current threshold of 936mA.
This circuit is suitable for high voltage applications (in this case with +28V) and voltage is at least twice the VCC voltage. In addition, VCC is limited to a maximum of +5.5V, which is within the limits of the comparator output. The difference between the values ​​of the resistors R1 and R2 introduces a certain gain, which can provide a higher MOSFET driving voltage, thereby reducing the loss of the MOSFET on-resistance.
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