23340458AN805_0| Application Note

23340458AN805_0| Application Note

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AN-805 APPLICATION NOTE

Altering the ADM1073 Current Limit Level
by Alan Moloney

INTRODUCTION The ADM1073 48 V hot swap controller accurately limits the current drawn from a supply by dynamically controlling the GATE voltage on an external N-channel FET placed in the path of that supply. An internal sense amplifier detects the voltage across a sense resistor connected between its VEE and SENSE pins. This voltage level gives an indication of the load current level. The sense amplifier has a preset control loop threshold of 100 mV (3%). This means the current control loop will regulate the load current when a voltage of 100 mV is detected across the sense resistor. In this way the value of the sense resistor sets the current level that causes the loop to regulate. 100 mV divided by RSENSE yields the current value at which the sense resistor causes the loop to regulate.

In addition to the 100 mV threshold, there is a separate circuit breaker threshold below this. It is nominally 90 mV (minimum 86 mV). If the load current is at a level that exceeds the circuit breaker level then the internal counter begins timing the event as a fault. Note that the device does not begin to regulate current until the current limit control loop threshold is reached. Table I shows the current limit levels for different values of sense resistor. ILOAD(MAX) is the maximum load current level that may trigger a current fault. This corresponds to a voltage of 86 mV across the sense resistor. ILIMIT(MIN) and ILIMIT(MAX) are the minimum and maximum load current levels at which the current limit loop will regulate. These correspond to voltages of 97 mV and 103 mV across the sense resistor, respectively.

Figure 1. ADM1073 Application Diagram

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