Datasheet MAX5902, MAX5903 (Maxim) - 9

+72V, SOT23/TDFN, Simple Swapper Hot-Swap Controllers MAX5902/MAX5903 Selecting a Circuit-Breaker Threshold
The MAX5902A/MAX5903A and the MAX5902L/ MAX5903L offer a circuit-breaker function to protect the GROUND external MOSFET and the load from the potentially damaging effects of excessive current. As load current U1 flows through the external MOSFET, a voltage, VSD, is SOT23-6 MAX5902/MAX5903 generated from source to drain due to the MOSFET’s on-resistance, RDS(ON). The MAX5902A/MAX5903A POWER IN and MAX5902L/MAX5903L monitor VSD when the exter- S D G nal MOSFET is fully enhanced. If V M1 POWER SD exceeds the SOT-223 OUT circuit-breaker threshold the external MOSFET is turned off, and PGOOD (PGOOD) is deasserted. To accom- modate different MOSFETs and different load currents the MAX5902/MAX5903 are available with circuit-break- Figure 7. Circuit Board Layout Example. er threshold voltages of 300mV, 400mV, and 500mV. To determine the proper circuit-breaker threshold for an application use:
Determining Inrush Current
Determining a circuit’s inrush current is necessary to VCB > (RDS(ON)) ✕ (IOUT(MAX)) help choose the proper MOSFET. The MAX5902/ MAX5903 regulate the inrush current by means of con- where RDS(ON) is the on-resistance of the MOSFET and trolling the load voltage slew rate, but inrush current is IOUT(MAX) is the maximum expected output current. also a function of load capacitance. Determine inrush The MAX5902N/MAX5903N have no circuit-breaker current using: function. For these parts choose an external MOSFET which meets the load requirements. dV I = C = C × SR The circuit-breaker function is intended to protect dt against gross overcurrent or short-circuit conditions. During a gross overcurrent or short-circuit condition, where C is the load capacitance, and SR is the the MAX5902/MAX5903 disconnect the load by dis- MAX5902/MAX5903 Load Voltage Slew-Rate abling the external MOSFET. For calculating the circuit- Magnitude from the Electrical Characteristics table. For breaker threshold use the MOSFET’s RON at the worst example, assuming a load capacitance of 100µF, and possible operating condition, and add a 20% overcur- using the typical value of 9V/ms for the slew rate, the rent margin to the maximum circuit current. For inrush current is 0.9A typical. instance, if a MOSFET has an RON of 0.06Ω at TA = If the maximum possible Load Voltage Slew Rate is +25°C, and a normalized on-resistance factor of 1.75 at used, the maximum inrush current calculates to 1.8A. TA = +130°C (from the MOSFET data sheet), the RON Choose a MOSFET with a maximum pulsed current used for calculation is the product of these two num- specification that exceeds the maximum inrush current. bers, or (0.06Ω) x (1.75) = 0.105Ω. Then, if the maxi- mum current is expected to be 2A, using a 20% margin, the current for calculation is (2A) x (1.2) = 2.4A.
SUGGESTED
The resulting minimum circuit-breaker threshold is then
MAXIMUM SUGGESTED MAXIM EXTERNAL
a product of these two results, or (0.105Ω) x (2.4A) =
ILOAD (A) PART MOSFET
0.252V. The next highest minimum available threshold is 0.280V of the MAX5902ABEUT, which is an ideal 0.5 IRF9540NS MAX5902AAEUT choice given these parameters. Using this method to 1 IRF9540NS MAX5902ABEUT choose a circuit-breaker threshold allows the circuit to 2 IRF5210S MAX5902ABEUT operate under worst case conditions without causing a circuit-breaker fault, but the circuit-breaker function will 3 IRF5210S MAX5902ACEUT still operate if a short circuit, or a gross overcurrent VIN = +9V to +72V condition exists.
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