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MAX5902ABEUT Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX5902ABEUT Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 14 page +72V, SOT23/TDFN, Simple Swapper Hot-Swap Controllers _______________________________________________________________________________________ 9 Selecting a Circuit-Breaker Threshold The MAX5902A/MAX5903A and the MAX5902L/ MAX5903L offer a circuit-breaker function to protect the external MOSFET and the load from the potentially damaging effects of excessive current. As load current flows through the external MOSFET, a voltage, VSD, is generated from source to drain due to the MOSFET’s on-resistance, RDS(ON). The MAX5902A/MAX5903A and MAX5902L/MAX5903L monitor VSD when the exter- nal MOSFET is fully enhanced. If VSD exceeds the 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- er threshold voltages of 300mV, 400mV, and 500mV. To determine the proper circuit-breaker threshold for an application use: VCB > (RDS(ON)) ✕ (IOUT(MAX)) where RDS(ON) is the on-resistance of the MOSFET and IOUT(MAX) is the maximum expected output current. The MAX5902N/MAX5903N have no circuit-breaker function. For these parts choose an external MOSFET which meets the load requirements. The circuit-breaker function is intended to protect against gross overcurrent or short-circuit conditions. During a gross overcurrent or short-circuit condition, the MAX5902/MAX5903 disconnect the load by dis- abling the external MOSFET. For calculating the circuit- breaker threshold use the MOSFET’s RON at the worst possible operating condition, and add a 20% overcur- rent margin to the maximum circuit current. For instance, if a MOSFET has an RON of 0.06Ω at TA = +25°C, and a normalized on-resistance factor of 1.75 at TA = +130°C (from the MOSFET data sheet), the RON used for calculation is the product of these two num- 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. The resulting minimum circuit-breaker threshold is then a product of these two results, or (0.105Ω) x (2.4A) = 0.252V. The next highest minimum available threshold is 0.280V of the MAX5902ABEUT, which is an ideal choice given these parameters. Using this method to choose a circuit-breaker threshold allows the circuit to operate under worst case conditions without causing a circuit-breaker fault, but the circuit-breaker function will still operate if a short circuit, or a gross overcurrent condition exists. Determining Inrush Current Determining a circuit’s inrush current is necessary to help choose the proper MOSFET. The MAX5902/ MAX5903 regulate the inrush current by means of con- trolling the load voltage slew rate, but inrush current is also a function of load capacitance. Determine inrush current using: where C is the load capacitance, and SR is the MAX5902/MAX5903 Load Voltage Slew-Rate Magnitude from the Electrical Characteristics table. For example, assuming a load capacitance of 100µF, and using the typical value of 9V/ms for the slew rate, the inrush current is 0.9A typical. If the maximum possible Load Voltage Slew Rate is used, the maximum inrush current calculates to 1.8A. Choose a MOSFET with a maximum pulsed current specification that exceeds the maximum inrush current. IC dV dt CSR == × GROUND POWER IN POWER OUT M1 SOT-223 SDG U1 SOT23-6 MAX5902/MAX5903 Figure 7. Circuit Board Layout Example. MAXIMUM ILOAD (A) SUGGESTED EXTERNAL MOSFET SUGGESTED MAXIM PART 0.5 IRF9540NS MAX5902AAEUT 1 IRF9540NS MAX5902ABEUT 2 IRF5210S MAX5902ABEUT 3 IRF5210S MAX5902ACEUT VIN = +9V to +72V |
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