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MAX5900ABETT-T Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX5900ABETT-T Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 14 page Power-Good Output The power-good output, PGOOD (PGOOD), is open- drain and asserts when the external MOSFET is fully enhanced and VDS is less than VPG (75% of the circuit- breaker threshold, VCB). For versions without the circuit- breaker function (MAX5900N/MAX5901N), PGOOD (PGOOD) asserts when the external MOSFET is fully enhanced. PGOOD (PGOOD) deasserts within 2µs when a circuit- breaker event occurs or if the die temperature exceeds +125°C. PGOOD (PGOOD) deasserts if |VEE| < |VUVLO| for longer than 20ms or ON/OFF is held low for longer than 20ms. The MAX5900 PGOOD is active-low and the MAX5901 PGOOD is active-high. Both are open-drain N-channel MOSFETs with their sources connected to VEE, and can withstand up to 100V. Selecting a Circuit-Breaker Threshold The MAX5900A/MAX5901A and the MAX5900L/ MAX5901L 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, VDS, is generated from drain to source due to the MOSFET’s on-resistance RDS(ON). The MAX5900A/MAX5901A and MAX5900L/MAX5901L monitor VDS when the external MOSFET is fully enhanced. If VDS exceeds the circuit- breaker threshold, the external MOSFET is turned off and PGOOD (PGOOD) is deasserted. To accommodate different MOSFETs and different load currents, the MAX5900/MAX5901 are available with cir- cuit-breaker threshold voltages of 200mV, 300mV, and 400mV. The circuit-breaker function is intended to disconnect the load if a gross overcurrent or short-circuit condition occurs. For calculating the circuit-breaker threshold, use the MOSFET’s RON at the worst possible operating condition, and add a 25% overcurrent margin to the maximum circuit current. For instance, if a MOSFET has an RON of 0.06 Ω 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 numbers, or (0.06 Ω) x (1.75) = 0.105 Ω. Then, if the maximum current is expected to be 2A, using a 25% margin, the current for calculation is (2A) x (1.25) = 2.5A. The resulting minimum circuit- breaker threshold is then the product of these two results, or (0.105 Ω) x (2.5A) = 0.263V. The next highest minimum available threshold is 0.265V of the MAX590_ _BEUT, 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 gross overcurrent condition occurs. See Table 1 for MOSFET suggestions. The MAX5900N/ MAX5901N have no circuit-breaker function. For these parts choose an external MOSFET that meets the load requirements. Determining Inrush Current Determining a circuit’s inrush current is necessary to help choose the proper MOSFET. The MAX5900/ MAX5901 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 MAX5900/MAX5901 Load Voltage Slew-Rate Magnitude from the Electrical Characteristics table. For example, assuming a load capacitance of 100µF, and using the typical value of 10V/ms for the slew rate, the inrush current is 1A typical. If the maximum possible Load Voltage Slew Rate is used, the maximum inrush current calculates to 1.7A. Choose a MOSFET with a maximum pulsed current specification that exceeds the maximum inrush current. -100V, SOT23/TDFN, Simple Swapper Hot-Swap Controllers 8 _______________________________________________________________________________________ MAXIMUM ILOAD (A) SUGGESTED EXTERNAL MOSFET SUGGESTED MAXIM PART 0.25 IRFL110 MAX590_ _CEUT 0.5 IRFL4310 MAX590_ _BEUT 1 IRFR3910 MAX590_ _CEUT 2 IRF540NS MAX590_ _BEUT 3 IRF1310NS MAX590_ _BEUT 4 IRF1310NS MAX590_ _CEUT Suggested External MOSFETs VIN = -9V to -90V IS == × C dV dt CR |
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