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SC4250LISTRT Datasheet(PDF) 6 Page - Semtech Corporation |
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SC4250LISTRT Datasheet(HTML) 6 Page - Semtech Corporation |
6 / 12 page 6 2005 Semtech Corp. www.semtech.com SC4250 POWER MANAGEMENT Applications Information Insertion of a power circuit board into a live backplane would draw enormous inrush currents. This is mostly due to the charging of the bulk electrolytic capacitors at the input of the power module being plugged in. The transient currents would send glitches all over the power system and could cause corruption of the signals and even a power down if the source isn’t able to handle these high surges. This section describes the components selection needed for a typical application utilizing the SC4250. Let’s assume the following requirements for a representative system: Input voltage range: 36V to 72V Nominal current: 2A typ. Over-current condition: 5A Bulk capacitance: Cload = 150µF The schematic in Figure 2 combines internal function blocks along with the external components of the application circuit. Resistors R1, R2 and R3 make up a voltage divider to set the Under-Voltage (UV) and Over-Voltage (OV) trip points. When the input power supply ramps up the UV trips at 1.273V and OV trips at 1.223V; during the ramp down transition the UV trips at 1.223V and OV trips at 1.198V. The 50mV hysteresis for UV and 25mV hysteresis for OV provide the necessary guard-bands to prevent false tripping during power up and power down conditions. As an additional noise killing and stabilizing measure, the capacitor C1 should be placed at the OV terminal with the value in range from 1,000 to 10,000pF. For the UV=38V and OV=70V the values of the resistor can be calculated as follows: Vuv = 1.273V · (R1+R2+R3) ÷ (R2+R3) Vov = 1.223V · (R1+R2+R3) ÷ R3 With the input bias current of the UV and OV comparators in the range of 20-30nA, let’s choose the R1 to be 562k Ω. This yields the values of R2=9.31kΩ and R3 = 10.2k Ω. With these values the accuracy is about 1% which is quite acceptable for those functions. Figure 2 60mV + _ _ + _ + _ + + _ +7V Delay Q1 DRAIN GATE R4 SENSE Vee Vcc 12.5V Reg 1.223V 1.75V PWRGD 50uA Cload 150uF C3 R5 R1 R2 R3 C2 UV OV -48V +48V C1 R6 |
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