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MAX5940AESA Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX5940AESA Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 15 page Inrush Current Limit The MAX5940A/MAX5940B charge the gate of the internal MOSFET with a constant current source (10µA, typ). The drain-to-gate capacitance of the MOSFET lim- its the voltage rise rate at the drain, thereby limiting the inrush current. Add an external capacitor from GATE to OUT to further reduce the inrush current. Use the fol- lowing equation to calculate the inrush current: PGOOD/PGOOD Outputs (MAX5940A/MAX5940B) PGOOD is an open-drain, active-high logic output. PGOOD goes high impedance when VOUT is within 1.2V of VEE and when GATE is 5V above VEE. Otherwise, PGOOD is pulled to VOUT (given that VOUT is at least 5V below GND). Connect PGOOD to the ON pin of a down- stream DC-DC converter. Connect a 100k Ω pullup resis- tor from PGOOD to GND if needed. (MAX5940B only) PGOOD is an open-drain, active-low logic output. PGOOD is pulled to VEE when VOUT is within 1.2V of VEE and when GATE is 5V above VEE. Otherwise, PGOOD goes high impedance. Connect PGOOD to the ON pin of a downstream DC-DC converter. Connect a 100k Ω pullup resistor from PGOOD to GND if needed. Thermal Dissipation During classification mode, if the PSE applies the maxi- mum DC voltage, the maximum voltage drop from GND to VRCLASS will be 13V. If the maximum classification current of 42mA flows through the MAX5940A/ MAX5940B, then the maximum DC power dissipation will be 546mW, which is slightly higher than the maximum DC power dissipation of the IC at maximum operating temperature. However, according to the IEEE 802.3af standard, the duration of the classification mode is limit- ed to 75ms (max). The MAX5940A/MAX5940B handle the maximum classification power dissipation for the maximum duration time without sustaining any internal damage. If the PSE violates the IEEE 802.3af standard by exceeding the 75ms maximum classification duration, it may cause internal damage to the IC. II x C C INRUSH G OUT GATE = IEEE 802.3af PD Interface Controller For Power-Over-Ethernet _______________________________________________________________________________________ 9 |
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