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LTC4357CDCB-TRMPBF Datasheet(PDF) 7 Page - Linear Technology |
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LTC4357CDCB-TRMPBF Datasheet(HTML) 7 Page - Linear Technology |
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7 / 12 page  LTC4357 7 4357fb MOSFET Selection The LTC4357 drives an N-channel MOSFET to conduct the load current. The important features of the MOSFET are on-resistance, RDS(ON), the maximum drain-source voltage, VDSS, and the gate threshold voltage. Gate drive is compatible with 4.5V logic-level MOSFETs in low voltage applications (VDD = 9V to 20V). At higher volt- ages (VDD = 20V to 80V) standard 10V threshold MOSFETs may be used. An internal clamp limits the gate drive to 15V between the GATE and IN pins. An external zener clamp may be added between GATE and IN for MOSFETs with a VGS(MAX) of less than 15V. The maximum allowable drain-source voltage, BVDSS, must be higher than the power supply voltage. If an input is connected to GND, the full supply voltage will appear across the MOSFET. ORing Two Supply Outputs Where LTC4357s are used to combine the outputs of two power supplies, the supply with the highest output voltage sources most or all of the load current. If this supply’s output is quickly shorted to ground while delivering load current, the flow of current temporarily reverses and flows backwards through the LTC4357’s MOSFET. When the reverse current produces a voltage drop across the MOSFET of more than –25mV, the LTC4357’s fast pull-down activates and quickly turns off the MOSFET. If the other, initially lower, supply was not delivering load current at the time of the fault, the output falls until the body diode of its ORing MOSFET conducts. Meanwhile, the LTC4357 charges its MOSFET gate with 20μA until the forward drop is reduced to 25mV. If instead this supply was delivering load current at the time of the fault, its associ- ated ORing MOSFET was already driven at least partially on, and the LTC4357 will simply drive the MOSFET gate harder in an effort to maintain a drop of 25mV. When the capacitances at the input and output are very small, rapid changes in current can cause transients that exceed the 100V Absolute Maximum Rating of the IN, OUT, and VDD pins. A surge suppressor (TransZorb or TVS) con- nected from OUT to ground clamps the output and prevents APPLICATIONS INFORMATION damage by limiting the magnitude of the peak voltage. In the absence of a surge suppressor, an output capacitance of 10μF is sufficient in most applications to prevent the transient from exceeding 100V. In lower voltage applica- tions, the MOSFET’s drain-source breakdown voltage may be sufficient to protect the LTC4357 provided BVDSS + VIN < 100V, making a surge suppressor unnecessary. Load Sharing The application in Figure 1 combines the outputs of multiple, redundant supplies using a simple technique known as droop sharing. Load current is first taken from the highest output, with the low outputs contributing as the output voltage falls under increased loading. The 25mV regulation technique ensures smooth load sharing between outputs without oscillation. The degree of sharing is a function of RDS(ON), the output impedance of the supplies and their initial output voltages. Figure 1. Droop Sharing Redundant Supplies 4357 F01 LTC4357 GND IN OUT VDD GATE FDB3632 48V BUS LTC4357 GND IN OUT VDD GATE FDB3632 LTC4357 GND IN OUT VDD PS1 VINA 48V RTNA GATE FDB3632 PS2 VINB 48V RTNA PS3 VINC 48V RTNA |
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