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LTC4355IDE-TRPBF Datasheet(PDF) 7 Page - Linear Technology |
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LTC4355IDE-TRPBF Datasheet(HTML) 7 Page - Linear Technology |
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7 / 16 page  LTC4355 4355fa operaTion High availability systems often employ parallel-connected power supplies or battery feeds to achieve redundancy and enhance system reliability. ORing diodes have been a popular means of connecting these supplies at the point of load. The disadvantage of this approach is the forward voltagedropandresultingefficiencyloss.Thisdropreduces the available supply voltage and dissipates significant power. Using N-channel MOSFETs to replace Schottky diodes reduces the power dissipation and eliminates the need for costly heat sinks or large thermal layouts in high power applications. The LTC4355 is a positive voltage diode-OR controller that drives two external N-channel MOSFETs as pass transis- tors to replace ORing diodes. The IN and OUT pins form the anodes and cathodes of the ideal diodes. The source pins of the external MOSFETs are connected to the IN pins. The drains of the MOSFETs are connected together at the OUT pin, which is the positive supply of the device. The gates of the external MOSFETs are driven by the LTC4355 to regulate the voltage drop across the pass transistors. At power-up, the initial load current flows through the body diode of the MOSFET with the higher INx voltage. TheassociatedGATExpinimmediatelyrampsupandturns on the MOSFET. The amplifier tries to regulate the voltage drop across the source and drain connections to 25mV. If the load current causes more than 25mV of drop, the MOSFET gate is driven fully on and the voltage drop is equal to RDS(ON) • ILOAD. When the power supply voltages are nearly equal, this regulation technique ensures that the load current is smoothly shared between the MOSFETs without oscil- lation. The current flowing through each pass trans- istor depends on the RDS(ON) of each MOSFET and the output impedances of the supplies. In the event of a supply failure, such as if the supply that is conducting most or all of the current is shorted to GND, reversecurrentflowstemporarilythroughtheMOSFETthat is on. This current is sourced from any load capacitance and from the second supply through the body diode of the other MOSFET. The LTC4355 quickly responds to this condition, turning off the MOSFET in about 500ns. This fast turn-off prevents the reverse current from ramping up to a damaging level. In the case where the forward voltage drop exceeds the configurable fault threshold, DVSD(FLT), the VDSFLT pin pulls low. Using this pin to shunt current away from an LED or optocoupler provides an indication that a pass transistorhaseitherfailedorhasexcessiveforwardcurrent. Additionally, in this condition the PWRFLT1 or PWRFLT2 pin pulls low to identify the faulting channel. The PWRFLT pins also indicate if an input supply is within regulation. When VMON1 < 1.23V or VMON2 < 1.23V, the corresponding PWRFLT pin pulls low to indicate that the input supply is low, turning off an optional LED or optocoupler. The FUSEFLT pins indicate the status of input fuses. If the voltage at one of the IN pins is less than 3.5V, the corresponding FUSEFLT pin pulls low. The IN pins sink a minimum of 0.5mA to guarantee that the IN pin will pull low when the input fuse is blown open. Note that the FUSEFLT pin will activate if the input supply is less than 3.5V even if the fuse is intact. |
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