LTC4357 Ver la hoja de datos (PDF) - Linear Technology

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LTC4357

Positive High Voltage Ideal Diode Controller

Linear Technology 

LTC4357

APPLICATIONS INFORMATION

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

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.

VINA

48V

PS1

RTNA

FDB3632

IN

GATE

OUT

LTC4357

VDD

GND

48V BUS

VINB

48V

PS2

RTNA

FDB3632

IN

GATE

OUT

LTC4357

VDD

GND

VINC

48V

PS3

RTNA

FDB3632

IN

GATE

OUT

LTC4357

GND

VDD

4357 F01

Figure 1. Droop Sharing Redundant Supplies

4357fb

7

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