LTC4371 datasheet(8/22 Pages) LINER

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LTC4371

4371f

For more information www.linear.com/LTC4371

applicaTions inForMaTion

Figure 1. –36V to –72V/25A Ideal Diode-OR Controller

Figure 2. Simplest Solution: VDD Connected Directly to VZ

4371 F01

LTC4371

GA GB

VDD

SB VSS

2.2F

RDA

20k

RDB

20k

IPT020N10N3

–36V TO

–72V

–36V TO

–72V

VOUT

25A LOAD

GREEN LED

30k

33k

RTN

FAULTB

LTC4371

VDD

VSS

2.2F

RTN

VOUT

4371 F02

Highavailabilitysystemsemployparallelconnectedpower

supplies or battery feeds to achieve redundancy and

enhance system reliability. Schottky diodes are a popular

meansofORingthesesuppliestogetheratthepointofload.

The chief disadvantage of Schottky diodes is their sig-

nificant forward voltage drop and resulting power and

efficiency loss. This drop reduces the available supply

voltage and dissipates significant power. The LTC4371

solves these problems by using an N-channel MOSFET

as a low loss pass element to emulate the behavior of a

diode (see Figure 1).

The MOSFET is turned on when power passes in the

forward direction (positive current flow from source to

drain), allowing for a low voltage drop from load to sup-

ply. In the reverse direction, the MOSFET is turned off to

block current flow. By these means, the MOSFET is made

to approach the function and performance of an ideal

diode. The MOSFET voltage drop, ∆VSD, is sensed by the

DA and SA, or DB and SB pins.

Powering VDD

The LTC4371 is fundamentally a low voltage device op-

erating over a range of 4.5V to 16V at the VDD pin, with

respect to VSS. The gate amplifiers are powered from the

VDD pin and pull-up to within 300mV of VDD.

In low voltage applications such as –5V or –12V, the VDD

pin can be powered directly from return, with VSS con-

nected to VOUT.

An internal 12.4V shunt regulator at the VZ pin provides

a means of operating the LTC4371 from higher voltage

supplies. It regulates over a range of 50μA to 10mA. In the

simplestconfigurationshowninFigure 2,VDDisconnected

directly to VZ and biased by resistor RZ from the return.

A 2.2μF decoupling capacitor is required to stabilize the

VZ shunt regulator, and to momentarily provide the 5mA

fast pull-up current at the gate pins as needed.

Bias resistor RZ is chosen to bias the shunt regulator

and provide the maximum VDD current at the expected

minimum input voltage according to:

RZ <

VIN(MIN) – VZ(MIN)

IDD(MAX) + 50µA

(1)

Maximum bias resistor dissipation is calculated from:

PD(RZ) =

(VIN(MAX) – VZ(MIN))2

(2)

The maximum shunt regulator current must not exceed

10mA such that:

RZ >

VIN(MAX) – VZ(MIN)

10mA

(3)

In –48V applications a single 1206 size 30kΩ resistor is

adequate to power the LTC4371. In the application shown

in Figure 1, at 100V (a commonly specified maximum

transient condition) peak dissipation in RZ just exceeds

250mW, while the maximum VZ current is slightly less

than 3mA.

Dissipation rises in certain applications so that a larger

package or multiple series units are necessary to imple-

ment RZ. Examples include AC applications where the

gatedriversdemandadditionalcurrenttosupplyrepetitive

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