LTC4350



4350fb

applicaTions inForMaTion

INTRODUCTION

Many system designers find it economically feasible to

parallelpowersuppliestoachieveredundancy.Thesecond

trend is providing some load sharing between the many

supplies. In some cases, a failure in any one supply will

trigger a sequence that disconnects the faulty supply and

sends a flag to the system. Then, a service technician will

swap in a good supply. For systems that are continuously

powered, there is Hot Swap circuitry to prevent glitches on

the power buses when power cards are swapped. A block

diagram of this system is shown in Figure 1.

By combining the features of a load share and a Hot Swap

controller into one IC, the LTC4350 simplifies the design of

redundant power supplies. A complete redundant power

supplyisacombinationofapowermoduleandtheLTC4350

as shown in Figure 2. Note that the power module must

have accessible feedback network or a remote sensing

The LTC4350 provides a means for paralleling power

supplies. It also provides for load sharing, fault isolation

and power supply hot insertion and removal. The power

supply current is accurately measured and then compared

to a share bus signal. The power supply’s output voltage

is adjusted until the load current matches the share bus,

whichresultsinloadsharing.Therearetwooptionalpower

FETs in series with the load that provide a quick disconnect

between a load and a failed power supply. These same

power FETs allow a power supply to be connected into a

powered backplane in a controlled manner or removed

without disruption.

CURRENT SHARING

The current sharing components will now be discussed.

Figure3showsasimplifiedblockdiagramofthesecompo-

by amplifying the voltage drop across the sense resistor.

An external resistor on the GAIN pin determines the gain

resistor is divided by a precision 1k resistor to produce a

current at the GAIN pin. For example, a 10mV sense volt-

age translates to a 10µA current. If a 10k resistor is on the

GAIN pin, then the voltage gain is 10k/1k or 10.

The voltage at the GAIN pin is compared to the current

share bus using the E/A2 block. The output of E/A2 is used

to adjust the output voltage of the power supply using the

GAIN pin voltage to equal the SB pin voltage. When the

GAIN pin voltages of all the LTC4350s in the system equal

the SB pin voltage, the load current is shared.

VOLTAGE MONITOR

UniquetotheLTC4350istightoutputvoltageregulation.This

is handled by the LTC4350’s error amplifier and reference

andnotthepowersupply’serroramplifierandreference.The

E/A1 amplifier monitors the output voltage via the feedback

divider connected to the FB pin. The FB pin is compared

to the internal reference of the LTC4350. If the FB pin is at

or below the reference, then the output of E/A1 drives the

OUTPUT

BUS

SHARE

BUS

INPUT

BUS

CONNECTOR

LOAD

4350 F01

HOT

SWAP

LOAD

SHARE

POWER

SUPPLY

CONNECTOR

HOT

SWAP

LOAD

SHARE

POWER

SUPPLY

4350 F02

POWER

MODULE

INPUT BUS

OUTPUT BUS

SHARE BUS

LTC4350

Figure 1. Redundant Power Card System

Figure 2. Redundant Power Supply