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LTC4350

4350fa

INTRODUCTION

Many system designers find it economically feasible to

parallel power supplies to achieve redundancy. The sec-

ond 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 continu-

ously 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

supply is a combination of a power module and the

LTC4350 as shown in Figure 2. Note that the power

APPLICATIO S I FOR ATIO

module must have accessible feedback network or a

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,

which results in load sharing. There are two optional

power 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 man-

ner or removed without disruption.

CURRENT SHARING

The current sharing components will now be discussed.

Figure 3 shows a simplified block diagram of these com-

current by amplifying the voltage drop across the sense

resistor. An external resistor on the GAIN pin determines

sense resistor is divided by a precision 1k resistor to

produce a current at the GAIN pin. For example, a 10mV

sense voltage 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

Unique to the LTC4350 is tight output voltage regulation.

This is handled by the LTC4350’s error amplifier and

reference and not the power supply’s error amplifier and

reference. 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

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