8

LTC4350

4350fa

APPLICATIO S I FOR ATIO

GATE

DRIVE

REF

SHARE BUS

4350 F03

PIN

LOAD

+

–

TO A VOLTAGE TO MODIFY THE

REMOTE SENSE INPUT OF THE

CREATES AN ARTIFICIAL

ADJUSTS THE POWER

SUPPLY’S OUTPUT VOLTAGE

UP OR DOWN

THIS AMPLIFIER CONVERTS

THE E/A2 VOLTAGE OUTPUT TO

THIS AMPLIFIER FORCES THE POWER

SUPPLY CURRENT TO EQUAL THE

REFERENCE CURRENT VALUE

(i.e., SHARE BUS)

THIS VOLTAGE REPRESENTS

THE

REFERENCE CURRENT

NEEDED TO FORCE THE OUTPUT

VOLTAGE TO EQUAL THE REF

THIS VOLTAGE REPRESENTS

THE POWER SUPPLY CURRENT

MEASURED USING A SENSE

RESISTOR

PASS-FET USED TO

DISCONNECT A BAD POWER

SUPPLY AND TO HOT-SWAP A

POWER SUPPLY

–

+

–

+

–

+

E/A1

COMP1 PIN

FB PIN

COMP2 PIN

E/A2

–

+

4

3

2

1

20k

Figure 3. Simplified Block Diagram

drives the SB pin (or share bus). If the FB pin is above the

reference, the COMP1 pin is grounded and the SB pin is

disconnected from the COMP1 pin using the series diode.

The LTC4350 with the highest reference will drive the SB

pin and the 20k loads connected to the SB pin. All of the

other LTC4350’s COMP1 pins are pulled low because their

FB pins are at a higher voltage than their references. The

series diode between the COMP1 pin and the SB pin is

actually a low impedance buffer amplifier with a diode in

the output stage. Therefore, the master LTC4350’s E/A1

drives the share bus to the proper value that keeps the

output voltage tightly regulated. The buffer amplifier is

capable of driving at least fifty 20k loads (each 20k load

represents an LTC4350).

OUTPUT VOLTAGE ADJUSTMENT

The LTC4350 is designed to work with supplies featuring

remote sense. The output voltage of each power supply

needs to be adjusted below the final output voltage at the

common load. For example, a 5V system would require the

power supply output be set to 4.90V or some value below

5V. This is normally done using the trim pin of the module.

The power supply output is then increased by artificially

reducing the positive sense voltage by a small amount.

The LTC4350 would then adjust the output voltage to 5V,

an increase of 2%. The maximum range of adjustment can

be set from 2% to 5% to compensate for voltage drops in

the wiring, but no more than 300mV.

In most power supplies, the voltage sense is tied directly

placed in series with the power supply sense line, a voltage

requires the power supply to increase its output voltage to

compensate. Thus, the LTC4350 exercises complete con-

trol of the final output voltage.