Datasheet

LTC4350
9
4350fb
applicaTions inForMaTion
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).
O
UTPUT
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
to the output voltage. If a small valued resistor, R
OUT
, is
placed in series with the power supply sense line, a voltage
drop across R
OUT
appears as a lower sensed voltage. This
requires the power supply to increase its output voltage
to compensate. Thus, the LTC4350 exercises complete
control of the final output voltage.
The I
OUT
block converts the E/A2 output (COMP2 pin)
to a current that flows through R
OUT
(see Figure 3). As
the voltage at COMP2 increases, the current in R
OUT
Figure 3. Simplified Block Diagram
GATE
DRIVE
R
SENSE
REF
SHARE BUS
4350 F03
I
ADJ
SENSE
+
I
OUT
PIN
OUT
+
C
G
LOAD
R
OUT
+–
THIS RESISTOR CONVERTS I
ADJ
TO A VOLTAGE TO MODIFY THE
REMOTE SENSE INPUT OF THE
POWER SUPPLY (SENSE
+
). IT
CREATES AN ARTIFICIAL
SENSE
+
VOLTAGE THAT
ADJUSTS THE POWER
SUPPLY’S OUTPUT VOLTAGE
UP OR DOWN
THIS AMPLIFIER CONVERTS
THE E/A2 VOLTAGE OUTPUT TO
A CURRENT OUTPUT (I
ADJ
)
THIS AMPLIFIER FORCES THE POWER
SUPPLY CURRENT TO EQUAL THE
REFERENCE CURRENT VALUE
(i.e., SHARE BUS)
THIS VOLTAGE REPRESENTS
THE REFERENCE CURRENT
VALUE (i.e., SHARE BUS)
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
–
+
–
+
–
+
I
SENSE
R
GAIN
E/A1
COMP1 PIN
FB PIN
COMP2 PIN
E/A2
–
+
I
OUT
R
SET
PIN
R
SET
4
3
21
20k
Ω
g
m
= 1m