LTC4350
8
4350fb
applicaTions inForMaTion
INTRODUCTION
Many system designers find it economically feasible to
parallel power supplies to achieve redundancy. The second
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
supply is a combination of a power module and the LTC4350
as shown in Figure 2. Note that the power module must
have accessible feedback network or a remote sensing
pin (SENSE
+
) to interface to the LTC4350.
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 manner or removed
without disruption.
C
URRENT SHARING
The current sharing components will now be discussed.
Figure 3 shows a simplified block diagram of these compo-
nents. The I
SENSE
block measures the power supply current
by amplifying the voltage drop across the sense resistor.
An external resistor on the GAIN pin determines the gain
of the I
SENSE
block. The voltage drop across the sense
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
I
OUT
block. The objective of the E/A2 block is forcing 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.
V
OLTAGE 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 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
OUT
+
OUT
–
SENSE
+
INPUT BUS
OUTPUT BUS
SHARE BUS
SENSE
–
LTC4350
Figure 1. Redundant Power Card System
Figure 2. Redundant Power Supply
