LTC4350
1
4350fb
Typical applicaTion
FeaTures DescripTion
Hot Swappable
Load Share Controller
The LTC
®
4350 is a load share controller that allows sys-
tems to equally load multiple power supplies connected
in parallel. The output voltage of each supply is adjusted
using the SENSE+ input until all currents match the share
bus. The LTC4350 also isolates supply failures by turning
off the series pass transistors and identifying the failed
supply. The failed supply can then be removed and replaced
with a new unit without turning off the system power. The
LTC4350 is available in a 16-pin narrow SSOP package.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and Hot
Swap and ThinSOT are trademarks of Linear Technology Corporation. All other trademarks are
the property of their respective owners.
5V Load Share (5A per Module)
applicaTions
n Build N + 1 Redundant Supply
n Hot Swap™ Power Supplies
n Isolates Supply Failures from Output
n Eliminates ORing Diodes
n Identifies and Localizes Output Low, Output High
and Open-Circuit Faults
n Output Voltages from 1.5V to 12V
n 16-Lead Narrow SSOP Package
n Servers and Network Equipment
n Telecom and Base Station Equipment
n Distributed Power Systems
GAIN COMP1 GND COMP2
GATE
0.010Ω
SUD50N03-07
R+R–
RSET
FB
IOUT
STATUS STATUS
SHARE BUS
SB
TIMER
VCC
VOUT+VOUT–
UV
OV
0.1µF
51Ω 100Ω 37.4k
12.1k
100Ω
4.7µF
1000pF
150Ω
0.1µF
LTC4350
0.1µF
34k
470k
12.1k
121k
43.2k
274k
0.1µF
VICOR*
VI-J30-CY
SENSE+
OUT+
SENSE–
TRIM
OUT–
GAIN COMP1 GND COMP2
GATE
0.010Ω
SUD50N03-07
R+R–
RSET
FB
IOUT
STATUS STATUS
SB
TIMER
VCC
UV
OV
0.1µF
51Ω 100Ω 37.4k
12.1k
100Ω
4350 TA01
4.7µF
1000pF
150Ω
0.1µF
LTC4350
34k
*LOAD SHARING CIRCUIT WORKS WITH MOST POWER SUPPLIES THAT HAVE A SENSE+ OR FB PIN
470k
0.1µF
VICOR*
VI-J30-CY
SENSE+
OUT+
SENSE–
TRIM
OUT–
0.1µF 12.1k121k
43.2k
274k
NOT RECOMMENDED FOR NEW DESIGNS
Contact Linear Technology for Potential Replacement
LTC4350
2
4350fb
pin conFiguraTionabsoluTe MaxiMuM raTings
Supply Voltage (VCC) ................................................17V
Input Voltage
TIMER ................................................... –0.3V to 1.2V
R+, R– (Note 2) ...................................... –0.3V to 17V
FB ......................................................... –0.3V to 5.3V
OV, UV ....................................................... –0.3V to 17V
Output Voltage
COMP1 .................................................... –0.3V to 6V
COMP2 .................................................... –0.3V to 3V
GAIN, SB ............................................... –0.3V to 5.6V
GATE (Note 3) ............................................ –0.3V to 20V
IOUT, STATUS ............................................. –0.3V to 17V
RSET ............................................................. –0.3V to 1V
Operating Temperature Range
LTC4350C ................................................ 0°C to 70°C
LTC4350I .............................................–40°C to 85°C
Storage Temperature Range .................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec)................... 300°C
(Note 1)
GN PACKAGE
16-LEAD PLASTIC SSOP
1
2
3
4
5
6
7
8
TOP VIEW
16
15
14
13
12
11
10
9
UV
OV
TIMER
GAIN
COMP2
COMP1
SB
GND
VCC
STATUS
GATE
R+
R–
IOUT
RSET
FB
TJMAX = 150°C, θJA = 135°C/W
orDer inForMaTion
LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE
LTC4350CGN#PBF LTC4350CGN#TRPBF 4350 16-Lead Plastic SSOP 0°C to 70°C
LTC4350IGN#PBF LTC4350IGN#TRPBF 4350I 16-Lead Plastic SSOP –40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
elecTrical characTerisTics
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
DC Characteristics
ICC VCC Supply Current UV = VCC l1.0 1.6 2.0 mA
VLKOH VCC Undervoltage Lockout High l2.36 2.45 2.52 V
VLKOL VCC Undervoltage Lockout Low l2.24 2.34 2.44 V
VFB FB Pin Voltage 0°C to 85°C (LTC4350I) or 0°C to 70°C (LTC4350C)
–40°C to 85°C (LTC4350I)
1.208
1.196
1.220
1.220
1.236
1.244
V
V
VFBLIR FB Line Regulation VCC = 3.3V to 12V, COMP1 = 1.240V 0.02 0.05 %/V
VFBLOR FB Load Regulation COMP1 = 2V
COMP1 = 0.64V
l
l
–0.0008
0.003
–0.1
0.1
%
%
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 5V unless otherwise noted.
LTC4350
3
4350fb
elecTrical characTerisTics
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 5V unless otherwise noted.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
DC Characteristics
VUVTH UV Pin Threshold High Going Threshold
Low Going Threshold
l
l
1.215
1.205
1.244
1.220
1.258
1.237
V
V
VOVTH OV Pin Threshold High Going Threshold
Low Going Threshold
l
l
1.203
1.180
1.220
1.205
1.250
1.229
V
V
VTM TIMER Pin Threshold l1.18 1.22 1.26 V
ITM TIMER Pin Current TIMER On, VTIMER = 0V
TIMER On, VTIMER = 0V, VOV > VOVTH
l
l
–1.75
–5.30
–2
–6
–2.3
–6.7
µA
µA
VGGAIN Pin Voltage RGAIN = 25k, (VR+ – VR–) = 100mV l2.3 2.5 2.7 V
VGO GAIN Pin Offset RGAIN = 25k, (VR+ – VR–) = 0mV l0 0.02 0.20 V
VSB(MIN) SB Pin Minimum Voltage 2 8 mV
VSB(MAX) SB Pin Maximum Voltage VCC = 3.3V
VCC = 12V
l
l
2.4
5.6
2.7
7.8
2.9
10.5
V
V
ISB(MAX) SB Pin Maximum Current VSB = 0V l–8 –33 –41 mA
RSB SB Pin Resistor Value l14 20 33 kΩ
VE/A2OFF E/A2 Offset VSB – VGAIN l8 25 50 mV
VRSET(MAX) RSET Pin Maximum Voltage VCC = 3.3V, RSET = 100Ω
VCC = 12V, RSET = 100Ω
l
l
0.94
0.94
1
1
1.03
1.03
V
V
VRSET(MIN) RSET Pin Minimum Voltage VCC = 5V, RSET = 1000Ω
VCC = 5V, RSET = 100Ω
l
l
0.001
0.001
0.5
0.5
V
V
IRSET(MAX) RSET Pin Maximum Current RSET = 50Ω, VIOUT = 1.1V l18 20 21 mA
VRCTH Reverse Current Threshold VR+ – VR+l10 30 40 mV
∆VGATE External N-Channel Gate Drive VGATE – VCC l10.8 12 12.7 V
IGATE GATE Pin Current Gate On, VGATE = 0V l–8 –10 –12 µA
VSOL STATUS Pin Output Low IOUT = 3mA l0.1 0.3 1.2 V
Note 2: R+ and R– could be at 17V while VCC = 0V.
Note 3: An internal clamp limits the GATE pin to a minimum of 10.8V
above VCC. Driving this pin to voltages beyond the clamp may damage the
part.
LTC4350
4
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Typical perForMance characTerisTics
UV Threshold vs Temperature OV Threshold vs VCC OV Threshold vs Temperature
FB vs VCC FB vs Temperature Gain PIn Voltage vs VCC
ICC vs VCC ICC vs Temperature UV Threshold vs VCC
VCC (V)
0 2
1.0
ICC (mA)
2.0
3.5
4810
4350 G01
1.5
3.0
2.5
612 14
TA = 25°C
TEMPERATURE (°C)
–50
1.66
1.64
1.62
1.60
1.58
1.56
1.54 25 75
4350 G02
–25 0 50 100
ICC (mA)
VCC = 5V
VCC (V)
0 2
UV THRESHOLD (V)
1.250
1.245
1.240
1.235
1.230
1.225
1.220
1.215 4810
4350 G03
612 14
TA = 25°C
TEMPERATURE (°C)
–50
UV THRESHOLD (V)
1.255
1.250
1.245
1.240
1.235
1.230
1.225
1.220
1.215 –25 0 25 50
4350 G04
75 100
VCC = 5V
VCC (V)
0 2
OV THRESHOLD (V)
1.222
1.220
1.218
1.216
1.214
1.212
1.210
1.208
1.206
1.204
1.202 4810
4350 G05
612 14
TA = 25°C
TEMPERATURE (°C)
–50
OV THRESHOLD (V)
1.225
1.220
1.215
1.210
1.205
1.200 –25 0 25 50
4350 G06
75 100
VCC = 5V
TEMPERATURE (°C)
–50
1.210
FB (V)
1.215
1.220
1.225
1.230
–25 0 25 50
4350 G08
75 100
VCC = 5V
VCC (V)
0 2
2.2
GAIN (V)
2.4
2.7
4810
4350 G09
2.3
2.6
2.5
612 14
RGAIN = 25k
(VR+ – VR–) = 100mV
TA = 25°C
LTC4350
5
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Typical perForMance characTerisTics
Gain Pin Voltage vs Temperature ∆VGATE vs VCC ∆VGATE vs Temperature
pin FuncTions
UV (Pin 1): Undervoltage Pin. The threshold is set at 1.244V
with a 24mV hysteresis. When the UV pin is pulled high,
the charge pump ramps the GATE pin. When the UV pin
is pulled low, the GATE pin will be pulled low.
OV (Pin 2): Overvoltage Pin. The threshold is set at 1.220V
with a 15mV hysteresis. When the OV pin is pulled high,
the GATE pin is pulled low. After a timer cycle, the STATUS
pin is pulled low until the OV pin is pulled low.
TIMER (Pin 3): Analog System Timing Generator Pin.
This pin is used to set the delay before the load sharing
turns on after the UV pin goes high. The other use for
the TIMER pin is to delay the indication of a fault on the
STATUS pin.
When the timer is off, an internal N-channel shorts the
TIMER pin to ground. When the timer is turned on, a 2µA
or 6µA timer current (ITIMER) from VCC is connected to the
TIMER pin and the voltage starts to ramp up with a slope
given by: dV/dt = ITIMER/CT
. When the voltage reaches the
trip point (1.220V), the timer will be reset by pulling the
TIMER pin back to ground. The timer period is given by:
(1.220V • CT)/ITIMER.
GAIN (Pin 4): Analog Output Pin. The voltage across the
R+ and R– pins is divided by a 1k resistor and sourced as
a current from the GAIN pin. An external resistor on the
GAIN pin determines the voltage gain from the current
sense resistor to the GAIN pin.
COMP2 (Pin 5): Analog Output Pin. This pin is the output
of the share bus error amplifier E/A2. (A compensation
capacitor between this pin and ground sets the crossover
frequency for the power supply adjustment loop.) In most
cases, this pin operates between 0.5V to 1.5V and repre-
sents a diode voltage up from the voltage at the RSET pin.
It is clamped at 3V. During start-up, this pin is clamped to
ground. After a timer cycle (and if the GATE pin is high),
the COMP2 pin is released.
COMP1 (Pin 6): Analog Output Pin. This pin is the output
of the voltage regulating error amplifier E/A1. A com-
pensation capacitor between this pin and ground sets
the crossover frequency of the share bus loop. This pin
operates a diode voltage up from the voltage at the SB
pin and is clamped at 8.4V.
SB (Pin 7): Analog Output Pin. This pin drives the share
bus used to communicate the value of shared load current
between several power supplies. There is an amplifier that
drives this pin a diode below the COMP1 pin using an internal
NPN as a pull-up and a 20k resistor as a pull-down.
TEMPERATURE (°C)
–50
2.2
GAIN (V)
2.3
2.4
2.5
2.6
2.7
–25 0 25 50
4350 G10
75 100
RGAIN = 25k
(VR+ – VR–) = 100mV
VCC = 5V
VCC (V)
0
10
11
13
6 10
4350 G11
9
8
2 4 8 12 14
7
6
12
∆VGATE (V)
TA = 25°C
TEMPERATURE (°C)
–50
11.0
∆VGATE (V)
11.5
12.0
12.5
13.0
–25 0 25 50
4350 G12
75 100
VCC = 5V
LTC4350
6
4350fb
pin FuncTions
GND (Pin 8): Chip Ground.
FB (Pin 9): Analog Error Amplifier Input (E/A1). This pin is
used to monitor the output supply voltage with an external
resistive divider. The FB pin voltage is compared to 1.220V
reference. The difference between the FB pin voltage and
the reference is amplified and output on the COMP1 pin.
RSET (Pin 10): Analog Output Pin. The IOUT amplifier
converts the voltage at the COMP2 pin (down a diode
voltage) to the RSET pin. Therefore, the current through
the external resistor (RSET) placed between the RSET pin
and ground is (COMP2 – VDIODE)/RSET. This current is
used to adjust the output voltage.
IOUT (Pin 11): Analog Output Pin. The current flowing into
the IOUT pin is equal to the current flowing out of the RSET
pin that was set by the external resistor RSET. This current
is used to adjust the output supply voltage by modifying
the voltage sensed by the power supply’s internal voltage
feedback circuitry.
R– (Pin 12): Analog Input Pin. With a sense resistor placed
in the supply path between the R+ and R– pins, the power
supply current is measured as a voltage drop between R+
and R–. This voltage is measured by the ISENSE block and
multiplied at the GAIN pin.
R+ (Pin 13): Analog Input Pin. With a sense resistor placed
in the supply path between the R+ and R– pins, the power
supply current is measured as a voltage drop between R+
and R–. This voltage is measured by the ISENSE block and
multiplied at the GAIN pin.
GATE (Pin14): The high side gate drive for the external
N-Channel power FET. An internal charge pump provides
the gate drive necessary to drive the FETs. The slope of
the voltage rise or fall at the GATE is set by an external
capacitor connected between GATE and GND, and the
10µA charge pump output current. When the undervoltage
lockout circuit monitoring VCC trips, the OV pin is pulled
high or the UV pin is pulled low, the GATE pin is immedi-
ately pulled to GND.
STATUS (Pin 15): Open-Drain Digital Output. The STATUS
pin has an open-drain output to GND. This pin is pulled
low to indicate a fault has occurred in the system. There
are three types of faults. The first is a undervoltage lockout
on VCC or the UV pin is low while the output voltage is
active. The second is when the COMP2 pin is above 1.5V
or below 0.5V and the voltage on the GAIN pin is greater
than 100mV. The final failure is when the OV pin is high.
The three faults will activate the pull-down on the STATUS
pin after a timing cycle.
VCC (Pin 16): The Positive Supply Input, Ranging from
3.3V to 12V for Normal Operation. ICC is typically 1.6mA.
An undervoltage lockout circuit disables the chip until
the voltage at VCC is greater than 2.47V. A 0.1µF bypass
capacitor is required on the VCC pin. If the VCC pin is tied
to the same power supply output that is being adjusted,
then a 51Ω decoupling resistor is needed to hold up the
supply during a short to ground on the supply output.
VCC must be greater than or equal to the supply that is
connected to the R+ and R– pins.
LTC4350
7
4350fb
block DiagraM
REF
GATE
GAIN
COMP1
FB
VCC
REVERSE CURRENT
9
UV
1
OV
2
6
CHARGE
PUMP
1416
GND
8
R+
13
R–
12
4
SB
20k
7
COMP2 5
IOUT 11
RSET
TIMER
2µA/6µA
4350 BD
R+
30mV
R–
REF
REF
3
STATUS 15
OVER/UNDER
CURRENT
10
–
+
–
+
–
+–
+
–
+
–
+
–+
ISENSE
Ω
gm = 1m
IOUT
LOGIC
E/A1
E/A2
+–
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.
CURRENT SHARING
The current sharing components will now be discussed.
Figure 3 shows a simplified block diagram of these compo-
nents. The ISENSE 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 ISENSE 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
IOUT 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.
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 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
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).
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
to the output voltage. If a small valued resistor, ROUT, is
placed in series with the power supply sense line, a voltage
drop across ROUT 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 IOUT block converts the E/A2 output (COMP2 pin)
to a current that flows through ROUT (see Figure 3). As
the voltage at COMP2 increases, the current in ROUT
Figure 3. Simplified Block Diagram
GATE
DRIVE
RSENSE
REF
SHARE BUS
4350 F03
IADJ
SENSE+
IOUT
PIN
OUT+
CG
LOAD
ROUT+–
THIS RESISTOR CONVERTS IADJ
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 (IADJ)
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
–
+
–
+
–
+
ISENSE
RGAIN
E/A1
COMP1 PIN
FB PIN
COMP2 PIN
E/A2
–
+
IOUT
RSET PIN
RSET
4
3
21
20k
Ω
gm = 1m
LTC4350
10
4350fb
increases. The output voltage will then increase by an
amount equal to the voltage drop across ROUT
. The external
resistor, RSET
, sets the voltage to current relationship in
the IOUT block. The current in ROUT is defined as IADJ =
(VCOMP2 – 0.58V)/RSET.
The maximum voltage that can be applied across RSET is
1V. The range of the output voltage adjustment is set to
be VMAXADJ = ROUT/RSET. This sets the worst-case output
voltage if the share bus is accidentally shorted to VCC. As
mentioned previously, this range is set to be 2% to 10%
in value.
The compensation elements, CCP1 and CCP2, are used
to set the crossover frequencies of the two error ampli-
fiers E/A1 and E/A2. In the Design Example section, the
calculations for choosing all of the components will be
discussed.
Output Adjust Soft-Start
In the LTC4350, there is soft-start circuitry that holds the
COMP2 pin at ground until both the GATE pin is 4V above
the VCC pin and a timer cycle is completed following the
UV pin becoming active.
Upon power-up, most of the circuitry is active including
the circuits that monitor and adjust the output voltage.
The external power FETs are initially open circuit when
power is applied. It takes about 10ms to 100ms for the
FETs to transition from the off to the fully on state (as
discussed in the following Hot Swapping section). During
this time the FB pin is near ground which forces the SB
to the positive rail. The COMP2 pin is then forced to the
positive rail, which forces the RSET pin to 1V. The volt-
age at the output of the power supply is now adjusted to
its maximum adjusted value, which can be 10% above
nominal. Once the power FETs are turned on, the load
will see this adjusted output voltage. This appears to be
a voltage overshoot at the load that exists until the loop
can correct itself. The dominant pole in the loop exists
on the COMP2 pin. Therefore, the overshoot duration is
determined by the discharge time of the COMP2 pin.
In order to eliminate this overshoot, the COMP2 pin is
clamped at ground until the GATE pin is 4V above the VCC
pin (power FETs are turned on). Now, the COMP2 pin will
begin to charge up until the FB pin regulates at 1.220V.
In cases where the power FETs are turned on but the
power supply is still ramping up, the load voltage may
overshoot. For these cases, the COMP2 pin is clamped to
ground during one timing cycle. If the UV pin is greater
than 1.244V, the chip begins the timer cycle. The timer
cycle uses a 2µA current source into an external capacitor
on the TIMER pin. As soon as the voltage at the TIMER
pin exceeds 1.220V, the timer cycle is over. The time-out
is defined as t = CT • 1.220V/2µA. At the end of the timer
cycle, the power supply ramping should be complete.
Faults
There are several types of power supply output faults.
Shorts from the output to ground or to a positive voltage
greater than the normal output voltage are considered
“hard faults.” These faults require the bad power supply
to be immediately disconnected from the load in order to
prevent disruption of the system. “Soft faults” include
power supply failed open-circuit or load current sharing
failure where the output voltage is normal but load sharing
between several supplies is not equal. The LTC4350 can
isolate soft and hard faults and signal a system controller
using the STATUS pin.
HARD FAULTS
The LTC4350 can identify faults in the power supply and
isolate them from the load if optional external power FETs
are included between the power supply and the load. In
the case of a power supply output short to ground, the
reverse current block will sense that the voltage across
the current sense resistor has changed directions and has
exceeded 30mV for more than 5µs. The gate of the external
power FETs is immediately pulled low disconnecting the
short from the load. The gate is allowed to ramp-up and
turn-on the power FETs as soon as the reverse voltage
across the sense resistor is less than 30mV.
The condition where a power supply output shorts to a
high voltage is referred to as an overvoltage fault. In this
case, the gate of the power FETs is pulled low disconnect-
ing the overvoltage from the load. This feature uses the
OV pin to monitor the power supply output voltage. Once
the voltage on the OV pin exceeds the 1.220V threshold,
the gate of the external power FETs is pulled low.
applicaTions inForMaTion
LTC4350
11
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applicaTions inForMaTion
A timer is started as soon as the OV pin exceeds 1.220V.
The timer consists of a 6µA current source into an external
capacitor on the TIMER pin. As soon as the voltage on the
TIMER pin exceeds 1.220V, the STATUS pin is pulled low.
There are two external power FETs in Figure 3. The FET
with its drain on the power supply side (left) and its source
on the load side (right) is used to block high voltage faults
from the load. If overvoltage protection is not needed, this
FET is omitted. Likewise, the FET with its drain on the load
side (right) can be eliminated if protection from a ground
short is not needed. The other use for the power FETs is
to allow hot swapping of the power supply. Hot swapping
will be discussed in a later section.
SOFT FAULTS
The existence of a share bus that forces tight regulation
of the system output voltage allows the system to detect
if the load current is not sharing properly. As mentioned
previously, the output of E/A2 will adjust until the measured
current equals the share bus value. If the power supply
output fails to share properly, the E/A2 output will hit the
plus or minus supply. The LTC4350 uses the over/under
current block to monitor the E/A2 output. This block
signals the logic that a soft fault has occurred if the E/A2
output goes out of the normal 0.5V to 1.5V range where
the IOUT block is active. After a timer cycle, the STATUS pin
indicates a soft fault. The timer consists of a 2µA current
source into an external capacitor on the TIMER pin. As
soon as the voltage on the TIMER pin exceeds 1.220V, the
STATUS pin is pulled low.
The fault indication at the STATUS pin is disabled under
one condition. The E/A2 output can be less than 0.5V
when the load currents are low. In this case, it is desired
to disable the soft fault indication until the current is
higher. Higher current is defined as when the GAIN pin is
greater than 100mV.
The most common situations for soft faults are a discon-
nected power supply and the share bus shorts to VCC or
ground.
HOT SWAPPING
The LTC4350 controls external power FETs to allow power
supplies to be hot swapped in and out of the powered
system without disturbing the power buses. The gate of
the power FETs are slowly ramped up. This slowly charges
the power supply input and output capacitors, preventing
the large inrush currents associated with capacitors being
hot plugged into power buses.
When power is first applied to the VCC pin, the gate of
the power FET is pulled low. As soon as VCC rises above
the undervoltage lockout threshold, the chip’s UV pin is
functional. A 0.1µF bypass capacitor is required on the VCC
pin. If the VCC pin is tied to the same power supply output
that is being adjusted, then a 51Ω decoupling resistor is
needed to hold up the supply during a short to ground
on the supply output.
If the UV pin is greater than 1.244V, the gate of the external
FETs is charged with a 10µA current source. The voltage at
the GATE pin begins to rise with a slope equal to 10µA/CG
(Figure 4), where CG is the external capacitor connected
between the GATE pin and GND. This slow charging al-
lows the power supply output to begin load sharing in a
nondisruptive manner.
Figure 4. Supply Turn-On
VCC + 10V
VCC
4350 F04
t1t2
GATE
VOUT
SLOPE = 10µA/CG
LTC4350
12
4350fb
Figure 5. 5V Load Share (20A per Module)
When the power supply is disconnected, the UV pin will
drop below 1.220V if the supply is loaded. The LTC4350
then discharges the gate of the power FET isolating the
load from the power supply.
DESIGN EXAMPLE
Load Share Components
This section demonstrates the calculations involved in
selecting the component values. The design example in
Figure 5 is a 5V output. This design can be extended to
each of the parallel sections.
The first step is to determine the final output voltage and
the amount of adjustment on the output voltage. The power
supply voltage before the load sharing needs to be lower
than the final output voltage. If the load is expecting to see
a 5V output, then all of the shared power supplies need
to be trimmed to 4.90V or lower. This allows 2% variation
in component and reference tolerances so that the output
always starts below 5V.
Now that the output voltage is preset below the desired
output, the LTC4350 will be responsible for increasing the
output utilizing the SENSE+ input to the power supply. If
a SENSE+ line is not available, then the feedback divider
at the module’s error amplifier can be used. The next step
is to determine the maximum positive adjustment needed
for each power supply. This adjustment includes any I • R
drops across sense resistors, power FETs, wiring and
connectors in the supply path between the power supply
and the load. For example, if the maximum current is 10A
and the parasitic resistance between the power supply and
load is 0.01Ω, then the positive adjustment range for I • R
drops is 0.1V. Since the starting voltage is 4.9V ± 0.1V,
then the lowest starting voltage can be 4.8V. This voltage
is 0.2V below the target. The total adjustment range that
the LTC4350 will need for this example is 0.1V + 0.2V =
0.3V. Note that the lowest starting voltage should not be
lower than 300mV below the target voltage.
The I • R drops should be designed to be low to elimi-
nate the need for additional bulk capacitance at the load.
In most cases the bulk capacitance exists at the power
supply output before the I • R drops. If a 0.002Ω sense
resistor is used and the FET resistance is below 0.003Ω,
then a total 0.005Ω series resistance is acceptable for
loads to 20A. Obviously, the FB pin compensates for the
DC output impedance, but the AC output impedance is the
I • R drops plus the ESR of the capacitors.
4
3
21
RGAIN
86.6k
RSET
100Ω
GAIN
RSET
IOUT R+
R–
FB
TIMER CT
0.1µF
STATUS STATUS
4350 F05
SB
COMP1
VCC GATE
GND
COMP2
LTC4350
UV
OV CP1
1000pF
CP2
1µF
RP1
150Ω
43.2k274k
12.1k121k
CG
0.1µF
0.1µF
OUT+
SENSE+
4.9V NOMINAL, 5.3V MAXIMUM
RG
100Ω
RSENSE
0.002Ω
37.4k
5V
BUS
12.1k
ROUT
30Ω 51Ω
4s SUD50N03-07
(0.007Ω EACH)
SHARE
BUS
CUV
0.1µF
applicaTions inForMaTion
LTC4350
13
4350fb
applicaTions inForMaTion
The resistors ROUT and RSET set the adjustment range.
The voltage on RSET is translated to a voltage on ROUT
by the ratio of ROUT/RSET
. Therefore, the adjustment on
the output voltage will track the voltage at the RSET pin
which is also the voltage on the COMP2 pin minus a diode
voltage. The expression is VADJ = (VRSET) • ROUT/RSET =
(VCOMP2 – VDIODE) • ROUT/RSET. The maximum voltage at
VRSET is limited to 1V. The maximum adjustment on the
output is expressed as VADJMAX = ROUT/RSET. A normal
value for RSET is in the 50Ω to 100Ω range.
If we set RSET to be 100Ω, then an ROUT of 100Ω allows
the output voltage a full 1V adjustment. For the 0.3V range
in this example, the ROUT is 30Ω. In some power modules,
there already exists a resistor between the SENSE+ line
and the power output. In this case, the value of ROUT is the
parallel combination of two resistors, one in the module
and one placed between the SENSE+ and output terminals
of the module.
The value of the gain setting resistor, RGAIN, depends on
the maximum voltage drop across the sense resistor and
the supply voltage VCC for the chip. The highest possible
voltage at the GAIN pin is 1.5V from the VCC voltage.
The maximum voltage on the GAIN pin is expressed as:
VGAINMAX = RSENSE • IMAX • RGAIN/1k = VCC – 1.5V. The
expression for RGAIN: RGAIN = (VCC – 1.5V) • 1k/(RSENSE •
IMAX). In this example, VCC is 5V, IMAX is 20A and RSENSE
is 0.002Ω. Therefore, RGAIN is 87.5k but using 1% values
results in 86.6k.
The FB pin divider provides a 1.220V output for a 5V
input. The precision of the FB pin divider resistors will
impact the accuracy of the final output voltage. The UV
resistive divider in this example, turns on the gate when
VCC increases above 4V. This corresponds to the UV pin at
1.220V. The capacitor CUV prevents false activation during
load steps. The OV set point needs to occur above the
adjustment max for VCC. The power supply output (which
also is VCC), can start as high as 5V and adjust upwards
to 5.3V. The OV set point in this example is 5.5V on VCC
when the OV pin is at 1.220V.
The timer capacitor CT is set to be 0.1µF for a 61ms
timer cycle. The expression is t = CT • 1.22V/2µA. The
gate capacitor CG is set to be 0.1µF which sets a slope
of 10µA/CG or 1V every 10ms. In this case, the GATE pin
must charge up to 9V before the output can ramp to 5V
which happens in 90ms. In this case, the output adjust
soft-start turns on when the gate ramps above 9V. The
soft-start circuitry releases the COMP2 pin allowing the
load sharing loop to function. A 100Ω resistor RG prevents
high frequency oscillations from the power FETs at their
turn-on threshold. A 0.1µF bypass capacitor is required on
the VCC pin. If the VCC pin is tied to the same power sup-
ply output that is being adjusted, then a 51Ω decoupling
resistor is needed to hold up the supply during a short to
ground on the supply output.
COMPENSATION
The compensation capacitor, CP1, is needed to set the
crossover frequency of the feedback error amplifier E/A1.
The crossover frequency of 200kHz is adequate for most
applications and requires CP1 to be 1000pF (0.001µF).
The design of the other compensation capacitor will require
some knowledge about the power supply’s bandwidth. The
bandwidth can be measured easily. First, use a storage
oscilloscope to monitor the power supply output voltage.
Then place a 1A resistive fixed load and switch in a second
resistive load that increases the total load current close
to rated maximum. Tapping the second resistor (with
the correct power rating) to the power supply output
creates this load step. Trigger the scope on the falling
edge of the output voltage as it drops more than 100mV
(for example from 5V to 4.8V). The recovery time, tR,
from the step needs to be measured. tR is defined as
the 10% to 90% time measurement (see Figure 6). The
VOUT (t)
4350 F06
t
tr
0.1∆V
∆V
0.1∆V
90%
10%
Figure 6. tR Measurement
LTC4350
14
4350fb
compensation capacitor CP2 can be looked up in Table 1
using tR. The value for the zero setting resistor, RP1, is
150Ω. This value guarantees the zero is at or above the
crossover frequency.
Table 1
tRfC = 0.35/tRCP2
5µs 70kHz 0.1µF
10µs 35kHz 0.22µF
20µs 17.5kHz 0.47µF
40µs 8.8kHz 1µF
60µs 5.8kHz 1.5µF
80µs 4.4kHz 2.2µF
100µs 3.5kHz 2.7µF
150µs 2.3kHz 3.3µF
200µs 1.8kHz 4.7µF
300µs 1.2kHz 6.8µF
400µs 0.9kHz 10µF
500µs 0.7kHz 12µF
OTHER APPLICATIONS
The application shown on the first page of this data sheet
assumes that the power supplies and the load reside on
one main board. If the system is a true N + 1 hot swappable
power supply, then the LTC4350 will reside with the power
supply on a daughter card that plugs into the main board.
In this case, the input and output capacitors need to be
hot swapped (see Figure 7). The output capacitors are Hot
Swap protected by the LTC4350. The input capacitors are
Hot Swap protected using the LT
®
4250. Other Hot Swap
parts are described in Table 2.
Table 2
VOLTAGE RANGE PART NUMBER
3.3V to 12V LTC1422 Single Channel
LTC1645 Dual Chanel
3.3V to 15V LTC1642 Overvoltage Protection
2.7V to 16.5V LTC1647 Dual Channel
9V to 80V LT1641 Positive High Voltage
–20V to –80V LT4250 Negative High Voltage
In some cases, the output voltage is below the undervoltage
lockout of the LTC4350. In this case, an external supply
of 3.3V or greater needs to provide for the chip. Figure 8
shows a 1.5V output redundant power supply that uses
24V to 1.5V switching power supplies. The VCC pin of
the LTC4350 can be driven from the INTVCC pin of the
LTC1629.
applicaTions inForMaTion
LTC4350
15
4350fb
applicaTions inForMaTion
4
3
21
RGAIN
RSET
GAIN
RSET
IOUT R+
R–
FB
TIMER
CT
STATUS
SB
COMP1
VCC GATE
GND
COMP2
LTC4350
UV
OV
CP1
CP2
RP2
CG
RG
RSENSE
ROUT
RTN–48V RTN–48V RTN
–48V
LOAD
–48V
GND
SB
STATUS
3.3VOUT
–48V
CONNECTOR
4350 F07
3.3V
POWER SUPPLY 1
OUT+
VIN+
VIN–
VDD
LT4250L DRAIN
OV
UV
GATESENSEVEE SENSE+
SENSE–
OUT–
43
21
4
3
21
RGAIN
RSET
GAIN
RSET
IOUT R+
R–
FB
TIMER
CT
STATUS
SB
COMP1
VCC GATE
GND
COMP2
LTC4350
UV
OV
CP1
CP2
RP2
CG
RG
RSENSE
ROUT
RTN–48V RTN
–48V
GND
SB
STATUS
3.3VOUT
–48V
CONNECTOR
3.3V
POWER SUPPLY 2
OUT+
VIN+
VIN–
ON/OFF
PWRGD
VDD
LT4250L DRAIN
OV
UV
GATESENSEVEE SENSE+
SENSE–
OUT–
43
21
CUV
CUV
ON/OFF
PWRGD
Figure 7. –48V to 3.3V Hot Swap Power Supply
LTC4350
16
4350fb
package DescripTion
GN16 (SSOP) 0204
1 2 345678
.229 – .244
(5.817 – 6.198)
.150 – .157**
(3.810 – 3.988)
16 15 14 13
.189 – .196*
(4.801 – 4.978)
12 11 10 9
.016 – .050
(0.406 – 1.270)
.015 ± .004
(0.38 ± 0.10) × 45°
0° – 8° TYP
.007 – .0098
(0.178 – 0.249)
.0532 – .0688
(1.35 – 1.75)
.008 – .012
(0.203 – 0.305)
TYP
.004 – .0098
(0.102 – 0.249)
.0250
(0.635)
BSC
.009
(0.229)
REF
.254 MIN
RECOMMENDED SOLDER PAD LAYOUT
.150 – .165
.0250 BSC.0165 ±.0015
.045 ±.005
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
INCHES
(MILLIMETERS)
NOTE:
1. CONTROLLING DIMENSION: INCHES
2. DIMENSIONS ARE IN
3. DRAWING NOT TO SCALE
GN Package
16-Lead Plastic SSOP (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1641)
LTC4350
17
4350fb
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
revision hisTory
REV DATE DESCRIPTION PAGE NUMBER
B 3/10 Not Recommended for New Designs 1
(Revision history begins at Rev B)
LTC4350
18
4350fb
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com
LINEAR TECHNOLOGY CORPORATION 2001
LT 0310 REV B • PRINTED IN USA
relaTeD parTs
RGAIN
RSET
GAIN
RSET
IOUT R+
R–
FB
TIMER
CT
STATUS
SB
COMP1
VCC GATE
GND
CUV COMP2
LTC4350
UV
OV
CP1
CP2
RP1
CG
RG
RSENSE
ROUT
CIN COUT
CUV
VIN
24VIN
GND
VIN
24V
LOAD
GND
SB
1.5VOUT
STATUS
CONNECTOR
1.5V
LTC1629
VOS+
INTVCC
VCC SENSE
LT1641
ON
GATE
TIMER GND
RGAIN
RSET
GAIN
RSET
IOUT R+
R–
FB
TIMER
CT
STATUS
SB
COMP1
VCC GATE
GND
COMP2
LTC4350
UV
OV
CP1
CP2
RP1
CG
RG
RSENSE
ROUT
CIN COUT
VIN
24VIN
GND GND
SB
VOUT
STATUS
CONNECTOR
1.5V
LTC1629
VOS+
INTVCC
VCC SENSE
LT1641
ON
GATE
TIMER GND
PART NUMBER DESCRIPTION COMMENTS
LTC1421 Hot Swap Controller Multiple Supplies from 3V to 12V and –12V
LT1640AL/LT1640AH Negative Voltage Hot Swap Controllers Negative High Voltage Supplies from –10V to –80V
LT1641 Positive Voltage Hot Swap Controller Positive High Voltage Supplies from 9V to 90V
LTC1645 2-Channel Hot Swap Controller Operates from 1.2V to 12V, Power Sequencing
LTC1646 Dual CompactPCI Hot Swap Controller 3.3V/5V Only with Precharge and Local Reset Logic
LTC1647-1/LTC1647-2 Dual Hot Swap Controllers Dual ON Pins, Operates from 2.7V to 16.5V
LTC4211 Hot Swap Controller with Multifunction Circuit Breaker 2.5V to 16.5V Supplies and RESET Output
LTC4251 –48V Hot Swap Controller in ThinSOT™ Active Current Limiting, –15V to –100V Supplies
Figure 8. 24V to 1.5V Hot Swap Power Supply
applicaTions inForMaTion