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LTC4354
1
4354f
TYPICAL APPLICATIO
U
APPLICATIO S
U
FEATURES
DESCRIPTIO
U
The LTC
4354 is a negative voltage diode-OR controller
that drives two external N-channel MOSFETs. It replaces
two Schottky diodes and the associated heatsink, saving
power and area. The power dissipation is greatly reduced
by using N-channel MOSFETs as the pass transistors.
Power sources can easily be ORed together to increase
total system power and reliability.
When first powered up, the MOSFET body diode conducts
the load current until the pass transistor is turned on. The
LTC4354 servos the voltage drop across the pass transis-
tors to ensure smooth transfer of current from one tran-
sistor to the other without oscillation.
The MOSFETs are turned off in less than 1s whenever the
corresponding power source fails or is shorted. Fast turn-
off prevents the reverse current from reaching a level that
could damage the pass transistors.
A fault detection circuit with an open drain output capable
of driving an LED or opto-coupler indicates either MOSFET
short, MOSFET open or supply failed.
AdvancedTCA
Systems
48V Distributed Power Systems
Computer Systems/Servers
Telecom Infrastructure
Optical Networks
, LTC and LT are registered trademarks of Linear Technology Corporation.
Controls N-Channel MOSFETs
Replaces Power Schottky Diodes
1s Turn-off Time Limits Peak Fault Current
80V Operation
Smooth Switchover Without Oscillation
No Reverse DC Current
Fault Output
Selectable Fault Thresholds
Available in 8-Pin (3mm 2mm) DFN and 8-Pin SO
Packages
Negative Voltage
Diode-OR Controller
and Monitor
48V Diode-OR
LTC4354
LOAD
DB
GA
DA
GB
V
SS
V
B
= 48V
V
A
= 48V
48V_RTN
FAULT
IRF3710
IRF3710
4354 TA01
V
CC
33k
12k
2k
2k
LED
1F
Power Dissipation vs Load Current
CURRENT (A)
0
0
POWER DISSIPATION (W)
1
2
3
4
6
5
2
4
6
4354 TA01b
8
10
DIODE (MBR10100)
FET (IRF3710)
POWER
SAVED
AdvancedTCA is a registered trademark of the PIC Industrial Computer Manufacturers Group.
LTC4354
2
4354f
TOP VIEW
DDB8 PACKAGE
8-LEAD (3mm 2mm) PLASTIC DFN
EXPOSED PAD (PIN 9)
CONNECTION TO PCB OPTIONAL
5
6
7
8
9
4
3
2
1
DA
V
SS
V
CC
GA
DB
FAULT
GB
V
SS
I
CC
(100s duration) ............................................. 50mA
Output Voltages
GA, GB ....................................... 0.3V to V
CC
+ 0.3V
FAULT .................................................... 0.3V to 7V
Input Voltages
DA, DB .................................................. 0.3V to 80V
Input Current
DA, DB, Reverse Current ................................. 20mA
ORDER PART
NUMBER
T
JMAX
= 125C,
JA
= 76C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
LTC4354CDDB
LTC4354IDDB
ABSOLUTE AXI U
RATI GS
W
W
W
U
PACKAGE/ORDER I FOR ATIO
U
U
W
(Note 1)
Operating Temperature Range
LTC4354C ............................................... 0C to 70C
LTC4354I ............................................. 40C to 85C
Storage Temperature Range ................. 65C to 150C
Lead Temperature (Soldering, 10 sec.)................. 300C
ORDER PART
NUMBER
T
JMAX
= 125C,
JA
= 150C/W
LTC4354CS8
LTC4354IS8
1
2
3
4
8
7
6
5
TOP VIEW
DB
FAULT
GB
V
SS
DA
V
SS
V
CC
GA
S8 PACKAGE
8-LEAD PLASTIC SO
DDB8 PART
MARKING
LBBK
LBMB
S8 PART
MARKING
4354
4354I
LTC4354
3
4354f
ELECTRICAL CHARACTERISTICS
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: I
CC
is defined as the current level where the V
CC
voltage is lower
by 100mV from the value with 2mA of current.
The
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25C. I
CC
= 5mA, V
SS
= 0V unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
Z
Internal Shunt Regulator Voltage
I
CC
= 5mA
10.25
11
11.75
V
V
Z
Internal Shunt Regulator Load Regulation
I
CC
= 2mA to 10mA
200
300
mV
V
CC
Operating Voltage Range
4.5
V
Z
V
I
CC
V
CC
Supply Current
V
CC
= (V
Z
0.1V), Note 2
1.2
2
mA
V
CC
= 5V
0.5
0.8
1.1
mA
V
GATE
GATE Pins Output High Voltage
V
CC
= 10.25V
10
10.25
V
V
CC
= 5V
4.75
V
I
GATE
GATE Pins Pull-Up Current
V
SD
= 60mV; V
GATE
= 5.5V
15
30
60
A
GATE Pins Pull-Down Current
V
SD
= 0V; V
GATE
= 5.5V
15
30
60
A
V
SD
Source Drain Sense Threshold Voltage
(V
SS
V
DX
)
10
30
55
mV
V
SD(FLT)
Source Drain Fault Detection Threshold
(V
SS
V
DX
)
200
260
320
mV
t
OFF
Gate Turn-Off Time in Fault Condition
C
GATE
= 3300pF; V
GATE
2V; V
OVERDR
= 0.5V
0.7
1.2
s
V
FAULT
FAULT Pin Output Low
I
FAULT
= 5mA
200
400
mV
I
FAULT
FAULT Pin Leakage Current
V
FAULT
= 5V
1
A
I
D
Drain Pin Input Current
V
DX
= 0V
3.5
2.5
1.5
A
V
DX
= 80V
1.1
1.5
1.9
mA
Note 3: An internal shunt regulator limits the V
CC
pin to less than 12V
above V
SS
. Driving this pin to voltages beyond the clamp may damage the
part.
Note 4: All currents into pins are positive; all voltages are referenced to
V
SS
unless otherwise specified.
LTC4354
4
4354f
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
Specifications are at T
A
= 25C, I
CC
= 5mA, V
SS
= 0V
unless otherwise noted.
Shunt Regulator Voltage vs Input
Current
Shunt Regulator Voltage vs Input
Current at Temperature
Source Drain Sense Voltage vs
Supply Voltage
Source Drain Sense Voltage vs
Temperature
Gate Turn-Off Time vs
Temperature
Fault Threshold Voltage vs
Temperature
I
GATE(UP)
vs V
SD
Drain Pin Current vs Temperature
Drain Pin Current vs Temperature
I
CC
(mA)
0
V
Z
(V) 11.0
20
4354 G01
10.0
11.5
10.5
10
5
15
12.0
TEMPERATURE (C)
50
V
Z
(V)
11.4
11.0
10.6
11.2
10.8
25
0
25
4354 G02
50
75
100
125
I
CC
= 10mA
I
CC
= 5mA
I
CC
= 2mA
V
CC
(V)
5
20
V
SD
(mV)
30
25
35
7
9
11
12
4354 G03
40
6
8
10
TEMPERATURE (C)
50
V
SD
(mV)
40
30
20
35
25
25
0
25
4354 G04
50
75
100
125
TEMPERATURE (C)
50
t
OFF
(ns)
740
700
660
720
680
25
0
25
4354 G05
50
75
100
125
TEMPERATURE (C)
50
V
SD(FLT)
(mV)
290
250
210
270
230
25
0
25
4354 G06
50
75
100
125
V
SD
(mV)
30
I
GATE(UP)
(
A)
100
40
60
0
80
20
40
50
60
4354 G07
70
80
90
TEMPERATURE (C)
50
I
D
(
A)
3.2
2.8
2.4
3.0
2.6
25
0
25
4354 G08
50
75
100
125
V
DX
= 0V
TEMPERATURE (C)
50
I
D
(mA)
1.60
1.50
1.55
1.45
1.40
25
0
25
4354 G09
50
75
100
125
V
DX
= 80V
LTC4354
5
4354f
PI FU CTIO S
U
U
U
DA, DB (Pins 1, 8): Drain Voltage Sense Inputs. These
pins sense source-drain voltage drop across the N-Chan-
nel MOSFETs. An external resistor is recommended to
protect these pins from transient voltages exceeding 80V
in extreme fault conditions. For Kelvin sensing, connect
these pins as close to the drains as possible. Connect to
V
SS
if unused.
V
CC
(Pin 3): Positive Supply Voltage Input. Connect this
pin to the positive side of the supply through a resistor. An
internal shunt regulator that can sink up to 20mA typically
clamps V
CC
at 11V. Bypass this pin with a 1F capacitor to
V
SS
.
GA, GB (Pins 4, 6): Gate Drive Outputs. Gate pins pull high
to 10V minimum, fully enhancing the N-Channel MOSFET,
when the load current creates more than 30mV of drop
across the FET. When the load current is small, the gates
are actively servoed to maintain a 30mV drop across the
MOSFET. If reverse current develops more than 60mV of
voltage drop across the MOSFET, the pins pull low to V
SS
in less than 1s. Quickly turning off the pass transistors
prevents excessive reverse currents. Leave the pins open
if unused.
V
SS
(Pins 2, 5): Negative Supply Voltage Input. This is the
device negative supply input and connects to the common
source connection of the N-Channel MOSFETs. It also
connects to the source voltage sense input of the servo
amplifiers. For Kelvin sensing, connect pin 5 as close to
the common source terminal of the MOSFETs as possible.
FAULT (Pin 7): Fault Output. Open drain output that
normally pulls the FAULT pin to V
SS
and shunts current to
turn off an external LED or optocoupler. In the fault
condition, where the pass transistor is fully on and the
voltage drop across it is higher than the fault threshold, the
FAULT pin goes high impedance, turning on the LED or
optocoupler. This indicates that one or both of the pass
transistors have failed open or failed short creating a cross
conduction current in between the two power supplies.
Connect to V
SS
if unused.
EXPOSED PAD (Pin 9): Exposed pad is common to V
SS
and may be left open or connected to pins 2 and 5.
FU CTIO AL DIAGRA
U
U
W
DB
GB
DA
GA
30mV
BV = 11V
30mV
V
SS
V
CC
V
SS
4354 FD01
V
SS
FAULT
+
+
+
+
FAULT DETECTION
AMP B
AMP A
5
4
1
6
8
2
7
3
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