LTC4354
Negative Voltage
Diode-OR Controller
and Monitor
FEATURES
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DESCRIPTIO
Controls N-Channel MOSFETs
Replaces Power Schottky Diodes
1µs 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
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 1µs 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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
AdvancedTCA is a registered trademark of the PIC Industrial Computer Manufacturers Group.
APPLICATIO S
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AdvancedTCA
®
Systems
–48V Distributed Power Systems
Computer Systems/Servers
Telecom Infrastructure
Optical Networks
TYPICAL APPLICATIO
–48V_RTN
12k
–48V Diode-OR
6
5
POWER DISSIPATION (W)
33k
V
CC
LTC4354
DA
2k
V
A
= –48V
IRF3710
V
B
= –48V
IRF3710
DB
2k
GA
GB
FAULT
V
SS
1µF
LED
4354 TA01
Power Dissipation vs Load Current
4
3
2
1
FET (IRF3710)
0
0
2
4
6
CURRENT (A)
8
10
4354 TA01b
LOAD
U
DIODE (MBR10100)
POWER
SAVED
4354f
U
U
1
LTC4354
ABSOLUTE
(Note 1)
AXI U RATI GS
Operating Temperature Range
LTC4354C ............................................... 0°C to 70°C
LTC4354I ............................................. –40°C to 85°C
Storage Temperature Range ................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec.)................. 300°C
I
CC
(100µs 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
PACKAGE/ORDER I FOR ATIO
TOP VIEW
DA 1
V
SS
2
V
CC
3
GA 4
9
8 DB
7 FAULT
6 GB
5 V
SS
ORDER PART
NUMBER
LTC4354CDDB
LTC4354IDDB
DDB8 PART
MARKING
LBBK
LBMB
DA 1
V
SS
2
V
CC
3
GA 4
DDB8 PACKAGE
8-LEAD (3mm
×
2mm) PLASTIC DFN
EXPOSED PAD (PIN 9)
CONNECTION TO PCB OPTIONAL
T
JMAX
= 125°C,
θ
JA
= 76°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
2
U
U
W
W W
U
W
TOP VIEW
8
7
6
5
DB
FAULT
GB
V
SS
ORDER PART
NUMBER
LTC4354CS8
LTC4354IS8
S8 PART
MARKING
4354
4354I
S8 PACKAGE
8-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 150°C/W
4354f
LTC4354
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. I
CC
= 5mA, V
SS
= 0V unless otherwise noted.
SYMBOL
V
Z
∆V
Z
V
CC
I
CC
V
GATE
I
GATE
∆V
SD
∆V
SD(FLT)
t
OFF
V
FAULT
I
FAULT
I
D
PARAMETER
Internal Shunt Regulator Voltage
Internal Shunt Regulator Load Regulation
Operating Voltage Range
V
CC
Supply Current
GATE Pins Output High Voltage
GATE Pins Pull-Up Current
GATE Pins Pull-Down Current
Source Drain Sense Threshold Voltage
Source Drain Fault Detection Threshold
Gate Turn-Off Time in Fault Condition
FAULT Pin Output Low
FAULT Pin Leakage Current
Drain Pin Input Current
V
CC
= (V
Z
– 0.1V), Note 2
V
CC
= 5V
V
CC
= 10.25V
V
CC
= 5V
V
SD
= 60mV; V
GATE
= 5.5V
V
SD
= 0V; V
GATE
= 5.5V
(V
SS
– V
DX
)
(V
SS
– V
DX
)
C
GATE
= 3300pF; V
GATE
≤
2V; V
OVERDR
= 0.5V
I
FAULT
= 5mA
V
FAULT
= 5V
V
DX
= 0V
V
DX
= 80V
●
●
●
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
I
CC
= 5mA
I
CC
= 2mA to 10mA
●
●
●
●
MIN
10.25
4.5
0.5
10
4.75
–15
15
10
200
TYP
11
200
1.2
0.8
MAX
11.75
300
V
Z
2
1.1
10.25
UNITS
V
mV
V
mA
mA
V
V
µA
µA
mV
mV
µs
mV
µA
µA
mA
–30
30
30
260
0.7
200
–60
60
55
320
1.2
400
±1
–1.5
1.9
–3.5
1.1
–2.5
1.5
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.
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.
4354f
3
LTC4354
TYPICAL PERFOR A CE CHARACTERISTICS
Shunt Regulator Voltage vs Input
Current
12.0
11.4
11.5
11.0
11.0
∆V
SD
(mV)
V
Z
(V)
V
Z
(V)
10.5
10.0
0
5
10
I
CC
(mA)
15
Source Drain Sense Voltage vs
Temperature
40
100
35
I
GATE(UP)
(µA)
∆V
SD
(mV)
30
t
OFF
(ns)
25
20
–50 –25
50
75
0
25
TEMPERATURE (°C)
Fault Threshold Voltage vs
Temperature
290
–3.2
270
∆V
SD(FLT)
(mV)
250
–2.8
I
D
(mA)
I
D
(µA)
230
210
–50 –25
0
25
50
75
TEMPERATURE (°C)
4
U W
20
4354 G01
Specifications are at T
A
= 25°C, I
CC
= 5mA, V
SS
= 0V
unless otherwise noted.
Source Drain Sense Voltage vs
Supply Voltage
40
Shunt Regulator Voltage vs Input
Current at Temperature
11.2
35
I
CC
= 10mA
I
CC
= 5mA
30
10.8
I
CC
= 2mA
10.6
–50 –25
25
20
0
25
50
75
TEMPERATURE (°C)
100
125
5
6
7
9
8
V
CC
(V)
10
11
12
4354 G02
4354 G03
I
GATE(UP)
vs
∆V
SD
740
Gate Turn-Off Time vs
Temperature
80
720
60
700
40
20
680
0
100
125
30
40
50
70
60
∆V
SD
(mV)
80
90
4354 G07
660
–50 –25
50
75
0
25
TEMPERATURE (°C)
100
125
4354 G04
4354 G05
Drain Pin Current vs Temperature
V
DX
= 0V
1.60
Drain Pin Current vs Temperature
V
DX
= 80V
–3.0
1.55
1.50
–2.6
1.45
100
125
–2.4
–50 –25
0
25
50
75
TEMPERATURE (°C)
100
125
1.40
–50 –25
0
25
50
75
TEMPERATURE (°C)
100
125
4354 G06
4354 G08
4354 G09
4354f
LTC4354
PI FU CTIO S
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 1µF 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 1µs. 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
W
U
U
U
U
U
V
CC
3
BV = 11V
5
V
SS
+
–
+
–
30mV
30mV
+
AMP A
4
GA
–
1
DA
+
AMP B
6
GB
–
FAULT
7
8
DB
FAULT DETECTION
V
SS
2
V
SS
4354 FD01
4354f
5
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