-48V -VOUT
INP/VSS
OFF
INN GATE
LM5051
Shutdown
GND GND
nFGDStatus
VCC
LINE
+
LOAD
-
LM5051
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SNVS702D –OCTOBER 2011–REVISED MARCH 2013
LM5051 Low Side OR-ing FET Controller
Check for Samples: LM5051
1FEATURES DESCRIPTION
The LM5051 Low Side OR-ing FET Controller
2• Wide operating input voltage range: -6V to - operates in conjunction with an external MOSFET as
100V an ideal diode rectifier when connected in series with
• -100V Transient Capability a power source. This OR-ing controller allows
• Gate drive for external N-Channel MOSFET MOSFETs to replace diode rectifiers in power
distribution networks thus reducing both power loss
• MOSFET diagnostic test mode and voltage drops.
• Fast 50ns response to current reversal The LM5051 controller provides gate drive for an
• 2A peak gate turn-off current external N-Channel MOSFET and a fast response
• Package: 8-Lead SOIC comparator to turn off the FET when current flows in
the reverse direction. The LM5051 can connect
APPLICATIONS power supplies ranging from -6V to -100V and can
withstand transients up to -100V.
• Active OR-ing of Redundant (N+1) Power
Supplies The LM5051 also provides a FET test diagnostic
mode which allows the system controller to test for
shorted MOSFETs.
Typical Application Circuits
Figure 1. Full Application with MOSFET Diagnostic
1Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
2All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date. Copyright © 2011–2013, Texas Instruments Incorporated
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
LINE
VCC
nFGD
OFF
GATE
INP/VSS
INP/VSS
INN
1
2
3
45
6
7
8
LM5051
MA
PS1
PS2 INN GATE INP/VSS
INN GATE INP/VSS
RLOAD
LM5051
LINE
LINE
VCC
D
D
S
S
VCC
LM5051
LM5051
SNVS702D –OCTOBER 2011–REVISED MARCH 2013
www.ti.com
Figure 2. Typical Redundant Supply Configuration
Connection Diagram
Top View
Device Pin 5 (INP) is internally connected to Device Pin 7 (VSS)
Figure 3. LM5051MA
8-Lead SOIC D Package
PIN DESCRIPTIONS
Pin # Name Function
Power supply pin to bias the internal 12V zener shunt regulator at the
1 LINE VCC pin through an internal 50 kΩ(typical) series resistor. See the
APPLICATION INFORMATION section.
Connection to the internal 12V zener shunt voltage regulator. Bypass
this pin with minimum 0.1μF capacitor to the VSS pin. This pin can be
2 VCC biased via an external resistor rather than via the internal resistor from
the LINE pin (pin 1). See the APPLICATION INFORMATION section.
FET Test Mode control input. Logic low or open state at the OFF pin
will deactivate the FET Test Mode and allow normal operation. A logic
high state at the OFF pin will pull the GATE pin low and turn off the
3 OFF external MOSFET. If the body diode forward voltage of the MOSFET
(from source to drain) is greater than 260mV the nFGD pin will indicate
that the MOSFET is not shorted by pulling to the active low state.
Open drain output for the FET Test circuit. An active low state on nFGD
indicates that the forward voltage (from source to drain) of the external
4 nFGD MOSFET is greater than 260 mV typical. The nFGD pin requires an
external pull-up resistor to a voltage not higher than VSS + 5.5V.
5 INP/VSS See device Pin 7.
6 INN Voltage sense connection to the external MOSFET Drain pin
Internally connected to device Pin 5. Negative supply voltage
connection and MOSFET voltage sense connected to the external
7 INP/VSS MOSFET common source connection. All device voltages and currents
are referenced to this pin, unless otherwise stated. See the INP/VSS
PINS section.
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SNVS702D –OCTOBER 2011–REVISED MARCH 2013
PIN DESCRIPTIONS (continued)
Pin # Name Function
8 GATE Connection to the external MOSFET Gate.
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
Absolute Maximum Ratings (1)
LINE Pin to INP/VSS -0.3V to 103V
INN Pin to INP/VSS -2V to 103V
OFF Pin to INP/VSS -0.3V to 7V
VCC Pin Sink to INP/VSS -0.1mA to 20mA
nFGD Pin to INP/VSS (Off) -0.3V to 7V
Storage Temperature Range −65°C to 150°C
ESD (HBM) (2) ±2 kV
Peak Reflow Temperature (3) 260°C, 30sec
(1) Absolute Maximum Ratings indicate limits beyond which damage to the device may occur, including in-operability and degradation of
device reliability and/or performance. Functional operation of the device and/or non-degradation at the Absolute Maximum Ratings or
other conditions beyond those indicated in the Recommended Operating Conditions is not implied. Operating Range conditions indicate
the conditions at which the device is functional and the device should not be operated beyond such conditions. For ensured
specifications and conditions, see Electrical Characteristics.
(2) The Human Body Model (HBM) is a 100 pF capacitor discharged through a 1.5 kΩresistor into each pin. Applicable test standard is
JESD-22-A114-C.
(3) For soldering specifications see the LM5051 Product Folder at www.national.com, general information at
www.national.com/analog/packaging/, and reflow information at www.national.com/ms/MS/MS-SOLDERING.pdf .
Operating Ratings (1)
Relative to VSS pin
LINE Pin Voltage 36V to 100V
INN Pin Voltage -1V to 100V
VCC Pin Current 1 mA to 10 mA
OFF Pin Voltage 0.0V to 5.0V
nFGD Voltage (Off) 0.0V to 5.0V
nFGD Sink Current (On) 0 mA to 2 mA
Junction Temperature Range (TJ)−40°C to +125°C
(1) Absolute Maximum Ratings indicate limits beyond which damage to the device may occur, including in-operability and degradation of
device reliability and/or performance. Functional operation of the device and/or non-degradation at the Absolute Maximum Ratings or
other conditions beyond those indicated in the Recommended Operating Conditions is not implied. Operating Range conditions indicate
the conditions at which the device is functional and the device should not be operated beyond such conditions. For ensured
specifications and conditions, see Electrical Characteristics.
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SNVS702D –OCTOBER 2011–REVISED MARCH 2013
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Electrical Characteristics
Limits in standard type are for TJ= 25°C only; limits in boldface type apply over the operating junction temperature (TJ)
range of -40°C to +125°C. Minimum and Maximum limits are ensured through test, design, or statistical correlation. Typical
values represent the most likely parametric norm at TJ= 25°C, and are provided for reference purposes only. Unless
otherwise stated all conditions and measurements are referenced to device pin 7 (INP/VSS), and the following conditions
apply: VLINE= 48.0V, VINN= -150 mV, VOFF= 0.0V, CGATE= 47 nF, CVCC= 0.1 µF, and TJ= 25°C.
Symbol Parameter Conditions Min Typ Max Unit
LINE Pin
VLINE = 48.0V
ILINE LINE Pin current - 690 780 μA
VCC Pin = Open
VCC Pin
VCC Operating Voltage Range LINE Pin = Open 4.50 -VZV
IVCC = 2 mA 11.9 13.0 14.3
VZVCC Shunt Zener Voltage V
IVCC = 10 mA 12.5 13.5 14.5
ΔVZShunt Zener Regulation IVCC = 2 mA to 10 mA - 0.50 1.11 V
VVCC = VZ- 100mV - 1.0 1.50
IVCC Supply Current mA
VVCC = 5.0V - 0.4 1.10
INN Pin
VINN = 0.0V - 3.1 -
IINN INN Pin Current μA
VINN = 90V - 0.04 -
GATE
VGATE = 5.5V
GATE Charge Current 0.28 0.66 0.95 mA
VINN = -150mV
IGATE VGATE = 5.5V
GATE Discharge Current VINN = -150 mV to +300 mV 2.4 3.5 - A
t≤10 ms
VLINE = 48.0V - 13.0 -
VGATE GATE Pin High Voltage VVCC = 10.25V, LINE = Open 9.98 10.2 - V
VVCC = 5.0V, LINE= Open 4.70 4.95 -
VINN going negative until Gate
VSD(REV) Reverse Threshold -112.2 -45 +11.4 mV
Drive Turns ON
ΔVSD(REV) Reverse Threshold Hysteresis VINN going positive from
VSD(REV) Threshold until Gate - 50 - mV
Drive Turns OFF
VSD(REG) Regulated VINP/VSS to VINN Threshold -10.8 12 30.8 mV
CGATE = 0 (1) - 34 50
Gate Capacitance Discharge Time at
tGATE(REV) Forward to Reverse Transition CGATE = 10 nF (1) - 60 - ns
See Figure 6 CGATE = 47 nF (1) - 90 230
Gate Capacitance Discharge Time at
tGATE(OFF) OFF pin Low to High Transition CGATE = 47 nF (2) - 120 - ns
See Figure 7
OFF Pin
VINN = -400 mV
VOFF(IH) OFF Input High Threshold Voltage 1.28 1.50 1.65
VOFF Rising until Gate is Low V
VINN = -400 mV
VOFF(IL) OFF Input Low Threshold Voltage - 1.48 -
VOFF Falling until Gate is High
ΔVOFF OFF Threshold Voltage Hysteresis VOFF(IH) - VOFF(IL) - 20 - mV
IOFF(IH) OFF Pin Internal Pull-down VOFF = 5.0V - 4.6 6.00 µA
IOFF(IL) VOFF = 0.0V - -0.03 - µA
(1) Time from VINN voltage transition from -200 mV to +500 mV until Gate pin voltage falls to ≤1.00V. See Figure 6
(2) Time from VOFF voltage transition from 0.0V to 5.0V until GATE pin voltage falls to ≤1.0V. See Figure 7
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VINN
VnFGD
0.0V
0.0 mV
VnFGD
VSD(TST)
ûVSD(TST)
VINN
VGATE
0.0V
0.0 mV
VGATE
VSD(REV)
ûVSD(REV)
VSD(REG)
LM5051
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SNVS702D –OCTOBER 2011–REVISED MARCH 2013
Electrical Characteristics (continued)
Limits in standard type are for TJ= 25°C only; limits in boldface type apply over the operating junction temperature (TJ)
range of -40°C to +125°C. Minimum and Maximum limits are ensured through test, design, or statistical correlation. Typical
values represent the most likely parametric norm at TJ= 25°C, and are provided for reference purposes only. Unless
otherwise stated all conditions and measurements are referenced to device pin 7 (INP/VSS), and the following conditions
apply: VLINE= 48.0V, VINN= -150 mV, VOFF= 0.0V, CGATE= 47 nF, CVCC= 0.1 µF, and TJ= 25°C.
Symbol Parameter Conditions Min Typ Max Unit
FET Test Comparator
VOFF = 5.0V
FET Test Threshold Voltage
VSD(TST) VINN/VSS going negative from -360 -260 -183 mV
VINP - VINN VINP until nFGD pin goes Hi-Z
VOFF = 5.0V
VINN going positve from
ΔVSD(TST) FET Test Threshold Voltage Hysteresis - 6.5 - mV
VSDT(ST) until nFGD pin goes
Lo-Z
nFGD Pin
nFGD Output Low Voltage VOFF = 5V
nFGDVOL - 285 450 mV
nFGD Output = On InFGD = 1mA Sinking
nFGD Output Leakage Current VOFF = 0V
nFGDIOL - 0.01 0.7 µA
nFGD Output = Off VnFGD = 5.0V
Figure 4. VSD(REV) Threshold Definitions
Figure 5. VSD(TST) Threshold Definitions
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VOFF
VGATE
0.0V
1.0V
VOFF(IH)
0.0V
5.0V
VGATE
tGATE(OFF)
VOFF(IL)
ûVOFF
VINN
VGATE
0.0V
1.0V
-200 mV
+500 mV
VGATE
tGATE(REV)
VSD(REV)
ûVSD(REV)
0 mV
LM5051
SNVS702D –OCTOBER 2011–REVISED MARCH 2013
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Figure 6. Gate Off Timing for VSD(REV) Transition
Figure 7. Gate Off Timing for VOFF Transition
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0 10 20 30 40 50
0
20
40
60
80
100
120
140
160
180
200
GATE CHARGE TIME (ns)
GATE CAPACITANCE, CGATE(nF)
+125°C
+25°C
-40°C
-50 -25 0 25 50 75 100 125
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
GATE CHARGE CURRENT (mA)
TEMPERATURE (°C)
-50 -25 0 25 50 75 100 125
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
GATE DISCHARGE CURRENT (A)
TEMPERATURE (°C)
-50 -25 0 25 50 75 100 125
0
20
40
60
80
100
120
140
160
180
200
GATE DISCHARGE TIME (ns)
TEMPERATURE (°C)
Cgate = 47 nF
Cgate = 10 nF
-1 0 1 2 3 4
-2
0
2
4
6
8
10
12
14
VOLTS (V)
TIME (ms)
INN
INP/VSS
GATE
-50 0 50 100 150 200 250
-2
0
2
4
6
8
10
12
14
VOLTS (V)
TIME (ns)
INN
INP/VSS
GATE
LM5051
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SNVS702D –OCTOBER 2011–REVISED MARCH 2013
Typical Performance Characteristics
Unless otherwise stated: All conditions and measurements are referenced to device pin 7 (INP/VSS), VLINE = 48V, VOFF =
0.0V, VINN = -150 mV, CVCC = 0.1 µF, CGATE = 47 nF, and TJ= 25°C
Gate Charge Time, CGATE = 47 nF GATE Discharge Time, tGATE(REV), CGATE = 47 nF
Figure 8. Figure 9.
IGATE Discharge Current Gate Discharge Time
vs vs
Temperature, VGATE = 5.5V Temperature
Figure 10. Figure 11.
Gate Discharge Time IGATE Charge Current
vs vs
CGATE Temperature, VGATE = 5.5V
Figure 12. Figure 13.
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20 30 40 50 60 70 80 90 100
4
5
6
7
8
9
10
11
12
13
14
VZ(V)
VLINE(V)
+125°C
+25°C
-40°C
20 30 40 50 60 70 80 90 100
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
ILINE(mA)
VLINE(V)
-40°C
+25°C
+125°C
-50 -25 0 25 50 75 100 125
1.25
1.30
1.35
1.40
1.45
1.50
1.55
1.60
VOFFTHRESHOLDS (V)
TEMPERATURE (°C)
Voff Falling
Voff Rising
-50 -25 0 25 50 75 100 125
10.0
10.5
11.0
11.5
12.0
12.5
13.0
13.5
14.0
14.5
15.0
VZ(V)
TEMPERATURE (°C)
Ivcc = 10 mA
Ivcc = 2mA
-50 -25 0 25 50 75 100 125
0.0
0.1
0.2
0.3
0.4
0.5
0.6
GATE CHARGE TIME (ms)
TEMPERATURE (°C)
Cgate = 47 nF
Cgate = 10 nF
-50 -25 0 25 50 75 100 125
-400
-350
-300
-250
-200
-150
-100
-50
0
VSD(TST)THRESHOLD (mV)
TEMPERATURE (°C)
Vsd(tst) Rising Threshold
Vsd(tst) Falling Threshold
LM5051
SNVS702D –OCTOBER 2011–REVISED MARCH 2013
www.ti.com
Typical Performance Characteristics (continued)
Unless otherwise stated: All conditions and measurements are referenced to device pin 7 (INP/VSS), VLINE = 48V, VOFF =
0.0V, VINN = -150 mV, CVCC = 0.1 µF, CGATE = 47 nF, and TJ= 25°C
Gate Charge Time VSD(TST) Thresholds
vs vs
Temperature, VGATE= 0.0V to 5.5V Temperature
Figure 14. Figure 15.
OFF Thresholds VZ
vs vs
Temperature Temperature, VINN= –100 mV
Figure 16. Figure 17.
VZILINE
vs vs
VLINE, VINN= –100 mV VLINE
Figure 18. Figure 19.
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-1.0 -0.8 -0.6 -0.4 -0.2 0.0
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
IINN(mA)
VINN(V)
+25°C
+125°C
-2 -1 0 1 2 3
-1
1
3
5
7
9
11
13
15
TIME (s)
VGATE
VOFF
-50 -25 0 25 50 75 100 125
0.0
0.1
0.2
0.3
0.4
0.5
NFGDVOL(V)
TEMPERATURE (°C)
1 mA
-50 -25 0 25 50 75 100 125
-100
-80
-60
-40
-20
0
20
40
VSD(REV) THRESHOLDS(mV)
TEMPERATURE (°C)
Vsd(rev) + ûVsd(rev)
Vsd(rev)
20 30 40 50 60 70 80 90 100
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
IGATE(mA)
VLINE(V)
-40°C
+25°C
+125°C
20 30 40 50 60 70 80 90 100
5
6
7
8
9
10
11
12
13
14
15
VGATE(V)
VLINE(V)
-40°C
+25°C
+125°C
LM5051
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SNVS702D –OCTOBER 2011–REVISED MARCH 2013
Typical Performance Characteristics (continued)
Unless otherwise stated: All conditions and measurements are referenced to device pin 7 (INP/VSS), VLINE = 48V, VOFF =
0.0V, VINN = -150 mV, CVCC = 0.1 µF, CGATE = 47 nF, and TJ= 25°C
IGATE Charge Current VGATE
vs vs
VLINE , VGATE = 0.0V VLINE
Figure 20. Figure 21.
nFGDVOL VSD(REV) Thresholds
vs vs
Temperature Temperature
Figure 22. Figure 23.
IINN VGATE
vs vs
VINN VOFF , VINN = -100mV
Figure 24. Figure 25.
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INN GATE
1.5V
OFF nFGD
LINE
LM5051
Reverse
Comparator
LOAD (-)
-48V
Forward
Comparator
260 mV
45 mV
5 µA
- +
-+
- +
VCC
INP/VSS
VZ
13V Bias
Circuitry
50 k:
D
LOAD (+)
S
GINP/VSS
INP VSS
8 56 7
3
1 2
4
0.1 µF
LM5051
SNVS702D –OCTOBER 2011–REVISED MARCH 2013
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BLOCK DIAGRAM
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PS1
PS2 INN GATE INP/VSS
INN GATE INP/VSS
RLOAD
LM5051
LM5051
LINE
LINE
VCC
VCC
LM5051
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SNVS702D –OCTOBER 2011–REVISED MARCH 2013
APPLICATION INFORMATION
FUNCTIONAL DESCRIPTION
Systems that require high availability often use multiple, parallel-connected redundant power supplies to improve
reliability. Schottky OR-ing diodes are typically used to connect these redundant power supplies to a common
point at the load. The disadvantage of using OR-ing diodes is the forward voltage drop, which reduces the
available voltage, and the associated power losses as load currents increase. Using an N-channel MOSFET to
replace the OR-ing diode requires a small increase in the level of complexity, but reduces, or eliminates, the
need for diode heat sinks or large thermal copper area in circuit board layouts for high power applications.
Figure 26. Traditional OR-ing with Diodes
The LM5051 is a negative voltage (i.e. low-side) OR-ing controller that will drive an external N-channel MOSFET
to replace an OR-ing diode. The voltage across the MOSFET source and drain pins is monitored by the LM5051
at the IN and OUT pins, while the GATE pin drives the MOSFET to control its operation based on the monitored
source-drain voltage. The resulting behavior is that of an ideal rectifier with source and drain pins of the
MOSFET acting as the anode and cathode pins of a diode respectively.
Figure 27. OR-ing with MOSFETs
INP/VSS PINS
The INP input is internally connected to the both device pin 5 and 7. Typical applications will use device pin 7
only, with a single common connection to the source connection of the N-Channel MOSFET array.
If pins 5 and 7 are both used, it is recommended that the two pins be externally connected together at the
package, with a single common connection routed to the source connection of the N-Channel MOSFET array.
Current should not be allowed flow through the internal connection between pin 5 and pin 7.
INN and GATE PINS
When power is initially applied, the load current will flow from source to drain through the body diode of the
MOSFET. The resulting voltage across the body diode will be detected across the LM5051 INN and INP/VSS
pins which then begins charging the MOSFET gate through a 0.66 mA (typical) current source.
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012345
0
20
40
60
80
100
120
140
160
0
2
4
6
8
10
12
14
16
VSD(mV)
ISD(A)
VGATE(V)
VSD(REG)
Q1 = IRF7495PDF
VGATE
VSD
LM5051
SNVS702D –OCTOBER 2011–REVISED MARCH 2013
www.ti.com
The LM5051 is designed to regulate the MOSFET gate to source voltage if the voltage across the MOSFET
source and drain pins falls below the VSD(REG) voltage of 20 mV (typical). If the MOSFET current decreases to the
point that the voltage across the MOSFET falls below the VSD(REG) voltage regulation point of 12 mV (typical), the
GATE pin voltage will be decreased until the voltage across the MOSFET is regulated at 12 mV (typical). If the
drain to source voltage is greater thanVSD(REG) voltage the gate voltage will increase.
Figure 28. VSD and VGATE vs ILOAD with IRF7495PDF
When the power supply voltages are within a few milli-volts of each other, this regulation method ensures that
the load current transitions between them without any abrupt on and off oscillations. The current flowing through
the MOSFET in each OR-ing circuit depends on the RDS(ON) of the MOSFETs, how close the power supply
voltages are set, and the load regulation of the supplies.
If the MOSFET current reverses, possibly due to failure of the input supply, such that the voltage across the
LM5051 INN pin is 5 mV (typical) more positive than INP/VSS pin (VSD(REV) +ΔVSD(REV)) the LM5051 will quickly
discharge the MOSFET gate through a strong GATE pin to INP/VSS pin discharge path. A reverse current
though the MOSFET is required to turn the gate drive off. If a single operating supply is removed from the OR-
ing array, the gate drive will not be discharged since there is no reverse current through the MOSFET to trip the
reverse comparator.
If the input supply fails abruptly, as would occur if the supply was shorted directly to ground, a reverse current
will temporarily flow through the MOSFET until the gate can be fully discharged. This reverse current is sourced
from the load capacitance and from the parallel connected supplies. The LM5051 responds to a voltage reversal
condition typically within 34 ns. The actual time required to turn off the MOSFET will depend on the charge held
by gate capacitance of the MOSFET being used. A MOSFET with 47 nF of effective gate capacitance can be
turned off in typically 90 ns. This fast turn-off time minimizes voltage disturbances at the output, as well as the
current transients from the redundant supplies.
OFF PIN
The OFF pin is used to disable the active OR-ing control circuitry, and to discharge the MOSFET Gate. The OFF
pin has an internal pull-down (4.6 μA typical) which will, by default, keep the active OR-ing control circuitry
enabled. If the OFF pin function is not needed, this pin can be left open or connected to the INP/VSS pin. Pulling
the OFF pin above the VOFF(IH) threshold of 1.50V (typical) will disable the active OR-ing control circuitry and
discharge the MOSFET Gate. The VOFF threshold has a typical hysteresis of 20mV. It is recommended that the
OFF pin be pulled cleanly, and promptly, through the VOFF(IH) threshold region to prevent any aberrant behavior.
The OFF pin must not be pulled higher than 5.5V above the INP/VSS pin.
nFGD PIN
The nFGD pin is an open Drain output pin and is controlled by status of the Forward comparator. When the
voltage on INN pin is more negative than the VSD(TST) threshold voltage (285 mV typical) the nFGD pin will
conduct current to the INP/VSS pin. During normal Active OR-ing, when the MOSFET is ON, the INN pin voltage
should be less than approximately -100mV and the nFGD pin will be logic high. When the MOSFET is OFF and
current is flowing through the body diode of the MOSFET, the INN pin voltage will be approximately -600 mV and
the nFGD pin will be logic low.
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INN GATE INP/VSS
LM5051
LINE VCC
RBIAS
0.1 F
LM5051
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SNVS702D –OCTOBER 2011–REVISED MARCH 2013
Several factors can prevent the nFGD pin from indicating that the external MOSFET is operating normally. If the
LM5051 is used to connect parallel, redundant power supplies, one of the connected supplies may hold the
INP/VSS pin voltage close enough to the LM5051 INN pin voltage that the VSD(TST) threshold is not exceeded.
Additionally, operating with a high output capacitance value and low output load current may require a significant
amount of time before the output load capacitance is discharged to the point where the VSD(TST) threshold is
crossed and the nFGD pin switches.
The status of the nFGD pin does not depend on the status of the OFF pin. The status of the nFGD pin depends
only on the voltage at the INN pin relative to the INP/VSS pin being above, or below, the VSD(TST) threshold
voltage.
The nFGD output pin requires pull-up to an external voltage source, and must not be pulled higher than 5.5V
above the INP/VSS pin. It is recommended that the nFGD pin is not required to sink more than 2mA.
VCC PIN
The VCC pin is connected to the cathode of the internal shunt (zener) voltage regulator. The anode of the shunt
regulator is connected to the INP/VSS pin. The VCC pin provides bias for internal circuitry, as well as gate drive
to the external MOSFET. The VCC pin should always be bypassed with a 0.1 μF ceramic capacitor to the
INP/VSS pin.
Typically, the VCC pin is biased from the LINE pin, through the internal 50 kΩseries resistor, when the available
VLINE voltage is not less than the 36V minimum operating voltage.
If the available LINE voltage is less than less than the 36V minimum operating voltage the VCC pin can be
biased through the use of an external resistor to an appropriate bias supply that is referenced to the INP/VSS
pin.
A minimum VCC pin bias current of 1 mA is recommended, with a recommended 10 mA maximum.
A design example for calculating the external resistor where the VCC pin will be biased from an 18V to 36V
supply (relative to the INP/VSS pin):
RBIAS = (VBIAS(MIN) - VZ) / IBIAS(MIN) (1)
RBIAS = (18V - 13V) / 1 mA (2)
RBIAS = 5.0 kΩ(3)
Next, using the calculated RBIAS resistor value, verify that the VCC pin current will be no more than 10mA at the
maximum VBIAS voltage:
ICC = (VBIAS(MAX) - 13V) / RBIAS (4)
ICC = (36V - 13V) / 5.0 kΩ(5)
ICC = 4.6 mA (6)
Since the calculated 4.6 mA is less than the 10 mA maximum, the 5 kΩvalue for RBIAS is acceptable.
Figure 29. Using an External Resistor to Bias the VCC Pin
Copyright © 2011–2013, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Links: LM5051
-2 0 2 4 6 8 10 12 14
-1
1
3
5
7
9
11
13
15
VCCAND VGATE(V)
TIME (ms)
VGATE
VVCC
VOFF
INN GATE INP/VSS
LM5051
LINE VCC
RBIAS
LM5051
SNVS702D –OCTOBER 2011–REVISED MARCH 2013
www.ti.com
Alternately, an external bias supply can be connected directly to the VCC pin, as long as the applied voltage is
below the minimum VZbreakdown voltage (11.9V) and above the minimum VCC operating voltage (4.50V). In
this case, it is important to pay close attention to the VGS rating of the external MOSFET as the gate drive voltage
will be affected by the lower voltage on the VCC pin.
Figure 30. Using an External Zener to Bias the VCC Pin
In the case where the OFF pin is high (i.e. OR-ing is disabled, and the Gate is discharged) and the voltage at the
INN pin is more negative than the VSD(REV) threshold voltage the internal current increases, and the voltage on
the VCC pin may drop.. Since the LM5051 is in the OFF state, this voltage drop does not affect any operation.
However, when the OFF pin is taken low to resume normal operation, the initial Gate charge time may be
extended slightly if the capacitor on the VCC pin has not had adequate time to fully recharge through either the
external RBIAS resistor, or through the internal 50 kΩresistor.
Figure 31. VCC and VGATE vs VOFF, VINN = –100 mV
HIGH SIDE OR-ing
Because the INP and VSS functions are internally connected, the LM5051 cannot be configured as a High-Side
(i.e. Positive) OR-ing controller. Please refer to the LM5050-1 and LM5050-2 High-Side OR-ing controllers.
MOSFET FAILURE
Typically, the INN pin maximum negative voltage will be defined by the body diode of the external MOSFET. In
the even that the external MOSFET has a catastrophic failure that results in an open body diode, the voltage
between the INP/VSS pin and the INN pin may cause current through the LM5051 substrate diode at the INN
pin. The voltage at the INN pin must be limited to a safe level ( -1V) to prevent damage to the LM5051. The
voltage on the INN pin can be limited with the use of a Schottky diode and a current limiting resistor. Note that
the power dissipation of the current limiting resistor should allow for any anticipated worst case condition. See
Figure 32.
14 Submit Documentation Feedback Copyright © 2011–2013, Texas Instruments Incorporated
Product Folder Links: LM5051
INN GATE INP/VSS CLOAD
LM5051
LINE
COUT
Parasitic
Inductance
Parasitic
Inductance
Shorted
Input
VCC
INN GATE INP/VSS
LM5051
LINE VCC
1k
LM5051
www.ti.com
SNVS702D –OCTOBER 2011–REVISED MARCH 2013
Figure 32. Protecting the INN Pin
SHORT CIRCUIT FAILURE OF AN INPUT SUPPLY
An abrupt zero ohm short circuit across the input supply will cause the highest possible reverse current to flow
while the internal LM5051 control circuitry discharges the gate of the MOSFET. During this time, the reverse
current is limited only by the RDS(ON) of the MOSFET, along with parasitic wiring resistances and inductances.
Worst case instantaneous reverse current would be limited to:
ID(REV) = (VOUT - VIN) / RDS(ON) (7)
The internal Reverse Comparator will react, and will start the process of discharging the Gate, when the reverse
current reaches:
ID(REV) = VSD(REV) / RDS(ON) (8)
When the MOSFET is finally switched off, the energy stored in the parasitic wiring inductances will be transferred
to the rest of the circuit.
Figure 33. Input Supply Fault Transients
MOSFET SELECTION
The important MOSFET electrical parameters are the maximum continuous Drain current ID, the maximum
Source current (i.e. body diode), the maximum drain-to-source voltage VDS(MAX), the gate-to-source threshold
voltage VGS(TH), the drain-to-source reverse breakdown voltage V(BR)DSS, and the drain-to-source On resistance
RDS(ON).
The maximum continuous drain current, ID, rating must be exceed the maximum continuous load current. The
rating for the maximum current through the body diode, IS, is typically rated the same as, or slightly higher than
the drain current, but body diode current only flows while the MOSFET gate is being charged to VGS(TH):
Gate Charge Time = Qg/ IGATE(ON) (9)
The maximum drain-to-source voltage, VDS(MAX), must be high enough to withstand the highest differential voltage
seen in the application. This would include any anticipated fault conditions.
The drain-to-source reverse breakdown voltage, V(BR)DSS, may provide some transient protection to the OUT pin
in low voltage applications by allowing conduction back to the IN pin during positive transients at the OUT pin.
The gate-to-source threshold voltage, VGS(TH), should be compatible with the LM5051 gate drive capabilities.
Logic level MOSFETs are recommended, but sub-Logic level MOSFETs can also be used.
Copyright © 2011–2013, Texas Instruments Incorporated Submit Documentation Feedback 15
Product Folder Links: LM5051
LM5051
SNVS702D –OCTOBER 2011–REVISED MARCH 2013
www.ti.com
The dominate MOSFET loss for the LM5051 active OR-ing controller is conduction loss due to source-to-drain
current to the output load, and the RDS(ON) of the MOSFET. This conduction loss could be reduced by using a
MOSFET with the lowest possible RDS(ON). However, contrary to popular belief, arbitrarily selecting a MOSFET
based solely on having low RDS(ON) may not always give desirable results for several reasons:
1) Reverse transition detection. Higher RDS(ON) will provide increased voltage information to the LM5051 Reverse
Comparator at a lower reverse current level. This will give an earlier MOSFET turn-off condition should the input
voltage become shorted to ground. This will minimize any disturbance of the redundant bus.
2) Reverse current leakage. In cases where multiple input supplies are closely matched it may be possible for
some small current to flow continuously through the MOSFET drain to source (i.e. reverse) without activating the
LM5051 Reverse Comparator. Higher RDS(ON) will reduce this reverse current level.
3) Cost. Generally, as the RDS(ON) rating goes lower, the cost of the MOSFET goes higher.
Selecting a MOSFET with an RDS(ON) that is too large will result in excessive power dissipation.
As a guideline, it is suggest that RDS(ON) be selected to provide at least 20 mV, and no more than 100 mV, at the
nominal load current.
(20 mV / ID)≤RDS(ON) ≤(100mV / ID) (10)
The thermal resistance of the MOSFET package should also be considered against the anticipated dissipation in
the MOSFET in order to ensure that the junction temperature (TJ) is reasonably well controlled, since the RDS(ON)
of the MOSFET increases as the junction temperature increases.
PDISS = ID2x (RDS(ON)) (11)
Operating with a maximum ambient temperature (TA(MAX)) of 35°C, a load current of 10A, and an RDS(ON) of 10
mΩ, and desiring to keep the junction temperature under 100°C, the maximum junction-to-ambient thermal
resistance rating (θJA) would need to be:
θJA≤(TJ(MAX) - TA(MAX))/(ID2x RDS(ON)) (12)
θJA≤(100°C - 35°C)/(10A x 10A x 0.01Ω) (13)
θJA≤65°C/W (14)
16 Submit Documentation Feedback Copyright © 2011–2013, Texas Instruments Incorporated
Product Folder Links: LM5051
-VIN
-48V -VOUT
LINE
INP/VSS
INN GATE
LM5051
GND
0V GND
Q1
IRF7495PDF
+
CIN
1 PF
100V
X7R
D1
SS16T3
1A/60V COUT
22 PF
100V
D2
SMB5945
68V/3W
VCC
CVCC
0.1 PF
25V
X7R
SD
GD3
SS16T3
R1
1 k:
-VIN
-48V -VOUT
LINE
INP/VSS
INN GATE
LM5051
GND
0V GND
Q1
IRF7495PDF
VCC
CVCC
0.1 PF
SD
G
LM5051
www.ti.com
SNVS702D –OCTOBER 2011–REVISED MARCH 2013
TYPICAL APPLICATIONS
Figure 34. Basic Application
Typical –48V Application with Input and Output Transient Protection and Open MOSFET Protection
Figure 35. Typical –48V Application
Copyright © 2011–2013, Texas Instruments Incorporated Submit Documentation Feedback 17
Product Folder Links: LM5051
LM5051
SNVS702D –OCTOBER 2011–REVISED MARCH 2013
www.ti.com
REVISION HISTORY
Changes from Revision C (March 2013) to Revision D Page
• Changed layout of National Data Sheet to TI format .......................................................................................................... 17
18 Submit Documentation Feedback Copyright © 2011–2013, Texas Instruments Incorporated
Product Folder Links: LM5051
PACKAGE OPTION ADDENDUM
www.ti.com 11-Apr-2013
Addendum-Page 1
PACKAGING INFORMATION
Orderable Device Status
(1)
Package Type Package
Drawing Pins Package
Qty Eco Plan
(2)
Lead/Ball Finish MSL Peak Temp
(3)
Op Temp (°C) Top-Side Markings
(4)
Samples
LM5051MA/NOPB ACTIVE SOIC D 8 95 Green (RoHS
& no Sb/Br) CU SN Level-1-260C-UNLIM -40 to 125 L5051
MA
LM5051MAE/NOPB ACTIVE SOIC D 8 250 Green (RoHS
& no Sb/Br) CU SN Level-1-260C-UNLIM -40 to 125 L5051
MA
LM5051MAX/NOPB ACTIVE SOIC D 8 2500 Green (RoHS
& no Sb/Br) CU SN Level-1-260C-UNLIM -40 to 125 L5051
MA
(1) The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2) Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability
information and additional product content details.
TBD: The Pb-Free/Green conversion plan has not been defined.
Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that
lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.
Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between
the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above.
Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight
in homogeneous material)
(3) MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature.
(4) Multiple Top-Side Markings will be inside parentheses. Only one Top-Side Marking contained in parentheses and separated by a "~" will appear on a device. If a line is indented then it is a
continuation of the previous line and the two combined represent the entire Top-Side Marking for that device.
Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information
provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and
continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals.
TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release.
In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis.
TAPE AND REEL INFORMATION
*All dimensions are nominal
Device Package
Type Package
Drawing Pins SPQ Reel
Diameter
(mm)
Reel
Width
W1 (mm)
A0
(mm) B0
(mm) K0
(mm) P1
(mm) W
(mm) Pin1
Quadrant
LM5051MAE/NOPB SOIC D 8 250 178.0 12.4 6.5 5.4 2.0 8.0 12.0 Q1
LM5051MAX/NOPB SOIC D 8 2500 330.0 12.4 6.5 5.4 2.0 8.0 12.0 Q1
PACKAGE MATERIALS INFORMATION
www.ti.com 11-Oct-2013
Pack Materials-Page 1
*All dimensions are nominal
Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm)
LM5051MAE/NOPB SOIC D 8 250 210.0 185.0 35.0
LM5051MAX/NOPB SOIC D 8 2500 367.0 367.0 35.0
PACKAGE MATERIALS INFORMATION
www.ti.com 11-Oct-2013
Pack Materials-Page 2
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