LM25069
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SNVS607E – FEBRUARY 2011 – REVISED MARCH 2013
LM25069 Positive Low Voltage Power Limiting Hot Swap Controller
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FEATURES
APPLICATIONS
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•
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1
2
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Operating R ange: +2.9V to +17V
In-rush Current Limit for Safe Board Insertion
into Live Power Sources
Programmable Maximum Power Dissipation in
the External Pass Device
Adjustable Current Limit
Circuit Breaker Function for Severe OverCurrent Events
Internal High Side Charge Pump and Gate
Driver for External N-channel MOSFET
Adjustable Under-Voltage Lockout (UVLO) and
Hysteresis
Adjustable Over-Voltage Lockout (OVLO) and
Hysteresis
Initial Insertion Timer Allows Ringing and
Transients to Subside After System
Connection
Programmable Fault Timer Avoids Nuisance
Trips
Active High Open Drain POWER GOOD Output
Available in Latched Fault and Automatic
Restart Versions
Server Backplane Systems
Base Station Power Distribution Systems
Solid State Circuit Breaker
PACKAGE
•
VSSOP-10
DESCRIPTION
The LM25069 positive hot swap controller provides
intelligent control of the power supply voltage to the
load during insertion and removal of circuit cards from
a live system backplane or other "hot" power sources.
The LM25069 provides in-rush current control to limit
system voltage droop and transients. The current limit
and power dissipation in the external series pass NChannel MOSFET are programmable, ensuring
operation within the Safe Operating Area (SOA). The
POWER GOOD output indicates when the output
voltage is within 1.3V of the input voltage. The input
under-voltage and over-voltage lockout levels and
hysteresis are programmable, as well as the initial
insertion delay time and fault detection time. The
LM25069-1 latches off after a fault detection, while
the LM25069-2 automatically restarts at a fixed duty
cycle. The LM25069 is available in a 10 pin VSSOP
package.
Typical Application
VSYS
VOUT
VIN
SENSE GATE
UVLO/EN
OUT
LM25069
PGD
OVLO
TIMER
GND
Power
Good
PWR
Figure 1. Positive Power Supply Control
1
2
Please 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.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2011–2013, Texas Instruments Incorporated
LM25069
SNVS607E – FEBRUARY 2011 – REVISED MARCH 2013
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Connection Diagram
SENSE
1
10
VIN
2
9
UVLO/EN
3
8
PGD
OVLO
4
7
PWR
GND
5
6
TIMER
GATE
OUT
Figure 2. Top View
10-Lead VSSOP
PIN DESCRIPTIONS
Pin #
Name
Description
Applications Information
1
SENSE
Current sense input
The voltage across the current sense resistor (RS) is measured from VIN to this pin. If
the voltage across RS reaches 50mV the load current is limited and the fault timer
activates.
2
VIN
Positive supply input
A small ceramic bypass capacitor close to this pin is recommended to suppress
transients which occur when the load current is switched off.
3
UVLO/EN
Under-voltage lockout
An external resistor divider from the system input voltage sets the under-voltage turnon threshold. An internal 20 µA current source provides hysteresis. The enable
threshold at the pin is 1.17V. This pin can also be used for remote shutdown control.
4
OVLO
Over-voltage lockout
An external resistor divider from the system input voltage sets the over-voltage turn-off
threshold. An internal 20 µA current source provides hysteresis. The disable threshold
at the pin is 1.16V.
5
GND
Circuit ground
6
TIMER
Timing capacitor
7
PWR
Power limit set
An external resistor connected to this pin, in conjunction with the current sense resistor
(RS), sets the maximum power dissipation allowed in the external series pass
MOSFET.
8
PGD
Power Good indicator
An open drain output. When the external MOSFET VDS decreases below 1.3V, the
PGD indicator is active (high). When the external MOSFET VDS increases above 1.9V
the PGD indicator switches low.
9
OUT
Output feedback
Connect to the output rail (external MOSFET source). Internally used to determine the
MOSFET VDS voltage for power limiting, and to control the PGD indicator.
10
GATE
Gate drive output
Connect to the external MOSFET’s gate. This pin's voltage is limited at 19.5V above
ground.
An external capacitor connected to this pin sets the insertion time delay and the Fault
Timeout Period. The capacitor also sets the restart timing of the LM25069-2.
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.
2
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Absolute Maximum Ratings (1) (2) (3)
VIN to GND (4)
-0.3V to 20V
SENSE, OUT, PGD to GND
-0.3V to 20V
UVLO to GND
-0.3V to 20V
OVLO to GND
-0.3V to 20V
VIN to SENSE
-0.3V to +0.3V
ESD Rating (5)
Human Body Model
2kV
Storage Temperature
-65°C to +150°C
Junction Temperature
+150°C
Lead Temperature (soldering 4 sec)
+260°C
(1)
(2)
(3)
(4)
(5)
Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for
which the device is intended to be functional, but do not ensure specific performance limits. For ensured specifications and conditions
see the Electrical Characteristics.
If Military/Aerospace specified devices are required, please contact the TI Sales Office/Distributors for availability and specifications.
For detailed information on soldering plastic VSSOP packages refer to the Packaging Databook available from Texas Instruments.
Current out of a pin is indicated as a negative number.
The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin.
Operating Ratings
VIN Supply Voltage
+2.9V to 17V
PGD Off Voltage
0V to 17V
Junction Temp. Range
−40°C to +85°C
Electrical Characteristics
Limits in standard type are for TJ = 25°C only; limits in boldface type apply over the junction temperature (TJ) range of -40°C
to +85°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 the
following conditions apply: VIN = 12V.
Symbol
Parameter
Conditions
Min.
Typ.
Max.
Units
Input (VIN pin)
IIN-EN
Input Current, enabled
UVLO = 2V and OVLO = 0.7V, VIN = 14V
1.6
2.4
mA
IIN-DIS
Input Current, disabled
UVLO = 0.7V or OVLO = 2V
1.0
1.6
mA
POR
Power On Reset threshold at VIN
VIN Increasing
2.6
2.8
V
POR hysteresis
VIN decreasing
150
mV
OUT = VIN, Normal operation
0.30
µA
Disabled, OUT = 0V, SENSE = VIN
-12
PORHYS
OUT pin
IOUT-EN
IOUT-DIS
OUT bias current, enabled
OUT bias current, disabled
(1)
UVLO, OVLO pins
UVLOTH
UVLO threshold
UVLOHYS
UVLO hysteresis current
UVLO = 1V
UVLODEL
UVLO delay
Delay to GATE high
15
Delay to GATE low
8.3
UVLOBIAS
(1)
UVLO bias current
1.154
1.17
1.183
V
15
20
26
µA
UVLO = 3V
µs
1
OVLOTH
OVLO threshold
OVLOHYS
OVLO hysteresis current
OVLO = 2V
OVLODEL
OVLO delay
Delay to GATE high
16
Delay to GATE low
8.2
OVLOBIAS
OVLO bias current
µA
1.142
1.16
1.185
V
-26
-20
-15
µA
OVLO = 1V
µs
1
µA
OUT bias current (disabled) due to leakage current through an internal 1.0 MΩ resistance from SENSE to VOUT.
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Electrical Characteristics (continued)
Limits in standard type are for TJ = 25°C only; limits in boldface type apply over the junction temperature (TJ) range of -40°C
to +85°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 the
following conditions apply: VIN = 12V.
Symbol
Parameter
Conditions
Min.
Typ.
Max.
Units
SENSE-OUT = 12V, RPWR = 69.8 kΩ
19
25
31
mV
SENSE-OUT = 6V, RPWR = 34.8 kΩ
19
25
31
mV
Power Limit (PWR pin)
PWRLIM-1
Power limit sense voltage (VIN-SENSE)
PWRLIM-2
IPWR
RSAT(PWR)
PWR pin current
VPWR = 2.5V
-15
µA
PWR pin impedance when disabled
UVLO = 0.7V
140
Ω
Gate Control (GATE pin)
IGATE
Source current
Normal Operation
-27
-20
-13
µA
Sink current
UVLO = 1V
1.5
2
2.7
mA
VIN - SENSE = 150 mV or VIN < POR,
VGATE = 5V
160
260
375
mA
Gate output voltage in normal operation
GATE voltage with respect to ground
18
19.5
21
V
VCL
Threshold voltage
VIN-SENSE voltage
45
50
55
mV
tCL
Response time
VIN-SENSE stepped from 0 mV to 80 mV
15
µs
SENSE input current
Enabled, SENSE = OUT
23
µA
Disabled, OUT = 0V
12
Enabled, OUT = 0V
62
VGATE
Current Limit
ISENSE
Circuit Breaker
VCB
Threshold voltage
VIN - SENSE
tCB
Response time
VIN - SENSE stepped from 0 mV to 150
mV, time to GATE low, no load
75
95
110
mV
0.19
0.36
µs
1.6
1.72
1.85
V
0.9
1.0
1.1
V
Timer (TIMER pin)
VTMRH
Upper threshold
VTMRL
Lower threshold
ITIMER
Restart cycles (LM25069-2)
End of 8th cycle (LM25069-2)
0.3
Re-enable Threshold (LM25069-1)
0.3
Insertion time current
-7.5
Sink current, end of insertion time
TIMER pin = 2V
-5.5
V
V
-3.5
µA
1.5
2
2.5
mA
Fault detection current
-110
-80
-50
µA
Fault sink current
1.6
2.5
3.4
µA
DCFAULT
Fault Restart Duty Cycle
LM25069-2 only
0.67
%
tFAULT
Fault to GATE low delay
TIMER pin reaches the upper threshold
20
µs
Decreasing
1.3
Threshold Hysteresis
0.6
15
Power Good (PGD pin)
PGDTH
4
Threshold measured at SENSE-OUT
PGDVOL
Output low voltage
ISINK = 2 mA
PGDIOH
Off leakage current
VPGD = 17V
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1.9
V
30
mV
1
µA
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Typical Performance Characteristics
Unless otherwise specified the following conditions apply: TJ = 25°C, VIN = 12V
VIN Pin Input Current
vs.
VIN
SENSE Pin Input Current
Figure 3.
Figure 4.
OUT Pin Input Current
GATE Pin Voltage
Figure 5.
Figure 6.
GATE Pin Source Current
MOSFET Power Dissipation Limit
Figure 7.
Figure 8.
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Typical Performance Characteristics (continued)
Unless otherwise specified the following conditions apply: TJ = 25°C, VIN = 12V
6
PGD Pin Low Voltage
vs.
Sink Current
Input Current, Enabled
vs.
Temperature
Figure 9.
Figure 10.
UVLO Threshold
vs.
Temperature
UVLO Hysteresis Current
vs.
Temperature
Figure 11.
Figure 12.
OVLO Threshold
vs.
Temperature
OVLO Hysteresis Current
vs.
Temperature
Figure 13.
Figure 14.
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Typical Performance Characteristics (continued)
Unless otherwise specified the following conditions apply: TJ = 25°C, VIN = 12V
Current Limit Threshold
vs.
Temperature
Circuit Breaker Threshold
vs.
Temperature
Figure 15.
Figure 16.
Power Limit Threshold
vs.
Temperature
GATE Output Voltage
vs.
Temperature
Figure 17.
Figure 18.
GATE Source Current
vs.
Temperature
PGD Low Voltage
vs.
Temperature
Figure 19.
Figure 20.
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Block Diagram
LM25069
Charge
Pump
LDO
Bias
Circuit
Breaker
95 mV
20 PA
ID
VIN
GATE
2 mA
SENSE
50 mV
Current
Limit
260 mA 19.5V
Gate
Control
Current Limit/
Power Limit
Control
1M
VDS
Power
Limit
OUT
5.5 PA
Insertion
Timer
PGD
1.3V/
1.9V
80 PA
Fault
Discharge
TIMER
Timer and Gate Logic Control
15 PA
PWR
20 PA
OVLO
1.16V
1.17V
2 mA
End
Insertion
Time
2.5 PA
Fault
Discharge
1.72V
1.0V
UVLO/EN
2.6V
20 PA
0.3V
VIN
POR
GND
Q1
VSYS
CIN
VOUT
RS
CL
R1
SENSE GATE
UVLO/EN
RPG
LM25069
R2
Power
Good
PGD
OVLO
R3
OUT
TIMER
GND
PWR
CT
RPWR
Figure 21. Basic Application Circuit
8
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FUNCTIONAL DESCRIPTION
The LM25069 is designed to control the in-rush current to the load upon insertion of a circuit card into a live
backplane or other "hot" power source, thereby limiting the voltage sag on the backplane’s supply voltage, and
the dV/dt of the voltage applied to the load. Effects on other circuits in the system are minimized, preventing
possible unintended resets. A controlled shutdown when the circuit card is removed can also be implemented
using the LM25069. In addition to a programmable current limit, the LM25069 monitors and limits the maximum
power dissipation in the series pass device to maintain operation within the device Safe Operating Area (SOA).
Either current limiting or power limiting for an extended period of time results in the shutdown of the series pass
device. In this event, the LM25069-1 latches off until the circuit is re-enabled by external control, while the
LM25069-2 automatically restarts with defined timing. The circuit breaker function quickly switches off the series
pass device upon detection of a severe over-current condition. The Power Good (PGD) output pin indicates
when the output voltage is within 1.3V of the system input voltage (VSYS). Programmable under-voltage lock-out
(UVLO) and over-voltage lock-out (OVLO) circuits enable the LM25069 when the system input voltage is
between the desired thresholds. The typical configuration of a circuit card with LM25069 hot swap protection is
shown in Figure 22.
RS
VSYS
+12V
LIVE POWER
SOURCE
VOUT
Q1
VIN
OUT
LM25069
CL
LOAD
PGD
GND
GND
PLUG - IN BOARD
Figure 22. LM25069 Application
Power Up Sequence
The VIN operating range of the LM25069 is +2.9V to +17V, with a transient capability to 20V. Referring to the
Block Diagram and Figure 21 and Figure 23, as the voltage at VIN initially increases, the external N-channel
MOSFET (Q1) is held off by an internal 260 mA pull-down current at the GATE pin. The strong pull-down current
at the GATE pin prevents an inadvertent turn-on as the MOSFET’s gate-to-drain (Miller) capacitance is charged.
Additionally, the TIMER pin is initially held at ground. When the VIN voltage reaches the POR threshold the
insertion time begins. During the insertion time, the capacitor at the TIMER pin (CT) is charged by a 5.5 µA
current source, and Q1 is held off by a 2 mA pull-down current at the GATE pin regardless of the VIN voltage.
The insertion time delay allows ringing and transients at VIN to settle before Q1 is enabled. The insertion time
ends when the TIMER pin voltage reaches 1.72V. CT is then quickly discharged by an internal 2 mA pull-down
current. The GATE pin then switches on Q1 when VSYS exceeds the UVLO threshold. If VSYS is above the UVLO
threshold at the end of the insertion time, Q1 switches on at that time. The GATE pin charge pump sources 20
µA to charge Q1’s gate capacitance. The maximum voltage at the GATE pin is limited by an internal 19.5V zener
diode.
As the voltage at the OUT pin increases, the LM25069 monitors the drain current and power dissipation of
MOSFET Q1. In-rush current limiting and/or power limiting circuits actively control the current delivered to the
load. During the in-rush limiting interval (t2 in Figure 23) an internal 80 µA fault timer current source charges CT.
If Q1’s power dissipation and the input current reduce below their respective limiting thresholds before the
TIMER pin reaches 1.72V the 80 µA current source is switched off, and CT is discharged by the internal 2.5 µA
current sink (t3 in Figure 23). The in-rush limiting interval is complete when the voltage at the OUT pin increases
to within 1.3V of the input voltage (VSYS), and the PGD pin switches high.
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If the TIMER pin voltage reaches 1.72V before in-rush current limiting or power limiting ceases (during t2), a fault
is declared and Q1 is turned off. See the Fault Timer & Restart section for a complete description of the fault
mode.
VSYS
UVLO
V IN
POR
5.5 PA
TIMER
Pin
GATE
Pin
1.72V
80 PA
2.5 PA
2 mA
260 mA
pull-down
2 mA pull-down
20 PA source
I LIMIT
Load
Current
Output
Voltage
(OUT Pin)
1.3V
PGD
t1
Insertion Time
t2
In- rush
Limiting
t3
Normal Operation
Figure 23. Power Up Sequence (Current Limit only)
Gate Control
A charge pump provides the voltage at the GATE pin to enhance the N-Channel MOSFET’s gate. During normal
operating conditions (t3 in Figure 23) the gate of Q1 is held charged by an internal 20 µA current source. The
voltage at the GATE pin (with respect to ground) is limited by an internal 19.5V zener diode. See the graph
GATE Pin voltage. Since the gate-to-source voltage applied to Q1 could be as high as 19.5V during various
conditions, a zener diode with the appropriate voltage rating must be added between the GATE and OUT pins if
the maximum VGS rating of the selected MOSFET is less than 19.5V. The external zener diode must have a
forward current rating of at least 260 mA.
When the system voltage is initially applied, the GATE pin is held low by a 260 mA pull-down current. This helps
prevent an inadvertent turn-on of the MOSFET through its drain-gate capacitance as the applied system voltage
increases.
During the insertion time (t1 in Figure 23) the GATE pin is held low by a 2 mA pull-down current. This maintains
Q1 in the off-state until the end of t1, regardless of the voltage at VIN or UVLO.
Following the insertion time, during t2 in Figure 23, the gate voltage of Q1 is modulated to keep the current or
power dissipation level from exceeding the programmed levels. While in the current or power limiting mode the
TIMER pin capacitor is charging. If the current and power limiting cease before the TIMER pin reaches 1.72V the
TIMER pin capacitor then discharges, and the circuit enters normal operation.
10
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If the in-rush limiting condition persists such that the TIMER pin reached 1.72V during t2, the GATE pin is then
pulled low by the 2 mA pull-down current. The GATE pin is then held low until either a power up sequence is
initiated (LM25069-1), or until the end of the restart sequence (LM25069-2). See the Fault Timer & Restart
section.
If the system input voltage falls below the UVLO threshold, or rises above the OVLO threshold, the GATE pin is
pulled low by the 2 mA pull-down current to switch off Q1.
Current Limit
The current limit threshold is reached when the voltage across the sense resistor RS (VIN to SENSE) reaches 50
mV. In the current limiting condition, the GATE voltage is controlled to limit the current in MOSFET Q1. While the
current limit circuit is active, the fault timer is active as described in the Fault Timer & Restart section. If the load
current falls below the current limit threshold before the end of the Fault Timeout Period, the LM25069 resumes
normal operation. For proper operation, the RS resistor value should be no larger than 200 mΩ. Higher values
may result in instability in the current limit control loop.
Circuit Breaker
If the load current increases rapidly (e.g., the load is short-circuited) the current in the sense resistor (RS) may
exceed the current limit threshold before the current limit control loop is able to respond. If the current exceeds
approximately twice the current limit threshold (95 mV/RS), Q1 is quickly switched off by the 260 mA pull-down
current at the GATE pin, and a Fault Timeout Period begins. When the voltage across RS falls below 95 mV the
260 mA pull-down current at the GATE pin is switched off, and the gate voltage of Q1 is then determined by the
current limit or the power limit functions. If the TIMER pin reaches 1.72V before the current limiting or power
limiting condition ceases, Q1 is switched off by the 2 mA pull-down current at the GATE pin as described in the
Fault Timer & Restart section.
Power Limit
An important feature of the LM25069 is the MOSFET power limiting. The Power Limit function can be used to
maintain the maximum power dissipation of MOSFET Q1 within the device SOA rating. The LM25069 determines
the power dissipation in Q1 by monitoring its drain-source voltage (SENSE to OUT), and the drain current
through the sense resistor (VIN to SENSE). The product of the current and voltage is compared to the power
limit threshold programmed by the resistor at the PWR pin. If the power dissipation reaches the limiting threshold,
the GATE voltage is modulated to regulate the current in Q1. While the power limiting circuit is active, the fault
timer is active as described in the Fault Timer & Restart section.
Fault Timer & Restart
When the current limit or power limit threshold is reached during turn-on or as a result of a fault condition, the
gate-to-source voltage of Q1 is modulated to regulate the load current and power dissipation in Q1. When either
limiting function is activated, an 80 µA fault timer current source charges the external capacitor (CT) at the
TIMER pin as shown in Figure 25 (Fault Timeout Period). If the fault condition subsides during the Fault Timeout
Period before the TIMER pin reaches 1.72V, the LM25069 returns to the normal operating mode and CT is
discharged by the 2.5 µA current sink. If the TIMER pin reaches 1.72V during the Fault Timeout Period, Q1 is
switched off by a 2 mA pull-down current at the GATE pin. The subsequent restart procedure then depends on
which version of the LM25069 is in use.
The LM25069-1 latches the GATE pin low at the end of the Fault Timeout Period. CT is then discharged to
ground by the 2.5 µA fault current sink. The GATE pin is held low by the 2 mA pull-down current until a power up
sequence is externally initiated by cycling the input voltage (VSYS), or momentarily pulling the UVLO pin below its
threshold with an open-collector or open-drain device as shown in Figure 24. The voltage at the TIMER pin must
be less than 0.3V for the restart procedure to be effective.
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VSYS
R1
VIN
UVLO/EN
Restart
Control
LM25069-1
R2
OVLO
GND
R3
Figure 24. Latched Fault Restart Control
The LM25069-2 provides an automatic restart sequence which consists of the TIMER pin cycling between 1.72V
and 1V seven times after the Fault Timeout Period, as shown in Figure 25. The period of each cycle is
determined by the 80 µA charging current, and the 2.5 µA discharge current, and the value of the capacitor CT.
When the TIMER pin reaches 0.3V during the eighth high-to-low ramp, the 20 µA current source at the GATE pin
turns on Q1. If the fault condition is still present, the Fault Timeout Period and the restart cycle repeat.
The Fault Timeout Period during restart cycles is approximately 18% shorter than the initial fault timeout period
which initiated the restart cycle. This is due to the fact that the TIMER pin transitions from 0.3V to 1.72V after
each restart time, rather than from ground.
Fault
Detection
ILIMIT
Load
Current
20 PA
Gate Charge
2 mA
pulldown
GATE
Pin
2.5 PA
1.72V
80 PA
TIMER
Pin
1V
1
Fault Timeout
Period
2
3
7
8
0.3V
tRESTART
Figure 25. Restart Sequence (LM25069-2)
Under-Voltage Lock-Out (UVLO)
The series pass MOSFET (Q1) is enabled when the input supply voltage (VSYS) is within the operating range
defined by the programmable under-voltage lockout (UVLO) and over-voltage lock-out (OVLO) levels. Typically
the UVLO level at VSYS is set with a resistor divider (R1-R3) as shown in Figure 21. Refering to the Block
Diagram when VSYS is below the UVLO level, the internal 20 µA current source at UVLO is enabled, the current
source at OVLO is off, and Q1 is held off by the 2 mA pull-down current at the GATE pin. As VSYS is increased,
raising the voltage at UVLO above its threshold the 20 µA current source at UVLO is switched off, increasing the
voltage at UVLO, providing hysteresis for this threshold. With the UVLO pin above its threshold, Q1 is switched
on by the 20 µA current source at the GATE pin if the insertion time delay has expired. See the Applications
Section for a procedure to calculate the values of the threshold setting resistors (R1-R3). The minimum possible
UVLO level at VSYS can be set by connecting the UVLO pin to VIN. In this case Q1 is enabled after the insertion
time.
Over-Voltage Lock-Out (OVLO)
The series pass MOSFET (Q1) is enabled when the input supply voltage (VSYS) is within the operating range
defined by the programmable under-voltage lockout (UVLO) and over-voltage lock-out (OVLO) levels. If VSYS
raises the OVLO pin voltage above its threshold Q1 is switched off by the 2 mA pull-down current at the GATE
pin, denying power to the load. When the OVLO pin is above its threshold, the internal 20 µA current source at
OVLO is switched on, raising the voltage at OVLO to provide threshold hysteresis. When VSYS is reduced below
the OVLO level Q1 is enabled. See the Applications Section for a procedure to calculate the threshold setting
resistor values.
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Shutdown Control
The load current can be remotely switched off by taking the UVLO pin below its threshold with an open collector
or open drain device, as shown in Figure 26. Upon releasing the UVLO pin the LM25069 switches on the load
current with in-rush current and power limiting.
VSYS
R1
VIN
UVLO/EN
Shutdown
Control
LM25069
R2
OVLO
R3
GND
Figure 26. Shutdown Control
Power Good Pin
The Power Good indicator pin (PGD) is connected to the drain of an internal N-channel MOSFET capable of
sustaining 17V in the off-state, and transients up to 20V. An external pull-up resistor is required at PGD to an
appropriate voltage to indicate the status to downstream circuitry. The off-state voltage at the PGD pin can be
higher or lower than the voltages at VIN and OUT. PGD is switched high when the voltage from SENSE to OUT
(the external MOSFET’s VDS) decreases below 1.3V. PGD switches low when the MOSFET’s VDS is increased
past 1.9V. If the UVLO pin is taken below its threshold or the OVLO pin taken above its threshold, to disable the
LM25069, PGD switches low within 10 µs without waiting for the voltage at OUT to fall. The PGD output pin is
high when the voltage at VIN is less than 1.6V.
Application Information (Refer to Figure 21)
CURRENT LIMIT, RS
The LM25069 monitors the current in the external MOSFET (Q1) by measuring the voltage across the sense
resistor (RS), connected from VIN to SENSE. The required resistor value is calculated from:
RS =
50 mV
ILIM
where
•
ILIM is the desired current limit threshold
(1)
If the voltage across RS reaches 50 mV, the current limit circuit modulates the gate of Q1 to regulate the current
at ILIM. While the current limiting circuit is active, the fault timer is active as described in the Fault Timer & Restart
section. For proper operation, RS must be no larger than 200 mΩ.
While the maximum load current in normal operation can be used to determine the required power rating for
resistor RS, basing it on the current limit value provides a more reliable design since the circuit can operate near
the current limit threshold continuously. The resistor’s surge capability must also be considered since the circuit
breaker threshold is approximately twice the current limit threshold. Connections from RS to the LM25069 should
be made using Kelvin techniques. In the suggested layout of Figure 27 the small pads at the lower corners of the
sense resistor connect only to the sense resistor terminals, and not to the traces carrying the high current. With
this technique, only the voltage across the sense resistor is applied to VIN and SENSE, eliminating the voltage
drop across the high current solder connections.
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HIGH CURRENT PATH
FROM
SYSTEM
INPUT
VOLTAGE
TO MOSFET'S
DRAIN
SENSE
RESISTOR
RS
SENSE
VIN
3
4
10
LM25069
5
9
8
7
6
Figure 27. Sense Resistor Connections
POWER LIMIT THRESHOLD
The LM25069 determines the power dissipation in the external MOSFET (Q1) by monitoring the drain current
(the current in RS), and the VDS of Q1 (SENSE to OUT pins). The resistor at the PWR pin (RPWR) sets the
maximum power dissipation for Q1, and is calculated from the following equation:
RPWR = 2.32 x 105 x RS x PFET(LIM)
where
•
•
PFET(LIM) is the desired power limit threshold for Q1
RS is the current sense resistor described in the Current Limit section
(2)
For example, if RS is 10 mΩ , and the desired power limit threshold is 20W, RPWR calculates to 46.4 kΩ. If Q1’s
power dissipation reaches the threshold Q1’s gate is modulated to regulate the load current, keeping Q1’s power
from exceeding the threshold. For proper operation of the power limiting feature, RPWR must be ≤150 kΩ. While
the power limiting circuit is active, the fault timer is active as described in the Fault Timer & Restart section.
Typically, power limit is reached during startup, or if the output voltage falls due to a severe overload or short
circuit.
The programmed maximum power dissipation should have a reasonable margin from the maximum power
defined by the FET's SOA chart if the LM25069-2 is used since the FET will be repeatedly stressed during fault
restart cycles. The FET manufacturer should be consulted for guidelines.
If the application does not require use of the power limit function the PWR pin can be left open.
The accuracy of the power limit function at turn-on may degrade if a very low value power dissipation limit is set.
The reason for this caution is that the voltage across the sense resistor, which is monitored and regulated by the
power limit circuit, is lowest at turn-on when the regulated current is at minimum. The voltage across the sense
resistor during power limit can be expressed as follows:
RS x PFET(LIM)
RPWR
VSENSE = IL x RS =
=
5
VDS
2.32 x 10 x VDS
where
•
•
IL is the current in RS
VDS is the voltage across Q1
(3)
For example, if the power limit is set at 20W with RS = 10 mohms, and VDS = 15V the sense resistor voltage
calculates to 13.3 mV, which is comfortably regulated by the LM25069. However, if a lower power limit is set
lower (e.g., 2W), the sense resistor voltage calculates to 1.33 mV. At this low level noise and offsets within the
LM25069 may degrade the power limit accuracy. To maintain accuracy, the sense resistor voltage should not be
less than 5 mV.
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TURN-ON TIME
The output turn-on time depends on whether the LM25069 operates in current limit, or in both power limit and
current limit, during turn-on.
A) Turn-on with current limit only: The current limit threshold (ILIM) is determined by the current sense resistor
(RS). If the current limit threshold is less than the current defined by the power limit threshold at maximum VDS
the circuit operates at the current limit threshold only during turn-on. Referring to Figure 30a, as the load current
reaches ILIM, the gate-to-source voltage is controlled at VGSL to maintain the current at ILIM. As the output voltage
reaches its final value, (VDS ≊ 0V) the drain current reduces to its normal operating value. The time for the OUT
pin voltage to transition from zero volts to VSYS is equal to:
VSYS x CL
tON =
ILIM
where
•
CL is the load capacitance
(4)
For example, if VSYS = 12V, CL = 1000 µF, and ILIM = 1A, tON calculates to 12 ms. The maximum instantaneous
power dissipated in the MOSFET is 12W. This calculation assumes the time from t1 to t2 in Figure 30a is small
compared to tON, and the load does not draw any current until after the output voltage has reached its final value,
and PGD switches high (Figure 28). If the load draws current during the turn-on sequence (Figure 29), the turnon time is longer than the above calculation, and is approximately equal to:
tON = -(RL x CL) x In
(ILIM x RL) - VSYS
(ILIM x RL)
where
•
RL is the load resistance
(5)
The Fault Timeout Period must be set longer than tON to prevent a fault shutdown before the turn-on sequence is
complete.
RS
Q1
VSYS
OUT
PGD
VIN
LM25069
CL
RL
GND
GND
Figure 28. No Load Current During Turn-On
RS
Q1
VSYS
OUT
VIN
CL
PGD
LM25069
RL
GND
GND
Figure 29. Load Draws Current During Turn-On
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B) Turn-on with power limit and current limit: The maximum allowed power dissipation in Q1 (PFET(LIM)) is
defined by the resistor at the PWR pin, and the current sense resistor RS. See the POWER LIMIT THRESHOLD
section. If the current limit threshold (ILIM) is higher than the current defined by the power limit threshold at
maximum VDS (PFET(LIM)/VSYS) the circuit operates initially in the power limit mode when the VDS of Q1 is high,
and then transitions to current limit mode as the current increases to ILIM and VDS decreases. See Figure 30b.
Assuming the load (RL) is not connected during turn-on, the time for the output voltage to reach its final value is
approximately equal to:
CL x VSYS2 CL x PFET(LIM)
tON =
+
2 x PFET(LIM)
2 x ILIM2
(6)
For example, if VSYS = 12V, CL = 1000 µF, ILIM = 1A, and PFET(LIM) = 10W, tON calculates to ≊12.2 ms, and the
initial current level (IP) is approximately 0.83A. The Fault Timeout Period must be set longer than tON.
VSYS
VSYS
VDS
VDS
Drain Current
ILIM
Drain Current
ILIM
IP
0
0
VGATE
VGATE
Gate- to - Source Voltage
VGSL
VGSL
VTH
VTH
t ON
0
0
t3
t1 t2
a) Current Limit Only
t ON
0
0
b) Power Limit and Current Limit
Figure 30. MOSFET Power Up Waveforms
MOSFET SELECTION
It is recommended that the external MOSFET (Q1) selection be based on the following criteria:
• The BVDSS rating should be greater than the maximum system voltage (VSYS), plus ringing and transients
which can occur at VSYS when the circuit card, or adjacent cards, are inserted or removed.
• The maximum continuous current rating should be based on the current limit threshold (50 mV/RS), not the
maximum load current, since the circuit can operate near the current limit threshold continuously.
• The Pulsed Drain Current spec (IDM) must be greater than the current threshold for the circuit breaker function
(95 mV/RS).
• The SOA (Safe Operating Area) chart of the device, and the thermal properties, should be used to determine
the maximum power dissipation threshold set by the RPWR resistor. The programmed maximum power
dissipation should have a reasonable margin from the maximum power defined by the FET's SOA chart if the
LM25069-2 is used since the FET will be repeatedly stressed during fault restart cycles. The FET
manufacturer should be consulted for guidelines.
• RDS(on) should be sufficiently low that the power dissipation at maximum load current (IL(max)2 x RDS(on)) does
not raise its junction temperature above the manufacturer’s recommendation.
If the circuit’s input voltage is at the low end of the LM25069’s operating range (14V), the gate-to-source voltage applied to the MOSFET by the LM25069 is less than 5V, and
can approach 1V in a worst case situation. See the graph “ GATE Pin voltage”. The selected device must have a
suitable Gate-to-Source Threshold Voltage.
The gate-to-source voltage provided by the LM25069 can be as high as 19.5V at turn-on when the output voltage
is zero. At turn-off the reverse gate-to-source voltage will be equal to the output voltage at the instant the GATE
pin is pulled low. If the device chosen for Q1 is not rated for these voltages, an external zener diode must be
added from its gate to source, with the zener voltage less than the device maximum VGS rating. The zener
diode’s working voltage protects the MOSFET during turn-on, and its forward voltage protects the MOSFET
during shutoff. The zener diode’s forward current rating must be at least 260 mA to conduct the GATE pull-down
current when a circuit breaker condition is detected.
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TIMER CAPACITOR, CT
The TIMER pin capacitor (CT) sets the timing for the insertion time delay, fault timeout period, and restart timing
of the LM25069-2.
A) Insertion Delay - Upon applying the system voltage (VSYS) to the circuit, the external MOSFET (Q1) is held
off during the insertion time (t1 in Figure 23) to allow ringing and transients at VSYS to settle. Since each
backplane’s response to a circuit card plug-in is unique, the worst case settling time must be determined for each
application. The insertion time starts when VIN reaches the POR threshold, at which time the internal 5.5 µA
current source charges CT from 0V to 1.72V. The required capacitor value is calculated from:
t1 x 5.5 PA
CT =
= t1 x 3.2 x 10-6
1.72V
(7)
For example, if the desired insertion delay is 250 ms, CT calculates to 0.8 µF. At the end of the insertion delay,
CT is quickly discharged by a 2 mA current sink.
B) Fault Timeout Period - During in-rush current limiting or upon detection of a fault condition where the current
limit and/or power limit circuits regulate the current through Q1, the fault timer current source (80 µA) switches on
to charge CT. The Fault Timeout Period is the time required for the voltage at the TIMER pin to transition from
ground to 1.72V, at which time Q1 is switched off. If the LM25069-1 is in use, the required capacitor value is
calculated from:
tFAULT x 80 PA
= tFAULT x 4.65 x 10-5
CT =
1.72V
(8)
For example, if the desired Fault Timeout Period is 17 ms, CT calculates to 0.8 µF. When the Fault Timeout
Period expires, the LM25069-1 latches the GATE pin low until a power-up sequence is initiated by external
circuitry. If the LM25069-2 is in use, the Fault Timeout Period during restart cycles is approximately 18% shorter
than the initial fault timeout period which initiated the restart cycles since the voltage at the TIMER pin transitions
from 0.3V to 1.72V. Since the Fault Timeout Period must always be longer than the turn-on-time, the required
capacitor value for the LM25069-2 is calculated using this shorter time period:
CT =
tFAULT x 80 PA
1.42V
-5
= tFAULT x 5.63 x 10
(9)
For example, if the desired Fault Timeout Period is 17 ms, CT calculates to 0.96 µF. When the Fault Timeout
Period of the LM25069-2 expires, a restart sequence starts as described below (Restart Timiing). Since the
LM25069 normally operates in power limit and/or current limit during a power-up sequence, the Fault Timeout
Period MUST be longer than the time required for the output voltage to reach its final value. See the TURN-ON
TIME section
C) Restart Timing For the LM25069-2, after the Fault Timeout Period described above, CT is discharged by the
2.5 µA current sink to 1.0V. The TIMER pin then cycles through seven additional charge/discharge cycles
between 1V and 1.72V as shown in Figure 25. The restart time ends when the TIMER pin voltage reaches 0.3V
during the final high-to-low ramp. The restart time, after the Fault Timeout Period, is equal to:
tRESTART = CT x
7 x 0.72V 7 x 0.72V 1.42V
+
+
2.5 PA
80 PA
2.5 PA
= CT x 2.65 x 106
(10)
For example, if CT = 0.8 µF, tRESTART = 2.12 seconds. At the end of the restart time, Q1 is switched on. If the fault
is still present, the fault timeout and restart sequence repeats. The on-time duty cycle of Q1 is approximately
0.67% in this mode.
UVLO, OVLO
By programming the UVLO and OVLO thresholds the LM25069 enables the series pass device (Q1) when the
input supply voltage (VSYS) is within the desired operational range. If VSYS is below the UVLO threshold, or above
the OVLO threshold, Q1 is switched off, denying power to the load. Hysteresis is provided for each threshold.
Option A: The configuration shown in Figure 31 requires three resistors (R1-R3) to set the thresholds.
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VSYS
VIN
R1
LM25069
UVLO
1.17V
R2
1.16V
R3
20 PA
TIMER AND GATE
LOGIC CONTROL
OVLO
GND
20 PA
Figure 31. UVLO and OVLO Thresholds Set By R1-R3
The procedure to calculate the resistor values is as follows:
• Choose the upper UVLO threshold (VUVH), and the lower UVLO threshold (VUVL).
• Choose the upper OVLO threshold (VOVH).
• The lower OVLO threshold (VOVL ) cannot be chosen in advance in this case, but is determined after the
values for R1-R3 are determined. If VOVL must be accurately defined in addition to the other three thresholds,
see Option B below.
The resistors are calculated as follows:
VUVH - VUVL VUV(HYS)
=
20 PA
20 PA
(11)
1.16V x R1 x VUVL
R3 =
VOVH x (VUVL ± 1.17V)
(12)
R1 =
1.17V x R1
- R3
VUVL - 1.17V
R2 =
(13)
The lower OVLO threshold is calculated from:
VOVL = [(R1 + R2) x ((1.16V) - 20 PA)] + 1.16V
R3
(14)
As an example, assume the application requires the following thresholds: VUVH = 8V, VUVL = 7V, VOVH = 15V.
R1 =
8V - 7V
1V
= 50 k:
=
20 PA
20 PA
(15)
1.16V x 50 k: x 7V
R3 =
= 4.64 k:
15V x (7V - 1.17V)
R2 =
(16)
1.17V x 50 k:
- 4.64 k: = 5.39 k:
(7V ± 1.17V)
(17)
The lower OVLO threshold calculates to 13.9V, and the OVLO hysteresis is 1.1V. Note that the OVLO hysteresis
is always slightly greater than the UVLO hysteresis in this configuration. When the R1-R3 resistor values are
known, the threshold voltages and hysteresis are calculated from the following:
VUVH = 1.17V + [R1 x (20 PA +
VUVL =
1.17V
)]
(R2 + R3)
1.17V x (R1 + R2 + R3)
R2 + R3
(19)
(20)
VUV(HYS) = R1 x 20 µA
18
(18)
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VOVH =
1.16V x (R1 + R2 + R3)
R3
(21)
VOVL = [(R1 + R2) x ((1.16V) - 20 PA)] + 1.16V
R3
(22)
(23)
VOV(HYS) = (R1 + R2) x 20 µA
Option B: If all four thresholds must be accurately defined, the configuration in Figure 32 can be used.
VSYS
VIN
20 PA
LM25069
R1
UVLO
R3
1.17V
R2
1.16V
TIMER AND GATE
LOGIC CONTROL
OVLO
R4
GND
20 PA
Figure 32. Programming the Four Thresholds
The four resistor values are calculated as follows:
• Choose the upper and lower UVLO thresholds (VUVH) and (VUVL).
R1 =
R2 =
•
VUVH - VUVL VUV(HYS)
=
20 PA
20 PA
(24)
1.17V x R1
(VUVL - 1.17V)
(25)
Choose the upper and lower OVLO threshold (VOVH) and (VOVL).
R3 =
VOV(HYS)
VOVH - VOVL
=
20 PA
20 PA
(26)
1.16V x R3
(VOVH - 1.16V)
(27)
R4 =
As an example, assume the application requires the following thresholds: VUVH = 8V, VUVL = 7V, VOVH = 15.5V,
and VOVL = 14V. Therefore VUV(HYS) = 1V, and VOV(HYS) = 1.5V. The resistor values are:
R1 = 50 kΩ, R2 = 10 kΩ
R3 = 75 kΩ, R4 = 6.07 kΩ
Where the R1-R4 resistor values are known, the threshold voltages and hysteresis are calculated from the
following:
VUVH = 1.17V + [R1 x (1.17V + 20 PA)]
R2
VUVL =
(28)
1.17V x (R1 + R2)
R2
(29)
(30)
VUV(HYS) = R1 x 20 µA
VOVH =
1.16V x (R3 + R4)
R4
(31)
VOVL = 1.16V + [R3 x (1.16V - 20 PA)]
R4
(32)
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VOV(HYS) = R3 x 20 µA
(33)
Option C: The minimum UVLO level is obtained by connecting the UVLO pin to VIN as shown in Figure 33. Q1
is switched on when the VIN voltage reaches the POR threshold (≊2.6V). The OVLO thresholds are set using
R3, R4. Their values are calculated using the procedure in Option B.
VSYS
VIN
20 A
LM25069
10k
UVLO
1.17V
Shutdown/
Restart
Control
TIMER AND GATE
LOGIC CONTROL
R3
1.16V
R4
OVLO
20 A
GND
Figure 33. UVLO = POR with Shutdown/Restart Control
Option D: The OVLO function can be disabled by grounding the OVLO pin. The UVLO thresholds are set as
described in Option B or Option C.
POWER GOOD PIN
During turn-on, the Power Good pin (PGD) is high until the voltage at VIN increases above ≊ 1.6V. PGD then
switches low, remaining low as the VIN voltage increases. When the voltage at OUT increases to within 1.3V of
the SENSE pin (VDS
