LTC1422
Hot Swap Controller
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DESCRIPTIO
FEATURES
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The LTC®1422 is an 8-pin Hot SwapTM controller that
allows a board to be safely inserted and removed from a
live backplane. Using an external N-channel pass transistor, the board supply voltage can be ramped up at a
programmable rate. A high side switch driver controls the
N-channel gate for supply voltages ranging from 2.7V to
12V.
Allows Safe Board Insertion and Removal
from a Live Backplane
System Reset Output with Programmable Delay
Programmable Electronic Circuit Breaker
User-Programmable Supply Voltage Power-Up Rate
High Side Driver for an External N-Channel FET
Controls Supply Voltages from 2.7V to 12V
Undervoltage Lockout
Soft Reset Input
Glitch Filter on RESET
Available in 8-Pin Narrow PDIP and SO Packages
A programmable electronic circuit breaker protects
against shorts. The RESET output can be used to generate
a system reset when the supply voltage falls below a
programmable voltage. The ON pin can be used to cycle
the board power or to generate a soft reset.
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APPLICATIO S
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The LTC1422 is available in 8-pin PDIP and SO packages.
Hot Board Insertion
Electronic Circuit Breaker
, LTC and LT are registered trademarks of Linear Technology Corporation.
Hot Swap is a trademark of Linear Technology Corporation.
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TYPICAL APPLICATIO
5V Hot Swap
Q1
R1
0.005Ω MTB50N06V
VCC
CONNECTOR 1
CONNECTOR 2
R2
10Ω
5%
ON/RESET
VOUT
5V
+
8
2
7
VCC
FB
ON
5
LTC1422
RESET
TIMER
3
GND
C2
0.33µF
4
R3
6.81k
1%
C1
0.1µF
6
SENSE GATE
CLOAD
1
R4
2.43k
1%
µP
RESET
GND
1422 TA01
BACKPLANE
PLUG-IN CARD
1422fb
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LTC1422
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Supply Voltage (VCC) ........................................... 13.2V
Input Voltage (TIMER, SENSE) ... – 0.3V to (VCC + 0.3V)
Input Voltage (FB, ON) ........................... – 0.3V to 13.2V
Output Voltage (RESET) ........................... – 0.3V to 20V
Output Voltage (GATE) ............................. – 0.3V to 20V
Operating Temperature Range
LTC1422C ............................................... 0°C to 70°C
LTC1422I ........................................... – 40°C to 85°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
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(Note 1)
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ABSOLUTE MAXIMUM RATINGS
PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
TOP VIEW
RESET 1
8 VCC
ON 2
6 GATE
GND 4
N8 PACKAGE
8-LEAD PDIP
LTC1422CN8
LTC1422CS8
LTC1422IN8
LTC1422IS8
7 SENSE
TIMER 3
5 FB
S8 PACKAGE
8-LEAD PLASTIC SO
TJMAX = 150°C, θJA = 130°C/W (N)
TJMAX = 150°C, θJA = 150°C/W (S)
S8 PART MARKING
1422
1422I
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 5V unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
0.65
1.00
mA
2.47
2.55
DC Characteristics
ICC
VCC Supply Current
ON = VCC
VLKO
VCC Undervoltage Lockout
VLKH
VCC Undervoltage Lockout Hysteresis
VFB
FB Pin Voltage Threshold
∆VFB
FB Pin Threshold Line Regulation
VFBHST
FB Pin Voltage Threshold Hysteresis
VTM
TIMER Pin Voltage Threshold
∆VTM
TIMER Pin Threshold Line Regulation
VTMHST
TIMER Pin Voltage Threshold Hystersis
ITM
TIMER Pin Current
Timer On, GND ≤ VTIMER ≤ 1.5V
Timer Off, VTIMER = 1.5V
●
– 2.5
– 2.0
10
– 1.5
µA
mA
VCB
Circuit Breaker Trip Voltage
VCB = (VCC – VSENSE)
●
44
50
64
mV
ICP
GATE Pin Output Current
Charge Pump On, VGATE = GND
Charge Pump Off, VGATE = VCC
●
– 12
– 10
10
–8
µA
mA
∆VGATE
External N-Channel Gate Drive
VGATE – VCC
●
10
12
14
V
VONHI
ON Pin Threshold High
●
1.25
1.30
1.35
V
VONLO
ON Pin Threshold Low
●
1.20
1.23
1.26
V
VONHYST
ON Pin Hysteresis
VOL
Output Low Voltage
RESET, IO = 3mA
IPU
Logic Output Pull-Up Current
RESET = GND
tRST
Soft Reset Time
●
●
2.40
120
●
3V ≤ VCC ≤ 12V
1.220
●
1.232
1.244
0.5
2.5
2.0
●
3V ≤ VCC ≤ 12V
1.208
●
1.232
1.256
2
15
22
30
V
mV
mV
0.4
V
µA
– 12
●
mV
mV
80
0.14
V
mV
45
●
V
mV
48
µs
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
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LTC1422
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TYPICAL PERFORMANCE CHARACTERISTICS
Supply Current vs Supply Voltage
1200
30
775
VCC = 5V
TA = 25°C
750
800
600
400
TA = 25°C
IG = 0A
25
725
GATE VOLTAGE (V)
SUPPLY CURRENT (µA)
1000
SUPPLY CURRENT (µA)
Gate Voltage (VGATE)
vs Supply Voltage
Supply Current vs Temperature
700
675
650
625
20
15
10
600
200
5
575
0
2
4
6
8
10
SUPPLY VOLTAGE (V)
12
550
– 55 – 35 –15
14
0
5 25 45 65 85 105 125
TEMPERATURE (°C)
1422 G01
12
Gate Current vs Supply Voltage
VCC = 5V
IG = 0A
14
1422 G03
Gate Current vs Temperature
16
18.2
10.4
TA = 25°C
VG = 0V
14
VCC = 5V
VG = 0V
10.2
10.0
17.6
17.4
17.2
17.0
GATE CURRENT (µA)
GATE CURRENT (µA)
17.8
GATE VOLTAGE (V)
6
8
10
SUPPLY VOLTAGE (V)
4
1422 G02
Gate Voltage (VGATE)
vs Temperature
18.0
2
12
10
8
9.8
9.6
9.4
9.2
9.0
16.8
6
8.8
16.6
16.4
– 55 – 35 –15
4
5 25 45 65 85 105 125
TEMPERATURE (°C)
2
4
6
8
10
SUPPLY VOLTAGE (V)
12
Feedback Threshold Voltage
vs Supply Voltage
1422 G06
Feedback Threshold Voltage
vs Temperature
1.2350
Glitch Filter Time
vs Feedback Transient
70
1.237
HIGH THRESHOLD
1.2340
1.2335
LOW THRESHOLD
1.2330
1.2325
1.2320
2
4
6
8
10
SUPPLY VOLTAGE (V)
12
14
1422 G07
TA = 25°C
1.236
60
HIGH THRESHOLD
1.235
1.234
1.233
LOW THRESHOLD
1.232
1.231
1.230
GLITCH FILTER TIME (µs)
FEEDBACK THRESHOLD VOLTAGE (V)
TA = 25°C
1.2345
5 25 45 65 85 105 125
TEMPERATURE (°C)
1422 G05
1422 G04
FEEDBACK THRESHOLD VOLTAGE (V)
8.6
– 55 – 35 –15
14
50
40
30
20
1.229
1.228
– 55 – 35 –15
10
5 25 45 65 85 105 125
TEMPERATURE (°C)
1422 G08
0
40
80
120
160
200
FEEDBACK TRANSIENT (mV)
240
1422 G09
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LTC1422
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TYPICAL PERFORMANCE CHARACTERISTICS
TIMER Threshold Voltage
vs Supply Voltage
TIMER Threshold Voltage
vs Temperature
1.244
TA = 25°C
TA = 25°C
1.241
1.240
1.238
1.236
1.234
VCC = 12V
1.240
2.5
TIMER CURRENT (µA)
1.242
TIMER THRESHOLD VOLTAGE (V)
TIMER THRESHOLD VOLTAGE (V)
TIMER Current vs Supply Voltage
2.6
1.242
1.239
VCC = 5V
1.238
1.237
VCC = 3V
1.236
2.4
2.3
2.2
1.235
2.1
1.234
1.232
4
6
8
10
SUPPLY VOLTAGE (V)
12
14
1.233
– 55 – 35 –15
2.0
5 25 45 65 85 105 125
TEMPERATURE (°C)
1422 G10
ON PIN THRESHOLD VOLTAGE (V)
TA = 25°C
TIMER CURRENT (µA)
2.25
2.20
2.15
2.10
1.36
1.30
HIGH THRESHOLD
1.28
1.26
1.24
LOW THRESHOLD
1.22
2.05
2.00
– 55 – 35 –15
1.20
5 25 45 65 85 105 125
TEMPERATURE (°C)
2
4
6
8
10
SUPPLY VOLTAGE (V)
14
12
14
VCC = 5V
1.34
1.32
INPUT HIGH
1.30
1.28
1.26
1.24
INPUT LOW
1.22
1.20
– 55 – 35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1422 G14
1422 G13
1422 G15
RESET Pull-Up Current
vs Temperature
Current Limit Threshold
vs Temperature
22
85
VCC = 5V
VCC = 5V
20
80
RESET PULL-UP CURRENT (µA)
CURRENT LIMIT THRESHOLD (mV)
12
1.38
VCC = 5V
2.30
6
8
10
SUPPLY VOLTAGE (V)
ON Pin Threshold Voltage
vs Temperature
1.32
2.45
2.35
4
1422 G12
ON Pin Threshold Voltage
vs Supply Voltage
TIMER Current vs Temperature
2.40
2
1422 G11
ON PIN THRESHOLD VOLTAGE (V)
2
75
70
65
60
55
50
16
14
12
10
8
6
45
40
– 55 – 35 –15
18
5 25 45 65 85 105 125
TEMPERATURE (°C)
1422 G17
4
– 55 – 35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1422 G18
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LTC1422
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TYPICAL PERFORMANCE CHARACTERISTICS
ON Pin Pulse (Soft Reset) Time
vs Temperature
RESET Voltage vs Temperature
0.24
0.22
55
VCC = 5V
3mA PULL-UP
50
ON PIN PULSE TIME (µs)
RESET VOLTAGE (V)
0.20
0.18
0.16
0.14
0.12
0.10
0.08
45
40
35
VCC = 3V
30
25
20
VCC = 5V
VCC = 12V
15
0.06
– 55 – 35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
10
– 55 – 35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1422 G19
1422 G20
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PIN FUNCTIONS
RESET (Pin 1) : Open drain output to GND with a 12µA
pull-up to VCC. This pin is pulled low when the voltage at
the FB (Pin 5) goes below the FB pin threshold. The RESET
pin will go high one timing cycle after the voltage at the FB
pin goes above the FB pin threshold. An external pull-up
resistor can be used to speed up the rising edge on the
RESET pin or pull the pin to a voltage higher or lower than
VCC.
ON (Pin 2): Analog Input Pin. The threshold is set at 1.30V
with 80mV hysteresis. When the ON pin is pulled high, the
timer turns on for one cycle, then the charge pump turns
on. When the ON pin is pulled low longer than 40µs, the
GATE pin will be pulled low and remain off until the ON pin
is pulled high.
If the ON pin is pulled low for less than 15µs a soft reset
will occur. The charge pump remains on, and the RESET
pin is pulled low for one timing cycle starting 30µs from
the falling edge of the ON pin.
The ON pin is also used to reset the electronic circuit
breaker. If the ON pin is cycled low and high following the
trip of the circuit breaker, the circuit breaker is reset and
a normal power-up sequence will occur.
TIMER (Pin 3): Analog system timing generator pin. This
pin is used to set the delay before the charge pump turns
on after the ON pin goes high. It also sets the delay before
the RESET pin goes high, after the output supply voltage
is good, as sensed by the FB pin.
When the timer is off, an internal N-channel shorts the
TIMER pin to ground. When the timer is turned on, a 2µA
current from VCC is connected to the TIMER pin and the
voltage starts to ramp up with a slope given by: dV/dt =
2µA/CTIMER. When the voltage reaches the trip point
(1.232V), the timer will be reset by pulling the TIMER pin
back to ground. The timer period is given by: (1.232V •
CTIMER )/2µA.
GND (Pin 4): Chip Ground.
FB (Pin 5): Analog comparator input used to monitor the
output supply voltage with an external resistive divider.
When the voltage on the FB pin is lower than the 1.232V,
the RESET pin will be pulled low. An internal filter helps
prevent negative voltage glitches from triggering a reset.
When the voltage on the FB pin rises above the trip point,
the RESET pin will go high after one timing cycle.
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LTC1422
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PIN FUNCTIONS
GATE (Pin 6): The high side gate drive for the external
N-Channel. An internal charge pump guarantees at least
10V of gate drive when VCC is 5V. The slope of the voltage
rise or fall at the GATE is set by an external capacitor
connected between GATE and GND, and the 10µA charge
pump output current. When the circuit breaker trips, the
undervoltage lockout circuit monitoring VCC trips, or the
ON pin is pulled low for more than 40µs, the GATE pin is
immediately pulled to GND.
the circuit breaker will trip when the voltage across the
resistor exceeds 50mV for more than 10µs. If the circuit
breaker trip current is set to twice the normal operating
current, only 25mV is dropped across the sense resistor
during normal operation. To disable the circuit breaker,
VCC and SENSE can be shorted together.
VCC (Pin 8): The positive supply input, ranging from 2.7V
to 13.2V for normal operation. ICC is typically 0.6mA. An
undervoltage lockout circuit disables the chip until the
voltage at VCC is greater than 2.47V.
SENSE (Pin 7) : Circuit Breaker Set Pin. With a sense
resistor placed in the supply path between VCC and SENSE,
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BLOCK DIAGRA
VCC
SENSE
8
GATE
7
6
50mV
+
–
Q3
CHARGE
PUMP
–
+
COMP 3
ON
2
+
COMP 1
REF
2.47V
UVL
10µs
FILTER
–
1.232V
REFERENCE
REF
–
LOGIC
20µs GLITCH
FILTER
COMP 2
5
FB
+
2µA
TIMER
3
+
COMP 4
REF
Q1
12µA
–
1
RESET
Q2
4
GND
1422 BD
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LTC1422
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APPLICATIONS INFORMATION
Hot Circuit Insertion
VCC + 10V
When circuit boards are inserted into a live backplane, the
supply bypass capacitors on the board can draw huge
transient currents from the backplane power bus as they
charge up. The transient currents can cause permanent
damage to the connector pins and cause glitches on the
system supply, causing other boards in the system to
reset.
The LTC1422 is designed to turn a board’s supply voltage
on and off in a controlled manner, allowing the board to be
safely inserted or removed from a live backplane. The chip
also provides a system reset signal to indicate when board
supply voltage drops below a programmable voltage.
Power Supply Ramping
The onboard power supply is controlled by placing an
external N-channel pass transistor in the power path
(Figure 1). R1 provides current fault detection and R2
prevents high frequency oscillation. By ramping up the
gate of the pass transistor at a controlled rate, the transient
surge current (I = C • dV/dt) drawn from the main backplane
supply can be limited to a safe value when the board makes
connection.
R1
GATE
SLOPE = 10µA/C1
VOUT
VCC
t1
t2
Figure 2. Supply Turn-On
equal to 10µA/C1 (Figure 2), where C1 is the external
capacitor connected between the GATE pin and GND.
The ramp time for the supply is equal to: t = (VCC • C1)/
10µA. After the ON pin has been pulled low for more than
40µs, the GATE is immediately pulled to GND.
Voltage Monitor
The LTC1422 uses a 1.232V bandgap reference, precision
voltage comparator and a resistive divider to monitor the
output supply voltage (Figure 3).
R1
Q1
Q1
VCC
VOUT
VCC
1422 F02
+
+
8
2
7
SENSE GATE
ON
FB
R3
C1
8
5
RESET
1
6
SENSE
R4
ON
GND
3
VCC
LTC1422
7
GATE
R3
LTC1422
TIMER
R2
C1
6
VCC
CLOAD
CLOAD
R2
10Ω
VOUT
2
FB 5
+
R4
COMP 2
LOGIC
–
4
C2
12µA
1422F01
1.232V
REFERENCE
TIMER
Figure 1. Supply Control Circuitry
When power is first applied to the chip, the gate of the
N-channel (Pin 6) is pulled low. After the ON pin is held
high for at least one timing cycle, the charge pump is
turned on. The voltage at GATE begins to rise with a slope
µP
1
RESET
Q2
3
4
C2
1422 F03
Figure 3. Supply Monitor Block Diagram
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LTC1422
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APPLICATIONS INFORMATION
1
2
V2
3
V1
V2
4
V1
1
3 30µs (typ) 4
2
30µs
5
6
V2
VOUT
ON
1.232V
15µs
1.232V
TIMER
TIMER
RESET
GATE
1422 F04
Figure 4. Supply Monitor Waveforms
When the voltage at the FB pin rises above its reset
threshold (1.232V), the comparator COMP 2 output goes
high, and a timing cycle starts (Figure 4, time points 1 and
4). After a complete timing cycle, RESET is pulled high.
The 12µA pull-up current source to VCC on RESET has a
series diode so the pin can be pulled above VCC by an
external pull-up resistor without forcing current back into
supply.
When the supply voltage at the FB pin drops below its reset
threshold, the comparator Comp 2 output goes low. After
passing through a glitch filter, RESET is pulled low (time
point 2). If the FB pin rises above the reset threshold for
less than a timing cycle, the RESET output will remain low
(time point 3).
VOUT
RESET
20µs
Figure 5. Soft Reset Waveforms
If the ON pin is held low for longer than 30µs (typ), the gate
will turn off and the RESET pin will eventually go low (time
points 4, 5 and 6).
Timer
The system timing for the LTC1422 is generated by the
circuitry shown in Figure 6. The timer is used to set the
turn-on delay after the ON pin goes high and the delay
before the RESET pin goes high after the output supply
voltage is good as sensed by the FB pin.
Glitch Filter
The LTC1422 has a glitch filter to prevent RESET from
generating a system reset when there are transients on the
FB pin. The filter is 20µs for large transients (greater than
150mV) and up to 80µs for small transients. The relationship between glitch filter time and the transient voltage is
shown in Typical Performance curve: Glitch Filter Time vs
Feedback Transient.
R1
Q2
VCC
+
VOUT
CLOAD
R2
C1
8
LTC1422
ON
VCC
6
7
SENSE
GATE
R3
2
5
LOGIC
2µA
Soft Reset
In some cases a system reset is desired without a power
down. The ON pin can signal the RESET pin to go low
without turning off the external N-channel (a soft reset).
This is accomplished by holding the ON pin low for only
15µs or less (Figure 5, time point 1). At about 30µs from
the falling edge of the ON pin (time point 2) the RESET pin
goes low and stays low for one timing cycle.
1422 F05
R4
+
COMP 4
1.232V
SUPPLY
MONITOR
–
1
RESET
Q1
TIMER
3
1422 F06
4
C2
Figure 6. System Timing Block Diagram
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LTC1422
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APPLICATIONS INFORMATION
When the timer is off, the internal N-channel Q1 shorts the
TIMER pin to ground. When the timer is turned on, a 2µA
current from VCC is connected to the TIMER pin and the
voltage on the external capacitor C2 starts to ramp up with
a slope given by: dV/dt = 2µA/C2. When the voltage
reaches the trip point (1.232V), the timer will be reset by
pulling the TIMER pin back to ground. The timer period is
given by: (1.232V • C2)/2µA. For a 200ms delay, use a
0.33µF capacitor.
1 2
3
4
5
6
7
8
9
10
VCC
ON
VCC – VSENSE
TIMER
GATE
Electronic Circuit Breaker
The LTC1422 features an electronic circuit breaker function that protects against short circuits or excessive currents on the supply. By placing a sense resistor between
the supply input and SENSE pin, the circuit breaker will be
tripped whenever the voltage across the sense resistor is
greater than 50mV for more than 10µs. When the circuit
breaker trips, the GATE pin is immediately pulled to
ground and the external N-channel is quickly turned off.
When the ON pin is cycled off for greater than 40µs and
then on as shown in Figure 7, time point 7, the circuit
breaker is reset and another timing cycle is started.
At the end of the timer cycle (time point 8), the charge
pump will turn on again. If the circuit breaker feature is not
required, the SENSE pin should be shorted to VCC.
If more than 10µs of response time is needed to reject
supply noise, an external resistor and capacitor can be
added to the sense circuit as shown in Figure 8.
Connection Sense with ON Pin
The ON pin can be used to sense board connection to the
backplane as shown in Figure 9.
Using staggered connection pins, ground mates first to
discharge any static build up on the board, followed by the
VCC connection and all other pins. When VCC makes
connection, the bases of transistors Q3 and Q4 are pulled
high turning them on and pulling the ON pin to ground.
When the base connector pins of Q3 and Q4 finally mate
to the backplane, the bases are shorted to ground. This
turns off Q3 and Q4 and allows the ON pin to pull high and
start a power-up cycle. The base connection pins of Q3 and
Q4 should be located at opposite ends of the connector
VOUT
RESET
1422 F07
Figure 7. Current Fault Timing
R1
CF
Q1
RF
R2
8
VCC
7
SENSE
6
C1
GATE
LTC1422
1422 F08
Figure 8. Extending the Short-Circuit Protection Delay
because most people will rock the board back and forth to
get it seated properly.
A software-initiated power-down cycle can be started by
momentarily turning on transistor Q2, which will pull the
ON pin to ground. If the ON pin is held low for greater than
40µs, the GATE pin is pulled to ground. If the low pulse on
the ON pin is less than 15µs, a soft reset is generated.
Hot Swapping Two Supplies
With two external pass transistors, the LTC1422 can
switch two supplies. In some cases, it is necessary to bring
up the dominant supply first during power-up and ramp it
down last during the power-down phase. The circuit in
Figure 10 shows how to program two different delays for
the pass transistors. The 5V supply is powered up first. R1
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LTC1422
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APPLICATIONS INFORMATION
R1
Q1
VCC
VOUT
+
CLOAD
R2
10k
CONNECTOR 1
CONNECTOR 2
8
10k
10k
C1
6
7
SENSE
VCC
R3
GATE
FB
5
LOGIC
2 ON
R4
+
COMP 5
Q4
REF
Q3
1
RESET
LTC1422
TIMER
3
Q2
ON/RESET
–
4
C2
1422 F09
Q2: 2N7002LT1
Q3, Q4: MMBT3904LT1
Figure 9. ON Pin Circuitry
5V OUT
3.3V OUT
Q2
1/2 Si99436
VIN
3.3V
VOUT
3.3V
+
R2
0.01Ω
5%
CURRENT LIMIT: 5A
VIN
5V
CLOAD
R7
10Ω
5%
Q1
1/2 Si9436
VOUT
5V
+
LTC1422
RESET
ON
1
2
3
C1
0.33µF
16V
4
VCC
RESET
ON
SENSE
TIMER
GND
GATE
FB
8
7
6
R3
10Ω
5%
R1
10k
5%
D1
1N4148
CLOAD
R6
1M
5%
R4
2.74k
1%
TRIP POINT: 4.6V
5
C3
0.047µF
25V
C2
0.022µF
25V
R5
1k
1%
1422 F10
Figure 10. Switching 5V and 3.3V
and C3 are used to set the rise and fall delays on the 5V
supply. Next, the 3.3V supply ramps up with a 20ms delay
set by R6 and C2. On the falling edge, the 3.3V supply
ramps down first because R6 is bypassed by the diode D1.
Using the LTC1422 as a Linear Regulator
The LTC1422 can be used to Hot Swap the primary supply
and generate a secondary low dropout regulated supply.
Figure 11 shows how to switch a 5V supply and create a
3.3V supply using the reset comparator and one additional transistor. The FB pin is used to monitor the 3.3V
output. When the voltage on the gate of Q2 increases, the
3.3V increases. At the 3.3V threshold, the reset comparator will trip. The RESET pin goes high which turns on Q3.
This lowers the voltage on the gate of Q2. This feedback
loop is compensated by the capacitor C1 and the resistors
R6 and R7.
Hot Swapping 48V DC/DC Module with
Active Low On/Off Control Signal
Using a 7.5V Zener and a resistor, the LTC1422 can switch
supplies much greater than the 12V VCC pin rating. As
shown in Figure 12, the switching FET Q1 is connected as
a common source driver rather than the usual source
follower used in previous applications. This allows the
ground of the LTC1422 to sit at the negative terminal of the
48V input. The clamp circuit of R5 and D1 provides power
to the LTC1422. The resistive divider R1 and R2 at the ON
pin monitors the input supply. The switching FET Q1 is
prevented from turning on until the input supply is at least
38V. Using the reset comparator to monitor the gate
voltage allows the module to be turned on after the gate
has reached a minimum level plus one timing cycle. A high
voltage transistor Q2 is used to translate the RESET signal
to the module On/Off input.
1422fb
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APPLICATIONS INFORMATION
Since the pass transistor is in a common source configuration, care must be taken to limit the inrush current into
capacitor C3. One way is to precharge C3 using resistor
R4. As the input supply is ramping up, current is flowing
through R4 and charging the capacitor C3. Once the input
supply crosses 38V, there is a timing cycle followed by the
ramp-up of the GATE pin. By this time the capacitor C3 is
sufficiently charged, thereby limiting the inrush current.
Another method to limit the inrush current is to slow down
the ramp-up rate of the GATE pin.
plus a Zener voltage (D1) is more positive than the drain
of Q1 (in other words, when the switching FET Q1 has only
7.5V across its drain source).
Hot Swapping 48V Module with Isolated Controller
A power supervisory controller will sometimes reside on
an isolated supply with responsibility for other supplies.
Figure 15 shows how to Hot Swap a controller’s 5V supply
and a 48V module using two LTC1422s. Assuming the 5V
supply comes up first, the controller waits for a power
good signal from the 48V circuit. Once it receives the right
signals the controller activates the GATE IN pin of the Vicor
power module.
Hot Swapping 48V DC/DC Module with
Active High On/Off Control Signal
This application is identical to the previous except for the
polarity of the module’s on/off signal. The polarity reversal
is accomplished by transistor Q3 in Figure 13.
Power Supply Sequencer
A circuit that forces two supply voltages to power up
together is shown in Figure 16. The input supply voltages
may power up in any sequence, but both input voltages
must be within tolerance before Q1 and Q2 turn on. Backto-back transistors Q1 and Q2 ensure isolation between
the two supplies.
Hot Swapping Redundant 48V
In critical situations, redundant input supplies are necessary. In Figure 14 a redundant 48V input is switched to a
power module. Supplies 1 and 2 are wire OR’ed using two
diodes D2 and D3. This results in the most negative of
these two supplies being used to drive the power module.
If one of the supplies is disconnected or a fuse opens, the
fault signal will be activated via diodes D4 and D5 and the
reset comparator at the FB pin. The GATE IN signal on the
Vicor module is controlled using the high voltage PNP Q2.
Once the module’s minus input pin is more negative than
the base of Q2 plus a diode drop, Q2 will turn off and the
module will turn on. This occurs when the source of Q1
When the 5V input powers up before 3.3V, Q1 and Q2
remain off and the 5V output remains off until the 3.3V
input is within tolerance as sensed by resistors R1 and R2.
When the 3.3V input powers up before 5V, the diode D1
will pull up the 5V supply output with it. Once the 5V input
powers up and is within tolerance as sensed by R4 and R5,
Q1 and Q2 will turn on in about 1ms and pull the 5V output
up to its final voltage.
Q2
MMFT2N02ELT1
VOUT
3.3V
+
R1
0.02Ω
5%
VIN CURRENT LIMIT: 2.5A
5V
2
3
4
RESET
ON
TIMER
GND
VCC
SENSE
GATE
FB
CLOAD
R2
10Ω
5%
VOUT
5V
+
LTC1422
1
Q1
MMFT2N02ELT1
8
7
6
CLOAD
R3
10Ω
5%
R6
1.6M
5%
R7
360k
5%
Q3
PN2222
5
C2
0.1µF
16V
C1
0.0033µF
16V
R4
2.74k
1%
C3
0.1µF
16V
R5
1.62k
1%
1422 F11
Figure 11. Switching 5V and Generating 3.3V
1422fb
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APPLICATIONS INFORMATION
Power N-Channel and Sense Resistor Selection
The decision of which external power N-Channel to use is
dependent on its maximum current rating and the maximum allowed current times RDS(ON) drop across the
transistor. Table 1 lists some transistors that are available.
Table 2 lists some current sense resistors that can be
used with the circuit breaker. Since this information is
subject to change, please verify the part numbers with the
manufacturer. Table 3 lists the web sites of several manufacturers.
Table 1. N-Channel Selection Guide
CURRENT LEVEL (A)
PART NUMBER
DESCRIPTION
MANUFACTURER
0 to 2
MMDF3N02HD
Dual N-Channel SO-8
RDS(ON) = 0.1Ω
ON Semiconductor
2 to 5
MMSF5N02HD
Single N-Channel SO-8
RDS(ON) = 0.025Ω
ON Semiconductor
5 to 10
MTB50N06V
Single N-Channel DD Pak
RDS(ON) = 0.028Ω
ON Semiconductor
10 to 20
MTB75N05HD
Single N-Channel DD Pak
RDS(ON) = 0.0095Ω
ON Semiconductor
Table 2. Sense Resistor Selection Guide
CURRENT LIMIT VALUE
PART NUMBER
DESCRIPTION
MANUFACTURER
1A
LR120601R050
0.05Ω 0.25W 1% Resistor
IRC-TT
2A
LR120601R025
0.025Ω 0.25W 1% Resistor
IRC-TT
2.5A
LR120601R020
0.02Ω 0.25W 1% Resistor
IRC-TT
3.3A
WSL2512R015F
0.015Ω 1W 1% Resistor
Vishay-Dale
5A
LR120601R010
0.01Ω 0.25W 1% Resistor
IRC-TT
10A
WSR2R005F
0.005Ω 2W 1% Resistor
Vishay-Dale
Table 3. Manufacturers’ Web Sites
MANUFACTURER
WEB SITE
TEMIC Semiconductor
www.temic.com
International Rectifier
www.irf.com
ON Semiconductor
www.onsemiconductor.com
Harris Semiconductor
www.semi.harris.com
IRC-TT
www.irctt.com
Vishay-Dale
www.vishay.com
1422fb
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APPLICATIONS INFORMATION
AT&T
JW050A1-E
50W
R1
36k
5%
C3
100µF
100V
R5
10k
5%
+
VIN+
VOUT+
5V
SENSE +
SENSE –
VIN–
VOUT–
ON/OFF
+
LTC1422
48V
–
1
Q2
MMBT5551LT1
RESET
2
FUSE
SENSE
ON
3
R2
1.2k
5%
TIMER
4
VCC
GND
GATE
FB
8
7
6
5
C1
0.47µF
25V
D1
7.5V
1N755A
C4
1µF
25V
R6
1M
5%
R7
270k
5%
R3
10Ω
5%
C2
0.1µF
25V
Q1
IRF530
CIRCUIT TURNS ON WHEN VIN > 38V
CIRCUIT FOR ACTIVE LOW TURN-ON MODULES
R4
510Ω
5%
OPTIONAL
PRECHARGE RESISTOR
1422 F12
Figure 12. Switching 48V to an AT&T Module
R1
36k
5%
C3
100µF
100V
R5
10k
5%
+
+
–
1
Q2
MMBT5551LT1
2
3
R2
1.2k
5%
FUSE
4
C1
0.1µF
25V
RESET
ON
TIMER
GND
VCC
SENSE
GATE
FB
_
R4
5.1k
5%
LTC1422
48V
+
VICOR
VI-J30-CY
GATE IN
+
5V
_
Q3
MMBT5551LT1
8
7
6
5
D1
7.5V
1N755A
C4
1µF
25V
R6
1M
5%
R7
270k
5%
C2
0.1µF
25V
R3
10Ω
5%
Q1
IRF530
CIRCUIT TURNS ON WHEN VIN > 38V
CIRCUIT FOR ACTIVE HIGH TURN-ON MODULES
R8
510Ω
5%
OPTIONAL
PRECHARGE RESISTOR
1422 F13
Figure 13. Switching 48V to a Vicor Module
1422fb
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APPLICATIONS INFORMATION
COMMON
RETURN
R6
10k
5%
R7
10k
5%
D4
1N4148
R1
36k
5%
R5
10k
5%
R10
5.1k
5%
D5
1N4148
1
FAULT
FUSE 1
D2
MUR415
FUSE 2
D3
MUR415
VCC
RESET
SENSE
ON
3
TIMER
4
GATE
GND
4N25
FB
8
_
VICOR
VI-J30-CY
GATE IN
+
5V
_
Q2
MPSA56
7
6
5
R9
1k
5%
C1
0.33µF
16V
D1
7.5V
1N755A
C2
0.1µF
25V
C4
1µF
25V
– 48V
R3
10Ω
5%
Q1
IRF530
Q1 TURNS ON WHEN VIN > 38V
FAULT GOES LOW WHEN EITHER SUPPLY FAILS
– 48V
+
+
R4
10k
5%
LTC1422
2
R2
1.2k
5%
C3
100µF
100V
R8
510Ω
5%
OPTIONAL
PRECHARGE RESISTOR
1422 F14
Figure 14. Hot Swapping Redundant 48V Supplies
Q4
5V
R9
0.5Ω
5%
LTC1422
1
2
3
4
RESET
ON
VCC
SENSE
TIMER
GATE
GND
FB
8
6
C6
0.022µF
16V
R1
36k
5%
R4
5.1k
5%
48V
–
2
3
4
4N25
C7
47µF
16V
C1
0.1µF
25V
RESET
ON
TIMER
GND
VCC
SENSE
GATE
FB
R12
5.1k
5%
µP
RESET
ON
PWRGD
GND
C3
100µF
100V
+
LTC1422
1
R11
5.1k
5%
VCC
R14
10k
1%
R5
6.2k
5%
+
FUSE
+
5
C5
0.33µF
16V
R2
1.2k
5%
R13
28k
1%
R10
10Ω
5%
7
+
_
VICOR
VI-J30-CY
GATE IN
+
5V
_
4N25
8
7
6
5
D1
7.5V
1N755A
CIRCUIT TURNS ON WHEN VIN > 38V
CIRCUIT FOR ACTIVE HIGH TURN-ON MODULES
C4
1µF
25V
R6
1M
5%
R7
270k
5%
C2
0.1µF
25V
R3
10Ω
5%
Q1
IRF530
1422 F15
R8
510Ω
5%
OPTIONAL
PRECHARGE RESISTOR
Figure 15. Switching 48V to a Vicor Module with Isolated Controller
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APPLICATIONS INFORMATION
VIN
3.3V
Q1
1/2
MMDF 2N02E
VIN
5V
VOUT
3.3V
D1
MBRS120T3
VOUT
5V
Q1
1/2
MMDF 2N02E
R6 10k 5%
R1
1.3k
1%
1
RESET
VCC
8
7
SENSE
LTC1422
6
3
TIMER
GATE
5
4
GND
FB
2
R2
1k
1%
R4
2.74k
1%
ON
R3
10Ω
5%
C1
0.047µF
25V
R5
1k
1%
1422 F16
Figure 16. Power Supply Sequencer
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PACKAGE DESCRIPTION
N8 Package
8-Lead PDIP (Narrow .300 Inch)
(Reference LTC DWG # 05-08-1510)
.300 – .325
(7.620 – 8.255)
.045 – .065
(1.143 – 1.651)
.065
(1.651)
TYP
.008 – .015
(0.203 – 0.381)
(
+.035
.325 –.015
+0.889
8.255
–0.381
)
.400*
(10.160)
MAX
.130 ± .005
(3.302 ± 0.127)
8
7
6
5
1
2
3
4
.255 ± .015*
(6.477 ± 0.381)
.120
(3.048) .020
MIN (0.508)
MIN
.018 ± .003
(0.457 ± 0.076)
.100
(2.54)
BSC
N8 1002
NOTE:
1. DIMENSIONS ARE
INCHES
MILLIMETERS
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm)
S8 Package
8-Lead Plastic Small Outline (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1610)
.189 – .197
(4.801 – 5.004)
NOTE 3
.045 ±.005
.050 BSC
.010 – .020
× 45°
(0.254 – 0.508)
.008 – .010
(0.203 – 0.254)
.245
MIN
.053 – .069
(1.346 – 1.752)
0°– 8° TYP
8
7
6
5
.004 – .010
(0.101 – 0.254)
.160 ±.005
.016 – .050
(0.406 – 1.270)
.030 ±.005
TYP
RECOMMENDED SOLDER PAD LAYOUT
NOTE:
1. DIMENSIONS IN
.014 – .019
(0.355 – 0.483)
TYP
INCHES
(MILLIMETERS)
2. DRAWING NOT TO SCALE
3. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm)
.050
(1.270)
BSC
.150 – .157
(3.810 – 3.988)
NOTE 3
.228 – .244
(5.791 – 6.197)
1
2
3
4
SO8 0303
1422fb
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LTC1422
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TYPICAL APPLICATION
Current Sensing with 48V Applications
In the LTC1422, the SENSE pin threshold is 50mV below
the VCC pin. Typically, the current sense resistor is connected to the VCC pin, but in 48V applications the sense
resistor is connected to the negative terminal of the 48V
supply. The circuit in Figure 17 translates the current in the
sense resistor to a resistor connected to the LTC1422
SENSE pin.
The mirror current can be described as: IMIRROR = ILOAD •
RSENSE/RMIRROR. The mirror current flows through the
trip resistor RTRIP. When the mirror current generates
50mV across RTRIP, the LTC1422 will latch the GATE pin
low (50mV = IMIRROR • RTRIP = ILOAD • RSENSE/RMIRROR •
RTRIP). This example uses a 48V input but this translation
circuit can be used anywhere the current sense resistor is
not tied to VCC.
The voltage drop across the current sense resistor RSENSE
is proportional to the load current ILOAD. The voltage drop
across RSENSE is buffered by the op amp follower and is
forced on RMIRROR.
LTC1422
1
2
3
4
R2
1.2k
5%
+
ON
TIMER
–
SENSE
GATE
FB
GND
C5
0.22µF
100V
RTRIP
10Ω
5%
8
7
R4
10k
5%
6
R6
1M
5%
5
IMIRROR
C1
0.47µF
25V
48V
VCC
RESET
+
R5
15k
5%
+
R1
36k
5%
C4
1µF
25V
D1
7.5V
1N755A
7
+
Q2
VN2222L
3
R7
270k
5%
C3
100µF
100V
+
LOAD
–
R3
10Ω
5%
Q1
IRF530
C2
0.1µF
25V
LT1006
–
2
RSENSE
0.02Ω
5%
4
OPAMP
IMIRROR
FUSE
ILOAD
RMIRROR
39Ω
5%
1422 F17
Figure 17. Switching 48V with Current Sensing
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LTC1421
Hot Swap Controller
24-Pin Multiple Supplies
LT1640L/LT1640H
Negative Voltage Hot Swap Controller in SO-8
Operates from –10V to – 80V
LT1641
High Voltage Hot Swap Controller in SO-8
Operates from 9V to 80V
LT1642
Fault Protected Hot Swap Controller
Operates Up to 16.5V, Protected to 33V
LTC1643L/LTC1643H
PCI-Bus Hot Swap Controller
3.3V, 5V and ±12V in Narrow 16-Pin SSOP
LT1645
2-Channel Hot Swap Controller
Operates from 1.2V to 12V, Power Sequencing
LTC1647
Dual Hot Swap Controller in SO-8 or SSOP-16
Two ON Pins, Operates from 2.7V to 16.5V
1422fb
16 Linear Technology Corporation
LT/TP 0503 1K REV B • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
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