1
LTC1422
1422fb
Hot Swap Controller
■
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
5V Hot Swap
The LTC
®
1422 is an 8-pin Hot Swap
TM
controller that
allows a board to be safely inserted and removed from a
live backplane. Using an external N-channel pass transis-
tor, 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.
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.
The LTC1422 is available in 8-pin PDIP and SO packages.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Hot Swap is a trademark of Linear Technology Corporation.
■
Hot Board Insertion
■
Electronic Circuit Breaker
+
V
CC
SENSE GATE
LTC1422
GNDTIMER
ON
8
34
76
R2
10Ω
5%
R3
6.81k
1%
R4
2.43k
1%
R1
0.005ΩQ1
MTB50N06V
C1
0.1µF
C
LOAD
V
OUT
5V
V
CC
ON/RESET
GND
1422 TA01
PLUG-IN CARDBACKPLANE
C2
0.33µF
52
1
FB
RESET
RESET
µP
CONNECTOR 1
CONNECTOR 2
APPLICATIO S
U
FEATURES
TYPICAL APPLICATIO
U
DESCRIPTIO
U
2
LTC1422
1422fb
Consult LTC Marketing for parts specified with wider operating temperature ranges.
WU
U
PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
TJMAX = 150°C, θJA = 130°C/W (N)
TJMAX = 150°C, θJA = 150°C/W (S)
1
2
3
4
8
7
6
5
TOP VIEW
N8 PACKAGE
8-LEAD PDIP S8 PACKAGE
8-LEAD PLASTIC SO
RESET
ON
TIMER
GND
V
CC
SENSE
GATE
FB
Supply Voltage (V
CC
) ........................................... 13.2V
Input Voltage (TIMER, SENSE) ... –0.3V to (V
CC
+ 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
ABSOLUTE MAXIMUM RATINGS
W
WW
U
LTC1422CN8
LTC1422CS8
LTC1422IN8
LTC1422IS8
S8 PART MARKING
ELECTRICAL CHARACTERISTICS
1422
1422I
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
DC Characteristics
I
CC
V
CC
Supply Current ON = V
CC
●0.65 1.00 mA
V
LKO
V
CC
Undervoltage Lockout ●2.40 2.47 2.55 V
V
LKH
V
CC
Undervoltage Lockout Hysteresis 120 mV
V
FB
FB Pin Voltage Threshold ●1.220 1.232 1.244 V
∆V
FB
FB Pin Threshold Line Regulation 3V ≤ V
CC
≤ 12V ●0.5 2.5 mV
V
FBHST
FB Pin Voltage Threshold Hysteresis 2.0 mV
V
TM
TIMER Pin Voltage Threshold ●1.208 1.232 1.256 V
∆V
TM
TIMER Pin Threshold Line Regulation 3V ≤ V
CC
≤ 12V ●215 mV
V
TMHST
TIMER Pin Voltage Threshold Hystersis 45 mV
I
TM
TIMER Pin Current Timer On, GND ≤ V
TIMER
≤ 1.5V ●–2.5 –2.0 –1.5 µA
Timer Off, V
TIMER
= 1.5V 10 mA
V
CB
Circuit Breaker Trip Voltage V
CB
= (V
CC
– V
SENSE
)●44 50 64 mV
I
CP
GATE Pin Output Current Charge Pump On, V
GATE
= GND ●–12 –10 –8 µA
Charge Pump Off, V
GATE
= V
CC
10 mA
∆V
GATE
External N-Channel Gate Drive V
GATE
– V
CC
●10 12 14 V
V
ONHI
ON Pin Threshold High ●1.25 1.30 1.35 V
V
ONLO
ON Pin Threshold Low ●1.20 1.23 1.26 V
V
ONHYST
ON Pin Hysteresis 80 mV
V
OL
Output Low Voltage RESET, I
O
= 3mA ●0.14 0.4 V
I
PU
Logic Output Pull-Up Current RESET = GND –12 µA
t
RST
Soft Reset Time ●22 30 48 µs
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 5V unless otherwise noted.
(Note 1)
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
3
LTC1422
1422fb
TYPICAL PERFORMANCE CHARACTERISTICS
UW
SUPPLY VOLTAGE (V)
2
0
GATE VOLTAGE (V)
5
10
15
20
30
46810
1422 G03
12 14
25
TA = 25°C
IG = 0A
Gate Voltage (VGATE)
vs Supply Voltage
Gate Current vs Temperature
SUPPLY VOLTAGE (V)
2
FEEDBACK THRESHOLD VOLTAGE (V)
1.2350
1.2345
1.2340
1.2335
1.2330
1.2325
1.2320 46810
1422 G07
12 14
T
A
= 25°C
HIGH THRESHOLD
LOW THRESHOLD
Feedback Threshold Voltage
vs Supply Voltage
Glitch Filter Time
vs Feedback Transient
Gate Voltage (VGATE)
vs Temperature
TEMPERATURE (°C)
–55
16.4
GATE VOLTAGE (V)
16.6
17.0
17.2
17.4
65
18.2
1422 G04
16.8
5–35 85
25–15 105
45 125
17.6
17.8
18.0 V
CC
= 5V
I
G
= 0A
SUPPLY VOLTAGE (V)
2
0
SUPPLY CURRENT (µA)
200
400
600
800
1200
46810
1422 G01
12 14
1000
TA = 25°C
Supply Current vs Supply Voltage Supply Current vs Temperature
TEMPERATURE (°C)
–55
550
SUPPLY CURRENT (µA)
575
625
650
675
65
775
1422 G02
600
5–35 85
25–15 105
45 125
700
725
750 V
CC
= 5V
Gate Current vs Supply Voltage
SUPPLY VOLTAGE (V)
2
4
GATE CURRENT (µA)
6
8
10
12
16
46810
1422 G05
12 14
14
TA = 25°C
VG = 0V
FEEDBACK TRANSIENT (mV)
0
10
GLITCH FILTER TIME (µs)
20
30
40
50
70
40 80 120 160
1422 G09
200 240
60
TA = 25°C
Feedback Threshold Voltage
vs Temperature
TEMPERATURE (°C)
–55
FEEDBACK THRESHOLD VOLTAGE (V)
65
1.237
1.236
1.235
1.234
1.233
1.232
1.231
1.230
1.229
1.228
1422 G08
5–35 85
25–15 105
45 125
HIGH THRESHOLD
LOW THRESHOLD
TEMPERATURE (°C)
–55
8.6
GATE CURRENT (µA)
8.8
9.2
9.4
9.6
65
10.4
1422 G06
9.0
5–35 85
25–15 105
45 125
9.8
10.0
10.2 V
CC
= 5V
V
G
= 0V
4
LTC1422
1422fb
TYPICAL PERFORMANCE CHARACTERISTICS
UW
SUPPLY VOLTAGE (V)
2
TIMER THRESHOLD VOLTAGE (V)
1.244
1.242
1.240
1.238
1.236
1.234
1.232 46810
1422 G10
12 14
T
A
= 25°C
TIMER Threshold Voltage
vs Supply Voltage
SUPPLY VOLTAGE (V)
2
2.0
TIMER CURRENT (µA)
2.1
2.2
2.3
2.4
2.6
46810
1422 G12
12 14
2.5
TA = 25°C
TIMER Current vs Supply Voltage
TEMPERATURE (°C)
–55
TIMER THRESHOLD VOLTAGE (V)
65
1.242
1.241
1.240
1.239
1.238
1.237
1.236
1.235
1.234
1.233
1422 G11
5–35 85
25–15 105
45 125
V
CC
= 12V
V
CC
= 5V
V
CC
= 3V
TIMER Threshold Voltage
vs Temperature
ON Pin Threshold Voltage
vs Supply VoltageTIMER Current vs Temperature
TEMPERATURE (°C)
–55
TIMER CURRENT (µA)
65
2.45
2.40
2.35
2.30
2.25
2.20
2.15
2.10
2.05
2.00
1422 G13
5–35 85
25–15 105
45 125
V
CC
= 5V
SUPPLY VOLTAGE (V)
2
ON PIN THRESHOLD VOLTAGE (V)
1.32
1.30
1.28
1.26
1.24
1.22
1.20 46810
1422 G14
12 14
TA = 25°C
LOW THRESHOLD
HIGH THRESHOLD
ON Pin Threshold Voltage
vs Temperature
TEMPERATURE (°C)
–55
ON PIN THRESHOLD VOLTAGE (V)
65
1.38
1.36
1.34
1.32
1.30
1.28
1.26
1.24
1.22
1.20
1422 G15
5–35 85
25–15 105
45 125
V
CC
= 5V
INPUT HIGH
INPUT LOW
Current Limit Threshold
vs Temperature
TEMPERATURE (°C)
–55
40
CURRENT LIMIT THRESHOLD (mV)
45
55
60
65
65
85
1422 G17
50
5–35 85
25–15 105
45 125
70
75
80 V
CC
= 5V
RESET Pull-Up Current
vs Temperature
TEMPERATURE (°C)
–55
4
RESET PULL-UP CURRENT (µA)
6
10
12
14
65
22
1422 G18
8
5–35 85
25–15 105
45 125
16
18
20 V
CC
= 5V
5
LTC1422
1422fb
TYPICAL PERFORMANCE CHARACTERISTICS
UW
PIN FUNCTIONS
UUU
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 V
CC
is connected to the TIMER pin and the
voltage starts to ramp up with a slope given by: dV/dt =
2µA/C
TIMER
. 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 •
C
TIMER
)/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.
RESET (Pin 1) : Open drain output to GND with a 12µA
pull-up to V
CC
. 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
V
CC
.
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.
RESET Voltage vs Temperature
TEMPERATURE (°C)
–55
RESET VOLTAGE (V)
65
0.24
0.22
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
1422 G19
5–35 85
25–15 105
45 125
V
CC
= 5V
3mA PULL-UP
TEMPERATURE (°C)
–55
ON PIN PULSE TIME (µs)
65
55
50
45
40
35
30
25
20
15
10
1422 G20
5–35 85
25–15 105
45 125
V
CC
= 12V
V
CC
= 5V
V
CC
= 3V
ON Pin Pulse (Soft Reset) Time
vs Temperature
6
LTC1422
1422fb
PIN FUNCTIONS
UUU
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 V
CC
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 V
CC
trips, or the
ON pin is pulled low for more than 40µs, the GATE pin is
immediately pulled to GND.
SENSE (Pin 7) : Circuit Breaker Set Pin. With a sense
resistor placed in the supply path between V
CC
and SENSE,
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,
V
CC
and SENSE can be shorted together.
V
CC
(Pin 8): The positive supply input, ranging from 2.7V
to 13.2V for normal operation. I
CC
is typically 0.6mA. An
undervoltage lockout circuit disables the chip until the
voltage at V
CC
is greater than 2.47V.
–
+
1.232V
REFERENCE
20µs GLITCH
FILTER
10µs
FILTER
2.47V
UVL
+
–
50mV
8
REF
5FB
1RESET
4GND
COMP 3
V
CC
7
SENSE
6
GATE
–
+
COMP 1
REF
–
+
COMP 2
2µA
12µA
–
+
COMP 4
REF
TIMER
1422 BD
3
ON 2
Q1 Q2
LOGIC
CHARGE
PUMP
Q3
BLOCK DIAGRA
W
7
LTC1422
1422fb
APPLICATIONS INFORMATION
WUU U
Hot Circuit Insertion
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.
V
CC
+ 10V
V
CC
1422 F02
t
1
t
2
GATE
V
OUT
SLOPE = 10µA/C1
Figure 2. Supply Turn-On
Figure 3. Supply Monitor Block Diagram
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 = (V
CC
• 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).
Figure 1. Supply Control Circuitry
+
VCC SENSE
TIMER GND
GATE
LTC1422
ON
8
34
76 C1
CLOAD
C2
R3
VOUT
VCC
Q1
R1
52
1422F01
1
FB
RESET
R2
10Ω
R4
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
+
SENSE
12µA
R4
R3
7
VCC
VOUT
VCC
8
GATELTC1422
6
2
FB
Q2
ON
1422 F03
1
5
C2
34
C1
CLOAD
Q1
R1
LOGIC
TIMER 1.232V
REFERENCE
–
+
COMP 2
µP
RESET
R2
8
LTC1422
1422fb
APPLICATIONS INFORMATION
WUU U
V
OUT
TIMER
RESET
V2 V2 V2V1V1
1.232V 1.232V
1234
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 V
CC
on RESET has a
series diode so the pin can be pulled above V
CC
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).
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 relation-
ship between glitch filter time and the transient voltage is
shown in Typical Performance curve: Glitch Filter Time vs
Feedback Transient.
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.
Figure 5. Soft Reset Waveforms
RESET
V
OUT
GATE
1422 F05
TIMER
ON
30µs
12 30µs (typ)
3465
15µs
20µs
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.
+
SENSE
TIMER
2µAR4
R3
R2
7
V
CC
V
OUT
V
CC
8
GATELTC1422
6
2
ON
1422 F06
SUPPLY
MONITOR
1
5
C2
34
RESET
C1
C
LOAD
Q2
R1
LOGIC
1.232V
–
+
COMP 4
Q1
Figure 6. System Timing Block Diagram
9
LTC1422
1422fb
APPLICATIONS INFORMATION
WUU U
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 V
CC
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.
Electronic Circuit Breaker
The LTC1422 features an electronic circuit breaker func-
tion that protects against short circuits or excessive cur-
rents 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 V
CC
.
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
V
CC
connection and all other pins. When V
CC
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
RESET
V
OUT
GATE
1422 F07
TIMER
ON
V
CC
– V
SENSE
V
CC
1 2 3 4 5 6 7 8 9 10
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
Figure 7. Current Fault Timing
SENSE
7
V
CC
8
GATE
LTC1422
6C1
1422 F08
Q1
R1
R
F
C
F
R2
Figure 8. Extending the Short-Circuit Protection Delay
10
LTC1422
1422fb
1
2
3
4
8
7
6
5
V
CC
SENSE
GATE
FB
RESET
ON
TIMER
GND
LTC1422
1422 F10
D1
1N4148
Q1
1/2 Si9436
Q2
1/2 Si99436
R2
0.01Ω
5%
R3
10Ω
5%
C1
0.33µF
16V
R1
10k
5%
RESET
ON
R6
1M
5%
R7
10Ω
5%
R4
2.74k
1%
TRIP POINT: 4.6V
R5
1k
1%
C3
0.047µF
25V
C2
0.022µF
25V
C
LOAD
C
LOAD
V
OUT
3.3V
V
OUT
5V
V
IN
3.3V
V
IN
5V
5V OUT
3.3V OUT
CURRENT LIMIT: 5A
+
+
Figure 10. Switching 5V and 3.3V
+
SENSE
R4
R3
R2
7
V
CC
V
OUT
V
CC
8
10k GATE
LTC1422
6
2
REF
FB
1422 F09
1
5
C2
Q2: 2N7002LT1
Q3, Q4: MMBT3904LT1
Q2
ON/RESET 3
TIMER
ON
4
RESET
C1
C
LOAD
Q1
R1
LOGIC
–
+
COMP 5
10k 10k
Q3
Q4
CONNECTOR 1
CONNECTOR 2
Figure 9. ON Pin Circuitry
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 addi-
tional 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 compara-
tor 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 V
CC
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.
APPLICATIONS INFORMATION
WUU U
11
LTC1422
1422fb
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.
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. Back-
to-back transistors Q1 and Q2 ensure isolation between
the two supplies.
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.
Since the pass transistor is in a common source configu-
ration, 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.
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.
Hot Swapping Redundant 48V
In critical situations, redundant input supplies are neces-
sary. 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
1
2
3
4
8
7
6
5
V
CC
SENSE
GATE
FB
RESET
ON
TIMER
GND
LTC1422
1422 F11
Q1
MMFT2N02ELT1
CURRENT LIMIT: 2.5A
Q2
MMFT2N02ELT1
R1
0.02Ω
5%
R3
10Ω
5% R6
1.6M
5%
R7
360k
5%
R2
10Ω
5%
R4
2.74k
1%
R5
1.62k
1%
Q3
PN2222
C1
0.0033µF
16V
C2
0.1µF
16V
C
LOAD
C3
0.1µF
16V
C
LOAD
V
OUT
3.3V
V
OUT
5V
V
IN
5V
+
+
Figure 11. Switching 5V and Generating 3.3V
APPLICATIONS INFORMATION
WUU U
12
LTC1422
1422fb
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 maxi-
mum allowed current times R
DS(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 manu-
facturers.
Table 1. N-Channel Selection Guide
CURRENT LEVEL (A) PART NUMBER DESCRIPTION MANUFACTURER
0 to 2 MMDF3N02HD Dual N-Channel SO-8 ON Semiconductor
R
DS(ON)
= 0.1Ω
2 to 5 MMSF5N02HD Single N-Channel SO-8 ON Semiconductor
R
DS(ON)
= 0.025Ω
5 to 10 MTB50N06V Single N-Channel DD Pak ON Semiconductor
R
DS(ON)
= 0.028Ω
10 to 20 MTB75N05HD Single N-Channel DD Pak ON Semiconductor
R
DS(ON)
= 0.0095Ω
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
APPLICATIONS INFORMATION
WUU U
13
LTC1422
1422fb
Figure 13. Switching 48V to a Vicor Module
1
2
3
4
8
7
6
5
V
CC
SENSE
GATE
FB
VICOR
VI-J30-CY
Q3
MMBT5551LT1
5V
++
__
GATE IN
C3
100µF
100V
RESET
ON
TIMER
GND
LTC1422
Q1
IRF530
OPTIONAL
PRECHARGE RESISTOR
1422 F13
Q2
MMBT5551LT1
C1
0.1µF
25V
CIRCUIT TURNS ON WHEN V
IN
> 38V
CIRCUIT FOR ACTIVE HIGH TURN-ON MODULES
D1
7.5V
1N755A
C4
1µF
25V
R6
1M
5%
R7
270k
5%
C2
0.1µF
25V
R5
10k
5%
R1
36k
5%
R2
1.2k
5%
R3
10Ω
5%
R8
510Ω
5%
R4
5.1k
5%
48V
+
–
FUSE
+
1
2
3
4
8
7
6
5
VCC
SENSE
GATE
FB
VOUT+
SENSE+
SENSE–
VOUT–
VIN+
VIN–
5V
ON/OFF
C3
100µF
100V
AT&T
JW050A1-E
50W
RESET
ON
TIMER
GND
LTC1422
Q1
IRF530
OPTIONAL
PRECHARGE RESISTOR
1422 F12
Q2
MMBT5551LT1
C1
0.47µF
25V
CIRCUIT TURNS ON WHEN VIN > 38V
CIRCUIT FOR ACTIVE LOW TURN-ON MODULES
D1
7.5V
1N755A
C4
1µF
25V
R6
1M
5%
R7
270k
5%
C2
0.1µF
25V
R5
10k
5%
R1
36k
5%
R2
1.2k
5%
FUSE
R3
10Ω
5%
R4
510Ω
5%
48V
+
–
+
Figure 12. Switching 48V to an AT&T Module
APPLICATIONS INFORMATION
WUU U
14
LTC1422
1422fb
Figure 15. Switching 48V to a Vicor Module with Isolated Controller
1
2
3
4
8
7
6
5
V
CC
SENSE
GATE
FB
VICOR
VI-J30-CY
5V
GND
µP
PWRGD
RESET ON
V
CC
++
__
GATE IN
C3
100µF
100V
RESET
ON
TIMER
GND
LTC1422
Q1
IRF530
OPTIONAL
PRECHARGE RESISTOR
1422 F15
1
2
3
4
8
7
6
5
V
CC
SENSE
GATE
FB
RESET
ON
TIMER
GND
LTC1422
Q4
5V
C1
0.1µF
25V
CIRCUIT TURNS ON WHEN V
IN
> 38V
CIRCUIT FOR ACTIVE HIGH TURN-ON MODULES
D1
7.5V
1N755A
C4
1µF
25V
R6
1M
5%
R7
270k
5%
C2
0.1µF
25V
R5
6.2k
5%
R4
5.1k
5%
R1
36k
5%
R2
1.2k
5%
R3
10Ω
5%
R8
510Ω
5%
48V
+
–
4N25
4N25
R11
5.1k
5%
R12
5.1k
5%
C5
0.33µF
16V
R10
10Ω
5%
R9
0.5Ω
5%
C6
0.022µF
16V
R13
28k
1%
R14
10k
1%
C7
47µF
16V
FUSE
+
+
Figure 14. Hot Swapping Redundant 48V Supplies
1
2
3
4
8
7
6
5
V
CC
SENSE
GATE
FB
VICOR
VI-J30-CY
5V
++
__
GATE IN
C3
100µF
100V
RESET
ON
TIMER
GND
LTC1422
Q1
IRF530
OPTIONAL
PRECHARGE RESISTOR
1422 F14
C1
0.33µF
16V
Q1 TURNS ON WHEN V
IN
> 38V
FAULT GOES LOW WHEN EITHER SUPPLY FAILS
D1
7.5V
1N755A C4
1µF
25V
C2
0.1µF
25V
R5
10k
5%
R1
36k
5%
R7
10k
5%
R10
5.1k
5%
R9
1k
5%
R2
1.2k
5%
R3
10Ω
5%
Q2
MPSA56
R8
510Ω
5%
R4
10k
5%
FAULT
D2
MUR415
D4
1N4148
COMMON
RETURN
D5
1N4148
4N25
D3
MUR415
FUSE 2
FUSE 1
–48V
–48V
R6
10k
5%
+
APPLICATIONS INFORMATION
WUU U
15
LTC1422
1422fb
S8 Package
8-Lead Plastic Small Outline (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1610)
N8 Package
8-Lead PDIP (Narrow .300 Inch)
(Reference LTC DWG # 05-08-1510)
Figure 16. Power Supply Sequencer
V
CC
SENSE
GATE
FB
LTC1422
8
7
6
5
Q1
1/2
MMDF 2N02E
Q1
1/2
MMDF 2N02E
D1
MBRS120T3
V
OUT
5V
V
OUT
3.3V
V
IN
3.3V
V
IN
5V
1
2
3
4
1422 F16
RESET
ON
TIMER
GND
R1
1.3k
1%
R6 10k 5%
R2
1 k
1%
R3
10Ω
5%
R4
2.74k
1%
R5
1k
1%
C1
0.047µF
25V
PACKAGE DESCRIPTION
U
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 represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
APPLICATIONS INFORMATION
WUU U
N8 1002
.065
(1.651)
TYP
.045 – .065
(1.143 – 1.651)
.130 ± .005
(3.302 ± 0.127)
.020
(0.508)
MIN
.018 ± .003
(0.457 ± 0.076)
.120
(3.048)
MIN 12 34
8765
.255 ± .015*
(6.477 ± 0.381)
.400*
(10.160)
MAX
.008 – .015
(0.203 – 0.381)
.300 – .325
(7.620 – 8.255)
.325 +.035
–.015
+0.889
–0.381
8.255
()
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)
.100
(2.54)
BSC
.016 – .050
(0.406 – 1.270)
.010 – .020
(0.254 – 0.508)× 45°
0°– 8° TYP
.008 – .010
(0.203 – 0.254)
SO8 0303
.053 – .069
(1.346 – 1.752)
.014 – .019
(0.355 – 0.483)
TYP
.004 – .010
(0.101 – 0.254)
.050
(1.270)
BSC
1234
.150 – .157
(3.810 – 3.988)
NOTE 3
8765
.189 – .197
(4.801 – 5.004)
NOTE 3
.228 – .244
(5.791 – 6.197)
.245
MIN .160 ±.005
RECOMMENDED SOLDER PAD LAYOUT
.045 ±.005
.050 BSC
.030 ±.005
TYP
INCHES
(MILLIMETERS)
NOTE:
1. DIMENSIONS IN
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)
16
LTC1422
1422fb
LINEAR TECHNOLOGY CORPORATION 1997
LT/TP 0503 1K REV B • PRINTED IN USA
TYPICAL APPLICATION
U
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
RELATED PARTS
1
2
3
4
8
7
6
5
V
CC
SENSE
GATE
FB
LOAD
+
–
C3
100µF
100V
C5
0.22µF
100V
RESET
ON
TIMER
GND
LTC1422
Q1
IRF530
3
2
7
4
1422 F17
C1
0.47µF
25V D1
7.5V
1N755A
R6
1M
5%
OPAMP
R7
270k
5%
C2
0.1µF
25V
R4
10k
5%
R5
15k
5%
R1
36k
5%
R2
1.2k
5%
FUSE
R3
10Ω
5%
R
SENSE
0.02Ω
5%
R
MIRROR
39Ω
5%
I
MIRROR
I
MIRROR
I
LOAD
48V
+
–
Q2
VN2222L
–
+
LT1006
R
TRIP
10Ω
5%
+
C4
1µF
25V
+
Figure 17. Switching 48V with Current Sensing
Current Sensing with 48V Applications
In the LTC1422, the SENSE pin threshold is 50mV below
the V
CC
pin. Typically, the current sense resistor is con-
nected to the V
CC
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 voltage drop across the current sense resistor R
SENSE
is proportional to the load current I
LOAD
. The voltage drop
across R
SENSE
is buffered by the op amp follower and is
forced on R
MIRROR
.
The mirror current can be described as: I
MIRROR
= I
LOAD
•
R
SENSE
/R
MIRROR
. The mirror current flows through the
trip resistor R
TRIP
. When the mirror current generates
50mV across R
TRIP
, the LTC1422 will latch the GATE pin
low (50mV = I
MIRROR
• R
TRIP
= I
LOAD
• R
SENSE
/R
MIRROR
•
R
TRIP
). This example uses a 48V input but this translation
circuit can be used anywhere the current sense resistor is
not tied to V
CC
.
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507
●
www.linear.com