Datasheet LTC1422 Datasheet (Linear Technology)

Page 1
FEATURES
Allows Safe Board Insertion and Removal from a Live Backplane
System Reset Output with Programmable Delay
Programmable Electronic Circuit Breaker
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
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APPLICATIO S
LTC1422
Hot Swap Controller
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DESCRIPTIO
TM
The LTC®1422 is an 8-pin Hot Swap 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.
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.
controller that
Hot Board Insertion
Electronic Circuit Breaker
TYPICAL APPLICATIO
5V Hot Swap
Q1
R1
MTB50N06V
VCCSENSE GATE ON
CONNECTOR 1
0.005
R2 10 5%
8
76
FB
LTC1422
RESET
GNDTIMER
34
C2
0.33µF
52
1
V
ON/RESET
GND
CC
CONNECTOR 2
PLUG-IN CARDBACKPLANE
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C1
0.1µF
R3
6.81k 1%
R4
2.43k 1%
µP
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.
V
OUT
+
5V
C
LOAD
1422 TA01
1
Page 2
LTC1422
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
WW
W
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ABSOLUTE MAXIMUM RATINGS
PACKAGE
/
O
RDER I FOR ATIO
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(Note 1)
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 13.2V
Output Voltage (GATE) ............................. –0.3V to 20V
Operating Temperature Range
ORDER PART
NUMBER
LTC1422CN8 LTC1422CS8 LTC1422IN8 LTC1422IS8
LTC1422C ............................................... 0°C to 70°C
T
LTC1422I........................................... –40°C to 85°C
Storage Temperature Range ................ –65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
temperature range, otherwise specifications are at T
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
DC Characteristics
I
CC
V
LKO
V
LKH
V
FB
V V
FBHST
V
TM
V V
TMHST
I
TM
V
CB
I
CP
V V
ONHI
V
ONLO
V
ONHYST
V
OL
I
PU
t
RST
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired.
VCC Supply Current ON = V VCC Undervoltage Lockout 2.40 2.47 2.55 V VCC Undervoltage Lockout Hysteresis 120 mV FB Pin Voltage Threshold 1.220 1.232 1.244 V FB Pin Threshold Line Regulation 3V ≤ V
FB
FB Pin Voltage Threshold Hysteresis 2.0 mV TIMER Pin Voltage Threshold 1.208 1.232 1.256 V TIMER Pin Threshold Line Regulation 3V ≤ VCC 12V 215 mV
TM
TIMER Pin Voltage Threshold Hystersis 45 mV TIMER Pin Current Timer On, GND ≤ V
Timer Off, V Circuit Breaker Trip Voltage VCB = (VCC – V GATE Pin Output Current Charge Pump On, V
Charge Pump Off, V External N-Channel Gate Drive V
GATE
ON Pin Threshold High 1.25 1.30 1.35 V ON Pin Threshold Low 1.20 1.23 1.26 V ON Pin Hysteresis 80 mV Output Low Voltage RESET, IO = 3mA 0.14 0.4 V Logic Output Pull-Up Current RESET = GND –12 µA Soft Reset Time 22 30 38 µs
GATE
The denotes the specifications which apply over the full operating
= 25°C. V
A
CC
12V 0.5 2.5 mV
CC
– V
CC
= 5V unless otherwise noted.
CC
1.5V –2.5 –2.0 –1.5 µA
TIMER
= 1.5V 10 mA
TIMER
) 44 50 64 mV
SENSE
= GND –12 –10 –8 µA
GATE
= V
GATE
= 150°C, θJA = 130°C/W (N)
JMAX
= 150°C, θJA = 150°C/W (S)
T
JMAX
CC
S8 PART MARKING
1422 1422I
0.65 1.00 mA
10 mA
10 12 14 V
2
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TYPICAL PERFORMANCE CHARACTERISTICS
LTC1422
Supply Current vs Supply Voltage Supply Current vs Temperature
1200
TA = 25°C
1000
800
600
400
SUPPLY CURRENT (µA)
200
0
2
6810
4
SUPPLY VOLTAGE (V)
Gate Voltage vs Temperature
18.2 VCC = 5V
= 0A
I
18.0
G
17.8
17.6
17.4
17.2
17.0
GATE VOLTAGE (V)
16.8
16.6
16.4
–55
–35
5
–15
25
TEMPERATURE (°C)
45
12 14
1422 G01
125
85
105
65
1422 G04
775
VCC = 5V
750
725
700
675
650
625
SUPPLY CURRENT (µA)
600
575
550
–55
–35
5
–15
25
TEMPERATURE (°C)
45
Gate Current vs Supply Voltage
16
TA = 25°C
= 0V
V
G
14
12
10
8
GATE CURRENT (µA)
6
4
2
6810
4
SUPPLY VOLTAGE (V)
Gate Voltage vs Supply Voltage
30
TA = 25°C
= 0A
I
G
25
20
15
10
GATE VOLTAGE (V)
5
0
125
85
105
65
1422 G02
2
6810
4
SUPPLY VOLTAGE (V)
12 14
1422 G03
Gate Current vs Temperature
10.4 VCC = 5V
= 0V
V
10.2
G
10.0
9.8
9.6
9.4
9.2
GATE CURRENT (µA)
9.0
8.8
12 14
1422 G05
8.6 –55
–35
–15
5
25
45
TEMPERATURE (°C)
65
125
85
105
1422 G06
Feedback Threshold Voltage vs Supply Voltage
1.2350 TA = 25°C
1.2345
1.2340
1.2335
1.2330
1.2325
FEEDBACK THRESHOLD VOLTAGE (V)
1.2320
2
4
HIGH THRESHOLD
LOW THRESHOLD
6810
SUPPLY VOLTAGE (V)
12 14
1422 G07
Feedback Threshold Voltage vs Temperature
1.237
1.236
1.235
1.234
1.233
1.232
1.231
1.230
1.229
FEEDBACK THRESHOLD VOLTAGE (V)
1.228
HIGH THRESHOLD
–55
–35
5
–15
25
TEMPERATURE (°C)
LOW THRESHOLD
45
Glitch Filter Time vs Feedback Transient
70
60
50
40
30
GLITCH FILTER TIME (µs)
20
10
65
125
85
105
1422 G08
0
80 120 160
40
FEEDBACK TRANSIENT (mV)
TA = 25°C
200 240
1422 G09
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LTC1422
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TYPICAL PERFORMANCE CHARACTERISTICS
TIMER Threshold Voltage vs Supply Voltage
1.244
TA = 25°C
1.242
1.240
1.238
1.236
1.234
TIMER THRESHOLD VOLTAGE (V)
1.232
2
2.45 VCC = 5V
2.40
2.35
2.30
2.25
2.20
2.15
TIMER CURRENT (µA)
2.10
2.05
2.00
–55
–35
6810
4
SUPPLY VOLTAGE (V)
5
–15
25
TEMPERATURE (°C)
TIMER Threshold Voltage vs Temperature
1.242
12 14
1422 G10
1.241
1.240
1.239
1.238
1.237
1.236
1.235
TIMER THRESHOLD VOLTAGE (V)
1.234
1.233 –55
VCC = 12V
–35
–15
VCC = 5V
VCC = 3V
5
25
45
TEMPERATURE (°C)
65
125
85
105
1422 G11
ON Pin Threshold Voltage vs Supply VoltageTIMER Current vs Temperature
1.32
1.30
2
HIGH THRESHOLD
LOW THRESHOLD
6810
4
SUPPLY VOLTAGE (V)
1.28
1.26
1.24
1.22
ON PIN THRESHOLD VOLTAGE (V)
45
65
125
85
105
1422 G13
1.20
TA = 25°C
12 14
1422 G14
TIMER Current vs Supply Voltage
2.6 TA = 25°C
2.5
2.4
2.3
2.2
TIMER CURRENT (µA)
2.1
2.0
2
6810
4
SUPPLY VOLTAGE (V)
ON Pin Threshold Voltage vs Temperature
1.38 VCC = 5V
1.36
1.34
1.32
1.30
1.28
1.26
1.24
ON PIN THRESHOLD VOLTAGE (V)
1.22
1.20
–55
–35
INPUT HIGH
INPUT LOW
5
–15
25
TEMPERATURE (°C)
45
12 14
1422 G12
65
125
85
105
1422 G15
4
Current Limit Threshold vs Temperature
85
VCC = 5V
80
75
70
65
60
55
50
CURRENT LIMIT THRESHOLD (mV)
45
40
–35
–55
5
–15
25
TEMPERATURE (°C)
45
RESET Pull-Up Current vs Temperature
22
VCC = 5V
20
18
16
14
12
10
8
RESET PULL-UP CURRENT (µA)
6
4
–35
125
85
105
65
1422 G17
–55
5
–15
25
TEMPERATURE (°C)
45
65
125
85
105
1422 G18
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TYPICAL PERFORMANCE CHARACTERISTICS
RESET Voltage vs Temperature
0.24 VCC = 5V
0.22
3mA PULL-UP
0.20
0.18
0.16
0.14
0.12
RESET VOLTAGE (V)
0.10
0.08
0.06
–55
–35
–15
5
25
45
TEMPERATURE (°C)
65
125
85
105
1422 G19
55 50
45
40
35
30
25
ON PIN PULSE TIME (µs)
20
15
10
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PIN FUNCTIONS
ON Pin Pulse (Soft Reset) Time vs Temperature
VCC = 3V
VCC = 5V
VCC = 12V
5
25
–55
–35
–15
TEMPERATURE (°C)
45
65
85
LTC1422
125
105
1422 G20
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/C
. When the voltage reaches the trip point
TIMER
(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.
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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. 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 cur- rent. When the circuit breaker trips, the undervoltage lockout circuit monitoring V
trips, or the ON pin is pulled
CC
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
and SENSE,
CC
W
BLOCK DIAGRA
V
CC
8
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
and SENSE can be shorted together.
CC
V
(Pin 8): The positive supply input, ranging from 2.7V
CC
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.
GATE
7
– +
50mV
Q3
6
CHARGE
PUMP
ON
TIMER
COMP 3
2
3
Q1 Q2
REF
2µA
REF
+
+
COMP 1
COMP 4
2.47V UVL
LOGIC
10µs
FILTER
1.232V
REFERENCE
GLITCH
FILTER
COMP 2
REF
12µA
1422 BD
5
FB
1
RESET
4
GND
+
6
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LTC1422
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APPLICATIONS INFORMATION
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.
VCC + 10V
V
SLOPE = 10µA/C1
CC
t
1
t
2
GATE
V
OUT
1422 F02
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).
V
CC
R1
8 VCCSENSE ON
TIMER GND
34
76
LTC1422
C2
Q1
GATE
RESET
FB
R2 10
V
OUT
+
C
R3
C1
52
1
R4
LOAD
1422F01
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
ON
V
CC
2
LOGIC
R1
8
SENSE
V
CC
COMP 2
Q1
R2
C1
6
7
GATELTC1422
+
FB
R3
5
R4
V
OUT
+
C
LOAD
12µA
TIMER
34
C2
1.232V
REFERENCE
Q2
µP
1
RESET
1422 F03
Figure 3. Supply Monitor Block Diagram
7
Page 8
LTC1422
RESET
V
OUT
GATE
1422 F05
TIMER
ON
30µs
12
30µs
3465
15µs
20µs
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APPLICATIONS INFORMATION
1234
V2 V2 V2V1V1
V
OUT
1.232V
TIMER
RESET
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).
1.232V
1422 F04
Figure 5. Soft Reset Waveforms
If the ON pin is held low for longer than 40µs, 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 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.
ON
V
CC
2
2µA
R1
8
SENSE
V
CC
+
COMP 4
1.232V
TIMER 3
Q1
C2
7
LOGIC
Q2
R2
C1
6
GATELTC1422
5
SUPPLY
MONITOR
1
4
+
R3
R4
RESET
1422 F06
V
C
OUT
LOAD
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.
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
1 2 3 4 5 6 7 8 9 10
V
CC
ON
V
– V
CC
SENSE
TIMER
GATE
V
OUT
RESET
1422 F07
Figure 7. Current Fault Timing
R1
C
F
8
SENSE
V
CC
LTC1422
Figure 8. Extending the Short-Circuit Protection Delay
Q1
R
F
R2
6
7
GATE
C1
1422 F08
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
V
CC
10k 10k
10k
CONNECTOR 1
ON/RESET
CONNECTOR 2
V
IN
3.3V
CURRENT LIMIT: 5A
V
IN
5V
RESET
ON
C1
0.33µF 16V
3.3V OUT
LTC1422
1
RESET
2
ON
3
TIMER
4
GND
5V OUT
SENSE
GATE
V
CC
FB
Q4
Q2: 2N7002LT1 Q3, Q4: MMBT3904LT1
8 7 6 5
R1
8
V
CC
ON
2
Q3
TIMER
Q2
+
COMP 5
REF
3
C2
Figure 9. ON Pin Circuitry
1/2 Si99436
R2
0.01
5%
Q1
1/2 Si9436
R3 10 5%
R1 10k 5%
C3
0.047µF 25V
D1
1N4148
R6 1M 5%
SENSE
Q2
7
R7 10 5%
C2
0.022µF 25V
LOGIC
Q1
6
GATE
LTC1422
4
R2
5
FB
1
+
+
R4
2.74k 1%
TRIP POINT: 4.6V
R5 1k 1%
V
OUT
+
C
LOAD
C1
R3
R4
RESET
1422 F09
V
OUT
3.3V
C
LOAD
V
OUT
5V
C
LOAD
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.
10
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LTC1422
U
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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.
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 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
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. Back­to-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.
CURRENT LIMIT: 2.5A
V
IN
5V
1
RESET
2
ON
3
TIMER
4
GND
Q2
MMFT2N02ELT1
LTC1422
SENSE
V
GATE
R1
0.02
5%
8
CC
7 6 5
FB
Q1
MMFT2N02ELT1
R3 10 5%
C2
0.1µF 16V
R6
1.6M 5%
R2 10 5%
C1
0.0033µF 16V
Figure 11. Switching 5V and Generating 3.3V
R7 360k 5%
Q3 PN2222
+
+
R4
2.74k 1%
R5
1.62k 1%
C
C
LOAD
LOAD
C3
0.1µF 16V
1422 F11
V
OUT
3.3V
V
OUT
5V
11
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LTC1422
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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 R
drop across the
DS(ON)
transistor. Table 1 lists some transistors that are available.
Table 1. N-Channel Selection Guide
CURRENT LEVEL (A) PART NUMBER DESCRIPTION MANUFACTURER
0 to 2 MMDF3N02HD Dual N-Channel SO-8 ON Semiconductor
2 to 5 MMSF5N02HD Single N-Channel SO-8 ON Semiconductor
5 to 10 MTB50N06V Single N-Channel DD Pak ON Semiconductor
10 to 20 MTB75N05HD Single N-Channel DD Pak ON Semiconductor
Table 2. Sense Resistor Selection Guide
CURRENT LIMIT VALUE PART NUMBER DESCRIPTION MANUFACTURER
1A LR120601R050 0.05 0.25W 1% Resistor IRC-TT 2A LR120601R025 0.025Ω 0.25W 1% Resistor IRC-TT
2.5A LR120601R020 0.02 0.25W 1% Resistor IRC-TT
3.3A WSL2512R015F 0.015 1W 1% Resistor Vishay-Dale 5A LR120601R010 0.01 0.25W 1% Resistor IRC-TT 10A WSR2R005F 0.005 2W 1% Resistor Vishay-Dale
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.
= 0.1
R
DS(ON)
= 0.025
R
DS(ON)
= 0.028
R
DS(ON)
R
= 0.0095
DS(ON)
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
12
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LTC1422
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APPLICATIONS INFORMATION
R1 36k 5%
48V
+ –
FUSE
MMBT5551LT1
R2
1.2k 5%
Q2
C1
0.47µF 25V
CIRCUIT TURNS ON WHEN V CIRCUIT FOR ACTIVE LOW TURN-ON MODULES
1 2 3 4
LTC1422
RESET ON TIMER GND
V
SENSE
GATE
CC
FB
IN
8 7 6 5
> 38V
R5 10k 5%
D1
7.5V 1N755A
C4 1µF 25V
R6 1M 5%
R7 270k 5%
C2
0.1µF 25V
OPTIONAL
PRECHARGE RESISTOR
100µF
100V
Q1
IRF530
R4
510
5%
C3
R3 10 5%
AT&T
JW050A1-E
50W
+
+
V
V
OUT
+
IN
V
IN
SENSE SENSE
ON/OFF
+
V
OUT
5V
1422 F12
+
48V
FUSE
R1 36k 5%
MMBT5551LT1
R2
1.2k 5%
Figure 12. Switching 48V to an AT&T Module
++
+
C3
100µF
C2
0.1µF 25V
Q1
IRF530
R8
510
5%
100V
R3 10 5%
R5 10k 5%
LTC1422
Q2
1
RESET
2
ON
3
TIMER
4
GND
C1
0.1µF 25V
CIRCUIT TURNS ON WHEN V CIRCUIT FOR ACTIVE HIGH TURN-ON MODULES
V
SENSE
GATE
8
CC
7 6 5
FB
D1
7.5V 1N755A
> 38V
IN
C4 1µF 25V
R6 1M 5%
R7 270k 5%
VICOR
VI-J30-CY
__
GATE IN
R4
5.1k 5%
Q3 MMBT5551LT1
5V
PRECHARGE RESISTOR
Figure 13. Switching 48V to a Vicor Module
OPTIONAL
1422 F13
13
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LTC1422
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APPLICATIONS INFORMATION
COMMON
RETURN
–48V
–48V
FUSE 1
FUSE 2
R6 10k 5%
R7 10k 5%
MUR415
MUR415
D4
1N4148
D5
1N4148
D2
D3
R1 36k 5%
R2
1.2k 5%
R10
5.1k 5%
FAULT
4N25
Figure 14. Hop Swapping Redundant 48V Supplies
R5 10k 5%
LTC1422
1
RESET
2
ON
3
TIMER
4
GND
C1
0.33µF 16V
Q1 TURNS ON WHEN V FAULT GOES LOW WHEN EITHER SUPPLY FAILS
V
SENSE
GATE
IN
CC
FB
> 38V
8 7 6 5
R9 1k 5%
D1
7.5V 1N755A
C3
+
100µF
100V
R4 10k 5%
C2
0.1µF 25V
C4 1µF 25V
Q1
IRF530
R8
510
5%
OPTIONAL
PRECHARGE RESISTOR
++
VICOR
VI-J30-CY
__
GATE IN
Q2 MPSA56
R3 10 5%
5V
1422 F14
48V
5V
C5
0.33µF 16V
R1 36k 5%
R4
4N25
5.1k 5%
1 2 3 4
C1
0.1µF 25V
CIRCUIT TURNS ON WHEN V CIRCUIT FOR ACTIVE HIGH TURN-ON MODULES
+ –
R2
1.2k 5%
FUSE
1 2 3 4
LTC1422
RESET ON TIMER GND
LTC1422
RESET ON TIMER GND
SENSE
V
GATE
SENSE
GATE
CC
FB
V
8 7 6 5
IN
CC
FB
> 38V
8 7 6 5
R5
6.2k 5%
0.5
D1
7.5V 1N755A
R9
5%
C4 1µF 25V
Q4
R10 10 5%
C6
0.022µF 16V
R13 28k 1%
R14 10k 1%
R6 1M 5%
R7 270k 5%
C2
0.1µF 25V
OPTIONAL
PRECHARGE RESISTOR
+
100µF
IRF530
R8
510
5%
100V
Q1
C7 47µF 16V
C3
R3 10 5%
RESET
++
+
__
4N25
V
CC
µP
PWRGD
GND
VICOR
VI-J30-CY
GATE IN
1422 F15
ON
R11
R12
5.1k
5.1k
5%
5%
5V
Figure 15. Switching 48V to a Vicor Module with Isolated Controller
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LTC1422
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APPLICATIONS INFORMATION
V
IN
3.3V
V
IN
5V
R1
1.3k
1 2
3 4
RESET ON
LTC1422
TIMER GND
1%
R2 1 k 1%
Figure 16. Power Supply Sequencer
U
PACKAGE DESCRIPTION
0.300 – 0.325
(7.620 – 8.255)
Dimensions in inches (millimeters) unless otherwise noted.
8-Lead PDIP (Narrow 0.300)
0.045 – 0.065
(1.143 – 1.651)
Q1
1/2
MMDF 2N02E
8
V
CC
7
SENSE
GATE
FB
R4
2.74k 1%
6 5
R5 1k 1%
N8 Package
(LTC DWG # 05-08-1510)
0.130 ± 0.005
(3.302 ± 0.127)
R3 10 5%
C1
0.047µF 25V
Q1 1/2
MMDF 2N02E
V
OUT
3.3V
D1 MBRS120T3
V
OUT
5V
1422 F16
876
0.400*
(10.160)
MAX
5
0.065
(1.651)
0.009 – 0.015
(0.229 – 0.381)
+0.035
0.325
–0.015
+0.889
8.255
()
–0.381
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm)
TYP
0.100
(2.54)
BSC
8-Lead Plastic Small Outline (Narrow 0.150)
0.010 – 0.020
(0.254 – 0.508)
0.008 – 0.010
(0.203 – 0.254)
*
DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**
DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
× 45°
0.016 – 0.050
(0.406 – 1.270)
0.053 – 0.069
(1.346 – 1.752)
0°– 8° TYP
0.014 – 0.019
(0.355 – 0.483)
TYP
0.125
(3.175)
MIN
0.018 ± 0.003
(0.457 ± 0.076)
S8 Package
(LTC DWG # 05-08-1610)
0.004 – 0.010
(0.101 – 0.254)
0.050
(1.270)
BSC
0.020
(0.508)
MIN
0.228 – 0.244
(5.791 – 6.197)
0.255 ± 0.015* (6.477 ± 0.381)
0.189 – 0.197* (4.801 – 5.004)
7
8
1
2
12
6
3
4
3
N8 1098
5
0.150 – 0.157** (3.810 – 3.988)
4
SO8 1298
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
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LTC1422
TYPICAL APPLICATION
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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 voltage drop across the current sense resistor R is proportional to the load current I across R forced on R
+
48V
is buffered by the op amp follower and is
SENSE
R1 36k 5%
R2
1.2k 5%
.
LTC1422
1
RESET
2
ON
3
TIMER
4
GND
C1
0.47µF 25V
MIRROR
FUSE
. The voltage drop
LOAD
R5 15k 5%
8
V
CC
7
SENSE
6
GATE
5
FB
D1
7.5V
1N755A
I
MIRROR
VN2222L
I
MIRROR
Q2
SENSE
R
TRIP
10 5%
R
MIRROR
39 5%
The mirror current can be described as: I R
SENSE/RMIRROR
trip resistor R 50mV across R low (50mV = I R
). This example uses a 48V input but this translation
TRIP
. The mirror current flows through the
. When the mirror current generates
TRIP
, the LTC1422 will latch the GATE pin
TRIP
MIRROR
• R
TRIP
= I
LOAD
MIRROR
• R
SENSE/RMIRROR
= I
LOAD
circuit can be used anywhere the current sense resistor is not tied to VCC.
Q1 IRF530
R
SENSE
0.02 5%
I
LOAD
1422 F17
+
LOAD
C4 1µF 25V
7
LT1006
4
+
OPAMP
C5
0.22µF
100V
+
R4 10k 5%
R6 1M 5%
R7 270k 5%
3
2
C2
0.1µF 25V
R3
10
5%
100µF
100V
+
C3
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
1422fa LT/TP 0300 2K REV A • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 1997
16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
www.linear-tech.com
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