LTC1645
Dual-Channel Hot Swap
Controller/Power Sequencer
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FEATURES
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DESCRIPTIO
Allows Safe Board Insertion and Removal from a
Live Backplane
Programmable Power Supply Sequencing
Programmable Electronic Circuit Breaker
User-Programmable Supply Voltage Power-Up and
Power-Down Rate
High Side Drivers for External N-Channel FETs
Controls Supply Voltages from 1.2V to 12V
Ensures Proper Power-Up Behavior
Undervoltage Lockout
Glitch Filter Protects Against Spurious RESET Signals
8- and 14-Pin SO Packages
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APPLICATIO S
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Hot Board Insertion
Power Supply Sequencing
Electronic Circuit Breaker
The LTC®1645 is a 2-channel Hot SwapTM controller that
allows a board to be safely inserted and removed from a
live backplane. Using external N-channel pass transistors,
the supply voltages can be ramped at a programmable
rate. Two high side switch drivers control the N-channel
gates for supply voltages ranging from 1.2V to 12V. The
two channels can be set to ramp up and down separately,
or they can be programmed to rise and fall simultaneously,
ensuring power supply tracking at the two outputs.
Programmable electronic circuit breakers protect against
shorts at either output. The RESET output can be used to
generate a system reset when a supply voltage falls below
a user-programmed voltage. An additional spare comparator is available for monitoring a second supply
voltage.
The LTC1645 is available in the 8- and 14-pin SO packages.
, 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 and 3.3V Hot Swap
5V and 3.3V Hot Swap Waveforms
VOUT2
3.3V
5A
0.005Ω* IRF7413
VIN2
+
CLOAD2
0.005Ω*
VOUT1
5V
5A
IRF7413
VIN1
+
CLOAD1
10Ω
0.01µF
25V
0.01µF
25V
CONNECTOR 1
CONNECTOR 2
10Ω
SENSE1 GATE1
ON
GATEn
10V/DIV
VOUT2
5V/DIV
VCC2 SENSE2 GATE2
VCC1
ON
ON
5V/DIV
VOUT1
5V/DIV
LTC1645
(8-LEAD)
10k
GND
1645 TA01
GND
BACKPLANE PLUG-IN CARD
*LRF1206-01-R005-J (IRC)
1645fa
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LTC1645
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ABSOLUTE
RATI GS
(Note 1)
Supply Voltage (VCC1, VCC2) ................................. 13.2V
Input Voltage
FB, ON, COMP + ..................... – 0.3V to (VCC1 + 0.3V)
TIMER ................................................. – 0.3V to 2.5V
SENSE1 ..................... (VCC1 – 0.7V) to (VCC1 + 0.3V)
SENSE2 ...................... (VCC1 – 0.7V) to (VCC2 + 0.3V)
Output Voltage
RESET, COMPOUT, FAULT .....................– 0.3V to 16V
GATE1, GATE2 ................. Internally Limited (Note 3)
Output Current
GATE1, GATE2 ............................................... ±20mA
Operating Temperature Range
LTC1645C ............................................... 0°C to 70°C
LTC1645I ............................................ – 40°C to 85°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
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PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
TOP VIEW
VCC2 1
8
VCC1
SENSE2 2
7
SENSE1
GATE2 3
6
GATE1
GND 4
5
LTC1645CS8
LTC1645IS8
ON
S8 PACKAGE
8-LEAD PLASTIC SO
S8 PART MARKING
TJMAX = 125°C, θJA = 150°C/ W
1645
1645I
ORDER PART
NUMBER
TOP VIEW
VCC2 1
14 VCC1
SENSE2 2
13 SENSE1
GATE2 3
12 GATE1
FAULT 4
11 TIMER
RESET 5
10 ON
FB 6
9
COMPOUT
GND 7
8
COMP +
LTC1645CS
LTC1645IS
S PACKAGE
14-LEAD PLASTIC SO
TJMAX = 125°C, θJA = 110°C/ W
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.
2.375V ≤ VCC1 ≤ 12V, 1.2V ≤ VCC2 ≤ 12V unless otherwise noted (Note 2).
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
DC Characteristics
ICC1
VCC1 Supply Current
ON = VCC1 = 5V, VCC2 = 3.3V
●
1.1
2.0
mA
ICC2
VCC2 Supply Current
ON = VCC1 = 5V, VCC2 = 3.3V
●
0.28
0.4
mA
VLKO1
VCC1 Undervoltage Lockout
High to Low
●
2.16
2.23
2.3
V
VLKO2
VCC2 Undervoltage Lockout
High to Low
●
1.06
1.12
1.18
V
VLKHn
VCCn Undervoltage Lockout Hysteresis
VFB
FB Pin Voltage Threshold
High to Low
●
∆VFB
FB Pin Threshold Line Regulation
High to Low, VCC1 = 2.375V to 12V
●
VFBHST
FB Pin Voltage Threshold Hysteresis
VCOMP
COMP +
∆VCOMP
COMP + Pin Threshold Line Regulation
VCOMPHST
COMP + Pin Voltage Threshold Hysteresis
Pin Voltage Threshold
25
1.226
mV
1.238
1.250
1
4
5
High to Low
●
High to Low, VCC1 = 2.375V to 12V
●
1.226
mV
1.238
1.250
1
4
5
V
mV
V
mV
mV
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LTC1645
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.
2.375V ≤ VCC1 ≤ 12V, 1.2V ≤ VCC2 ≤ 12V unless otherwise noted (Note 2).
SYMBOL PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
1.212
1.230
1.255
1
9
mV
VTM
TIMER Pin Voltage Threshold
∆VTM
TIMER Pin Threshold Line Regulation
VCC1 = 2.375V to 12V
●
ITM
TIMER Pin Current
Timer On, VTIMER = 0.6V, VCC1 = 5V
Timer Off, VTIMER = 1.5V
●
– 2.3
–2
12
– 1.7
µA
mA
VCB1
Circuit Breaker Trip Voltage 1
VCB1 = (VCC1 – VSENSE1)
●
46
50
56
mV
VCB2
Circuit Breaker Trip Voltage 2
VCB2 = (VCC2 – VSENSE2)
●
46
50
56
mV
t CBDn
Circuit Breaker Trip Delay
VCBn = (VCCn – VSENSEn) > 60mV
ICP
GATEn Pin Output Current
ON = 2.2V, VGATEn = VCCn, VCC1 = 5V, VCC2 = 3.3V
ON = 0.7V, VGATEn = VCCn, VCC1 = 5V, VCC2 = 3.3V
ON = 0.3V, VGATEn = VCCn, VCC1 = 5V, VCC2 = 3.3V
●
●
– 12.5
30
ISENSE
SENSE Pin Current
VCCn – 0.3V ≤ VSENSEn ≤ VCCn + 0.3V
●
∆VGATEn
External N-Channel Gate Drive
∆VSENSE = VCC – 0.5V
●
VONFPD
ON Pin Fast Pull-Down Threshold
Low to High
High to Low, Fast Pull-Down Engaged
●
●
0.375
0.35
VON1
ON Pin Threshold #1
Low to High, GATE1 Turns On
High to Low, GATE1 Turns Off
●
●
VON2
ON Pin Threshold #2
Low to High, GATE2 Turns On
High to Low, GATE2 Turns Off
●
●
VONHYST
ON Pin Hysteresis
ION
ON Pin Input Current
VCC1 = 5V, VCC2 = 3.3V
●
±0.01
±2
µA
VOL
Output Low Voltage
RESET, FAULT, COMPOUT, IOUT = 1.6mA, VCC1 = 5V
●
0.16
0.4
V
●
V
µs
1.5
– 7.5
50
µA
µA
mA
–10
10
µA
4.5
16
V
0.4
0.375
0.425
0.4
V
V
0.8
0.775
0.825
0.8
0.85
0.825
V
V
2
1.975
2.025
2
2.050
2.025
V
V
– 10
40
12
25
mV
Note 3: An internal zener on the GATEn pins clamps the charge pump
voltage to a typical maximum operating voltage of 22V. External overdrive
of a GATE pin (for example, from capacitive coupling of VCCn glitches)
beyond the internal zener voltage may damage the device. If a lower
GATEn pin clamp voltage is desired, use an external zener diode.
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: All currents into device pins are positive; all currents out of device
pins are negative. All voltages are referenced to ground unless otherwise
specified.
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TYPICAL PERFOR A CE CHARACTERISTICS
VCC1 Supply Current vs Voltage
VCC2 Supply Current vs Voltage
3.5
TA = 25°C
2.5
ICC2 (mA)
ICC1 (mA)
2.0
1.5
VCC2 = 1.5V
1.0
TA = 25°C
3.0
1.2
2.5
1.0
2.0
VCC1 = 2.375V
1.5
1.0
VCC2 = 12V
0
4
5
6 7 8
VCC1 (V)
9
10 11 12
1645 G01
ICC1
0.8
0.6
0.4
ICC2
0.2
0.5
3
VCC1 = 5V
VCC2 = 3.3V
VCC1 = 12V
0.5
2
Supply Current vs Temperature
1.4
ICCn (mA)
3.0
0
1
2
3
4
5
7 8
VCC2 (V)
6
9 10 11 12
1645 G02
0
–40 –20
40
20
60
0
TEMPERATURE (°C)
80
100
1645 G03
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LTC1645
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TYPICAL PERFOR A CE CHARACTERISTICS
GATE Voltage vs Supply Voltage
Glitch Filter Time
vs Feedback Transient
GATE Voltage vs Temperature
16.0
25
100
VCC1 = 5V
15.8 VCC2 = 3.3V
TA = 25°C
GLITCH FILTER TIME (µs)
15.6
20
GATEn (V)
GATEn (V)
15.4
15
10
15.2
15.0
14.8
14.6
70
60
50
40
30
14.4
20
14.2
10
0
14.0
–40 – 20
4
3
5
6 7 8 9
HIGHEST VCC (V)
10 11 12
0
40
20
0
60
TEMPERATURE (°C)
800
VCC1 = 5V
400
OUTPUT VOLTAGE (mV)
250
200
150
SINK CURRENT = 1.6mA
100
600
500
SINK CURRENT = 3mA
400
300
200
100
50
SINK CURRENT = 1.6mA
0
20
40
60
TEMPERATURE (°C)
80
100
1645 G07
2
3
4
5
6 7 8
VCC1 (V)
280
80 120 160 200 240
FEEDBACK TRANSIENT (mV)
Fast Pull-Down Current vs VCC1
18
TA = 25°C
17 VCC2 = 1.5V
16
15
14
13
12
11
10
9
0
–20
40
1645 G06
FAST PULL-DOWN CURRENT (mA)
SINK CURRENT = 3mA
300
0
TA = 25°C
700
350
0
–40
100
RESET, FAULT, COMPOUT Output
Voltage vs VCC1
RESET, FAULT, COMPOUT Output
Voltage vs Temperature
450
80
1645 G05
1645 G04
OUTPUT VOLTAGE (mV)
80
5
2
TA = 25°C
90
9
10 11 12
1645 G08
8
2
3
4
5
6 7 8
VCC1 (V)
9
10 11 12
1645 G09
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LTC1645
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PI FU CTIO S
(14-Lead Package/8-Lead Package)
VCC2 (Pin 1/Pin 1): Positive Supply Input. VCC2 can range
from 1.2V to 12V for normal operation. ICC2 is typically
0.2mA. An undervoltage lockout circuit disables the
LTC1645 whenever the voltage at VCC2 is less than 1.12V.
SENSE2 (Pin 2/Pin 2): VCC2 Circuit Breaker Set Pin. With
a sense resistor placed in the supply path between VCC2
and SENSE2, the circuit breaker trips when the voltage
across the resistor exceeds 50mV for more than 1.5µs. 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, short VCC2 and SENSE2 together.
GATE2 (Pin 3/Pin 3): Channel 2 High Side Gate Drive.
Connect to the gate of an external N-channel MOSFET. An
internal charge pump guarantees at least 4.5V of gate
drive. The charge pump is powered by the higher of VCC1
and VCC2. When the ON pin exceeds 2V, GATE2 is turned
on by connecting a 10µA current source from the charge
pump output to the GATE2 pin and the voltage starts to
ramp up with a slope dv/dt = 10µA/CGATE2. While the ON
pin is below 2V but above 0.4V, a 40µA current source
pulls GATE2 toward ground. If the ON pin is below 0.4V,
the circuit breaker trips or the undervoltage lockout circuit
trips, the GATE2 pin is immediately pulled to ground with
a 12mA (typ) current source.
FAULT (Pin 4/NA): Circuit Breaker Fault. FAULT is an
open-drain output that pulls low when the circuit breaker
function trips. The circuit breaker is reset by pulling the ON
pin below 0.4V. An external pull-up is required to generate
a logic high at the FAULT pin. When the ON pin is low,
FAULT will release.
The circuit breaker can be programmed to automatically
reset by connecting the FAULT pin to the ON pin. In this
circuit configuration, if a logic device is driving the ON pin,
use a series resistor between the logic output and the ON
pin to prevent large currents from flowing.
RESET (Pin 5/NA): Open-Drain RESET Output. The RESET
pin is pulled low when the voltage at the FB pin goes below
1.238V or VCC1 is below the undervoltage lockout threshold. The RESET pin goes high one timing cycle after the
voltage at the FB pin goes above the FB pin threshold. The
ON pin must remain above 0.8V during this timing cycle.
An external pull-up is required to generate a logic high at
the RESET pin.
FB (Pin 6/NA): RESET Comparator Input. The FB pin is
used to monitor the output supply voltage with an external
resistive divider. When the voltage on the FB pin is lower
than 1.238V, the RESET pin is pulled low. A glitch filter on
the FB pin prevents fast transients from forcing RESET
low. When the voltage on the FB pin rises above the trip
point, the RESET pin goes high after one timing cycle.
GND (Pin 7/Pin 4): Ground. Connect to a ground plane for
optimum performance.
COMP+ (Pin 8/NA): Spare Comparator Noninverting Input. When the voltage on COMP+ is lower than 1.238V,
COMPOUT pulls low.
COMPOUT (Pin 9/NA): Open-Drain Spare Comparator
Output. COMPOUT pulls low when the voltage on COMP+
is below 1.238V or VCC1 is below the undervoltage lockout
threshold. An external pull-up is required to generate a
logic high at the COMPOUT pin.
ON (Pin 10/Pin 5): Analog Control Input. If the ON pin
voltage is below 0.4V, both GATE1 and GATE2 are immediately pulled to ground. While the voltage is between 0.4V
and 0.8V, both GATE1 and GATE2 are each pulled to
ground with a 40µA current source. While the voltage is
between 0.8V and 2V, the GATE1 pull-up is turned on after
one timing cycle, but GATE2 continues to be pulled to
ground with a 40µA current source. When the voltage
exceeds 2V, both the GATE1 and GATE2 pull-ups are
turned on one timing cycle after the voltage exceeds 0.8V.
The ON pin is also used to reset the electronic circuit
breaker. If the ON pin is brought below and then above
0.4V following the trip of the circuit breaker, the circuit
breaker resets, and a normal power-up sequence occurs.
TIMER: (Pin 11/NA): System Timing Pin. The TIMER pin
requires an external capacitor to ground to generate a
timing delay. The pin is used to set the delay before the
RESET pin goes high after the output supply voltage is
good as sensed by the FB pin. It is also used to set the delay
between the ON pin exceeding 0.8V and the GATE1 and
GATE2 pins turning on (GATE2 turns on only if the ON pin
exceeds 2V).
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LTC1645
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PI FU CTIO S
(14-Lead Package/8-Lead Package)
the circuit breaker trips or the undervoltage lockout circuit
trips, the GATE1 pin is immediately pulled to ground with
a 12mA (typ) current source.
Whenever the timer is inactive, an internal N-channel FET
shorts the TIMER pin to ground. Activating the timer
connects a 2µA current source from VCC1 to the TIMER pin
and the voltage starts to ramp up with a slope dv/dt = 2µA/
CTIMER. When the voltage reaches the trip point (1.23V),
the timer is reset by pulling the TIMER pin back to ground.
The timer period is (1.23V • CTIMER)/2µA.
SENSE1 (Pin 13/Pin 7): VCC1 Circuit Breaker Set Pin. With
a sense resistor placed in the supply path between VCC1
and SENSE1, the circuit breaker trips when the voltage
across the resistor exceeds 50mV for more than 1.5µs. 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, short VCC1 and SENSE1 together.
GATE1 (Pin 12/Pin 6): Channel 1 High Side Gate Drive.
Connect to the gate of an external N-channel MOSFET. An
internal charge pump guarantees at least 4.5V of gate
drive. The charge pump is powered by the higher of VCC1
and VCC2. When the ON pin exceeds 0.8V, GATE1 is turned
on by connecting a 10µA current source from the charge
pump output to the GATE1 pin and the voltage starts to
ramp up with a slope dv/dt = 10µA/CGATE1. While the ON
pin is below 0.8V but above 0.4V, a 40µA current source
pulls GATE1 toward ground. If the ON pin is below 0.4V,
VCC1 (Pin 14/Pin 8): Positive Supply Input. VCC1 can range
from 2.375V to 12V for normal operation. ICC1 is typically
1mA. An undervoltage lockout circuit disables the chip
whenever the voltage at VCC1 is less than 2.23V. All internal
logic is powered by VCC1.
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BLOCK DIAGRA
VCC1
SENSE1
VCC2
SENSE2
14
13
1
2
–
+
+
–
+
50mV
+
–
2V
–
2.23V
UVL
1.5µs
FILTER
1.12V
UVL
12
3
50mV
–
+
ON 10
GATE1 GATE2
1.5µs
FILTER
4× CHARGE
PUMP
+
0.8V
–
+
0.4V
LOGIC
1.238V
REFERENCE
REF
–
GLITCH
FILTER
2µA
–
+
+
TIMER 11
REF
FB
5
RESET
7
GND
8
COMP+
9
COMPOUT
–
–
+
FAULT 4
6
REF
1645 BD
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LTC1645
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APPLICATIO S I FOR ATIO
Hot Circuit Insertion
Power Supply Tracking and Sequencing
When a circuit board is 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. These transient currents can cause permanent
damage to the connector pins and produce glitches on the
system supply, resetting other boards in the system.
Some applications require that the potential difference
between two power supplies not exceed a certain voltage.
This requirement applies during power-up and powerdown as well as during steady state operation, often to
prevent latch-up in a dual supply ASIC. Other systems
require one supply to come up after another, for example,
if a system clock needs to start before a block of logic.
Typical dual supplies or backplane connections may come
up at arbitrary rates depending on load current, capacitor
size, soft-start rates, etc. Traditional solutions are cumbersome and require complex circuitry to meet the power
supply requirements.
The LTC1645 is designed to turn a board’s supply voltages
on and off in a controlled manner, allowing the board to be
safely inserted or removed from a live backplane. The chip
provides a system reset signal and a spare comparator to
indicate when board supply voltages drop below userprogrammable voltages, and a fault signal to indicate if an
overcurrent condition has occurred.
The LTC1645 provides a simple solution to power supply
tracking and sequencing needs. The LTC1645 guarantees
supply tracking by ramping the supplies up and down
together (see Figure 15). The sequencing capabilities of
the LTC1645 allow nearly any combination of supply
ramping (e.g., see Figure 17) to satisfy various sequencing specifications. See the Power Supply Tracking and
Sequencing Applications section for more information.
The LTC1645 can be located before or after the connector
as shown in Figure 1. A staggered PCB connector can
sequence pin connections when plugging and unplugging
circuit boards. Alternatively, the control signal can be
generated by processor control.
BACKPLANE
CONNECTOR
STAGGERED PCB
EDGE CONNECTOR
VCC
+
VCC
ON
SENSE
VOUT
CLOAD
GATE
ON
FAULT
FAULT
LTC1645
GND
(a) Hot Swap Controller on Motherboard
BACKPLANE
CONNECTOR
STAGGERED PCB
EDGE CONNECTOR
VCC
+
VOUT
CLOAD
FAULT
VCC
SENSE
GATE
ON
FAULT
LTC1645
GND
1645 F01
(b) Hot Swap Controller on Daughterboard
Figure 1. Staggered Pins Connection
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LTC1645
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APPLICATIO S I FOR ATIO
Power Supply Ramping
The power supplies on a board are controlled by placing
external N-channel pass transistors in the power paths as
shown in Figure 2. Consult Table 1 for a selection of
N-channel FETs suitable for use with the LTC1645. RSENSE1
and RSENSE2 provide current fault detection and R1 and R2
prevent high frequency oscillation. By ramping the gates
of the pass transistors up and down at a controlled rate,
the transient surge current (I = C • dv/dt) drawn from the
main backplane supply is limited to a safe value when the
board makes connection.
When power is first applied to the chip, the gates of the
N-channels (GATE1 and GATE2 pins) are pulled low. After
the ON pin is held above 0.8V for at least one timing cycle,
the voltage at GATE1 begins to rise with a slope equal to
dv/dt = 10µA/C1 (Figure 3), where C1 is the external
capacitor connected between the GATE1 pin and GND. If
the ON pin is brought above 2V (and the ON pin has been
held above 0.8V for at least one timing cycle), the voltage
at GATE2 begins to rise with a slope equal to dv/dt =
10µA/C2.
RSENSE1
Q1
VCC1
+
VOUT1
The ramp time for each supply is t = (VCCn • Cn)/10µA. If
the ON pin is pulled below 2V for GATE2 or 0.8V for GATE1
(but above 0.4V), a 40µA current source is connected from
GATEn to GND, and the voltage at the GATEn pin will ramp
down, as shown in Figure 4.
Ringing
Good engineering practice calls for bypassing the supply
rail of any circuit. Bypass capacitors are often placed at the
supply connection of every active device, in addition to one
or more large value bulk bypass capacitors per supply rail.
If power is connected abruptly, the bypass capacitors slow
the rate of rise of voltage and heavily damp any parasitic
resonance of lead or trace inductance working against the
supply bypass capacitors.
The opposite is true for LTC1645 Hot Swap circuits on a
daughterboard. In most cases, on the powered side of the
N-channel FET switches (VCCn) there is no supply bypass
capacitor present. An abrupt connection, produced by
plugging a board into a backplane connector, results in a
fast rising edge applied to the VCCn line of the LTC1645.
VCCn + ∆VGATE
GATEn
CLOAD1
R1
10Ω
SLOPE = 10µA/Cn
C1
RSENSE2
Q2
VCC2
+
VOUT2
VOUTn
VCCn
CLOAD2
R2
10Ω
14
13
1
12
VCC1 SENSE1 GATE1
VCC2 SENSE2 GATE2
4
ON
LTC1645
(14-LEAD)
COMPOUT
FAULT
FB
t2
8
VCCn + ∆VGATE
9
GATEn
6
SLOPE = 40µA/Cn
5
TIMER
11
CTIMER
GND
1645 F03
Figure 3. Supply Turning On
C2
3
2
COMP+
10
t1
RESET
VCCn
7
VOUTn
1645 F02
t3
Figure 2. Typical Hot Swap Connection
t4
1645 F04
Figure 4. Supply Turning Off
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LTC1645
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APPLICATIO S I FOR ATIO
No bulk capacitance is present to slow the rate of rise and
heavily damp the parasitic resonance. Instead, the fast
edge shock excites a resonant circuit formed by a combination of wiring harness, backplane and circuit board
parasitic inductances and FET capacitance. In theory, the
peak voltage should rise to 2X the input supply, but in
practice the peak can reach 2.5X, owing to the effects of
voltage dependent FET capacitance.
The absolute maximum VCCn potential for the LTC1645 is
13.2V; any circuit with an input of 5V or greater should be
scrutinized for ringing. A well-bypassed backplane should
not escape suspicion: circuit board trace inductances of as
little as 10nH can produce sufficient ringing to overvoltage
VCC.
Check ringing with a fast storage oscilloscope (such as a
LECROY 9314AL DSO) by attaching coax or a probe to VCC
and GND, then repeatedly inserting the circuit board into
the backplane. Figures 5a and 5b show typical results in a
12V application with different VCC lead lengths. The peak
amplitude reaches 22V, breaking down the ESD protection
diode in the process.
There are two methods for eliminating ringing: clipping
and snubbing. A transient voltage suppressor is an effective means of limiting peak voltage to a safe level.
Figure␣ 6 shows the effect of adding an ON Semiconductor,
1SMA12CAT3, on the waveform of Figure 5.
Figures 7a and 7b show the effects of snubbing with
different RC networks. The capacitor value is chosen as
10X to 100X the FET COSS under bias and R is selected for
best damping—1Ω to 50Ω depending on the value of
parasitic inductance.
R1
0.01Ω
8'
IRF7413
+
12V
+
–
POWER
LEADS
VOUT
CLOAD
10Ω
SCOPE
PROBE
0.1µF
LTC1645
1645 F05
4V/DIV
24V
4V/DIV
24V
0V
0V
1µs/DIV
1µs/DIV
1645 F05a
(a) Undamped VCC Waveform (48" Leads)
1645 F05b
(b) Undamped VCC Waveform (8" Leads)
Figure 5. Ring Experiment
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R1
0.01Ω
IRF7413
VOUT
12V
CLOAD
10Ω
D1*
2V/DIV
+
–
PCB EDGE CONNECTOR
12V
POWER
LEADS
BACKPLANE CONNECTOR
+
0.1µF
LTC1645
0V
1645 F06
1µs/DIV
ON SEMICONDUCTOR
* 1SMA12CAT3
1645 F06a
VCC Waveform Clamped by a Transient Suppressor
Figure 6. Transient Suppressor Clamp
R1
0.01Ω
IRF7413
+
–
PCB EDGE CONNECTOR
12V
POWER
LEADS
BACKPLANE CONNECTOR
+
10Ω
VOUT
CLOAD
10Ω
0.1µF
0.1µF
LTC1645
1645 F07
12V
2V/DIV
2V/DIV
12V
0V
0V
1µs/DIV
1µs/DIV
1645 F07a
(a) VCC Waveform Damped by a Snubber (15Ω, 6.8nF)
1645 F07b
(b) VCC Waveform Damped by a Snubber (10Ω, 0.1µF)
Figure 7. Snubber “Fixes”
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SUPPLY
GLITCH
12V
+
–
R1
0.01Ω
If a dead short occurs after a supply connection is made
(Figure 8), the sense resistor R1 and the RDS(ON) of the
fully enhanced FET provide a low impedance path for
IRF7413
2µH
10Ω
+
100µF
0.1µF
LTC1645
BOARD WITH POSSIBLE
SHORT-CIRCUIT FAULT
LTC1645 Hot Swap circuits on the backplane are generally
used to provide power-up/down sequence at insertion/
removal as well as overload/short-circuit protection. If a
short-circuit occurs at supply ramp-up, the circuit breaker
trips. The partially enhanced FET is easily disconnected
without any supply glitch.
BACKPLANE CONNECTOR
Supply Glitching
1645 F08
4V/DIV
25A/DIV
GATE
VCC
1µs/DIV
1µs/DIV
1645 F08a
(a) VCC Short-Circuit Supply Current Glitch Without Any Limiting
1645 F08b
(b) VCC Supply Glitch Without Any Limiting
4V/DIV
5A/DIV
GATE
1µs/DIV
VCC
1µs/DIV
1645 F08c
(c) VCC Short-Circuit Supply Current Glitch
with 2µH Series Inductor
1645 F08d
(d) VCC Supply Glitch with 2µH Series Inductor
Figure 8. Supply Glitch
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nearly unlimited current flow. The LTC1645 discharges
the GATE pin in a few microseconds, but during this
discharge time current on the order of 150 amperes flows
from the VCC power supply. This current spike glitches the
power supply, causing VCC to dip (Figure 8a and 8b).
On recovery from overload, some supplies may overshoot. Other devices attached to this supply may reset or
malfunction and the overshoot may also damage some
components. An inductor (1µH to 10µH) in series with the
FET’s source limits the short-circuit di/dt, thereby limiting
the peak current and the supply glitch (Figure 8c and 8d).
Additional power supply bypass capacitance also reduces
the magnitude of the VCC glitch.
Reset
The LTC1645 uses an internal 1.238V bandgap reference,
a precision voltage comparator, and a resistive divider to
monitor the output supply voltage (Figure 9).
Whenever the voltage at the FB pin rises above its reset
threshold (1.238V), the comparator output goes high, and
a timing cycle starts (see Figure 10, time points 1 and 4).
After a complete timing cycle, RESET is released. An
external pull-up is required for the RESET pin to rise to a
logic high.
Glitch Filter
The LTC1645 has a glitch filter to prevent RESET from
generating a spurious system reset in the presence of
transients on the FB pin. The filter is 20µs for large
transients (greater than 150mV) and up to 80µs for
smaller transients. The relationship between glitch filter
time and the transient voltage is shown in Typical Performance Characteristics: Glitch Filter Time vs Feedback
Transient.
Timer
The system timing for the LTC1645 is generated by the
circuitry shown in Figure 11. The timer is used to set the
turn-on delay after the ON pin goes high. It also sets the
delay before the RESET pin goes high after the FB pin
exceeds 1.238V.
Whenever the timer is off, the internal N-channel shorts
the TIMER pin to ground (Figure 11). Activating the timer
connects a 2µA current from VCC1 to the TIMER pin and the
3
V1
V2
ON
V2
1.23V
1.23V
TIMER
RESET
1645 F10
Figure 10. Supply Monitor Waveforms
ON
LOGIC
SUPPLY
MONITOR
2µA
+
4
V1
VOUT
When the voltage at the FB pin drops below its reset
threshold, the comparator 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 remains low (time
point 3).
VOUT
2
1
V2
+
FB
COMP
LOGIC
COMP
10k
–
1.238V
REFERENCE
TIMER
1.23V
–
µP
RESET
RESET
TIMER
CTIMER
TIMER
1645 F09
CTIMER
1645 F11
Figure 9. Supply Monitor Block Diagram
Figure 11. System Timing Block Diagram
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voltage on the external capacitor CTIMER starts to ramp up
with a slope dv/dt = 2µA/CTIMER. When the voltage reaches
the trip point (1.23V), the timer is reset by pulling the
TIMER pin back to ground. The timer period is
t = (1.23V • CTIMER)/2µA. For a 200ms delay, use a 0.33µF
capacitor.
Electronic Circuit Breaker
The LTC1645 features an electronic circuit breaker function that protects against short circuits or excessive output currents. By placing sense resistors between the
supply inputs and sense pins of the supplies, the circuit
breaker trips whenever the voltage across either sense
resistor is greater than 50mV for more than 1.5µs. If the
circuit breaker trips, both GATE pins are immediately
pulled to ground and the external N-channels FETs are
quickly turned off (time point 6 in Figure 12). The circuit
breaker resets and another timing cycle starts by taking
CURRENT
FAULT
RAMPING UP
1 2
3
4
5
6
RESET FAULT
AND RAMP UP
7
8
9
10
VCCn
ON
VCCn – VSENSEn
the ON pin below 0.4V and then high as shown at time
point 7.
At the end of the timer cycle (time point 8), the charge
pump turns on again. If the circuit breaker feature is not
required, short the SENSEn pin to VCCn.
If the 1.5µs response time is too fast to reject supply noise,
add external resistors and capacitors RF and CF to the
sense circuit as shown in Figure 13.
The ON Pin
The ON pin is used to control system operation as shown
in Figure 14. At time point 1, the board makes connection
and the supplies power up the chip. At time point 2, the ON
pin goes high and a timer cycle starts as long as both VCC
pins are higher than the undervoltage lockout trip point
(2.23V for VCC1 and 1.12V for VCC2) and an overcurrent
fault is not detected. At the end of the timer cycle (time
point 3), the charge pump is turned on and the GATEn pin
voltages start to ramp up with the output supply voltages,
VOUTn, following one gate-to-source voltage drop lower.
At time point 4, VOUT2 reaches its power-good trip level
(this example assumes the FB pin resistive divider is
connected to VOUT2) and a timing cycle starts. At the end
of the timing cycle (time point 5), RESET goes high and the
power-up process is complete.
TIMER
RAMPING UP AND
DOWN TOGETHER
RAMPING UP AND
DOWN SEPARATELY
RAMPING UP AND
TURNING OFF FAST
GATEn
12
VOUTn
3
4
5
6 7
8
9
10 11 12 13 14 15 16 17 18 19 20
VCCn
2V
RESET
1645 F12
Figure 12. Current Fault Timing
0.8V
ON 0.4V
0V
TIMER
GATE1
CF
RF
VOUT1
GATE2
VCCn SENSEn
GATEn
VOUT2
LTC1645
RESET
1645 G13
1645 F14
Figure 13. Extending the Short-Circuit Protection Delay
Figure 14. ON Pin Waveforms
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An external hard reset is initiated at time point 6. The ON
pin is forced below 0.8V but above 0.4V, and the GATEn
pin voltages start to ramp down. VOUTn also starts to ramp
down, and RESET goes low when VOUT2 drops below the
power-good trip level at time point 7.
Time points 16 to 19 show the same power-up sequence
as time points 2 to 5, while time point 20 demonstrates the
GATEn pins being pulled immediately to ground (instead
of ramping down) by the ON pin going below 0.4V.
Power Supply Tracking and Sequencing Applications
Time points 8 to 15 are similar to time points 1 to 7, except
the ON pin’s different voltage thresholds are used to ramp
VOUT1 and VOUT2 separately. At time point 8, the ON pin
goes above 0.8V but below 2V, and one timing cycle later
(time point 9) GATE1 begins to ramp up with VOUT1
following one gate-to-source voltage drop lower. At time
point 10, the ON pin goes above 2V and GATE2 immediately begins ramping up with VOUT2 following one gate-tosource voltage drop lower. As soon as VOUT2 reaches its
power-good trip level at time point 11, a timing cycle
starts. At the end of the timing cycle (time point 12),
RESET goes high and the power-up process is complete.
The LTC1645 is able to sequence VOUTn in a number of
ways, including ramping VOUT1 up first and down last;
ramping VOUT1 up first and down first; ramping VOUT1 up
first and VOUT1 and VOUT2 down together; and ramping
VOUT1 and VOUT2 up and down together.
Figure 15 shows an application ramping VOUT1 and VOUT2
up and down together. The ON pin must reach 0.8V to
ramp up VOUT1 and VOUT2. The spare comparator pulls the
ON pin low until VCC2 is above 2.3V, and the ON pin cannot
reach 0.8V before VCC1 is above 3V. Thus, both input
supplies must be within regulation before a timing cycle
can start. At the end of the timing cycle, the output voltages
ramp up together. If either input supply falls out of
regulation, the gates of Q1 and Q2 are pulled low together.
Figure 16 shows an oscilloscope photo of the circuit in
Figure 15.
The ON pin is forced below 2V but above 0.8V at time point
13 and the GATE2 pin voltage starts to ramp down. VOUT2
also starts to ramp down and RESET goes low when VOUT2
drops below the power-good trip level at time point 14.
When the ON pin goes below 0.8V but above 0.4V at time
point 15, GATE1 and VOUT1 ramp down.
BOTH CURRENT LIMITS: 5A
Q1
1/2 Si4920DY
0.01Ω*
VIN1
3.3V
D1
1N4002
10Ω
Q2
0.01Ω* 1/2 Si4920DY
VIN2
2.5V
D2
1N4002
+
CLOAD1
D3
MBR0530T1
VOUT1
3.3V
2.5A
VOUT2
2.5V
2.5A
+
CLOAD2
10k
10Ω
14
4.99k
1%
TRIP
POINT:
3V
1.37k
1%
12
VCC1 SENSE1 GATE1
10
1
VCC2 SENSE2 GATE2
COMP+
COMPOUT
LTC1645
(14-LEAD)
FB
8
9
1.18k
1%
6
5
FAULT
TIMER
11
0.33µF
GND
10k
0.1µF
25V
3
2
ON
1.82k
1%
4
1.18k
1%
13
1.37k
1%
RESET
7
µP RESET
1645 F15
*WSL1206-01-1% (VISHAY DALE)
Figure 15. Ramping 3.3V and 2.5V Up and Down Together
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VIN2
5V/DIV
VIN1
5V/DIV
VOUT2
5V/DIV
VOUT1
5V/DIV
TIMER
2V/DIV
RESET
5V/DIV
Figure 16. Ramping 3.3V and 2.5V Up and Down Together
This circuit guarantees that: (1) VOUT1 never exceeds
VOUT2 by more than 1.2V, and (2) VOUT2 is never greater
than VOUT1 by more than 0.4V. On power-up, VOUT1 and
VOUT2 ramp up together. On power-down, the LTC1645
turns off Q1 and Q2 simultaneously. Charge remains
stored on CLOAD1 and CLOAD2 and the output voltages will
vary depending on the loads. D1 and D2 turn on at ≈ 1V
(≈ 0.5V each), ensuring condition 1 is satisfied, while D3
prevents violations of condition 2. Different diodes may be
necessary for different output voltage configurations.
Barring an overvoltage condition at the input(s), the only
time these diodes might conduct current is during a
power-down event, and then only to discharge CLOAD1 or
CLOAD2. In the case of an input overvoltage condition that
causes excess current to flow, the circuit breaker will trip
if the current limit level is set appropriately.
Figure 17 shows an application circuit where VOUT1
ramps up before VOUT2. VOUT1 is initially discharged and
D1 is reverse-biased, thus the voltage at the ON pin is
determined only by VCC1 through the resistor divider R1
and R2. The voltage at the ON pin exceeds 0.8V if V CC1 is
above 4.6V and VOUT1 begins to ramp up after a timing
cycle. As VOUT1 ramps up, D1 becomes forward-biased
and pulls the ON pin above 2V when VOUT1 ≈ 4.5V. This
turns on GATE2 and VOUT2 ramps up. The FB comparator
monitors VOUT2, and the spare comparator monitors
VOUT1 with RHYST creating ≈50mV of hysteresis.
Power Supply Multiplexer
Using back-to-back FETs, the LTC1645 can Hot Swap two
supplies to the same output, automatically selecting the
primary supply if present or the secondary supply if the
primary supply is not available. Referring to Figure 18, a
diode-or circuit provides power to the LTC1645 if either
supply is up. Schottky diodes are used to prevent the
voltage at VCC1 from approaching the undervoltage lockout threshold. This application assumes that if a supply is
not present, the supply input is floating.
If only the 3.3V supply is present, the voltage at the COMP +
pin is below the trip point and COMPOUT pulls the base of
Q3 low, allowing the GATE1 pin to ramp up normally. The
voltage at the ON pin exceeds 0.8V if the 3.3V supply is
greater than 3V, ramping up GATE1 and turning on Q1A and
Q1B. The ON pin does not exceed 2V (unless the 3.3V supply
exceeds 7.5V!), keeping GATE2 low and Q2A and Q2B off.
If only the 5V supply is present or if both supplies are
present, the COMP + pin is above 1.238V and COMPOUT
allows the base of Q3 to be pulled high by R2. This turns
Q3 on, keeping GATE1 low and Q1A and Q1B off. The
voltage at the ON pin is pulled above 2V by R1 and GATE2
turns Q2A and Q2B on.
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0.005Ω*
BOTH CURRENT LIMITS: 10A
VIN2
3.3V
IRF7413
+
CLOAD2
0.005Ω*
VIN1
5V
IRF7413
+
CLOAD1
D1
1N4148
10Ω
13k
1%
10k
R1
47.5k
1%
13
0.01µF
25V
12
1
VCC1 SENSE1 GATE1
10
3
2
VCC2 SENSE2 GATE2
COMP+
ON
R2
10k
1%
LTC1645
(14-LEAD)
COMPOUT
4
RESET
FAULT
TIMER
10k
10k
28k
1%
8
14.7k
1%
RHYST
681k
FB
FAULT
VOUT1
5V
5A
10Ω
0.01µF
25V
14
VOUT2
3.3V
5A
10k
1%
9
µP RESET2
6
10k
1%
5
GND
11
µP RESET
7
1645 F17
0.33µF
*LRF1206-01-R005-J (IRC)
Figure 17. Ramping Up 5V Followed by 3.3V
Q1A
Q1B
IRF7313
VIN1
3.3V
D1
1/2 BAT54C
Q2A
Q2B
IRF7313
10Ω
VIN2
5V
R1
10k
14
10k
VCC1
10
10k
D2
1/2
BAT54C
22.6k
1%
13
12
1
10Ω
2
0.1µF
25V
3
R2
10k
SENSE1 GATE1 VCC2 SENSE2 GATE2
COMP+
ON
11.3k
1%
4
0.1µF
25V
VOUT
5V OR
3.3V
5A
COMPOUT
LTC1645
(14-LEAD)
FB
FAULT
TIMER
GND
11
7
RESET
8
9
Q3
PN2222
6
5
0.33µF
1645 F18
Figure 18. Power Supply Multiplexer
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goes high which turns on Q3. This lowers the voltage on
the gate of Q2. This feedback loop is compensated by
capacitors C1 and C2 and resistor R1. When power is first
applied, the FB pin is low and RESET holds one side of C2
low, slowing the ramp-up of VOUT2. As VOUT2 exceeds
2.75V, RESET releases to allow improved loop transient
response. Figure 20 shows the load transient response
and voltage ripple of the generated supply.
Using the LTC1645 as a Linear Regulator
This application uses the LTC1645 to Hot Swap one
primary supply and generate a secondary low dropout
regulated supply. Figure 19 shows how to switch a 5V
supply and create a 3.3V supply using the spare comparator and one additional transistor. The COMP+ pin is used
to monitor the 3.3V output. As the voltage on the gate of
Q2 increases, the 3.3V output increases. At the 3.3V
threshold the spare comparator trips. The COMPOUT pin
Q1
IRF7413
0.01Ω*
VIN
5V
VOUT1
5V
2.5A
BOTH CURRENT LIMITS: 5A
+
CLOAD1
10Ω
0.1µF
25V
0.01Ω*
1M
Q2
IRFZ24
+
10k
10Ω
14
13
VCC1 SENSE1 GATE1
10
1
12
VCC2 SENSE2 GATE2
COMP+
ON
LTC1645
(14-LEAD)
4
3
2
COMPOUT
FB
R1
200k
C2
0.1µF
25V
2.49k
1%
VOUT2
3.3V
2.5A
470µF**
6V
×2
C1
0.033µF
8
9
Q3
PN2222
6
1.5k
1%
12.1k
1%
5
FAULT
TIMER
GND
11
7
0.33µF
RESET
10k
1%
*LRF1206-01-R010-J (IRC)
**T510X477K006AS (KEMET)
1645 F19
Figure 19. Switching 5V and Generating 3.3V
VOUT2
0.1V/DIV
2.5A
IOUT2
1A/DIV
0.5A
Figure 20. Load Transient Response and Voltage Ripple
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Switching Regulator Supply Sequencing
Switching Regulator Hot Swapping
Figure 21 shows the LTC1645 sequencing two power
supplies, the lower of which is generated by the LTC1430A
switching regulator. Connecting the regulator’s FB pin
resistor divider (R1 and R2) to the other side of the pass
FET (Q1) allows the LTC1430A to compensate for the
voltage drop across RSENSE1 and Q1, assuring an accurate
voltage output. The spare comparator holds the LTC1645’s
ON pin low until the LTC1430A’s output is at least 3V, and
shuts both channels off if it drops below 3V. When the
ON/OFF signal is taken high to 5V (turn-on), the voltage at
the ON pin rises with an RC exponential characteristic,
reaching 0.8V first. This starts a timing cycle, and GATE1
begins to rise. GATE2 starts to ramp up after the ON pin
reaches 2V. As long as the timing cycle is shorter than the
time for the ON pin to rise from 0.8V to 2V, VOUT2 ramps
up after VOUT1. RESET goes high one timing cycle after
VOUT1 exceeds 3V. When the ON/OFF signal is brought
low, the voltage at the ON pin exponentially decays and
GATE2 ramps down before GATE1. RESET goes low as
soon as VOUT1 falls below 3V. Figure 22 shows the powerup and power-down sequences of the circuit in Figure 21.
High current switching regulators usually require large
bypass capacitors on both input and output for proper
operation. The application in Figure 23 controls the inrush
current to the LTC1649’s input bypass capacitors and
ramps the two output voltages up and down together. As
with the previous application, connecting the regulator’s
FB pin resistor divider to the other side of the output pass
FET (Q2) allows the LTC1649 to compensate for the
voltage drop across Q2, assuring an accurate voltage
output. The voltage at the LTC1645’s ON pin reaches 0.8V
when VIN exceeds 3V, and GATE1 begins to ramp up one
timing cycle later. As the regulator’s output rises, D2 pulls
the ON pin above 2V and GATE2 begins to rise, ramping
VOUT1 and VOUT2 up together. RESET goes high one timing
cycle after VOUT1 exceeds 3V and VOUT2 exceeds 2.35V.
Figure 24 shows the circuit in Figure 23 powering up.
Care should be taken connecting a switching regulator’s
FB or SENSE pins to a node other than its output. Depending on the regulator’s internal architecture, unusual behavior may occur as it tries in vain to raise the voltage at
ON
2V/DIV
VREGOUT
2V/DIV
VOUT1
2V/DIV
VOUT2
2V/DIV
RESET
5V/DIV
Figure 22. Switching Regulator Supply Sequencing
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18
FAULT
ON/OFF
10k
1µF
270pF
15µF
10V
+
4700pF
22k
1500µF
6.3V
×3
6
5
8
7
0.1µF
51Ω
COMP
SHDN
G2
PVCC2
GND
FB
G1
PVCC1
LTC1430ACS8
3
4
1
2
0.1µF
+
130k
1%
162k
1%
1500µF
6.3V
×2
VREGOUT
1µF
1µF
R2
16.9k
1%
*LRF1206-01-R010-J (IRC)
4700pF
1Ω
2.4µH
CDRH1272R4
R1
16.5k
1%
4
10
13
12
FAULT
ON
7
GND
LTC1645
(14-LEAD)
10Ω
RESET
FB
COMPOUT
COMP+
3
1
2
VCC2 SENSE2 GATE2
0.047µF
25V
11
0.33µF
TIMER
VCC1 SENSE1 GATE1
14
10Ω
RSENSE1*
Q1
0.01Ω 1/2 Si4920DY
Figure 21. Switching Regulator Supply Sequencing
MBRS130T3
Si4410DY
Si4410DY
680pF
MBR0530T1
5
6
9
8
0.047µF
25V
1.87k
1%
2.67k
1%
1.15k
1%
3.16k
1%
10k
+
+
1645 F21
CLOAD1
CLOAD2
RESET
VOUT1
3.3V
2.5A
VOUT2
5V
2.5A
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RSENSE2*
Q2
0.01Ω 1/2 Si4920DY
APPLICATIO S I FOR ATIO
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VIN
5V
LTC1645
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20
FAULT
10k
+
0.33µF
100k
0.1µF
0.1µF
14
0.1µF
1.82k
1%
4
FAULT
4.99k
VCC1
1%
10
ON
0.33µF
MBR0530LT1
1500µF
6.3V
×6
VREGIN
D2
MBR0530T1
10µF
*LRF2010-01-R003-J (IRC)
**MBRS340T (ON SEMICONDUCTOR)
†ETQP6F1R2HFA (PANASONIC)
GND
10Ω
+
12
C–
VIN
SS
3
7
GND
COMPOUT
COMP+
RESET
FB
9
8
5
6
+
18k
0.047µF
25V
100µF
0.1µF
10k
33k
0.015µF
1.87k
1%
2.67k
1%
220pF
IRF7801
IRF7801
Figure 23. Switching Regulator Hot Swap
0.01µF
2
10Ω
Q3
FDS6680
1µF
220Ω
1k
IRF7801
VCC2 SENSE2 GATE2
1
0.003Ω*
MBR0530LT1
0.01µF
25V
0.33µF
10
11
12
13
14
15
16
9
C+
CPOUT
COMP
IMAX
LTC1645
(14-LEAD)
11
TIMER
VCC
PVCC2
G2
LTC1649 IFB
SHDN
FB
GND
PVCC1
G1
SENSE1 GATE1
13
8
7
6
5
4
3
2
1
IRF7801
**
R2
1k
R1
1.8k
1µF
+
D1
1N4148
2200pF
5.1Ω
1.2µH†
0.01µF
1500µF
6.3V
×4
VREGOUT
1µF
1645 F23
10Ω
Q2
FDB8030L
1.13k
1%
1.02k
1%
+
3.09k
1%
3.01k
1%
+
RESET
CLOAD1
VOUT1
3.3V
10A
GND
CLOAD2
VOUT2
2.5V
15A
U U
W
Q1
0.003Ω* FDB8030L
APPLICATIO S I FOR ATIO
U
VIN
3.3V
LTC1645
1645fa
LTC1645
U
W
U U
APPLICATIO S I FOR ATIO
Table 1. N-Channel Selection Guide
ON
2V/DIV
CURRENT
LEVEL
PART
NUMBER
MANUFACTURER
DESCRIPTION
1A to 2A
NDH8503N
Fairchild
Dual N-Channel
RDS(ON) = 0.033
SuperSOT-8
1A to 2A
Si6928DQ
Vishay/Siliconix
Dual N-Channel
RDS(ON) = 0.035
TSSOP-8
2A to 5A
Si4920DY
Vishay/Siliconix
Dual N-Channel
RDS(ON) = 0.025
SO-8
2A to 5A
IRF7313
International
Rectifier
Dual N-Channel
RDS(ON) = 0.029
SuperSOT-8
5A to 10A
Si4420
Vishay/Siliconix
Single N-Channel
RDS(ON) = 0.009
SO-8
5A to 10A
FDS6680
Fairchild
Single N-Channel
RDS(ON) = 0.01
SO-8
5A to 10A
IRF7413
International
Rectifier
Single N-Channel
RDS(ON) = 0.011
SO-8
5A to 10A
MMSF3300
ON Semiconductor
Single N-Channel
RDS(ON) = 0.0125
SO-8
10A to 20A
FDB8030L
Fairchild
Single N-Channel
RDS(ON) = 0.0035
TO-263AB
10A to 20A
SUB75N03-04
Vishay/Siliconix
Single N-Channel
RDS(ON) = 0.004
D2PAK
VREGIN
2V/DIV
VREGOUT
2V/DIV
VOUT2
2V/DIV
VOUT1
2V/DIV
RESET
5V/DIV
Figure 24. Switching Regulator Hot Swap
its FB or SENSE pin. In the case of the LTC1649, large peak
currents result if the FB pin is at ground and not connected
directly to the output inductor and capacitors. To keep the
peak currents under control, R1, R2 and D1 hold the FB pin
above ground but below its normal regulated value until
VOUT2 ramps up and D1 reverse-biases.
Power N-Channel Selection
The RDS(ON) of the external pass transistors must be low
enough so that the voltage drop across them is 100mV or
less at full current. If the RDS(ON) is too high, the voltage
drop across the transistor can cause the output voltage to
trip the reset circuit. The transistors listed in Table 1 or
other similar transistors are recommended for use with
the LTC1645.
Low voltage applications may require the use of logic-level
FETs; ensure their maximum VGS rating is sufficient for the
application. GATE voltage as a function of VCC is illustrated
in the Typical Performance curves. If lower GATE drive is
desired, connect a diode in series with a zener between
GATE and VCC or between GATE and VOUT as shown in
Figure 25.
R1
Q1
VCC
VOUT
D1*
D2
1N4148
D2
1N4148
D4*
*USER SELECTED VOLTAGE CLAMP
1N4688 (5V)
1N4692 (7V): LOGIC-LEVEL MOSFET
1N4695 (9V)
1N4702 (15V): STANDARD-LEVEL MOSFET
1645 F25
Figure 25. Optional Gate Clamp
1645fa
21
LTC1645
U
PACKAGE DESCRIPTIO
S8 Package
8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
.189 – .197
(4.801 – 5.004)
NOTE 3
.045 ±.005
.050 BSC
8
.245
MIN
7
6
5
.160 ±.005
.150 – .157
(3.810 – 3.988)
NOTE 3
.228 – .244
(5.791 – 6.197)
.030 ±.005
TYP
1
RECOMMENDED SOLDER PAD LAYOUT
.010 – .020
× 45°
(0.254 – 0.508)
.008 – .010
(0.203 – 0.254)
0°– 8° TYP
.016 – .050
(0.406 – 1.270)
NOTE:
1. DIMENSIONS IN
.053 – .069
(1.346 – 1.752)
.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)
2
3
4
.004 – .010
(0.101 – 0.254)
.050
(1.270)
BSC
SO8 0303
1645fa
22
LTC1645
U
PACKAGE DESCRIPTIO
S Package
14-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
.337 – .344
(8.560 – 8.738)
NOTE 3
.045 ±.005
.050 BSC
14
N
12
11
10
9
8
N
.245
MIN
.160 ±.005
.150 – .157
(3.810 – 3.988)
NOTE 3
.228 – .244
(5.791 – 6.197)
1
.030 ±.005
TYP
13
2
3
N/2
N/2
RECOMMENDED SOLDER PAD LAYOUT
1
.010 – .020
× 45°
(0.254 – 0.508)
.008 – .010
(0.203 – 0.254)
2
3
4
5
6
.053 – .069
(1.346 – 1.752)
.004 – .010
(0.101 – 0.254)
0° – 8° TYP
.016 – .050
(0.406 – 1.270)
NOTE:
1. DIMENSIONS IN
.014 – .019
(0.355 – 0.483)
TYP
7
.050
(1.270)
BSC
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)
S14 0502
1645fa
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.
23
LTC1645
U
TYPICAL APPLICATIO
Dual Supply Hot Swap with Tracking Outputs
BOTH CURRENT LIMITS: 5A
Q1
1/2 Si4920DY
0.01Ω*
VIN1
3.3V
D1
1N4002
VIN2
2.5V
D2
1N4002
10Ω
Q2
0.01Ω* 1/2 Si4920DY
+
CLOAD1
D3
MBR0530T1
VOUT1
3.3V
2.5A
VOUT2
2.5V
2.5A
+
CLOAD2
10k
10Ω
14
4.99k
1%
TRIP
POINT:
3V
13
12
VCC1 SENSE1 GATE1
10
1
VCC2 SENSE2 GATE2
COMP+
ON
1.82k
1%
COMPOUT
LTC1645
(14-LEAD)
4
1.18k
1%
FB
8
9
1.18k
1%
6
5
FAULT
TIMER
11
0.33µF
1.37k
1%
GND
10k
0.1µF
25V
3
2
1.37k
1%
RESET
7
µP RESET
1645 F15
*WSL1206-01-1% (VISHAY DALE)
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LTC1421
Hot Swap Controller
Dual Supplies from 3V to 12V, Additionally – 12V
LTC1422
Hot Swap Controller
Single Supply Hot Swap in SO-8 from 3V to 12V
LT 1640AL/LT1640AH
Negative Voltage Hot Swap Controllers
Negative High Voltage Supplies from –10V to – 80V
LT1641-1/LT1641-2
Positive Voltage Hot Swap Controllers
Positive High Voltage Supplies From 9V to 80V
LTC1642
Fault Protected Hot Swap Controller
3V to 15V, Overvoltage Protection Up to 33V
LTC1643AL/LTC1643AL-1/
LTC1643AH
PCI-Bus Hot Swap Controllers
3.3V, 5V, 12V, –12V Supplies for PCI Bus
LTC1647
Dual Hot Swap Controller
Dual ON Pins for Supplies from 3V to 15V
LTC2921/LTC2922
Power Supply Tracker with Input Monitors
Includes 3 (LTC2921) or 5 (LTC2922) Remote Sense Switches
LTC2923
Power Supply Sequencing/Tracking Controller
Controls Two Supplies Without FETs, Tracks Up and Down
LTC4211
Single Channel Hot Swap Controller
8-Pin or 10-Pin with Overvoltage and Overcurrent Protection
LT4220
Dual Supply Hot Swap Controller
±2.7V to ±16.5V, Supply Tracking Mode
LTC4230
Triple Hot Swap Controller with Multifunction
Current Control
1.7V to 16.5V, Active Inrush Limiting, Fast Comparator
®
1645fa
24
Linear Technology Corporation
LT/TP 0404 1K REV A • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
LINEAR TECHNOLOGY CORPORATION 1999
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