LT3581 - 3.3A Boost/Inverting DC/DC Converter with Fault Protection

LT3581
3.3A Boost/Inverting DC/DC
Converter with Fault Protection
FEATURES
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DESCRIPTION
3.3A, 42V Combined Power Switch
Master/Slave (1.9A/1.4A) Switch Design
Output Short Circuit Protection
Wide Input Range:2.5V to 22V Operating,
40V Maximum Transient
Switching Frequency Up to 2.5MHz
Easily Configurable as a Boost, SEPIC, Inverting or
Flyback Converter
User Configurable Undervoltage Lockout
Low VCESAT Switch: 250mV at 2.75A (Typical)
Can be Synchronized to External Clock
Can Be Synchronized to other Switching Regulators
High Gain SHDN Pin Accepts Slowly Varying Input
Signals
14-Pin 4mm × 3mm DFN and 16-Lead MSE Packages
The LT®3581 is a PWM DC/DC converter with built-in fault
protection features to aid in protecting against output
shorts, input/output overvoltages, and overtemperature
conditions. The part consists of a 42V master switch, and
a 42V slave switch that can be tied together for a total
current limit of 3.3A.
The LT3581 is ideal for many local power supply designs. It
can be easily configured in Boost, SEPIC, Inverting or Flyback
configurations, and is capable of generating 12V at 830mA,
or –12V at 625mA from a 5V input. In addition, the LT3581’s
slave switch allows the part to be configured in high voltage,
high power charge pump topologies that are very efficient
and require fewer components than traditional circuits.
The LT3581’s switching frequency range can be set bet­ween
200kHz and 2.5MHz. The part may be clocked internally at
a frequency set by the resistor from the RT pin to ground,
or it may be synchronized to an external clock. A buffered
version of the clock signal is driven out of the CLKOUT
pin, and may be used to synchronize other compatible
switching regulator ICs to the LT3581.
APPLICATIONS
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Local Power Supply
Vacuum Fluorescent Display (VFD) Bias Supplies
TFT-LCD Bias Supplies
Automotive Engine Control Unit (ECU) Power
The LT3581 also features innovative SHDN pin circuitry that
allows for slowly varying input signals and an adjustable
undervoltage lockout function. Additional features such
as frequency foldback and soft-start are integrated. The
LT3581 is available in 14-Pin 4mm × 3mm DFN and 16Lead MSE packages.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
Protected by U.S. Patents, including 7579816.
TYPICAL APPLICATION
Efficiency and Power Loss vs
Load Current
Output Short Protected, 5V to 12V Boost Converter Operating at 2MHz
1.5µH
18.7k
4.7µF
10k
43.2k
100k
FB
VIN
FAULT
GATE
SHDN
CLKOUT
RT
LT3581
SYNC
VC
SS
GND
EFFICIENCY (%)
4.7µF
SW1 SW2
VOUT
12V
830mA
6.04k
130k
VIN
4.7µF
56pF
10.5k
0.1µF
1nF
2000
95
1800
90
1600
85
1400
80
1200
75
1000
70
800
65
600
60
400
55
200
50
3581 TA01
0
200
600
800
400
LOAD CURRENT (mA)
POWER LOSS (mW)
VIN
5V
100
0
1000
3581 TA01b
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1
LT3581
ABSOLUTE MAXIMUM RATINGS
(Note 1)
VIN Voltage.................................................. –0.3V to 40V
SW1/SW2 Voltage ...................................... –0.4V to 42V
RT Voltage.................................................... –0.3V to 5V
SS, FB Voltage........................................... –0.3V to 2.5V
VC Voltage..................................................... –0.3V to 2V
SHDN Voltage............................................. –0.3V to 40V
SYNC Voltage............................................. –0.3V to 5.5V
GATE Voltage.............................................. –0.3V to 80V
FAULT Voltage............................................. –0.3V to 40V
FAULT Current......................................................±500µA
CLKOUT Voltage........................................... –0.3V to 3V
CLKOUT Current.......................................................1mA
Operating Junction Temperature Range
LT3581E (Notes 2, 4).......................... –40°C to 125°C
LT3581I (Notes 2, 4)........................... –40°C to 125°C
LT3581H (Notes 2, 4).......................... –40°C to 150°C
Storage Temperature Range................... –65°C to 150°C
PIN CONFIGURATION
TOP VIEW
FB
1
14 SYNC
VC
2
13 SS
GATE
3
12 RT
FAULT
4
15
GND
TOP VIEW
FB
VC
GATE
FAULT
VIN
SW1
SW1
SW1
11 SHDN
VIN
5
SW1
6
10 CLKOUT
9 SW2
SW1
7
8 SW2
DE14 PACKAGE
14-PIN (4mm × 3mm) PLASTIC DFN
TJMAX = 125°C, θJA = 43°C/W, θJC = 4.3°C/W
EXPOSED PAD (PIN 15) IS GND, MUST BE SOLDERED TO PCB
1
2
3
4
5
6
7
8
17
GND
16
15
14
13
12
11
10
9
SYNC
SS
RT
SHDN
CLKOUT
SW2
SW2
SW2
MSE PACKAGE
16-LEAD PLASTIC MSOP
TJMAX = 125°C, θJA = 45°C/W, θJC = 10°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3581EDE#PBF
LT3581EDE#TRPBF
3581
14-Lead (4mm × 3mm) Plastic DFN
–40°C to 125°C
LT3581IDE#PBF
LT3581IDE#TRPBF
3581
14-Lead (4mm × 3mm) Plastic DFN
–40°C to 125°C
LT3581HDE#PBF
LT3581HDE#TRPBF
3581
14-Lead (4mm × 3mm) Plastic DFN
–40°C to 150°C
LT3581EMSE#PBF
LT3581EMSE#TRPBF
3581
16-Lead Plastic MSOP
–40°C to 125°C
LT3581IMSE#PBF
LT3581IMSE#TRPBF
3581
16-Lead Plastic MSOP
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
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LT3581
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 5V, VSHDN = VIN, VFAULT = VIN, unless otherwise noted. (Note 2).
PARAMETER
CONDITIONS
Minimum Input Voltage
MIN
TYP
MAX
2.3
2.5
V
22.1
23.5
25
V
V
l
VIN Overvoltage Lockout
UNITS
Positive Feedback Voltage
l
1.195
1.215
1.230
Negative Feedback Voltage
l
3
9
16
mV
µA
Positive FB Pin Bias Current
VFB = Positive Feedback Voltage, Current into Pin
l
81
83.3
85
Negative FB Pin Bias Current
VFB = Negative Feedback Voltage, Current out of Pin
l
81
83.3
85.5
Error Amp Transconductance
ΔI = 10μA
Error Amp Voltage Gain
Quiescent Current
Not Switching
Quiescent Current in Shutdown
VSHDN = 0V
Reference Line Regulation
2.5V ≤ VIN ≤ 20V
Switching Frequency, fOSC
RT = 34k
l
2.25
RT = 432k
l
180
Switching Frequency in Foldback
Compared to Normal fOSC
Switching Frequency Range
Free-Running or Synchronizing
200
SYNC High Level for Synchronization
l
1.3
SYNC Low Level for Synchronization
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SYNC Clock Pulse Duty Cycle
µmhos
70
V/V
1.9
2.3
mA
0
1
µA
0.01
0.05
%/V
2.5
2.75
MHz
200
220
kHz
1/6
l
VSYNC = 0V to 2V
µA
270
ratio
2500
kHz
V
20
0.4
V
80
%
Recommended Minimum SYNC Ratio
fSYNC/fOSC
3/4
Minimum Off-Time
45
ns
Minimum On-Time
55
ns
SW1 Current Limit
At All Duty Cycles
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1.9
Current Sharing (SW2/SW1)
2.4
3
78
3.3
4.3
A
%
5.4
A
SW1 + SW2 Current Limit
At All Duty Cycles, SW2/SW1 = 78% (Note 3)
Switch VCESAT
SW1 & SW2 Tied Together, ISW1 + ISW2 = 2.75A
250
SW1 Leakage Current
VSW1 = 5V
0.01
1
µA
SW2 Leakage Current
VSW2 = 5V
0.01
1
µA
l
mV
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3
LT3581
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 5V, VSHDN = VIN, VFAULT = VIN, unless otherwise noted. (Note 2).
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Soft-Start Charge Current
VSS = 30mV, Current Flows Out of SS pin
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5.7
8.7
11.3
µA
Soft-Start Discharge Current
Part in FAULT, VSS = 2.1V, Current Flows into SS Pin
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5.7
8.7
11.3
µA
Soft-Start High Detection Voltage
Part in FAULT
l
1.65
1.8
1.95
V
Soft-Start Low Detection Voltage
Part Exiting FAULT
l
30
50
85
SHDN Minimum Input Voltage High
Active Mode, SHDN Rising (LT3581E, LT3581I)
Active Mode, SHDN Rising (LT3581H)
Active Mode, SHDN Falling (LT3581E, LT3581I, LT3581H)
l
l
l
1.27
1.27
1.24
1.33
1.33
1.3
1.41
1.44
1.38
V
V
V
SHDN Input Voltage Low
Shutdown Mode
l
0.3
V
SHDN Pin Bias Current
VSHDN = 3V
VSHDN = 1.3V
VSHDN = 0V
9.7
40
11.4
0
60
13.4
0.1
µA
µA
µA
CLKOUT Output Voltage High
CCLKOUT = 50pF
1.9
2.1
2.3
V
CLKOUT Output Voltage Low
CCLKOUT = 50pF
5
200
mV
CLKOUT Duty Cycle
TJ = 25°C
42
%
CLKOUT Rise Time
CCLKOUT = 50pF
12
ns
CLKOUT Fall Time
CCLKOUT = 50pF
GATE Pull Down Current
VGATE = 3V (LT3581E, LT3581I)
VGATE = 3V (LT3581H)
VGATE = 80V (LT3581E, LT3581I, LT3581H)
GATE Leakage Current
VGATE = 50V, GATE Off
FAULT Output Voltage Low
50μA into FAULT Pin (LT3581E, LT3581I)
50μA into FAULT Pin (LT3581H)
FAULT Leakage Current
VFAULT = 40V, FAULT Off
8
l
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l
800
700
800
l
l
mV
ns
933
900
933
1100
1100
1100
µA
µA
µA
0.01
1
µA
100
100
300
400
mV
mV
0.01
1
µA
FAULT Input Voltage Low
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700
750
800
mV
FAULT Input Voltage High
l
950
1000
1050
mV
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LT3581E is guaranteed to meet performance specifications
from 0°C to 125°C. Specifications over the –40°C to 125°C junction
temperature range are assured by design, characterization and correlation
with statistical process controls. The LT3581I is guaranteed over the full
–40°C to 125°C operating junction temperature range. The LT3581H is
guaranteed over the full –40°C to 150°C operating junction temperature
range. Operating lifetime is derated at junction temperatures greater than
125°C.
Note 3: Current limit guaranteed by design and/or correlation to static test.
Note 4: This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 150°C when overtemperature protection is active.
Continuous operation over the specified maximum operating junction
temperature may impair device reliability.
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LT3581
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Fault Current Limit vs
Duty Cycle
Switch Saturation Voltage with
SW1 and SW2 Tied Together
4
3
2
450
100
400
90
350
300
250
200
150
100
1
50
0
20
60
50
40
DUTY CYCLE (%)
30
70
0
80
0
1
2
3
4
SW1 + SW2 CURRENT (A)
3581 G01
Switch Fault Current Limit vs
Temperature
80
70
60
50
40
30
20
10
0
5
0
1
2
4
3
SW1 + SW2 CURRENT (A)
3581 G02
Positive Feedback Voltage
vs Temperature
6
5
3581 G03
CLKOUT Duty Cycle
vs Temperature
1.2200
80
70
5
4
3
2
1.2175
60
CLKOUT DC (%)
POSITIVE FB VOLTAGE (V)
SW1 + SW2 FAULT CURRENT LIMIT (A)
Current Sharing Between SW1 and
SW2 When Tied Together
CURRENT SHARING = SW2/SW1 (%)
5
SATURATION VOLTAGE (mV)
SW1 + SW2 FAULT CURRENT LIMIT (A)
6
1.2150
50
40
30
1.2125
1
20
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1.2100
–50 –25
0
Oscillator Frequency
3581 G06
Frequency Foldback
Gate Current vs Gate Voltage
1000
RT = 34k
2400
2000
1600
1200
800
400
RT = 432k
0
25 50 75 100 125 150
TEMPERATURE (°C)
3581 G07
900
–40°C
800
GATE CURRENT (µA)
SWITCHING FREQUENCY RATIO (fSW/fOSC)
1
2800
0 25 50 75 100 125 150
TEMPERATURE (°C)
3581 G05
3200
0
–50 –25
10
–75 –50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
3581 G04
FREQUENCY (kHz)
TA = 25°C, unless otherwise noted.
1/2
1/3
1/4
1/5
1/6
0
25°C
700
125°C
600
500
400
300
200
INVERTING
CONFIGURATIONS
0
0.2
BOOSTING
CONFIGURATIONS
0.4
0.6
0.8
FB VOLTAGE (V)
1.0
100
1.2
3581 G08
0
0
20
40
60
GATE VOLTAGE (V)
80
3581 G09
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5
LT3581
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, unless otherwise noted.
Commanded Current Limit vs
SS Voltage
Gate Current vs SS Voltage
1000
SHDN Voltage Threshold with
Hysteresis
1.40
5
1.38
900
700
600
500
400
300
200
4
1.36
SHDN VOLTAGE (V)
SW1 + SW2 CURRENT (A)
GATE CURRENT (µA)
800
3
2
SHDN FALLING
1.28
1.26
0
1.20
–50 –25
1.22
0.25
0.50 0.75 1.00
SS VOLTAGE (V)
1.25
1.50
0
0.2
0.4
0.6
0.8
SS VOLTAGE (V)
25°C
20
16
12
8
25 50 75 100 125 150
TEMPERATURE (°C)
Internal UVLO
2.40
–40°C
2.38
200
SHDN PIN CURRENT (µA)
24
0
3581 G12
SHDN Pin Current
250
2.36
25°C
150
125°C
100
50
125°C
4
1.2
3580 G11
SHDN Pin Current
28
1.0
VIN VOLTAGE (V)
0
32
SHDN PIN CURRENT (µA)
1.30
1.24
3581 G10
0
1.32
1
100
0
SHDN RISING
1.34
2.34
2.32
2.30
2.28
2.26
2.24
2.22
–40°C
0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
SHDN VOLTAGE (V)
0
5
10
15 20 25 30
SHDN VOLTAGE (V)
35
2.20
–50 –25
40
0
25 50 75 100 125 150
TEMPERATURE (°C)
3581 G14
3581 G15
3581 G13
CLKOUT Rise Time at 1MHz
50
30
CLKOUT RISE TIME
26
30
25
20
15
CLKOUT FALL TIME
10
24
22
20
0
50
100
150
200
CLKOUT CAPACITIVE LOAD (pF)
250
3581 G16
16
–50 –25
0.75
FAULT FALLING
0.50
0.25
18
5
FAULT RISING
1.00
FAULT VOLTAGE (V)
35
0
1.25
28
40
VIN VOLTAGE (V)
CLKOUT RISE OR FALL TIME (ns)
45
FAULT Input Voltage Threshold
with Hysteresis
VIN OVLO
0
25 50 75 100 125 150
TEMPERATURE (°C)
3581 G17
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3581 G18
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LT3581
PIN FUNCTIONS
(DFN/MSOP)
FB (Pin 1/Pin 1): Positive and Negative Feedback Pin. For
a Boost or Inverting Converter, tie a resistor from the FB
pin to VOUT according to the following equations:
– 1.215V 
V
RFB =  OUT
; Boost or SEPIC Converter
 83.3•10 –6 
 | V | + 9mV 
;Inverting Converter
RFB =  OUT
 83.3•10 –6 
VC (Pin 2/Pin 2): Error Amplifier Output Pin. Tie external
com­pensation network to this pin.
GATE (Pin 3/Pin 3): PMOS Gate Drive Pin. The GATE pin
is a pull-down current source, used to drive the gate of
an external PMOS for output short circuit protection or
output disconnect. The GATE pin current increases linearly
with the SS pin’s voltage, with a maximum pull-down
current of 933µA at SS voltages exceeding 500mV. Note
that if the SS voltage is greater than 500mV and the GATE
pin voltage is less than 2V, then the GATE pin looks like
a 2kΩ impedance to ground. See the Appendix for more
information.
FAULT (Pin 4/Pin 4): Fault Indication Pin. This active low,
bidirectional pin can either be pulled low (below 750mV)
by an external source, or internally by the chip to indicate a
fault. When pulled low, this pin causes the power switches
to turn off, the GATE pin to become high impedance, the
CLKOUT pin to become disabled, and the SS pin to go
through a charge/discharge sequence. The end/absence
of a fault is indicated when the voltage on this pin exceeds
1V. A pull-up resistor or current source is needed on this
pin to pull it above 1V in the absence of a fault.
VIN (Pin 5/Pin 5): Input Supply Pin. Must be locally bypassed.
SW1 (Pins 6, 7/Pins 6,7, 8): Master Switch Pin. This is the
collector of the internal master NPN power switch.
Minimize the metal trace area connected to this pin to
minimize EMI.
CLKOUT (Pin 10/Pin 12): Clock Output Pin. Use this pin
to synchronize one or more other compatible switching
regulator ICs to the LT3581. The clock that this pin outputs
runs at the same frequency as the internal oscillator of the
part or as the SYNC pin. CLKOUT may also be used as a
temperature monitor since the CLKOUT pin’s duty cycle
varies linearly with the part’s junction temperature. Note
that the CLKOUT pin is only meant to drive capacitive
loads up to 50pF.
SHDN (Pin 11/Pin 13): Shutdown Pin. In conjunction
with the UVLO (undervoltage lockout) circuit, this pin is
used to enable/disable the chip and restart the soft-start
sequence. Drive below 300mV to disable the chip. Drive
above 1.33V (typical) to activate the chip and restart the
soft-start sequence. Do not float this pin.
RT (Pin 12/Pin 14): Timing Resistor Pin. Adjusts the
LT3581’s switching frequency. Place a resistor from this
pin to ground to set the frequency to a fixed free running
level. Do not float this pin.
SS (Pin 13/Pin 15): Soft-Start Pin. Place a soft-start
capacitor here. Upon start-up, the SS pin will be charged
by a (nominally) 250k resistor to about 2.1V. During a
fault, the SS pin will be slowly charged up and eventually
discharged as part of a timeout sequence (see the State
Diagram for more information on the SS pin’s role during
a fault event).
SYNC (Pin 14/Pin 16): To synchronize the switching
frequency to an outside clock, simply drive this pin with
a clock. The high voltage level of the clock must exceed
1.3V, and the low level must be less than 0.4V. Drive this
pin to less than 0.4V to revert to the internal free running
clock. See the Applications Information section for more
information.
GND (Exposed Pad Pin 15/Exposed Pad Pin 17): Ground.
Exposed pad must be soldered directly to local ground
plane.
SW2 (Pins 8, 9/Pins 9, 10, 11): Slave Switch Pin. This
is the collector of the internal slave NPN power switch.
Minimize the metal trace area connected to this pin to
minimize EMI.
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7
LT3581
BLOCK DIAGRAM
VIN
CIN
OPTIONAL
D1
L1
M1
COUT1
RFAULT
DIE TEMP
165°C
2.1V
+
–
–
+
1.8V
250k
750mV
STARTUP
AND FAULT
LOGIC
LDO
SS
VIN
**
+
–
+
–
SW2
**
+
–
ISW1
+
–
+
–
VC
22V
MIN
–
+
933µA
50mV
RGATE
FAULT
GATE
SOFTSTART
VOUT
COUT2
DRIVER
DISABLE
CSS
SW1
42V
MIN
SAMPLE MODE BLOCK
42V
MIN
+
–
FB
1.9A
MIN
TD ~ 30ns
SAMPLE
SHDN
1.33V
+
–
+
–
+
–
SW2
45mV
VBE • 0.9
RFB
Q2
SW1
COMPARATOR
SR1
–
UVLO
VIN
1.17V
R
A3
A3
1.215V
REFERENCE
+
S
Q1
Q
+
+
14.6k
∑
A1
–
FB
A4
FREQUENCY
FOLDBACK
A2
RS
20mΩ
–
RAMP
GENERATOR
+
14.6k
27mΩ
DRIVER
GND
÷N ADJUSTABLE
OSCILLATOR
–
SS
SYNC
BLOCK
VC
SYNC
RT
CLKOUT
RC
CC
RT
3581 BD
**SW OVERVOLTAGE PROTECTION IS NOT GUARANTEED TO PROTECT THE LT3581 DURING SW OVERVOLTAGE EVENTS
Figure 1. Block Diagram
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LT3581
STATE DIAGRAM
SHDN < 1.33V (TYPICAL)
or
VIN < 2.3V (TYPICAL)
CHIP OFF
• ALL SWITCHES DISABLED
• IGATE OFF
• FAULTS CLEARED
SHDN > 1.33V (TYPICAL)
AND
VIN > 2.3V (TYPICAL)
INITIALIZE
• SS PULLED LOW
FAULT1
FAULT2
FAULT DETECTED
SS < 50mV
SOFT START
• IGATE ENABLED
• SS CHARGES UP
• SWITCHER ENABLED
FAULT1
• SS CHARGES UP
• IGATE OFF
• FAULT PIN PULLED LOW
INTERNALLY BY LT3581
• SWITCHER DISABLED
• CLKOUT DISABLED
SS > 1.8V AND
NO FAULT1 CONDITIONS
STILL DETECTED
POST FAULT DELAY
• SS SLOWLY DISCHARGES
SAMPLE MODE
• Q1 & Q2 SWITCHES
FORCED ON EVERY CYCLE
FOR AT LEAST MINIMUM
ON TIME
• IGATE FULLY ACTIVATED
WHEN SS > 500mV
FAULT1
FAULT1
SS < 50mV
LOCAL FAULT OVER
IF |VOUT| DROPS CAUSING:
FB < 1.17V (BOOST)
OR
FB > 45mV (INVERTING)
• INTERNAL FAULT PIN PULLDOWN
RELEASED BY LT3581
• SS CONTINUES DISCHARGING
TO GND
NORMAL MODE
• NORMAL OPERATION
• CLKOUT ENABLED WHEN
SS > 1.8V
FAULT1 = OVER VOLTAGE PROTECTION ON VIN (VIN > 22V MIN)
OVER TEMPERATURE (TJUNCTION > 165°C)
OVER CURRENT ON SW1 (ISW1 > 1.9A MIN)
OVER VOLTAGE PROTECTION ON SW1 (VSW1 > 42V MIN)
OVER VOLTAGE PROTECTION ON SW2 (VSW2 > 42V MIN)
FAULT1
FAULT PIN > 1.0V
FAULT1
FAULT2 = FAULT PULLED LOW EXTERNALLY (FAULT < 0.75V)
3581 SD
Figure 2. State Diagram
3581fb
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9
LT3581
OPERATION
OPERATION – OVERVIEW
The LT3581 uses a constant-frequency, current mode control scheme to provide excellent line and load regulation.
The part’s undervoltage lockout (UVLO) function, together
with soft-start and frequency foldback, offers a controlled
means of starting up. Fault features are incorporated in the
LT3581 to aid in the detection of output shorts, over-voltage, and overtemperature conditions. Refer to the Block
Diagram (Figure 1) and the State Diagram (Figure 2) for
the following description of the part’s operation.
OPERATION – START-UP
Several functions are provided to enable a very clean
start-up for the LT3581:
Precise Turn-On Voltage
The SHDN pin is compared to an internal voltage reference
to give a precise turn on voltage level. Taking the SHDN pin
above 1.33V (typical) enables the part. Taking the SHDN pin
below 300mV shuts down the chip, resulting in extremely
low quiescent current. The SHDN pin has 30mV of hysteresis
to protect against glitches and slow ramping.
Undervoltage Lockout (UVLO)
The SHDN pin can also be used to create a configurable
UVLO. The UVLO function sets the turn on/off of the LT3581
at a desired input voltage (VINUVLO). Figure 3 shows how a
resistor divider (or single resistor) from VIN to the SHDN
pin can be used to set VINUVLO. RUVLO2 is optional. It may
be left out, in which case set it to infinite in the equation
below. For increased accuracy, set RUVLO2 ≤ 10k. Pick
RUVLO1 as follows:
RUVLO1 =
VINUVLO – 1.33V
 1.33V 
 R
 + 11.6µA
UVLO2 
VIN
VIN
1.33V
RUVLO1
SHDN
–
ACTIVE/
LOCKOUT
+
11.6µA
AT 1.33V
RUVLO2
(OPTIONAL)
GND
3581 F03
Figure 3. Configurable UVLO
The LT3581 also has internal UVLO circuitry that disables
the chip when VIN < 2.3V (typical).
Soft-Start of Switch Current
The soft-start circuitry provides for a gradual ramp-up
of the switch current (refer to Commanded Current Limit
vs SS Voltage in Typical Performance Characteristics).
When the part is brought out of shutdown, the external
SS capacitor is first discharged which resets the states
of the logic circuits in the chip. Then an integrated 250k
resistor pulls the SS pin to ~1.8V. The ramp rate of the SS
pin voltage is set by this 250k resistor and the external
capacitor connected to this pin. Once SS gets to 1.8V, the
CLKOUT pin is enabled, and an internal regulator pulls
the pin up quickly to ~2.1V. Typical values for the external
soft-start capacitor range from 100nF to 1μF.
Soft-Start of External PMOS (if used)
The soft-start circuitry also gradually ramps up the GATE
pin pull-down current which allows an external PMOS to
slowly turn on (M1 in Block Diagram). The GATE pin current
increases linearly with the SS voltage, with a maximum
current of 933µA when the SS voltage gets above 500mV.
Note that if the GATE pin voltage is less than 2V for SS
voltages exceeding 500mV, then the GATE pin impedance
to ground is 2kΩ. The soft turn on of the external PMOS
helps limit inrush current at start-up, making hot-plugs
of LT3581s feasible and safe.
3581fb
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LT3581
OPERATION
Sample Mode
Sample Mode is the mechanism used by the LT3581 to
aid in the detection of output shorts. It refers to a state of
the LT3581 where the master and slave power switches
(Q1 and Q2) are turned on for a minimum period of time
every clock cycle (or every few clock cycles in frequency
foldback) in order to “sample” the inductor current. If the
sampled current through Q1 exceeds the master switch current limit of 1.9A (min), the LT3581 triggers an overcurrent
fault internally (see Operation-Fault section for details).
Sample Mode is active when FB is out of regulation by
more than approximately 3.7% (45mV < FB < 1.17V).
Frequency Foldback
The frequency foldback circuit reduces the switching frequency when 350mV < FB < 900mV (typical). This feature
lowers the minimum duty cycle that the part can achieve,
thus allowing better control of the inductor current during start-up. When the FB voltage is pulled outside of this
range, the switching frequency returns to normal.
Note that the peak inductor current at start-up is a function
of many variables including load profile, output capacitance,
target VOUT, VIN, switching frequency, etc. Test each and
every application’s performance at start-up to ensure that
the peak inductor current does not exceed the minimum
fault current limit.
OPERATION – REGULATION
The following description of the LT3581’s operation assumes that the FB voltage is close enough to its regulation
target so that the part is not in sample mode. Use the
Block Diagram as a reference when stepping through the
following description of the LT3581 operating in regulation.
At the start of each oscillator cycle, the SR latch (SR1) is
set, which turns on the power switches Q1 and Q2. The
collector current through the master switch, Q1, is ~1.3
times the collector current through the slave switch, Q2,
when the collectors of the two switches are tied together.
Q1’s emitter current flows through a current sense resistor
(RS) generating a voltage proportional to the total switch
current. This voltage (amplified by A4) is added to a stabilizing ramp and the resulting sum is fed into the positive
terminal of the PWM comparator A3. When the voltage on
the positive input of A3 exceeds the voltage on the negative
input, the SR latch is reset, turning off the master and slave
power switches. The voltage on the negative input of A3
(VC pin) is set by A1 (or A2), which is simply an amplified
difference between the FB pin voltage and the reference
voltage (1.215V if the LT3581 is configured as a boost
converter, or 9mV if configured as an inverting converter).
In this manner, the error amplifier sets the correct peak
current level to maintain output regulation.
As long as the part is not in fault (see Operation – Fault
section) and the SS pin exceeds 1.8V, the LT3581 drives its
CLKOUT pin at the frequency set by the RT pin or the SYNC
pin. The CLKOUT pin can be used to synchronize other
compatible switching regulator ICs (including additional
LT3581s) with the LT3581. Additionally, CLKOUT’s duty
cycle varies linearly with the part’s junction temperature,
and may be used as a temperature monitor.
OPERATION – FAULT
The LT3581’s FAULT pin is an active low, bidirectional pin
that is pulled low to indicate a fault. Each of the following
events can trigger a fault in the LT3581:
A.FAULT1 events:
1.SW Overcurrent:
a.
ISW1 > 1.9A (minimum)
b.
(ISW1 + ISW2) > 3.3A (minimum)
2.VIN Voltage > 22V (minimum)
3.SW1 Voltage and/or SW2 Voltage > 42V
(minimum)
4.Die Temperature > 165°C
B.FAULT2 events:
1.Pulling the FAULT pin low externally
3581fb
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11
LT3581
OPERATION
Refer to the State Diagram (Figure 2) for the following
description of the LT3581’s operation during a fault event.
When a fault is detected, in addition to the FAULT pin being
pulled low internally, the LT3581 also disables its CLKOUT
pin, turns off its power switches, and the GATE pin becomes
high impedance. The external PMOS, M1, turns off when
the gate of M1 is pulled up to its source by the external
RGATE resistor (see Block Diagram). With the external
PMOS turned off, the power path from VIN to VOUT is cut
off, protecting power components downstream.
At the same time, a timeout sequence commences where
the SS pin is charged up to 1.8V (the SS pin will continue
charging up to 2.1V and be held there in the case of a
FAULT1 event that has still not ended), and then discharged
to 50mV. This timeout period relieves the part, the PMOS,
and other downstream power components from electrical
and thermal stress for a minimum amount of time as set
by the voltage ramp rate on the SS pin.
In the absence of faults, the FAULT pin is pulled high by the
external RFAULT resistor (typically 100k). Figure 4 shows
the events that accompany the detection of an output
short on the LT3581.
VOUT
10V/DIV
VFAULT
5V/DIV
VCLKOUT
2V/DIV
IL
2A/DIV
5µs/DIV
3581 F04
Figure 4. Output Short Circuit Protection of the LT3581
3581fb
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LT3581
APPLICATIONS INFORMATION
BOOST CONVERTER COMPONENT SELECTION
D1
20V, 2A
L1
1.5µH
VIN
5V
CIN
4.7µF
RFAULT
100k
RT
43.2k
VIN
FB
LT3581
FAULT
GATE
SHDN
CLKOUT
RT
VC
SYNC
SS
GND
PARAMETERS/EQUATIONS
OPTIONAL
PMOS
COUT1
4.7µF
SW1 SW2
Table 1. Boost Design Equations
RGATE
6.04k
VOUT
12V
IOUT < 0.83A
Step 1: Pick VIN, VOUT, and fOSC to calculate equations below.
Inputs
Step 2:
DC
RFB
130k
DC ≅
COUT2
4.7µF
CF
56pF
CSS
0.1µF
L TYP =
RC
10.5k
CC
1nF
3581 F05
Table 1 is a step-by-step set of equations to calculate
component values for the LT3581 when operating as a
boost converter. Input parameters are input and output
voltage, and switching frequency (VIN , VOUT and fOSC respectively). Refer to the Appendix for further information
on the design equations presented in Table 1.
Variable Definitions:
VIN = Input Voltage
VOUT = Output Voltage
DC = Power Switch Duty Cycle
fOSC = Switching Frequency
IOUT = Maximum Average Output Current
IRIPPLE = Inductor Ripple Current
RDSON_PMOS = RDSON of External PMOS (set to 0 if not
using PMOS)
( VIN – 0.3V ) • DC
(1)
fOSC • 1A
( VIN – 0.3V ) • (2 • DC – 1)
2.2A • fOSC • (1– DC)
( V – 0.3V ) • DC
= IN
LMIN =
Step 3:
L1
Figure 5. Boost Converter – The Component Values and Voltages
Given Are Typical Values for a 2MHz, 5V to 12V Boost
The LT3581 can be configured as a Boost converter as
in Figure 5. This topology allows for positive output voltages that are higher than the input voltage. An external
PMOS (optional) driven by the GATE pin of the LT3581 can
achieve input or output disconnect during a fault event.
A single feedback resistor sets the output voltage. For
output voltages higher than 40V, see the Charge Pump
Aided Regulators section.
VOUT – VIN + 0.5V
VOUT + 0.5V – 0.3V
LMAX
(2)
(3)
fOSC • 0.35A
• Pick L1 out of a range of inductor values where the minimum
value of the range is set by LTYP or LMIN, whichever is higher.
The maximum value of the range is set by LMAX. See appendix
on how to choose current rating for inductor value chosen.
Step 4:
IRIPPLE
IRIPPLE =
( VIN – 0.3V ) • DC
fOSC • L1
Step 5:
IOUT
I


IOUT =  3.3A – RIPPLE  • (1– DC)

2 
Step 6:
D1
VR > VOUT ; IAVG > IOUT
COUT1 = COUT2 ≥
Step 7:
COUT1,
COUT2
IOUT • DC
fOSC 0.01• VOUT – 0.50 • IOUT • RDSON _ PMOS 
• If PMOS is not used, then use just one capacitor where
COUT = COUT1 + COUT2.
CIN ≥ C VIN +CPWR ≥
Step 8:
CIN
IRIPPLE
3.3A • DC
+
45 • fOSC • 0.005 • VIN 8 • fOSC • 0.005 • VIN
• Refer to Input Capacitor Selection in Appendix for definition of
CVIN and CPWR.
Step 9:
RFB
Step 10:
RT
RFB =
RT =
VOUT – 1.215V
83.3µA
87.6
–1; fOSC in MHz and R T in kΩ
fOSC
Only needed for input or output disconnect. See PMOS Selection
Step 11:
in the Appendix for information on sizing the PMOS, RGATE and
PMOS
picking appropriae UVLO components.
Note 1: The maximum design target for peak switch current is 3.3A and is
used in this table.
Note 2: The final values for COUT1, COUT2 and CIN may deviate from the
above equations in order to obtain desired load transient performance.
3581fb
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LT3581
APPLICATIONS INFORMATION
SEPIC CONVERTER COMPONENT SELECTION
(COUPLED OR UN-COUPLED INDUCTORS)
C1
1µF
L1
3.3µH
CIN
22µF
RFAULT
100k
ENABLE
RT
124k
SW1 SW2
VIN
•
L2
3.3µH
PARAMETERS/EQUATIONS
Step 1:
Inputs
D1
30V, 2A
•
VIN
3V TO 16V
Table 2. SEPIC Design Equations
COUT
22µF
×2
RFB
45.3k
VOUT
5V
IOUT < 0.9A (VIN = 3V)
IOUT < 1.5A (VIN = 12V)
Pick VIN, VOUT, and fOSC to calculate equations below.
Step 2:
DC
DC ≅
FB
L TYP =
LT3581
FAULT
GATE
SHDN
CLKOUT
RT
VC
SYNC
SS
GND
CSS
1µF
RC
7.87k
CC
2.2nF
( VIN – 0.3V ) • DC
fOSC • 1A
( VIN – 0.3V ) • (2 • DC – 1)
2.2A • fOSC • (1– DC)
( V – 0.3V ) • DC
= IN
LMIN =
CF
100pF
3581 F06
The LT3581 can also be configured as a SEPIC as shown in
Figure 6. This topology allows for positive output voltages
that are lower, equal, or higher than the input voltage. Output disconnect is inherently built into the SEPIC topology,
meaning no DC path exists between the input and output
due to capacitor C1. This implies that a PMOS controlled
by the GATE pin is not required in the power path.
Table 2 is a step-by-step set of equations to calculate
component values for the LT3581 when operating as a
SEPIC converter. Input parameters are input and output
voltage, and switching frequency (VIN , VOUT and fOSC respectively). Refer to the Appendix for further information
on the design equations presented in Table 2.
LMAX
Step 3:
L
fOSC • 0.35A
(1)
(2)
(3)
• Pick L out of a range of inductor values where the minimum
value of the range is set by LTYP or LMIN, whichever is higher.
The maximum value of the range is set by LMAX. See
Appendix on how to choose current rating for inductor value
chosen.
• Pick L1 = L2 = L for coupled inductors.
• Pick L1L2 = L for un-coupled inductors.
Figure 6. SEPIC Converter – The Component Values and Voltages
Given Are Typical Values for a 700kHz, Wide Input Range (3V to
16V) SEPIC Converter with 5V Out
Step 4:
IRIPPLE
IRIPPLE =
( VIN – 0.3V ) • DC
fOSC • L
• L = L1 = L2 for coupled inductors.
• L = L1L2 for un-coupled inductors.
Step 5:
IOUT
I


IOUT =  3.3A – RIPPLE  • (1– DC)

2 
Step 6:
D1
VR > VIN + VOUT ; IAVG > IOUT
Step 7:
C1
C1 ≥ 1µF; VRATING ≥ VIN
Step 8:
COUT
Variable Definitions:
VIN = Input Voltage
VOUT = Output Voltage
DC = Power Switch Duty Cycle
fOSC = Switching Frequency
IOUT = Maximum Average Output Current
IRIPPLE = Inductor Ripple Current
VOUT + 0.5V
VIN + VOUT + 0.5V – 0.3V
COUT ≥
IOUT • DC
fOSC •0.005• VOUT
CIN ≥ C VIN +CPWR ≥
Step 9:
CIN
IRIPPLE
3.3A • DC
+
45 • fOSC • 0.005 • VIN 8 • fOSC • 0.005 • VIN
• Refer to Input Capacitor Selection in Appendix for definition
of CVIN and CPWR.
Step 10:
RFB
Step 11:
RT
RFB =
RT =
VOUT – 1.215V
83.3µA
87.6
– 1; fOSC in MHz and R T in kΩ
fOSC
Note 1: The maximum design target for peak switch current is 3.3A and is
used in this table.
Note 2: The final values for COUT, CIN and C1 may deviate from the above
equations in order to obtain desired load transient performance.
3581fb
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LT3581
APPLICATIONS INFORMATION
DUAL INDUCTOR INVERTING CONVERTER COMPONENT
SELECTION (COUPLED OR UN-COUPLED INDUCTORS)
C1
1µF
L1
3.3µH
RFAULT
100k
ENABLE
RT
43.2k
VOUT
–12V
IOUT < 625mA
•
CIN
3.3µF
VIN
COUT
4.7µF
D1
20V
1A
SW1 SW2
PARAMETERS/EQUATIONS
Step 1: Inputs Pick VIN, VOUT, and fOSC to calculate equations below.
L2
3.3µH
•
VIN
5V
Table 3. Dual Inductor Inverting Design Equations
DC ≅
Step 2: DC
RFB
143k
L TYP =
FB
LT3581
FAULT
GATE
SHDN
CLKOUT
RT
VC
SYNC
SS
GND
| VOUT | + 0.5V
VIN + | VOUT | +0.5V – 0.3V
( VIN – 0.3V ) • DC
(1)
fOSC • 1A
( VIN – 0.3V ) • (2 • DC – 1)
2.2A • fOSC • (1– DC)
( V – 0.3V ) • DC
= IN
LMIN =
CSS
100nF
RC
11k
CC
1nF
CF
47pF
Figure 7. Dual Inductor Inverting Converter – The Component
Values and Voltages Given Are Typical Values for a 2MHz, 5V to
–12V Inverting Topology Using Coupled Inductors
Table 3 is a step-by-step set of equations to calculate
component values for the LT3581 when operating as a
dual inductor inverting converter. Input parameters are
input and output voltage, and switching frequency (VIN ,
VOUT and fOSC respectively). Refer to the Appendix for
further information on the design equations presented
in Table 3.
IRIPPLE =
Step 4: IRIPPLE
( VIN – 0.3V ) • DC
fOSC • L
• L = L1 = L2 for coupled inductors.
• L = L1L2 for un-coupled inductors.
Step 5: IOUT
I


IOUT =  3.3A – RIPPLE  • (1– DC)

2 
Step 6: D1
VR > VIN +| VOUT | ; IAVG > IOUT
Step 7: C1
C1 ≥ 1µF; VRATING ≥ VIN + | VOUT |
Step 8: COUT
COUT ≥
IRIPPLE
8 • fOSC ( 0.005 • | VOUT |)
CIN ≥ C VIN +CPWR ≥
IRIPPLE
3.3A • DC
+
45 • fOSC • 0.005 • VIN 8 • fOSC • 0.005 • VIN
Step 9: CIN
Variable Definitions:
VIN = Input Voltage
VOUT = Output Voltage
DC = Power Switch Duty Cycle
fOSC = Switching Frequency
IOUT = Maximum Average Output Current
IRIPPLE = Inductor Ripple Current
(3)
fOSC • 0.35A
• Pick L out of a range of inductor values where the
minimum value of the range is set by LTYP or LMIN,
whichever is higher. The maximum value of the range
is set by LMAX. See Appendix on how to choose current
rating for inductor value chosen.
• Pick L1 = L2 = L for coupled inductors.
• Pick L1L2 = L for un-coupled inductors.
3581 F07
Due to its unique FB pin, the LT3581 can work in a Dual
Inductor Inverting configuration as in Figure 7. Changing
the connections of L2 and the Schottky diode in the SEPIC
topology results in generating negative output voltages.
This solution results in very low output voltage ripple
due to inductor L2 being in series with the output. Output
disconnect is inherently built into this topology due to the
capacitor C1.
LMAX
Step 3: L
(2)
• Refer to Input Capacitor Selection in Appendix for
definition of CVIN and CPWR.
RFB =
Step 10: RFB
Step 11: RT
RT =
| VOUT | + 5mV
83.3µA
87.6
– 1; fOSC in MHz and R T in kΩ
fOSC
Note 1: The maximum design target for peak switch current is 3.3A and is
used in this table.
Note 2: The final values for COUT, CIN and C1 may deviate from the above
equations in order to obtain desired load transient performance.
3581fb
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15
LT3581
APPLICATIONS INFORMATION
LAYOUT GUIDELINES FOR BOOST, SEPIC, AND DUAL
INDUCTOR INVERTING TOPOLOGIES
General Layout Guidelines
• To optimize thermal performance, solder the exposed
ground pad of the LT3581 to the ground plane, with
multiple vias around the pad connecting to additional
ground planes.
• A ground plane should be used under the switcher circuitry
to prevent interplane coupling and overall noise.
• High speed switching path (see specific topology for
more information) must be kept as short as possible.
• The VC , FB, and RT components should be placed as
close to the LT3581 as possible, while being as far
away as practically possible from the switch node. The
ground for these components should be separated from
the switch current path.
• Place the bypass capacitor for the VIN pin as close as
possible to the LT3581.
• Place the bypass capacitor for the inductor as close as
possible to the inductor.
• The load should connect directly to the positive and
negative terminals of the output capacitor for best load
regulation.
Boost Topology Specific Layout Guidelines
• Keep length of loop (high speed switching path) governing switch, diode D1, output capacitor COUT1, and
ground return as short as possible to minimize parasitic
inductive spikes at the switch node during switching.
SEPIC Topology Specific Layout Guidelines
• Keep length of loop (high speed switching path) governing switch, flying capacitor C1, diode D1, output
capacitor COUT, and ground return as short as possible
to minimize parasitic inductive spikes at the switch node
during switching.
Inverting Topology Specific Layout Guidelines
• Keep ground return path from the cathode of D1 (to
chip) separated from output capacitor COUT’s ground
return path (to chip) in order to minimize switching
noise coupling into the output.
• Keep length of loop (high speed switching path) governing switch, flying capacitor C1, diode D1, and ground
return as short as possible to minimize parasitic inductive spikes at the switch node during switching.
GND
GND
2
CIN
A
–
VIN
+
17
15
3
14
4
13
SHDN
5
12
CLKOUT
6
11
7
10
8
L1
SYNC
16
9
B
COUT1
M1
D1
A
–
VIN
+
16
2
15
17
SYNC
3
14
4
13
SHDN
5
12
CLKOUT
6
11
7
10
8
9
B
–
L2
L1
C1
D1
COUT
VOUT
+
•
D2
–
VOUT
+
COUT
CIN
1
•
1
RGATE
3581 F09
3581 F08
A: RETURN CIN GROUND DIRECTLY TO LT3581 EXPOSED PAD PIN 17. IT IS ADVISED TO NOT
COMBINE CIN GROUND WITH GND EXCEPT AT THE EXPOSED PAD.
B: RETURN COUT AND COUT1 GROUND DIRECTLY TO LT3581 EXPOSED PAD PIN 17. IT IS ADVISED
TO NOT COMBINE COUT AND COUT1 GROUND WITH GND EXCEPT AT THE EXPOSED PAD.
A: RETURN CIN AND L2 GROUND DIRECTLY TO LT3581 EXPOSED PAD PIN 17. IT IS ADVISED
TO NOT COMBINE CIN AND L2 GROUND WITH GND EXCEPT AT THE EXPOSED PAD.
B: RETURN COUT GROUNDS DIRECTLY TO LT3581 EXPOSED PAD PIN 17. IT IS ADVISED
TO NOT COMBINE COUT GROUND WITH GND EXCEPT AT THE EXPOSED PAD.
L1, L2: MOST COUPLED INDUCTOR MANUFACTURERS USE CROSS PINOUT FOR IMPROVED
PERFORMANCE.
Figure 8. Suggested Component Placement for Boost Topology
(MSOP Shown, DFN Similar, Not to Scale.) Pin 15 on DFN or
Pin 17 on MSOP Is the Exposed Pad Which Must Be Soldered
Directly to the Local Ground Plane for Adequate Thermal
Performance. Multiple Vias to Additional Ground Planes Will
Improve Thermal Performance
Figure 9. Suggested Component Placement for SEPIC Topology
(MSOP Shown, DFN Similar, Not to Scale.) Pin 15 on DFN or
Pin 17 on MSOP Is the Exposed Pad Which Must Be Soldered
Directly to the Local Ground Plane for Adequate Thermal
Performance. Multiple Vias to Additional Ground Planes Will
Improve Thermal Performance
3581fb
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LT3581
APPLICATIONS INFORMATION
GND
1
CIN
A
–
VIN
+
SYNC
16
2
15
17
3
14
4
13
SHDN
5
12
CLKOUT
6
11
7
10
8
9
B
GND
C
C1
COUT
L1
•
•
D1
– VOUT
L2
3581 F10
A: RETURN CIN GROUND DIRECTLY TO LT3581 EXPOSED PAD PIN 17. IT IS ADVISED
TO NOT COMBINE CIN GROUND WITH GND EXCEPT AT THE EXPOSED PAD.
B: RETURN COUT GROUND DIRECTLY TO LT3581 EXPOSED PAD PIN 17. IT IS ADVISED
TO NOT COMBINE COUT GROUND WITH GND EXCEPT AT THE EXPOSED PAD.
C: RETURN D1 GROUND DIRECTLY TO LT3581 EXPOSED PAD PIN 17. IT IS ADVISED
TO NOT COMBINE D1 GROUND WITH GND EXCEPT AT THE EXPOSED PAD.
L1, L2: MOST COUPLED INDUCTOR MANUFACTURERS USE CROSS PINOUT FOR
IMPROVED PERFORMANCE.
Figure 10. Suggested Component Placement for Dual Inductor
Inverting Topology (MSOP Shown, DFN Similar, Not to Scale.)
Pin 15 on DFN or Pin 17 on MSOP Is the Exposed Pad Which
Must Be Soldered Directly to the Local Ground Plane for
Adequate Thermal Performance. Multiple Vias to Additional
Ground Planes Will Improve Thermal Performance
the heat generated within the package. This can be
accomplished by taking advantage of the thermal pad on
the underside of the IC. It is recommended that multiple
vias in the printed circuit board be used to conduct heat
away from the IC and into a copper plane with as much
area as possible.
Power and Thermal Calculations
Power dissipation in the LT3581 chip comes from four
primary sources: switch I2R losses, switch dynamic
losses, NPN base drive DC losses, and miscellaneous
input current losses. These formulas assume continuous
mode operation, so they should not be used for calculating
thermal losses or efficiency in discontinuous mode or at
light load currents.
The following example calculates the power dissipation in the LT3581 for a particular boost application
(VIN = 5V, VOUT = 12V, IOUT = 0.83A, fOSC = 2MHz, VD = 0.45V,
VCESAT = 0.21V).
THERMAL CONSIDERATIONS
To calculate die junction temperature, use the appropriate
thermal resistance number and add in worst-case ambient
temperature:
Overview
TJ = TA + θJA • PTOTAL
For the LT3581 to deliver its full output power, it is imp­
erative that a good thermal path be provided to dissipate
Table 4. Power Calculations Example for Boost Converter with VIN = 5V, VOUT = 12V, IOUT = 0.83A, fOSC = 2MHz, VD = 0.45V, VCESAT = 0.21V
DEFINITION OF VARIABLES
DC = SWITCH DUTY CYCLE
IIN = Average Switch Current
η = Power Conversion Efficiency
(typically 88% at high currents)
EQUATIONS
DC =
VOUT – VIN + VD
VOUT + VD – VCESAT
IIN =
VOUT • IOUT
VIN • η
DESIGN EXAMPLE
DC =
12V – 5V + 0.45V
12V + 0.45V – 0.21V
12V •0.83A
5V • 0.88
IIN = 2.3A
IIN =
PSWDC = Switch I2R Loss (DC)
RSW = Switch Resistance (typically
90mΩ combined SW1 and SW2)
PSWDC = DC • IIN 2 • RSW
PSWDC = 0.609 • (2.3A)2 • 90mΩ
PSWAC = Switch Dynamic Loss (AC)
PSWAC =13ns • IIN • VOUT • fOSC
PSWAC = (13ns ) • 2.3A • 12V • ( 2MHz )
PBDC = Base Drive Loss (DC)
PINP = Input Power Loss
PBDC =
VIN • IIN • DC
45
PINP = 9mA • VIN
VALUE
DC = 60.9%
PBDC =
5V •2.3A • 0.609
45
PINP = 9mA • 5V
PSWDC = 290mW
PSWAC = 718mW
PBDC = 156mW
PINP = 45mW
PTOTAL = 1.209W
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17
LT3581
APPLICATIONS INFORMATION
where TJ = Die Junction Temperature, TA = Ambient Temperature, PTOTAL is the final result from the calculations
shown in Table 4, and θJA is the thermal resistance from
the silicon junction to the ambient air.
The published (http://www.linear.com/designtools/packaging/Linear_Technology_Thermal_Resistance_Table.
pdf) θJA value is 43°C/W for the 4mm × 3mm 14-pin DFN
package and 45°C/W for the 16-lead MSOP package. In
practice, lower θJA values are realizable if board layout is
performed with appropriate grounding (accounting for heat
sinking properties of the board) and other considerations
listed in the Layout Guidelines section. For instance, a
θJA value of ~24°C/W was consistently achieved for both
MSE and DFN packages of the LT3581 (at VIN = 5V, VOUT =
12V, IOUT = 0.83A, fOSC = 2MHz) when board layout was
optimized as per the suggestions in the Board Layout
Guidelines section.
Junction Temperature Measurement
The duty cycle of the CLKOUT signal is linearly proportional to die junction temperature, TJ. To get a temperature
reading, measure the duty cycle of the CLKOUT signal and
use the following equation to approximate the junction
temperature:
TJ =
DCCLKOUT – 35%
0.3%
where DCCLKOUT is the CLKOUT duty cycle in % and TJ
is the die junction temperature in °C. Although the actual
die temperature can deviate from the above equation by
±15°C, the relationship between change in CLKOUT duty
cycle and change in die temperature is well defined. Basically a 1% change in CLKOUT duty cycle corresponds to a
3.33°C change in die temperature. Note that the CLKOUT
pin is only meant to drive capacitive loads up to 50pF.
Thermal Lockout
A fault condition occurs when the die temperature exceeds
165°C (see Operation Section), and the part goes into
thermal lockout. The fault condition ceases when the die
temperature drops by ~5°C (nominal).
SWITCHING FREQUENCY
There are several considerations in selecting the operating frequency of the converter. The first is staying clear
of sensitive frequency bands, which cannot tolerate any
spectral noise. For example, in products incorporating RF
communications, the 455kHz IF frequency is sensitive to
any noise, therefore switching above 600kHz is desired.
Some communications have sensitivity to 1.1MHz and in
that case a 1.5MHz switching converter frequency may be
employed. The second consideration is the physical size
of the converter. As the operating frequency goes up, the
inductor and filter capacitors go down in value and size.
The tradeoff is efficiency, since the losses due to switching dynamics (see Thermal Considerations), Schottky
diode charge, and other capacitive loss terms increase
proportionally with frequency.
Oscillator Timing Resistor (RT)
The operating frequency of the LT3581 can be set by the
internal free-running oscillator. When the SYNC pin is driven
low (< 0.4V), the frequency of operation is set by a resistor
from the RT pin to ground. An internally trimmed timing
capacitor resides inside the IC. The oscillator frequency
is calculated using the following formula:
fOSC =
87.6
RT + 1
where fOSC is in MHz and RT is in k. Conversely, RT (in k)
can be calculated from the desired frequency (in MHz)
using:
RT =
87.6
–1
fOSC
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LT3581
APPLICATIONS INFORMATION
Clock Synchronization
2.2µF
1.5µH
The operating frequency of the LT3581 can be set by an
external source by simply providing a digital clock signal
into the SYNC pin (RT resistor still required). The LT3581
will revert to its internal free-running oscillator clock (set
by the RT resistor) when the SYNC pin is driven below
0.4V for a few free-running clock periods.
Driving SYNC high for an extended period of time effectively stops the operating clock and prevents latch SR1
from becoming set (see Block Diagram). As a result, the
switching operation of the LT3581 will stop and the CLKOUT
pin will be held at ground.
The duty cycle of the SYNC signal must be between 20%
and 80% for proper operation. Also, the frequency of the
SYNC signal must meet the following two criteria:
(1) SYNC may not toggle outside the frequency range of
200kHz to 2.5MHz unless it is stopped low (below
0.4V) to enable the free-running oscillator.
(2) The SYNC frequency can always be higher than the
free-running oscillator frequency (as set by the RT
resistor), fOSC , but should not be less than 25%
below fOSC.
CLOCK SYNCHRONIZATION OF ADDITIONAL
REGULATORS
The CLKOUT pin of the LT3581 can be used to synchronize
one or more other compatible switching regulator ICs as
shown in Figure 11.
The frequency of the master LT3581 is set by the external
RT resistor. The SYNC pin of the slave LT3581 is driven
by the CLKOUT pin of the master LT3581. Note that the
RT pin of the slave LT3581 must have a resistor tied to
ground. It takes a few clock cycles for the CLKOUT signal
to begin oscillating, and it’s preferable for all LT3581s to
have the same internal free-running frequency. Therefore,
in general, use the same value RT resistor for all of the
synchronized LT3581s.
VOUT
–12V
450mA
143k
4.7µF
SW1
GATE
SW2
FB
VIN
CLKOUT
LT3581
VC
SHDN SLAVE
SS
FAULT
RT
SYNC
GND
0.1µF
100pF
10k
2.2nF
43.2k
1.5µH
VIN
5V
6.8µF
6.8µF
10k
GATE
4.7µF
ENABLE
43.2k
LT3581
MASTER
FB
FAULT
VC
SHDN
SS
RT
130k
SW1 SW2
CLKOUT
VIN
100k
VOUT
12V
830mA
SYNC
GND
0.1µF
10.5k
1nF
56pF
3581 F11
Figure 11. A Single Inductor Inverting Topology Is Synchronized
with a Boost Regulator to Generate –12V and 12V Outputs. The
External PMOS Helps Disconnect the Input from the Power Paths
During Fault Events
Also, the FAULT pins can be tied together so that a fault
condition from one LT3581 causes all of the LT3581s to
enter fault, until the fault condition disappears.
CHARGE PUMP AIDED REGULATORS
Designing charge pumps with the LT3581 can offer efficient solutions with fewer components than traditional
circuits because of the master/slave switch configuration
on the IC. Although the slave switch, SW2, operates in
phase with the master switch, SW1, it is only the current
through the master switch (SW1) that is sensed by the
current comparator (A4 in Block Diagram) as part of the
current feedback loop. This method of operation by the
master/slave switches can offer the following benefits to
charge pump designs:
3581fb
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19
LT3581
APPLICATIONS INFORMATION
• The slave switch, by not performing a current sense
operation like the master switch, can sustain fairly large
current spikes when the flying capacitors charge up.
Since this current spike flows through SW2, it does
not affect the operation of the current comparator (A4
in Block Diagram).
• The master switch, immune from the capacitor current
spike (seen only by the slave switch) can sense the
inductor current more accurately.
• Since the slave switch can sustain large current spikes,
the diodes that feed current into the flying capacitors do
not need current limiting resistors, leading to efficiency
and thermal improvements.
2.2µF
VIN
12V
10µH
2.2µF
2.2µF
SW1 SW2
100k
8.06k
VIN
FB
FAULT
GATE
SHDN
CLKOUT
RT
370k
2.2µF
VC
LT3581
SYNC
43.2k
SS
GND
24k
100pF
1nF
0.47µF
3581 F12
Figure 12. High VOUT Charge Pump Topology Can Be Used to
Build VFD Bias Supplies
D1
L1
VIN
C1
D3
D2
CIN
SW1
GATE
Single Inductor Inverting Topology
VIN
If there is a need to use just one inductor to generate a
negative output voltage whose magnitude is greater than
VIN , the single inductor inverting topology (shown in Figure
13) can be used. Since the master and slave switches are
isolated by a Schottky diode, the current spike through C1
will flow only through the slave switch, thereby preventing
the current comparator, (A4 in the Block Diagram), from
falsely tripping. Output disconnect is inherently built into
the single inductor topology.
2.2µF
VOUT1
65V
70mA
2.2µF
High VOUT Charge Pump Topology
The LT3581 can be used in a charge-pump topology as
shown in Figure 12, multiplying the output of an inductive
boost converter. The master switch (SW1) can be used to
drive the inductive boost converter (first stage of charge
pump), while the slave switch (SW2) can be used to drive
one or more other charge pump stages. This topology is
useful for high voltage applications including VFD bias
supplies.
2.2µF
VOUT2
97V
140mA
100k
ENABLE
SW2
COUT
RFB
FB
CLKOUT
LT3581
FAULT
VC
SHDN
SS
RT
VOUT < 0V
AND |VOUT| > |VIN|
SYNC
GND
CSS
RVC
CVC2
CVC1
RT
3579 F13
Figure 13. Single Inductor Inverting Topology
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LT3581
APPLICATIONS INFORMATION
HOT-PLUG
The high inrush current associated with hot-plugging VIN
can be largely rejected with the use of an external PMOS. A
simple hot-plug controller can be designed by connecting
an external PMOS in series with VIN, with the gate of the
PMOS being driven by the GATE pin of the LT3581. Since
the GATE pin pull-down current is linearly proportional to
the SS voltage, and the SS charge up time is relatively slow,
the GATE pin pull-down current will increase gradually,
thereby turning on the external PMOS slowly. Controlled
in this manner, the PMOS acts as an input current limiter
when VIN hot-plugs or ramps up sharply.
Likewise, when the PMOS is connected in series with the
output, inrush currents into the output capacitor can be
limited during a hot-plug event. To illustrate this, the circuit
in Figure 18 was re-configured by adding a large 1500µF
capacitor to the output. An 18Ω resistive load was used
and a 2.2µF capacitor was placed on SS. Figure 14 shows
the results of hot-plugging this re-configured circuit. Notice
how the inductor current is well behaved.
VIN
5V/DIV
VOUT
10V/DIV
IL
5A/DIV
SS
1V/DIV
1s/DIV
3581 F14
Figure 14. Inrush Current Is Well Controlled in Spite Of HotPlugging the Re-configured Boost Converter in Figure 18
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21
LT3581
APPENDIX
SETTING THE OUTPUT VOLTAGE
The output voltage is set by connecting a resistor (RFB)
from VOUT to the FB pin. RFB is determined by using the
following equation:
RFB =
| VOUT – VFB |
83.3µA
POWER SWITCH DUTY CYCLE
In order to maintain loop stability and deliver adequate
current to the load, the power NPNs (Q1 and Q2 in the
Block Diagram) cannot remain “on” for 100% of each clock
cycle. The maximum allowable duty cycle is given by:
DCMAX
( T –MinOffTime) •100%
= P
TP
where TP is the clock period and MinOffTime (found in the
Electrical Characteristics) is typically 60ns.
Conversely, the power NPNs (Q1 and Q2 in the Block Diagram) cannot remain “off” for 100% of each clock cycle,
and will turn on for a minimum on time (MinOnTime) when
in regulation. This MinOnTime governs the minimum allowable duty cycle given by:
DCMIN =
(MinOnTime) •100%
TP
Where TP is the clock period and MinOnTime (found in
the Electrical Characteristics) is typically 100ns.
The application should be designed such that the operating
duty cycle is between DCMIN and DCMAX.
Duty cycle equations for several common topologies are given
below where VD is the diode forward voltage drop and VCESAT
is the collector to emitter saturation voltage of the switch.
VCESAT, with SW1 and SW2 tied together, is typically 250mV
when the combined switch current (ISW1 + ISW2) is 2.75A.
For the boost topology (see Figure 5):
DCBOOST ≅
DCSEPIC _&_INVERT ≅
VD +| VOUT |
VIN +| VOUT | + VD − VCESAT
For the Single Inductor Inverting topology (see Figure 13):
where VFB is 1.215V (typical) for non-inverting topologies
(i.e. boost and SEPIC regulators) and 5mV (typical) for
inverting topologies.
For the SEPIC or Dual Inductor Inverting topology (see
Figures 6 and 7):
DCSI_INVERT =
| VOUT | −VIN + VCESAT + 3• VD
| VOUT | + 3• VD
The LT3581 can be used in configurations where the duty
cycle is higher than DCMAX , but it must be operated in
the discontinuous conduction mode so that the effective
duty cycle is reduced.
INDUCTOR SELECTION
General Guidelines: The high frequency operation of the
LT3581 allows for the use of small surface mount inductors.
For high efficiency, choose inductors with high frequency
core material, such as ferrite, to reduce core losses. Also
to improve efficiency, choose inductors with more volume
for a given inductance. The inductor should have low
DCR (copper-wire resistance) to reduce I2R losses, and
must be able to handle the peak inductor current without
saturating. Note that in some applications, the current
handling requirements of the inductor can be lower, such
as in the SEPIC topology where each inductor only carries
one half of the total switch current. Molded chokes or chip
inductors usually do not have enough core area to support
peak inductor currents in the 2A to 6A range. To minimize
radiated noise, use a toroidal or shielded inductor. See
Table 5 for a list of inductor manufacturers.
Table 5. Inductor Manufacturers
Sumida
CDR6D28MN and CDR7D28MN
Series
www.sumida.com
Coilcraft
MSD7342 Series
www.coilcraft.com
Vishay
IHLP-1616BZ-01, IHLP-2020BZ-01
and IHLP-2525CZ-01 Series
www.vishay.com
Taiyo Yuden
NR Series
www.t-yuden.com
Wurth
WE-PD Series
www.we-online.com
TDK
VLF, SLF and RLF Series
www.tdk.com
VOUT – VIN + VD
VOUT + VD – VCESAT
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LT3581
APPENDIX
Minimum Inductance
Although there can be a tradeoff with efficiency, it is often
desirable to minimize board space by choosing smaller
inductors. When choosing an inductor, there are three
conditions that limit the minimum inductance: (1) providing adequate load current, (2) avoidance of subharmonic
oscillations and (3) supplying a minimum ripple current
to avoid false tripping of the current comparator.
Adequate Load Current
Small value inductors result in increased ripple currents and
thus, due to the limited peak switch current, decrease the
average current that can be provided to the load. In order
to provide adequate load current, L should be at least:
LBOOST >
DC • ( VIN − VCESAT )


| V |• I
2• fOSC •  IPK − OUT OUT 
VIN • η 

Boost
Topology
or
LDUAL >
DC • ( VIN − VCESAT )


| V |• I
2•fOSC•  IPK − OUT OUT −IOUT 
VIN • η


SEPIC
or
Inverting
Topologies
where:
LBOOST=L1 for Boost Topologies (see Figure 5)
LDUAL =L1 = L2 for Coupled Dual Inductor
Topologies (see Figures 6 and 7)
LDUAL =L1 || L2 for Uncoupled Dual Inductor
Topologies (see Figures 6 and 7)
DC
= Switch Duty Cycle (see Power Switch Duty Cycle section in Appendix)
IPK
= Maximum Peak Switch Current; should not
exceed 3.3A for a combined SW1 + SW2
current, or 1.9A of SW1 current if SW1 is
being used by itself.
η
= Power Conversion Efficiency (typically 88%
for Boost and 75% for Dual Inductor
Topologies at High Currents)
fOSC = Switching Frequency
IOUT = Maximum Output Current
Negative values of LBOOST or LDUAL indicate that the output load current, IOUT, exceeds the switch current limit
capability of the LT3581.
Avoiding Sub-Harmonic Oscillations
The LT3581’s internal slope compensation circuit will
prevent sub-harmonic oscillations that can occur when
the duty cycle is greater than 50%, provided that the
inductance exceeds a certain minimum value. In applications that operate with duty cycles greater than 50%, the
inductance must be at least:
LMIN =
( VIN − VCESAT ) • (2 • DC−1)
2.2A• fOSC • (1−DC)
where:
LMIN =L1 for Boost Topologies (see Figure 5)
LMIN =L1 = L2 for Coupled Dual Inductor
Topologies (see Figures 6 and 7)
LMIN =L1 || L2 for Uncoupled Dual Inductor
Topologies (see Figures 6 and 7)
Maximum Inductance
Excessive inductance can reduce ripple current to levels
that are difficult for the current comparator (A4 in the Block
Diagram) to cleanly discriminate, causing duty cycle jitter
and/or poor regulation. The maximum inductance can be
calculated by:
LMAX =
VIN − VCESAT DC
•
350mA
fOSC
where:
LMAX =L1 for Boost Topologies (see Figure 5)
LMAX =L1 = L2 for Coupled Dual Inductor
Topologies (see Figures 6 and 7)
LMAX =L1 || L2 for Uncoupled Dual Inductor
Topologies (see Figures 6 and 7)
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LT3581
APPENDIX
Inductor Current Rating
Inductors must have a rating greater than their peak
operating current, or else they could saturate and hence
contribute to losses in efficiency. The maximum inductor
current (considering start-up and steady-state conditions)
is given by:
IL _ PEAK = ILIM +
where:
VIN • TMIN _ PROP
L
IL_PEAK = Peak Inductor Current in L1 for a Boost
Topology, or the Peak of the sum of the
Inductor Currents in L1 and L2 for Dual
Inductor Topologies.
ILIM**
= 3.3A with SW1 and SW2 Tied Together,
or 1.9A with just SW1 (This assumes
usage of an inductor whose core
material soft-saturates such as
powdered iron core).
TMIN_PROP= 100ns (Propagation Delay through the
Current Feedback Loop).
**If using an inductor whose core material saturates
hard (e.g., ferrite), then pick ILIM to be 5.4A with SW1
and SW2 tied together, or 3A when just SW1 is used.
Note that these equations offer conservative results for
the required inductor current ratings. The current ratings
could be lower for applications with light loads, if the SS
capacitor is sized appropriately to limit inductor currents
at start-up.
Multilayer ceramic capacitors are an excellent choice, as
they have extremely low ESR and are available in very
small packages. X5R or X7R dielectrics are preferred, as
these materials retain their capacitance over wide voltage
and temperature ranges. A 10μF to 22μF output capacitor
is sufficient for most applications, but systems with very
low output currents may need only 2.2μF to 10μF. Always
use a capacitor with a sufficient voltage rating. Many
ceramic capacitors, particularly 0805 or 0603 case sizes,
have greatly reduced capacitance at the desired output
voltage. Tantalum Polymer or OS-CON capacitors can be
used, but it is likely that these capacitors will occupy more
board area than a ceramic, and will have higher ESR with
greater output ripple.
INPUT CAPACITOR SELECTION
Ceramic capacitors make a good choice for the input
decoupling capacitor, and should be placed such that it is
in close proximity to the VIN of the chip as well as to the
inductor connected to the input of the power path. If it is
not possible to optimally place a single input capacitor,
then use two separate capacitors—use one at the VIN of
the chip (see equation for CVIN in Tables 1, 2 and 3) and
one at the input to the power path (see equation for CPWR
in Tables 1, 2 and 3) A 4.7μF to 20μF input capacitor is
sufficient for most applications.
Table 6 shows a list of several ceramic capacitor man­
ufacturers. Consult the manufacturers for detailed infor­
mation on their entire selection of ceramic parts.
Table 6: Ceramic Capacitor Manufacturers
DIODE SELECTION
Schottky diodes, with their low forward voltage drops and
fast switching speeds, are recommended for use with the
LT3581. Choose a Schottky diode with low parasitic capacitance to reduce reverse current spikes through the power
switch of the LT3581. The Central Semiconductor Corp.
CMMSH2-40 diode is a very good choice with a 40V reverse
voltage rating and an average forward current of 2A.
OUTPUT CAPACITOR SELECTION
Low ESR (equivalent series resistance) capacitors should
be used at the output to minimize the output ripple voltage.
AVX
www.avxcorp.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com
PMOS SELECTION
An external PMOS, controlled by the LT3581’s GATE pin,
can be used to facilitate input or output disconnect. The
GATE pin turns on the PMOS gradually during start-up
(see Soft-Start of External PMOS in the Operation section),
and turns the PMOS off when the LT3581 is in shutdown
or in fault.
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LT3581
APPENDIX
The use of the external PMOS, controlled by the GATE pin,
is particularly beneficial when dealing with unintended
output shorts in a boost regulator. In a conventional boost
regulator, the inductor, Schottky diode, and power switches
are susceptible to damage in the event of an output short
to ground. Using an external PMOS in the boost regulator’s
power path (path from VIN to VOUT) controlled by the GATE
pin, will serve to disconnect the input from the output
when the output has a short to ground, thereby helping
save the IC, and the other components in the power path
from damage. Ensure that both, the diode and the inductor
can survive low duty cycle current pulses of 3 to 4 times
their steady state levels.
The PMOS chosen must be capable of handling the maximum input or output current depending on whether the
PMOS is used at the input (see Figure 11) or the output
(see Figure 18).
Ensure that the PMOS is biased with enough source to
gate voltage (VSG) to enhance the device into the triode
mode of operation. The higher the VSG voltage that biases
the PMOS into triode, the lower the RDSON of the PMOS,
thereby lowering power dissipation in the device during
normal operation, as well as improving the efficiency of
the application in which the PMOS is used. The following equations show the relationship between RGATE (see
Block Diagram) and the desired VSG that the PMOS is
biased with:

RGATE
if V
< 2V
 VIN
RGATE + 2kΩ GATE
VSG = 
 933µA•R
GATE if VGATE ≥ 2V

When using a PMOS, it is advisable to configure the specific
application for undervoltage lockout (see the Operations
section). The goal is to have VIN get to a certain minimum
voltage where the PMOS has sufficient headroom to attain
a high enough VSG, which prevents it from entering the
saturation mode of operation during start-up.
Figure 18 shows the PMOS connected in series with the
output to act as an output disconnect during a fault condition. The Schottky diode from the VIN pin to the GATE
pin is optional and helps turn off the PMOS quicker in the
event of hard shorts. The resistor divider from VIN to the
SHDN pin sets a UVLO of 4V for this application.
Connecting the PMOS in series with the output offers certain
advantages over connecting it in series with the input:
• Since the load current is always less than the input
current for a boost converter, the current rating of the
PMOS goes down.
• A PMOS in series with the output can be biased with
a higher overdrive voltage than a PMOS used in series
with the input, since VOUT > VIN. This higher overdrive
results in a lower RDSON rating for the PMOS, thereby
improving the efficiency of the regulator.
In contrast, an input connected PMOS works as a simple
hot-plug controller (covered in more detail in the Hot-Plug
section). The input connected PMOS also functions as an
inexpensive means of protecting against multiple output
shorts in boost applications that synchronize the LT3581
with other compatible ICs (see Figure 11).
Table 7 shows a list of several discrete PMOS manufa­
cturers. Consult the manufacturers for detailed information
on their entire selection of PMOS devices.
Table 7. Discrete PMOS Manufacturers
Vishay
www.vishay.com
Fairchild Semiconductor
www.fairchildsemi.com
COMPENSATION – ADJUSTMENT
To compensate the feedback loop of the LT3581, a series
resistor-capacitor network in parallel with an optional
single capacitor should be connected from the VC pin to
GND. For most applications, choose a series capacitor in
the range of 1nF to 10nF with 2.2nF being a good starting
value. The optional parallel capacitor should range in value
from 47pF to 160pF with 100pF being a good starting
value. The compensation resistor, RC , is usually in the
range of 5k to 50k with 10k being a good starting value.
A good technique to compensate a new application is to
use a 100k potentiometer in place of the series resistor RC.
With the series and parallel capacitors at 2.2nF and 100pF
respectively, adjust the potentiometer while observing the
transient response and the optimum value for RC can be
3581fb
For more information www.linear.com/LT3581
25
LT3581
APPENDIX
found. Figures 15a to 15c illustrate this process for the
circuit of Figure 18 with a load current stepped between
540mA and 800mA. Figure 15a shows the transient response with RC equal to 1k. The phase margin is poor as
evidenced by the excessive ringing in the output voltage
and inductor current. In Figure 15b, the value of RC is
increased to 3k, which results in a more damped response.
Figure 15c shows the results when RC is increased further
to 10.5k. The transient response is nicely damped and the
compensation procedure is complete.
VOUT
AC-COUPLED
500mV/DIV
IL
1A/DIV
50µs/DIV
3581 F15a
Figure 15a. Transient Response Shows Excessive Ringing
VOUT
AC-COUPLED
500mV/DIV
COMPENSATION – THEORY
Like all other current mode switching regulators, the LT3581
needs to be compensated for stable and efficient operation.
Two feedback loops are used in the LT3581: a fast current
loop which does not require compensation, and a slower
voltage loop which does. Standard Bode plot analysis can be
used to understand and adjust the voltage feedback loop.
As with any feedback loop, identifying the gain and
phase contribution of the various elements in the loop
is critical. Figure 16 shows the key equivalent elements
of a boost converter. Because of the fast current control
loop, the power stage of the IC, inductor and diode
have been replaced by a combination of the equivalent
transconductance amplifier gmp and the current controlled
current source (which converts IVIN to ηVIN/VOUT • IVIN).
gmp acts as a current source where the peak input current,
IVIN, is proportional to the VC voltage. η is the efficiency
of the switching regulator and is typically about 80%.
Note that the maximum output currents of the gmp and
gma stages are finite. The output of the gmp stage is
limited by the minimum switch current limit (see Electrical
Specifications) and the output of the gma stage is nominally
limited to about ±12μA.
–
IL
1A/DIV
+
gmp
IVIN
VOUT
η • VIN
VOUT
50µs/DIV
• IVIN
RESR
3581 F15b
COUT
Figure 15b. Transient Response is Better
+
VC
CF
VOUT
AC-COUPLED
500mV/DIV
RL
RC
gma
RO
CC
1.215V
REFERENCE
CPL
R1
R2
FB
–
R2
3581 F16
IL
1A/DIV
50µs/DIV
3581 F15c
Figure 15c. Transient Response is Well Damped
CC: COMPENSATION CAPACITOR
COUT: OUTPUT CAPACITOR
CPL: PHASE LEAD CAPACITOR
CF: HIGH FREQUENCY FILTER CAPACITOR
gma: TRANSCONDUCTOR AMPLIFIER INSIDE IC
gmp: POWER STAGE TRANSCONDUCTANCE AMPLIFIER
RC: COMPENSATION RESISTOR
RL: OUTPUT RESISTANCE DEFINED AS VOUT/ILOAD(MAX)
RO: OUTPUT RESISTANCE OF gma
R1, R2; FEEDBACK RESISTOR DIVIDER NETWORK
RESR: OUTPUT CAPACITOR ESR
Figure 16. Boost Converter Equivalent Model
3581fb
26
For more information www.linear.com/LT3581
LT3581
APPENDIX
From Figure 16, the DC gain, poles and zeros can be
calculated as follows:
DC Gain:
(Breaking loop at FB pin)
Table 8. Bode Plot Parameters
∂V ∂I
∂V
∂V
= A OL (0) = C • VIN • OUT • FB =
∂VFB ∂VC ∂IVIN ∂VOUT


2
(gma •RO ) •gmp •  η• VVIN • R2L  • R 0.5R
+
0.5R


1
2
OUT
Output Pole: P1=
2
2• π•RL •COUT
1
Error Amp Pole: P2 =
2• π• RO +RC  •CC
Error Amp Zero: Z1=
ESR Zero: Z2 =
RHP Zero: Z3 =
1
2• π•RC •CC
1
2• π•RESR •COUT
VIN 2 •RL
UNITS
COMMENT
RL
14.5
Ω
Application Specific
COUT
9.4
µF
Application Specific
1
mΩ
Application Specific
RO
305
kΩ
Not Adjustable
CC
1000
pF
Adjustable
RESR
CF
56
pF
Optional/Adjustable
CPL
0
pF
Optional/Adjustable
RC
10.5
kΩ
Adjustable
R1
130
kΩ
Adjustable
R2
14.6
kΩ
Not Adjustable
VREF
1.215
V
Not Adjustable
VOUT
12
V
Application Specific
VIN
5
V
Application Specific
gma
270
µmho
Not Adjustable
gmp
15.1
mho
Not Adjustable
L
1.5
µH
Application Specific
2
MHz
Adjustable
From Figure 17, the phase is –130° when the gain reaches
0dB giving a phase margin of 50°. The crossover frequency
is 17kHz, which is more than three times lower than the
frequency of the RHP zero Z3 to achieve adequate phase
margin.
f
High Frequency Pole: P3 > S
3
Phase Lead Pole: P4 =
VALUE
fOSC
2• π• VOUT 2 •L
Phase Lead Zero: Z4 =
PARAMETER
1
2• π•R1•CPL
1
Error Amp Filter Pole:
C
1
, CF < C
P5 =
R •R
10
2• π• C O •CF
R
+R
C
O
170
0
–40
150
PHASE
130
–80
110
–120
90
–160
70
–180
50
–200
30
–240
GAIN
10
–280
–10
The current mode zero (Z3) is a right half plane zero which
can be an issue in feedback control design, but is manageable with proper external component selection.
–30
PHASE (DEG)
R2
R1•
2 •C
2• π•
R2 PL
R1+
2
GAIN (dB)
ADC
Using the circuit in Figure 18 as an example, Table 8 shows
the parameters used to generate the Bode plot shown in
Figure 17.
–320
10
100
1k
10k
FREQUENCY (Hz)
100k
1M
–360
3851 F17
Figure 17. Bode Plot for Example Boost Converter
3581fb
For more information www.linear.com/LT3581
27
LT3581
TYPICAL APPLICATIONS
L1
1.5µH
VIN
5V
D1
SW1 SW2
18.7k
FB
VIN
100k
C1
4.7µF
FAULT
GATE
SHDN
CLKOUT
RT
10k
LT3581
SYNC
43.2k
VOUT
12V
830mA
M1
C2
4.7µF
6.04k
D2
130k
VIN
VC
56pF
SS
GND
C3
4.7µF
10.5k
0.1µF
1nF
3581 F18
C1: 4.7µF, 16V, X7R, 1206
C2, C3: 4.7µF, 25V, X7R, 1206
D1: DIODES INC. PD3S230H-7
D2: VISHAY MSS2P3
L1: SUMIDA CDR6D28MN-IR5
M1: VISHAY SILICONIX SI7123DN
Figure 18. 2MHz, 5V to 12V, 830mA Boost Converter with Output Short Circuit Protection
Transient Response with 430mA to 830mA to 430mA Load Step
Switching Waveforms with 830mA Load
VOUT
AC-COUPLED
200mV/DIV
IL
1A/DIV
VOUT
AC-COUPLED
1V/DIV
IL
1A/DIV
VSW
0.5A/DIV
LOAD
0.5A/DIV
VCLKOUT
(BW LIMIT)
2V/DIV
3581 TA02a
50µs/DIV
3581 TA02b
500ns/DIV
2MHz, 5V, 1.1A Boost Converter Operates from an Input Range of 2.8V to 4.2V
L1
0.68µH
VIN
2.8V TO 4.2V
D1
Efficiency and Power Loss at VIN = 3.3V
VOUT
5V
1.1A
SW1 SW2
43.2k
FAULT
GATE
SHDN
CLKOUT
RT
VC
SYNC
SS
GND
45.3k
68pF
0.1µF
C2
22µF
6.98k
1.5nF
3581 TA03a
C1: 3.3µF, 16V, X7R, 1206
C2: 22µF, 16V, X7R, 1210
D1: DIODES INC. PD3S230H-7
L1: VISHAY IHLP1616 BZ-01-OR68 (ONLY 4.1mm × 4.5mm × 2mm)
2000
85
1800
1600
80
1400
75
1200
70
1000
800
65
600
60
400
55
50
POWER LOSS (mW)
100k
FB
LT3581
EFFICIENCY (%)
VIN
C1
3.3µF
90
200
0
200
600
800
400
LOAD CURRENT (mA)
1000
0
1200
3581 TA03b
3581fb
28
For more information www.linear.com/LT3581
LT3581
TYPICAL APPLICATIONS
High Efficiency, VFD (Vacuum Fluorescent Display) Power Supply Switches at 2MHz to Avoid AM Band
DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY
D6
C7
2.2µF
D5
C5
2.2µF
D4
C6
2.2µF
D3
C4
2.2µF
D2
D7
L1
10µH
VIN
9V TO 16V
C1
2.2µF
32.6k
D1
FB
100k
FAULT
GATE
SHDN
CLKOUT
RT
10k
LT3581
SYNC
43.2k
GND
365k
8.06k
D9*
10V
D8*
VIN
VC
SS
VOUT1
65V
60mA (VIN = 9V)
70mA (VIN = 12V)
90mA (VIN = 16V)
M1*
C2
2.2µF
SW1 SW2
VIN
VOUT2
97V
90mA (VIN = 9V)
140mA (VIN = 12V)
180mA (VIN = 16V)
100pF
C3
2.2µF
24k
1nF
0.47µF
3581 TA04a
C1: 2.2µF, 25V, X7R, 1206
C2 TO C7: 2.2µF, 50V, X7R, 1206
D1 TO D7: CENTRAL SEMI SOD123F
D8: CENTRAL SEMI CMDSH-3TR
D9: CENTRAL SEMI CMHZ5240B-LTZ
L1: TAIYO YUDEN NR6045T100M
M1: VISHAY SILICONIX SI7611DN
*OPTIONAL,
FOR OUTPUT SHORT-CIRCUIT
PROTECTION
Transient Response with 60mA to 140mA to 60mA
Load Step on VOUT2 (VIN = 12V)
VOUT
AC-COUPLED
2V/DIV
Start-Up Waveforms
VOUT2
50V/DIV
VOUT1
50V/DIV
IL
0.5A/DIV
IL
0.5A/DIV
ILOAD
0.1A/DIV
VSS
1V/DIV
3581 TA04b
100ms/DIV
3581 TA04c
Efficiency and Power Loss at VIN = 12V
90
3.0
85
2.5
80
2.0
75
1.5
70
0
4
12
16
8
TOTAL OUTPUT POWER (W)
20
POWER LOSS (mW)
EFFICIENCY (%)
100µs/DIV
1.0
3581 TA04d
3581fb
For more information www.linear.com/LT3581
29
LT3581
TYPICAL APPLICATIONS
2MHz, 12V SEPIC Converter Can Accept Input Voltages from 9V to 16V
C2
2.2µF
L1
3.3µH
D1
VOUT
12V
1A (VIN = 9V)
1.1A (VIN = 12V)
1.3A (VIN = 16V)
•
VIN
9V TO 16V
SW1
VIN
C1
3.3µF
100k
43.2k
•
SW2
LT3581
L2
3.3µH
FB
SHDN
GATE
FAULT
CLKOUT
RT
VC
SYNC
SS
GND
130k
C3
10µF
100pF
10k
0.1µF
2.2nF
3581 TA06a
C1: 3.3µF, 25V, X7R, 1206
C2: 2.2µF, 50V, X7R, 1206
C3: 10µF, 25V, X7R, 1210
D1: CENTRAL SEMI CTLSH2-40M832
L1, L2: COILCRAFT MSD7342-332MLB
Efficiency
Line Regulation with No Load
12.000
100
LINE REGULATION ~0.0044%/V
95
11.998
VIN = 9V
85
VIN = 16V
80
75
VOUT (V)
EFFICIENCY (%)
90
VIN = 12V
70
11.996
11.994
65
60
11.992
55
50
0
300
1200
600
900
LOAD CURRENT (mA)
11.990
1500
8
9
3581 TA06b
10
11
12 13
VIN (V)
14
15
16
17
3581 TA06c
Load Regulation at VIN = 12
12.01
LOAD REGULATION ~0.25%/A
VOUT (V)
12.00
11.99
11.98
11.97
11.96
0
200
400
600 800 1000 1200 1400
ILOAD (mA)
3581 TA06d
3581fb
30
For more information www.linear.com/LT3581
LT3581
TYPICAL APPLICATIONS
Wide Input Range, 3.3V SEPIC Converter Can Operate from 3V to 36V
D3
C4
2.2µF
D2
VOUT
3.3V
0.9A (3V < VBAT < 9V)
1.5A (VBAT = 9V)
•
VBAT
3V TO 36V
(VBAT AT START-UP = 6V TO 16V)
L1
3.3µH
C1
10µF
200k
10k
SW1
M1
VIN
100k
D1
18V
C3
4.7µF
C2
10nF
174k
Q1
470pF
SW2
LT3581
FB
•
24.9k
SHDN
GATE
FAULT
VC
RT
SS
47pF
CLKOUT
GND
1µF
SYNC
L2
3.3µH
C5
47µF
×2
10k
2.2nF
10k
3581 TA07a
C1: 10µF, 50V, X7R, 1210
C2: 10nF, 25V, X7R, 0603
C3: 4.7µF, 25V, X7R, 1206
C4: 2.2µF, 50V, X7R, 1206
C5: 47µF, 10V, X7R, 1210
D1: CENTRAL SEMI CMHZ5248B-LTZ
Efficiency
Wide Input Range SEPIC Can Ride Through VBAT
Voltages that Are Higher than VIN_OVP
80
VBAT = 9V
VBAT = 12V
EFFICIENCY (%)
75
70
VBAT
10V/DIV
VBAT = 3V
VBAT = 17V
VOUT
2V/DIV
65
60
VBAT = 31V
VOUT = 3.3V
IL
2A/DIV
55
50
D2: CENTRAL SEMI CMMSH2-40
D3: DIODES INC. PD3S230H-7
L1, L2: COILCRAFT MSD7342-332MLB
M1: 2N7002
Q1: MMBT3904
1s/DIV
0
800
1200
400
LOAD CURRENT (mA)
3581 TA07c
1600
3581 TA07b
3581fb
For more information www.linear.com/LT3581
31
LT3581
TYPICAL APPLICATIONS
1MHz, ±12V Charge Pump Topology Uses Only Single Inductor
D5
C5
10µF
D4
VIN
5V
C2
1µF
D1
D3
C1
3.3µF
100k
86.6k
SW2
LT3581
VIN
FB
SHDN
GATE
FAULT
CLKOUT
RT
VC
SYNC
SS
1800
85
1600
130k
1200
75
1000
70
800
65
600
60
C4
10µF
100pF
16.9k
0.1µF
400
55
50
GND
1400
80
VOUT+
12V
0.27A
D2
SW1
90
POWER LOSS (mW)
L1
8.2µH
R1
*2.4k
VOUT–
–12V
0.27A
EFFICIENCY (%)
C3
1µF
Efficiency and Power Loss with
Symmetric Load
200
50
0
150
100
200
LOAD CURRENT (mA)
250
0
300
3581 TA08b
2.2nF
3581 TA08a
C1: 3.3µF, 25V, X7R, 1206
C2, C3: 1µF, 25V, X7R, 1206
C4, C5: 10µF, 50V, X7R, 1210
D1 TO D5: DIODES INC. PD3S230H-7
L1: VISHAY IHLP-2525CZ-01-8R2
R1: 2.4k, 2W
*IF DRIVING ASYMMMETRICAL LOADS,
PLACE A 2.4k, 2W RESISTOR FROM THE 12V
OUTPUT TO THE –12V OUTPUT FOR IMPROVED
LOAD REGULATION OF THE –12V OUTPUT
Efficiency
700kHz, –5V Inverting Converter Can Accept Input Voltages from 3V to 16V
90
VOUT
–5V
0.9A (VIN = 3.3V)
1.5A (VIN = 12V)
1.6A (VIN = 16V)
•
•
L2
3.3µH
D1
SW1
LT3581
VIN
C1
22µF
100k
SW2
FB
SHDN
GATE
FAULT
CLKOUT
RT
C3
22µF
VIN = 12V
VIN = 3.3V
80
75
VIN = 16V
70
65
60
VC
SYNC
124k
60.4k
85
EFFICIENCY (%)
C2
1µF
L1
3.3µH
VIN
3V TO 16V
56pF
SS
GND
6.19k
0.1µF
55
50
2.2nF
0
3581 TA09a
300
600
900 1200
LOAD CURRENT (mA)
1500
1800
3581 TA09b
C1: 22µF, 25V, X7R, 1210
C2: 1µF, 50V, X7R, 1206
C3: 22µF, 16V, X7R, 1210
D1: VISHAY SSB44
L1, L2: COILCRAFT MSD7342-332MLB
700kHz, 5V SEPIC Can Accept Input Voltages from 3V to 16V
L1
3.3µH
90
D1
VOUT
5V
0.9A (VIN = 3V)
1.5A (12V ≤ VIN ≤ 16V)
•
VIN
3V TO 16V
SW1
VIN
C1
22µF
100k
124k
•
SW2
LT3581
FB
SHDN
GATE
FAULT
CLKOUT
RT
VC
SYNC
SS
GND
L2
3.3µH
45.3k
100pF
1µF
Efficiency
C3
22µF
×2
85
VIN = 12V
VIN = 3.3V
80
EFFICIENCY (%)
C2
1µF
75
VIN = 16V
70
65
60
55
7.87k
2.2nF
3581 TA10a
50
0
800
1200
400
LOAD CURRENT (mA)
1600
3581 TA10b
C1: 22µF, 25V, X7R, 1210
C2: 1µF, 50V, X7R, 1206
C3: 22µF, 16V, X7R, 1210
D1: DIODES INC. B230LA
L1, L2: COILCRAFT MSD7342-332MLB
3581fb
32
For more information www.linear.com/LT3581
LT3581
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
MSE Package
16-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1667 Rev F)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.845 ±0.102
(.112 ±.004)
5.10
(.201)
MIN
2.845 ±0.102
(.112 ±.004)
0.889 ±0.127
(.035 ±.005)
8
1
1.651 ±0.102
(.065 ±.004)
1.651 ±0.102 3.20 – 3.45
(.065 ±.004) (.126 – .136)
0.305 ±0.038
(.0120 ±.0015)
TYP
16
0.50
(.0197)
BSC
4.039 ±0.102
(.159 ±.004)
(NOTE 3)
RECOMMENDED SOLDER PAD LAYOUT
0.254
(.010)
0.35
REF
0.12 REF
DETAIL “B”
CORNER TAIL IS PART OF
DETAIL “B” THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
9
NO MEASUREMENT PURPOSE
0.280 ±0.076
(.011 ±.003)
REF
16151413121110 9
DETAIL “A”
0° – 6° TYP
3.00 ±0.102
(.118 ±.004)
(NOTE 4)
4.90 ±0.152
(.193 ±.006)
GAUGE PLANE
0.53 ±0.152
(.021 ±.006)
DETAIL “A”
1.10
(.043)
MAX
0.18
(.007)
SEATING
PLANE
0.17 – 0.27
(.007 – .011)
TYP
1234567 8
0.50
(.0197)
BSC
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD SHALL
NOT EXCEED 0.254mm (.010") PER SIDE.
0.86
(.034)
REF
0.1016 ±0.0508
(.004 ±.002)
MSOP (MSE16) 0213 REV F
3581fb
For more information www.linear.com/LT3581
33
LT3581
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
DE Package
14-Lead Plastic DFN (4mm × 3mm)
(Reference LTC DWG # 05-08-1708 Rev B)
4.00 ±0.10
(2 SIDES)
R = 0.05
TYP
0.70 ±0.05
3.60 ±0.05
2.20 ±0.05
3.30 ±0.05
PACKAGE
OUTLINE
0.25 ± 0.05
0.50 BSC
1.70 ± 0.10
PIN 1 NOTCH
R = 0.20 OR
0.35 × 45°
CHAMFER
PIN 1
TOP MARK
(SEE NOTE 6)
0.200 REF
(DE14) DFN 0806 REV B
7
0.75 ±0.05
1
0.25 ± 0.05
0.50 BSC
3.00 REF
3.00 REF
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
0.40 ± 0.10
14
3.30 ±0.10
3.00 ±0.10
(2 SIDES)
1.70 ± 0.05
R = 0.115
TYP
8
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING PROPOSED TO BE MADE VARIATION OF VERSION (WGED-3) IN JEDEC
PACKAGE OUTLINE MO-229
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
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34
For more information www.linear.com/LT3581
LT3581
REVISION HISTORY
REV
DATE
DESCRIPTION
A
11/13
Added H-grade to Absolute Maximum Ratings table and Order Information table
PAGE NUMBER
2
Clarified Electrical Characteristics
4
B
07/14
Clarified FAULT Output Voltage Low on Electrical Characteristics table, Typ was 150, changed to 100
4
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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.
For more
information
www.linear.com/LT3581
35
LT3581
TYPICAL APPLICATION
5V to –12V Inverting Converter Switches at 2MHz
C2
1µF
L1
3.3µH
VOUT
–12V
625mA
•
•
VIN
5V
Efficiency and Power Loss
L2
3.3µH
D1
SW1 SW2
100k
43.2k
85
1800
1600
80
FB
SHDN
GATE
FAULT
CLKOUT
RT
VC
SYNC
SS
143k
C3
4.7µF
47pF
GND
0.1µF
11k
EFFICIENCY (%)
C1
3.3µF
LT3581
2000
1400
75
1200
70
1000
800
65
600
60
400
55
1nF
3581 TA05a
50
POWER LOSS (mW)
VIN
90
200
0
C1: 3.3µF, 16V, X7R, 1206
C2: 1µF, 25V, X7R, 1206
C3: 4.7µF, 25V, X7R, 1206
D1: DIODES INC. PD3S230H-7
L1, L2: COILCRAFT MSD7342-332MLB
125
375
500
250
LOAD CURRENT (mA)
0
625
3581 TA05b
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT3580
2A (ISW), 2.5MHz, High Efficiency Step-Up DC/DC Converter
VIN = 2.5V to 32V, VOUT(MAX) = 42V, IQ = 1mA, ISD < 1µA,
3mm × 3mm DFN-8, MSOP-8E Packages
LT3471
Dual Output 1.3A (ISW), 1.2MHz, High Efficiency Step-Up DC/DC
Converter
VIN = 2.4V to 16V, VOUT(MAX) = ±40V, IQ = 2.5mA, ISD < 1µA,
3mm × 3mm DFN-10 Package
LT3479
40V, 3A, Full Featured DC/DC Converter with Soft-Start and Inrush
Current Protection
VIN = 2.5V to 24V, VOUT(MAX) = 40V, IQ = Analog/PWM,
ISD < 1µA, DFN, TSSOP Packages
LT3477
40V, 3A, Full Featured DC/DC Converter
VIN = 2.5V to 25V, VOUT(MAX) = 40V, IQ = Analog/PWM,
ISD < 1µA, QFN, TSSOP-20E Packages
LT1946/LT1946A 1.5A (ISW), 1.2MHz/2.7MHz, High Efficiency Step-Up DC/DC Converter
VIN = 2.6V to 16V, VOUT(MAX) = 34V, IQ = 3.2mA, ISD < 1µA,
MS8E Package
LT1935
2A (ISW), 40V, 1.2MHz, High Efficiency Step-Up DC/DC Converter
VIN = 2.3V to 16V, VOUT(MAX) = 40V, IQ = 3mA, ISD < 1µA,
ThinSOT Package
LT1310
2A (ISW), 40V, 1.2MHz, High Efficiency Step-Up DC/DC Converter
VIN = 2.3V to 16V, VOUT(MAX) = 40V, IQ = 3mA, ISD < 1µA,
ThinSOT Package
LT3436
3A (ISW), 800kHz, 34V Step-Up DC/DC Converter
VIN = 3V to 25V, VOUT(MAX) = 34V, IQ = 0.9mA, ISD < 6µA,
TSSOP-16E Package
3581fb
36 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LT3581
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com/LT3581
LT 0714 REV B • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 2010