LT3971_15 [Linear]
38V, 1.2A, 2MHz Step-Down Regulator with 2.8A Quiescent Current;
| 型号： | LT3971_15 |
| 厂家： | 
Linear |
| 描述： | 38V, 1.2A, 2MHz Step-Down Regulator with 2.8A Quiescent Current |
| 文件： | 总28页 (文件大小：633K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT3971/LT3971-3.3/LT3971-5
38V, 1.2A, 2MHz
Step-Down Regulator with
2.8µA Quiescent Current
FEATURES
DESCRIPTION
The LT®3971 is an adjustable frequency monolithic buck
switchingregulatorthatacceptsawideinputvoltagerange
up to 38V. Low quiescent current design consumes only
2.8µAofsupplycurrentwhileregulatingwithnoload. Low
ripple Burst Mode operation maintains high efficiency at
low output currents while keeping the output ripple below
15mV in a typical application. An internally compensated
current mode topology is used for fast transient response
and good loop stability. A high efficiency 0.33Ω switch
is included on the die along with a boost Schottky diode
and the necessary oscillator, control and logic circuitry.
An accurate 1V threshold enable pin can be used to shut
down the LT3971, reducing the input supply current to
700nA. A capacitor on the SS pin provides a controlled
inrush current (soft-start). A power good flag signals
n
Ultralow Quiescent Current:
2.8µA I Regulating 12V to 3.3V
Q
IN
OUT
n
n
Fixed Output Voltages: 3.3V, 5V,
2.1µA I Regulating 12V to 3.3V
Q
IN
OUT
Low Ripple Burst Mode® Operation:
Output Ripple < 15mV
P-P
n
n
n
n
n
n
n
n
n
n
n
n
Wide Input Voltage Range: 4.3V to 38V
1.2A Maximum Output Current
Adjustable Switching Frequency: 200kHz to 2MHz
Synchronizable Between 250kHz to 2MHz
Fast Transient Response
Accurate 1V Enable Pin Threshold
Low Shutdown Current: I = 700nA
Q
Power Good Flag
Soft-Start Capability
Internal Compensation
when V
reaches 91% of the programmed output volt-
OUT
Output Voltage: 1.19V to 30V
Small Thermally Enhanced 10-Lead MSOP, 16-Lead
MSOP and (3mm × 3mm) DFN Packages
age. The LT3971 is available in small 10-lead MSOP and
3mm × 3mm DFN packages with exposed pads for low
thermal resistance. A 16-lead MSOP is also offered which
has enhanced pin-to-pin fault tolerance.
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
APPLICATIONS
n
Automotive Battery Regulation
n
Power for Portable Products
n
Industrial Supplies
TYPICAL APPLICATION
No Load Supply Current
3.3V Step Down Converter
3.0
OUTPUT IN REGULATION
V
IN
4.5V TO 38V
V
IN
EN
BOOST
SW
2.5
OFF ON
4.7µF
LT3971-5
0.47µF
4.7µH
PG
SS
2.0
LT3971-3.3
LT3971-3.3
RT
BD
V
3.3V
1.2A
OUT
1.5
1.0
V
49.9k
f = 800kHz
OUT
SYNC GND
22µF
3971 TA01
5
10
15
20
25
30
35
INPUT VOLTAGE (V)
3971 TA01b
3971fd
1
LT3971/LT3971-3.3/LT3971-5
ABSOLUTE MAXIMUM RATINGS (Note 1)
V , EN Voltage .........................................................38V
Operating Junction Temperature Range (Note 2)
LT3971E............................................. –40°C to 125°C
LT3971I.............................................. –40°C to 125°C
Storage Temperature Range .............. –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
IN
BOOST Pin Voltage ...................................................55V
BOOST Pin Above SW Pin.........................................30V
FB, V , RT, SYNC, SS Voltage .................................6V
OUT
PG, BD Voltage .........................................................30V
Boost Diode Current....................................................1A
(MSE Only) .......................................................300°C
PIN CONFIGURATION
LT3971
LT3971
LT3971
TOP VIEW
TOP VIEW
TOP VIEW
1
2
3
4
5
6
7
8
BD
NC
16 GND
15 SYNC
14 PG
13 RT
BD
BOOST
SW
1
2
3
4
5
10 SYNC
BD
BOOST
SW
1
2
3
4
5
10 SYNC
BOOST
NC
9
8
7
6
PG
RT
SS
FB
9
8
7
6
PG
RT
SS
FB
11
GND
11
17
GND
SW
12 SS
11 NC
10 FB
GND
V
IN
EN
V
IN
NC
V
IN
EN
EN
9
FB
MSE PACKAGE
10-LEAD PLASTIC MSOP
= 45°C, θ = 10°C/W
JC
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
MSE PACKAGE
DD PACKAGE
θ
JA
16-LEAD PLASTIC MSOP
10-LEAD (3mm × 3mm) PLASTIC DFN
= 45°C, θ = 10°C/W
JC
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
θ
= 40°C
JA
θ
JA
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
LT3971-3.3, LT3971-5
LT3971-3.3, LT3971-5
TOP VIEW
TOP VIEW
BD
BOOST
SW
1
2
3
4
5
10 SYNC
BD
BOOST
SW
1
2
3
4
5
10 SYNC
9
8
7
6
PG
RT
SS
9
8
7
6
PG
RT
SS
11
GND
11
GND
V
IN
V
IN
EN
V
OUT
EN
V
OUT
MSE PACKAGE
10-LEAD PLASTIC MSOP
DD PACKAGE
θ
= 45°C, θ = 10°C/W
JC
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
JA
10-LEAD (3mm × 3mm) PLASTIC DFN
θ
JA
= 45°C, θ = 10°C/W
JC
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT3971EDD#PBF
TAPE AND REEL
PART MARKING*
LFJF
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3971EDD#TRPBF
LT3971IDD#TRPBF
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
10-Lead (3mm × 3mm) Plastic DFN
10-Lead (3mm × 3mm) Plastic DFN
10-Lead Plastic MSOP
LT3971IDD#PBF
LFJF
LT3971EMSE#PBF
LT3971IMSE#PBF
LT3971EMSE16#PBF
LT3971IMSE16#PBF
LT3971EDD-3.3#PBF
LT3971IDD-3.3#PBF
LT3971EMSE-3.3#PBF
LT3971IMSE-3.3#PBF
LT3971EMSE#TRPBF
LT3971IMSE#TRPBF
LT3971EMSE16#TRPBF
LT3971IMSE16#TRPBF
LT3971EDD-3.3#TRPBF
LT3971IDD-3.3#TRPBF
LTFJG
LTFJG
10-Lead Plastic MSOP
3971
16-Lead Plastic MSOP
3971
16-Lead Plastic MSOP
LFRM
10-Lead (3mm × 3mm) Plastic DFN
10-Lead (3mm × 3mm) Plastic DFN
10-Lead Plastic MSOP
LFRM
LT3971EMSE-3.3#TRPBF LTFRN
LT3971IMSE-3.3#TRPBF LTFRN
10-Lead Plastic MSOP
3971fd
2
LT3971/LT3971-3.3/LT3971-5
ORDER INFORMATION
LEAD FREE FINISH
LT3971EDD-5#PBF
LT3971IDD-5#PBF
LT3971EMSE-5#PBF
LT3971IMSE-5#PBF
TAPE AND REEL
PART MARKING*
LFRP
PACKAGE DESCRIPTION
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
LT3971EDD-5#TRPBF
LT3971IDD-5#TRPBF
LT3971EMSE-5#TRPBF
LT3971IMSE-5#TRPBF
10-Lead (3mm × 3mm) Plastic DFN
10-Lead (3mm × 3mm) Plastic DFN
10-Lead Plastic MSOP
LFRP
LTFRQ
LTFRQ
10-Lead Plastic MSOP
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/
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VEN = 12V, VBD = 3.3V unless otherwise noted. (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
l
Minimum Input Voltage
Quiescent Current from V
(Note 4)
4
4.3
V
V
V
V
Low
High, V
High, V
0.7
1.7
1.2
2.7
4.5
μA
μA
μA
IN
EN
EN
EN
Low
Low
SYNC
SYNC
l
l
LT3971 FB Pin Current
V
= 1.19V
0.1
10
12
nA
FB
Internal Feedback Resistor Divider (LT3971-X)
Feedback Voltage
MΩ
1.175
1.165
1.19
1.19
1.205
1.215
V
V
l
l
l
LT3971-3.3 Output Voltage
LT3971-5 Output Voltage
3.25
3.224
3.3
3.3
3.35
3.376
V
V
4.93
4.89
5
5
5.07
5.11
V
V
FB Voltage Line Regulation
Switching Frequency
4.3V < V < 38V (Note 4)
0.0002
0.01
%/V
IN
R = 11k
1.6
0.8
160
2
1
200
2.4
1.2
240
MHz
MHz
kHz
T
R = 35.7k
T
R = 255k
T
Minimum Switch On Time
Minimum Switch Off Time
Switch Current Limit
80
110
2.4
330
0.02
770
0.02
1.4
20
ns
ns
150
3
1.8
A
Switch V
I
I
= 1A
mV
μA
mV
μA
V
CESAT
SW
Switch Leakage Current
Boost Schottky Forward Voltage
Boost Schottky Reverse Leakage
Minimum Boost Voltage (Note 3)
BOOST Pin Current
1
= 100mA
SH
V
V
= 12V
1
REVERSE
l
l
= 5V
1.8
28
IN
I
= 1A, V
= 15V
mA
V
SW
BOOST
EN Voltage Threshold
EN Rising
0.95
1.01
30
1.07
EN Voltage Hysteresis
mV
nA
mV
mV
%
EN Pin Current
0.2
100
20
20
LT3971 PG Threshold Offset from V
LT3971 PG Hysteresis
V
V
Rising
60
140
FB
FB
LT3971-X PG Threshold Offset from V
LT3971-X PG Hysteresis
Rising
5.5
9
12.5
OUT
OUT
1.3
%
3971fd
3
LT3971/LT3971-3.3/LT3971-5
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VEN = 12V, VBD = 3.3V unless otherwise noted. (Note 2)
PG Leakage
V
V
= 3V
0.02
570
0.8
0.1
1
1
µA
μA
V
PG
PG
l
PG Sink Current
SYNC Threshold
SYNC Pin Current
SS Source Current
= 0.4V
300
0.6
1.0
1.6
nA
μA
V
= 1V
0.6
SS
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.
temperature range. High junction temperatures degrade operating
lifetimes. Operating lifetime is derated at junction temperatures greater
than 125°C.
Note 3: This is the minimum voltage across the boost capacitor needed to
Note 2: The LT3971E is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the –40°C
to 125°C operating junction temperature range are assured by design,
characterization, and correlation with statistical process controls. The
LT3971I is guaranteed over the full –40°C to 125°C operating junction
guarantee full saturation of the switch.
Note 4: This is the minimum input voltage for operation with accurate FB
regulation. Minimum input voltage for output regulation depends on the
application circuit.
TA = 25°C, unless otherwise noted.
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency, VOUT = 5V
Efficiency, VOUT = 3.3V
Efficiency, VOUT = 5V
100
90
80
70
60
50
40
30
20
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
V
= 5V
OUT
R1 = 1M
V
= 12V
IN
V
= 12V
R2 = 309k
IN
V
= 12V
IN
V
= 36V
IN
V
= 24V
V
IN
= 36V
V
= 24V
IN
IN
V
= 24V
V
= 36V
IN
IN
V
= 5V
OUT
R1 = 1M
R2 = 309k
0
0.2
0.4
0.6
0.8
1
1.2
0.01
0.1
1
10
100
1000
0
0.2
0.4
0.6
0.8
1
1.2
LOAD CURRENT (A)
LOAD CURRENT (mA)
LOAD CURRENT (A)
3971 G01
3971 G03
3971 G02
Efficiency, VOUT = 3.3V
No Load Supply Current
No Load Supply Current
100
10
1
4.0
3.5
3.0
2.5
2.0
1.5
1.0
90
80
70
60
50
40
30
20
10
0
DIODES, INC.
DFLS2100
V
= 12V
IN
LT3971
V
IN
= 24V
V
= 3.3V
OUT
V
= 36V
IN
LT3971-5
LT3971-3.3
–55 –25
5
35
65
95 125 155
0.01
0.1
1
10
100
1000
5
10
15
20
25
30
35
TEMPERATURE (°C)
INPUT VOLTAGE (V)
LOAD CURRENT (mA)
3971 G05
3971 G06
3971 G04
3971fd
4
LT3971/LT3971-3.3/LT3971-5
TA = 25°C, unless otherwise noted.
TYPICAL PERFORMANCE CHARACTERISTICS
LT3971 Feedback Voltage
LT3971-3.3 Output Voltage
LT3971-5 Output Voltage
1.205
1.200
1.195
1.190
1.185
1.180
1.175
3.345
3.330
3.315
3.300
3.285
3.270
3.255
5.06
5.04
5.02
5.00
4.98
4.96
4.94
–55 –25
5
35
65
95 125 155
–55 –25
5
35
65
95 125 155
–55 –25
5
35
65
95 125 155
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
3971 G07
3971 G28
3971 G29
Maximum Load Current
Maximum Load Current
Load Regulation
0.30
0.25
0.20
0.15
0.10
0.05
0
3.0
2.5
2.0
1.5
1.0
0.5
0
2.5
2.0
1.5
1.0
0.5
0
V
= 5V
V
= 3.3V
OUT
OUT
TYPICAL
TYPICAL
MINIMUM
MINIMUM
–0.05
–0.10
–0.15
–0.20
–0.25
–0.30
REFERENCED FROM V
AT 0.5A LOAD
OUT
0
200
400
600
800 1000 1200
5
10
15
20
25
30
35
40
5
10
15
20
25
30
35
40
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
3971 G10
3971 G08
3971 G09
Switching Frequency
Switch Current Limit
Switch Current Limit
3.0
2.5
2.0
1.5
1.0
0.5
0
2.5
2.4
2.3
2.2
2.1
2.0
1.9
1.8
1.7
1.6
1.5
1000
950
900
850
800
750
700
650
600
DUTY CYCLE = 30%
0
20
40
60
80
100
–55 –25
5
35
65
95 125 155
–55 –25
5
35
65
95 125 155
DUTY CYCLE (%)
TEMPERATURE (°C)
TEMPERATURE (°C)
3971 G12
3971 G13
3971 G11
3971fd
5
LT3971/LT3971-3.3/LT3971-5
TA = 25°C, unless otherwise noted.
TYPICAL PERFORMANCE CHARACTERISTICS
Boost Pin Current
Frequency Foldback
Switch VCESAT
900
600
500
400
300
200
100
0
30
25
20
15
10
5
800
700
600
500
400
300
200
100
0
0
0
0.2
0.4
0.6
0.8
1
1.2
0
250
500
750 1000 1250 1500
0
250
500
750 1000 1250 1500
FB PIN VOLTAGE (V)
SWITCH CURRENT (mA)
SWITCH CURRENT (mA)
3971 G16
3971 G14
3971 G15
Minimum Switch On-Time/
Switch Off-Time
LT3971-X Frequency Foldback
Soft-Start
2.5
2.0
1.5
1.0
0.5
0
900
800
700
600
500
400
300
200
100
0
400
350
300
250
200
150
100
50
MIN T
1A LOAD
OFF
MIN T
0.5A LOAD
OFF
MIN T
ON
0
–55 –25
5
35
65
95 125 155
0
0.25 0.5 0.75
SS PIN VOLTAGE (V)
1
1.25 1.5 1.75
2
0
20
40
(% OF REGULATION VOLTAGE)
60
80
100
V
TEMPERATURE (°C)
OUT
3971 G18
3971 G30
3971 G17
Minimum Input Voltage
EN Threshold
Minimum Input Voltage
6.4
6.2
6.0
5.8
5.6
5.4
5.2
5.0
5.0
4.8
4.6
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
1.05
1.04
1.03
1.02
1.01
1.00
0.99
0.98
0.97
0.96
0.95
V
= 5V
V
= 3.3V
OUT
OUT
TO START
RISING THRESHOLD
TO START
TO RUN
FALLING THRESHOLD
TO RUN
0
200
400
600
800 1000 1200
–55 –25
5
35
65
95 125 155
0
200
400
600
800 1000 1200
LOAD CURRENT (mA)
LOAD CURRENT (mA)
TEMPERATURE (°C)
3971 G20
3971 G19
3971 G21
3971fd
6
LT3971/LT3971-3.3/LT3971-5
TA = 25°C, unless otherwise noted.
TYPICAL PERFORMANCE CHARACTERISTICS
Transient Load Response,
Load Current Stepped from 25mA
(Burst Mode Operation) to 525mA
Boost Diode Forward Voltage
Power Good Threshold
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
95
94
93
92
91
90
89
88
87
86
85
V
OUT
100mV/DIV
I
L
500mA/DIV
3971 G24
0
250
500
750 1000 1250 1500
10µs/DIV
–55 –25
5
35
65
95 125 155
BOOST DIODE CURRENT (mA)
V
C
= 12V, V
OUT
= 3.3V
OUT
TEMPERATURE (°C)
IN
3971 G22
3971 G23
= 47µF
Transient Load Response,
Load Current Stepped from
0.5A to 1A
Switching Waveforms;
Burst Mode Operation
Switching Waveforms; Full
Frequency Continuous Operation
V
OUT
V
SW
100mV/
DIV
V
SW
5V/DIV
5V/DIV
I
L
I
L
500mA/DIV
500mA/DIV
I
L
500mA/
DIV
V
V
OUT
20mV/DIV
OUT
20mV/DIV
3971 G27
3971 G26
3971 G25
1µs/DIV
= 3.3V
OUT
5µs/DIV
= 3.3V
10µs/DIV
= 3.3V
V
I
= 12V, V
= 1A
OUT
V
I
= 12V, V
V
C
= 12V, V
OUT
IN
LOAD
IN
OUT
IN
OUT
= 10mA
= 47µF
LOAD
C
= 22µF
C
= 22µF
OUT
3.3V Start-Up and Dropout
5V Start-Up and Dropout
3.3V Start-Up and Dropout
V
IN
V
V
IN
IN
V
V
V
OUT
OUT
OUT
3971 G31
3971 G32
3971 G33
0.5s/DIV
0.5s/DIV
0.5s/DIV
800kHz
3kΩ LOAD
800kHz
6.7Ω LOAD
800kHz
5kΩ LOAD
3971fd
7
LT3971/LT3971-3.3/LT3971-5
TA = 25°C, unless otherwise noted.
TYPICAL PERFORMANCE CHARACTERISTICS
5V Start-Up and Dropout
Minimum Input Voltage to Switch
Feedback Regulation Voltage
5
4
3
2
1.6
1.2
0.8
0.4
V
IN
V
OUT
1
0
3971 G34
0.5s/DIV
–55 –25
5
35
65
95 125 155
2
2.5
3
3.5
4
4.5
5
800kHz
10Ω LOAD
TEMPERATURE (°C)
INPUT VOLTAGE (V)
3971 G36
3971 G35
PIN FUNCTIONS (DFN, MSE10/MSE16)
SS (Pin 7/Pin 12): A capacitor is tied between SS and
ground to slowly ramp up the peak current limit of the
LT3971onstart-up.Thesoft-startcapacitorisonlyactively
discharged when EN is low. The SS pin is released when
the EN pin goes high. Float this pin to disable soft-start.
For applications with input voltages above 25V, add a 100k
resistor in series with the soft-start capacitor.
BD (Pin 1/Pin 1): This pin connects to the anode of the
boostdiode.TheBDpinisnormallyconnectedtotheoutput.
BOOST (Pin 2/Pin 3): This pin is used to provide a drive
voltage,higherthantheinputvoltage,totheinternalbipolar
NPN power switch.
SW (Pin 3/Pin 5): The SW pin is the output of an internal
powerswitch.Connectthispintotheinductor,catchdiode,
and boost capacitor.
RT(Pin8/Pin13):AresistoristiedbetweenRTandground
to set the switching frequency.
V
(Pin 4/Pin 7): The V pin supplies current to the
IN
IN
PG (Pin 9/Pin 14): The PG pin is the open-drain output of
an internal comparator. PGOOD remains low until the FB
pin is within 9% of the final regulation voltage. PGOOD is
LT3971’sinternalcircuitryandtotheinternalpowerswitch.
This pin must be locally bypassed.
EN (Pin 5/Pin 8): The part is in shutdown when this pin
is low and active when this pin is high. The hysteretic
threshold voltage is 1.005V going up and 0.975V going
valid when the LT3971 is enabled and V is above 4.3V.
IN
SYNC (Pin 10/Pin 15): This is the external clock synchro-
nization input. Ground this pin for low ripple Burst Mode
operation at low output loads. Tie to a clock source for
synchronization, which will include pulse-skipping at low
output loads. When in pulse-skipping mode, quiescent
current increases to 1.5mA.
down.TheENthresholdisonlyaccuratewhenV isabove
IN
4.3V. If V is lower than 4.2V, ground EN to place the part
IN
in shutdown. Tie to V if shutdown feature is not used.
IN
FB (Pin 6, LT3971 Only/Pins 9, 10): The LT3971 regulates
the FB pin to 1.19V. Connect the feedback resistor divider
taptothispin.Also,connectaphaseleadcapacitorbetween
GND (Exposed Pad Pin 11/Pin 16, Exposed Pad Pin 17):
Ground. The exposed pad must be soldered to PCB.
FB and V . Typically this capacitor is 10pF.
OUT
NC (None/Pins 2, 4, 6, 11): No Connect. These pins
are not connected to internal circuitry. Float these pins
to achieve FMEA fault tolerance. (See Fault Tolerance of
MS16E Package section.)
V
(Pin 6, LT3971-3.3 and LT3971-5 Only): The
OUT
LT3971-3.3andLT3971-5regulatetheV pinto3.3Vand
OUT
5V respectively. This pin connects to the internal 10MΩ
feedback divider that programs the fixed output voltage.
3971fd
8
LT3971/LT3971-3.3/LT3971-5
BLOCK DIAGRAM
V
IN
V
IN
–
+
C1
INTERNAL 1.19V REF
SHDN
BD
1V
+
–
SWITCH
SLOPE COMP
Σ
LATCH
BOOST
EN
RT
R
C3
OSCILLATOR
200kHz TO 2MHz
Q
S
L1
R
T
V
OUT
SW
Burst Mode
DETECT
SYNC
PG
D1
C2
ERROR AMP
V
CLAMP
C
V
+
–
+
–
1.09V
C
1µA
SS
C5
R1
R3
C4
SHDN
R2
GND
FB
V
OUT
LT3971-3.3
LT3971-5
ONLY
R2
R1
C5
3991 BD
LT3971-3.3: R1 = 6.39M, R2 = 3.61M
LT3971-5: R1 = 7.62M, R2 = 2.38M
LT3971
ONLY
3971fd
9
LT3971/LT3971-3.3/LT3971-5
OPERATION
The LT3971 is a constant frequency, current mode step-
down regulator. An oscillator, with frequency set by RT,
sets an RS flip-flop, turning on the internal power switch.
An amplifier and comparator monitor the current flowing
voltage at the BOOST pin that is higher than the input
supply. This allows the driver to fully saturate the internal
bipolar NPN power switch for efficient operation.
To further optimize efficiency, the LT3971 automatically
switches to Burst Mode operation in light load situations.
Between bursts, all circuitry associated with controlling
the output switch is shut down, reducing the input supply
current to 1.7μA. In a typical application, 2.8μA will be
consumed from the supply when regulating with no load.
between the V and SW pins, turning the switch off when
IN
this current reaches a level determined by the voltage at
V (see Block Diagram). An error amplifier measures the
C
output voltage through an external resistor divider tied to
the FB pin and servos the V node. If the error amplifier’s
C
output increases, more current is delivered to the output;
The oscillator reduces the LT3971’s operating frequency
when the voltage at the FB pin is low. This frequency
foldback helps to control the output current during start-
up and overload.
if it decreases, less current is delivered. An active clamp
on the V node provides current limit. The V node is
C
C
also clamped by the voltage on the SS pin; soft-start is
implemented by generating a voltage ramp at the SS pin
using an external capacitor.
The LT3971 contains a power good comparator which
trips when the FB pin is at 91% of its regulated value. The
PG output is an open-drain transistor that is off when the
output is in regulation, allowing an external resistor to pull
the PG pin high. Power good is valid when the LT3971 is
If the EN pin is low, the LT3971 is shut down and draws
700nA from the input. When the EN pin exceeds 1V, the
switching regulator will become active.
The switch driver operates from either V or from the
BOOST pin. An external capacitor is used to generate a
IN
enabled and V is above 4.3V.
IN
APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
1000
V
V
= 12V
IN
OUT
= 3.3V
To enhance efficiency at light loads, the LT3971 operates
inlowrippleBurstMode,whichkeepstheoutputcapacitor
charged to the desired output voltage while minimizing
the input quiescent current. In Burst Mode operation the
LT3971 delivers single pulses of current to the output ca-
pacitor followed by sleep periods where the output power
is supplied by the output capacitor. When in sleep mode
the LT3971 consumes 1.7μA, but when it turns on all the
circuitry to deliver a current pulse, the LT3971 consumes
1.5mA of input current in addition to the switch current.
Therefore, the total quiescent current will be greater than
1.7μA when regulating.
800
600
400
200
0
0
20
40
60
80
100
120
LOAD CURRENT (mA)
3971 F01
Figure 1. Switching Frequency in Burst Mode Operation
gets closer to the 1.7μA ideal. Therefore, to optimize the
quiescent current performance at light loads, the current
in the feedback resistor divider and the reverse current
in the catch diode must be minimized, as these appear
to the output as load currents. Use the largest possible
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure 1) and the percentage
of time the LT3971 is in sleep mode increases, resulting
in much higher light load efficiency. By maximizing the
time between pulses, the converter quiescent current
3971fd
10
LT3971/LT3971-3.3/LT3971-5
APPLICATIONS INFORMATION
feedback resistors and a low leakage Schottky catch diode
in applications utilizing the ultralow quiescent current
performanceoftheLT3971.Thefeedbackresistorsshould
preferably be on the order of MΩ and the Schottky catch
diode should have less than 1µA of typical reverse leak-
age at room temperature. These two considerations are
reiterated in the FB Resistor Network and Catch Diode
Selection sections.
programmed by the R resistor, and will be operating in
T
standardPWMmode.ThetransitionbetweenPWMandlow
ripple Burst Mode operation will exhibit slight frequency
jitter, but will not disturb the output voltage.
To ensure proper Burst Mode operation, the SYNC pin
must be grounded. When synchronized with an external
clock, the LT3971 will pulse skip at light loads. The qui-
escent current will significantly increase to 1.5mA in light
load situations when synchronized with an external clock.
Holding the SYNC pin high yields no advantages in terms
of output ripple or minimum load to full frequency, so is
not recommended.
It is important to note that another way to decrease the
pulse frequency is to increase the magnitude of each
single current pulse. However, this increases the output
voltage ripple because each cycle delivers more power to
the output capacitor. The magnitude of the current pulses
was selected to ensure less than 15mV of output ripple in
a typical application. See Figure 2.
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to:
V
SW
5V/DIV
VOUT
1.19V
R1= R2
− 1
I
L
500mA/DIV
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
V
OUT
20mV/DIV
3971 F02
5µs/DIV
The total resistance of the FB resistor divider should be
selected to be as large as possible to enhance low current
performance. The resistor divider generates a small load
on the output, which should be minimized to optimize the
low supply current at light loads.
V
V
LOAD
= 12V
IN
= 3.3V
OUT
I
= 10mA
Figure 2. Burst Mode Operation
While in Burst Mode operation, the burst frequency and
the charge delivered with each pulse will not change with
output capacitance. Therefore, the output voltage ripple
will be inversely proportional to the output capacitance.
In a typical application with a 22μF output capacitor, the
output ripple is about 10mV, and with a 47μF output ca-
pacitor the output ripple is about 5mV. The output voltage
ripple can continue to be decreased by increasing the
output capacitance.
WhenusinglargeFBresistors,a10pFphaseleadcapacitor
should be connected from V
to FB.
OUT
The LT3971-3.3 and LT3971-5 contain an internal 10M FB
resistor divider as well as an internal phase lead capacitor.
Setting the Switching Frequency
The LT3971 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 2MHz
by using a resistor tied from the RT pin to ground. A table
At higher output loads (above 92mA for the front page
application) the LT3971 will be running at the frequency
showing the necessary R value for a desired switching
T
frequency is in Table 1.
3971fd
11
LT3971/LT3971-3.3/LT3971-5
APPLICATIONS INFORMATION
switch off-time. These equations show that duty cycle
range increases when switching frequency is decreased.
Table 1. Switching Frequency vs RT Value
SWITCHING FREQUENCY (MHz)
R VALUE (kΩ)
T
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
255
118
A good choice of switching frequency should allow
adequate input voltage range (see Input Voltage Range
section) and keep the inductor and capacitor values small.
71.5
49.9
35.7
28.0
22.1
17.4
14.0
11.0
Input Voltage Range
The minimum input voltage is determined by either the
LT3971’s minimum operating voltage of 4.3V or by its
maximumdutycycle(seeequationinOperatingFrequency
Tradeoffs section). The minimum input voltage due to
duty cycle is:
Operating Frequency Tradeoffs
Selection of the operating frequency is a tradeoff between
efficiency,componentsize,minimumdropoutvoltage,and
maximum input voltage. The advantage of high frequency
operationisthatsmallerinductorandcapacitorvaluesmay
be used. The disadvantages are lower efficiency, lower
maximum input voltage, and higher dropout voltage. The
VOUT + VD
VIN(MIN)
=
− VD + VSW
1− fSW OFF(MIN)
t
where V
is the minimum input voltage, V
is
OUT
IN(MIN)
the output voltage, V is the catch diode drop (~0.5V),
D
highest acceptable switching frequency (f
given application can be calculated as follows:
) for a
SW(MAX)
V
is the internal switch drop (~0.5V at max load), f
SW
SW
is the switching frequency (set by R ), and t
is
OFF(MIN)
T
VOUT + VD
the minimum switch off-time. Note that higher switch-
ing frequency will increase the minimum input voltage.
If a lower dropout voltage is desired, a lower switching
frequency should be used.
fSW(MAX)
=
tON(MIN)(VIN − VSW + VD)
where V is the typical input voltage, V
is the output
OUT
IN
voltage, V is the catch diode drop (~0.5V), and V is
D
SW
The maximum input voltage for LT3971 applications
depends on switching frequency, the Absolute Maximum
theinternalswitchdrop(~0.5Vatmaxload).Thisequation
shows that slower switching frequency is necessary to
Ratings of the V and BOOST pins, and the operating
IN
safely accommodate high V /V
ratio. Also, as shown
IN OUT
mode. For a given application where the switching fre-
intheInputVoltageRangesection,lowerfrequencyallows
a lower dropout voltage. The input voltage range depends
ontheswitchingfrequencybecausetheLT3971switchhas
finite minimum on and off times. The minimum switch on
and off times are strong functions of temperature. Use
the typical minimum on and off curves to design for an
application’s maximum temperature, while adding about
30%forpart-to-partvariation.Theminimumandmaximum
duty cycles that can be achieved taking minimum on and
off times into account are:
quency and the output voltage are already selected, the
maximum input voltage (V
) that guarantees
IN(OP-MAX)
optimum output voltage ripple for that application can be
found by applying the following equation:
VOUT + VD
fSW • tON(MIN)
V
=
– VD + VSW
IN(OP-MAX)
where t
is the minimum switch on-time. Note that
ON(MIN)
a higher switching frequency will decrease the maximum
operating input voltage. Conversely, a lower switching
frequency will be necessary to achieve normal operation
at higher input voltages.
DCMIN = fSW ON(MIN)
t
DCMAX = 1− fSW OFF(MIN)
t
The circuit will tolerate inputs above the maximum op-
erating input voltage and up to the Absolute Maximum
where f is the switching frequency, the t
is the
ON(MIN)
istheminimum
SW
minimumswitchon-time,andthet
OFF(MIN)
3971fd
12
LT3971/LT3971-3.3/LT3971-5
APPLICATIONS INFORMATION
Table 2. Inductor Vendors
Ratings of the V and BOOST pins, regardless of chosen
IN
VENDOR
Murata
TDK
URL
PART SERIES
TYPE
switching frequency. However, during such transients
where V is higher than V
, the LT3971 will enter
IN(OP-MAX)
www.murata.com
LQH55D
Open
IN
pulse-skippingoperationwheresomeswitchingpulsesare
skipped to maintain output regulation. The output voltage
ripple and inductor current ripple will be higher than in
www.componenttdk.com SLF7045
SLF10145
Shielded
Shielded
Toko
www.toko.com
D62CB
D63CB
D73C
Shielded
Shielded
Shielded
Open
typical operation. Do not overload when V is greater
IN
D75F
than V
.
IN(OP-MAX)
Coilcraft
Sumida
www.coilcraft.com
www.sumida.com
MSS7341
MSS1038
Shielded
Shielded
Inductor Selection and Maximum Output Current
CR54
Open
A good first choice for the inductor value is:
CDRH74
CDRH6D38
CR75
Shielded
Shielded
Open
VOUT + VD
L =
fSW
When the switch is off, the potential across the inductor
is the output voltage plus the catch diode drop. This gives
the peak-to-peak ripple current in the inductor:
where f is the switching frequency in MHz, V
is the
OUT
SW
output voltage, V is the catch diode drop (~0.5V) and L
D
is the inductor value in μH.
(1− DC)•(VOUT + VD)
ΔIL =
Theinductor’sRMScurrentratingmustbegreaterthanthe
maximumloadcurrentanditssaturationcurrentshouldbe
about 30% higher. For robust operation in fault conditions
(start-up or short-circuit) and high input voltage (>30V),
the saturation current should be above 3.8A. To keep the
efficiency high, the series resistance (DCR) should be less
than 0.1Ω, and the core material should be intended for
high frequency applications. Table 2 lists several vendors
and suitable types.
L • fSW
Where f is the switching frequency of the LT3971, DC is
SW
the duty cycle and L is the value of the inductor. Therefore,
the maximum output current that the LT3971 will deliver
depends on the switch current limit, the inductor value,
and the input and output voltages. The inductor value may
have to be increased if the inductor ripple current does
not allow sufficient maximum output current (I
)
OUT(MAX)
giventheswitchingfrequency,andmaximuminputvoltage
Theinductorvaluemustbesufficienttosupplythedesired
used in the desired application.
maximum output current (I
), which is a function
OUT(MAX)
of the switch current limit (I ) and the ripple current.
The optimum inductor for a given application may differ
fromtheoneindicatedbythissimpledesignguide.Alarger
value inductor provides a higher maximum load current
and reduces the output voltage ripple. If your load is lower
than the maximum load current, than you can relax the
value of the inductor and operate with higher ripple cur-
rent. This allows you to use a physically smaller inductor,
or one with a lower DCR resulting in higher efficiency. Be
aware that if the inductance differs from the simple rule
above, then the maximum load current will depend on
the input voltage. In addition, low inductance may result
in discontinuous mode operation, which further reduces
LIM
ΔIL
2
IOUT(MAX) = ILIM
–
The LT3971 limits its peak switch current in order to
protect itself and the system from overload faults. The
LT3971’s switch current limit (I ) is at least 2.4A at low
LIM
duty cycles and decreases linearly to 1.75A at DC = 0.8.
3971fd
13
LT3971/LT3971-3.3/LT3971-5
APPLICATIONS INFORMATION
maximumloadcurrent.Fordetailsofmaximumoutputcur-
rentanddiscontinuousoperation,seeLinearTechnology’s
Application Note 44. Finally, for duty cycles greater than
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
withtheinductor,itfiltersthesquarewavegeneratedbythe
LT3971toproducetheDCoutput. Inthisroleitdetermines
the output ripple, so low impedance (at the switching
frequency) is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
LT3971’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
50%(V /V >0.5),aminimuminductanceisrequiredto
OUT IN
avoidsub-harmonicoscillations. SeeApplicationNote19.
One approach to choosing the inductor is to start with
the simple rule given above, look at the available induc-
tors, and choose one to meet cost or space goals. Then
use the equations above to check that the LT3971 will be
able to deliver the required output current. Note again
that these equations assume that the inductor current is
100
COUT
=
continuous. Discontinuous operation occurs when I
OUT
VOUT SW
f
is less than ΔI /2.
L
wheref isinMHz, andC
istherecommendedoutput
OUT
SW
Input Capacitor
capacitance in μF. Use X5R or X7R types. This choice will
provide low output ripple and good transient response.
Transient performance can be improved with a higher
value capacitor. Increasing the output capacitance will
also decrease the output voltage ripple. A lower value of
output capacitor can be used to save space and cost but
transient performance will suffer.
Bypass the input of the LT3971 circuit with a ceramic
capacitor of X7R or X5R type. Y5V types have poor
performance over temperature and applied voltage, and
should not be used. A 4.7μF to 10μF ceramic capacitor
is adequate to bypass the LT3971 and will easily handle
the ripple current. Note that larger input capacitance is
required when a lower switching frequency is used (due
to longer on-times). If the input power source has high
impedance, or there is significant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a low performance
electrolytic capacitor.
When choosing a capacitor, look carefully through the
data sheet to find out what the actual capacitance is under
operating conditions (applied voltage and temperature). A
physicallylargercapacitororonewithahighervoltagerating
may be required. Table 3 lists several capacitor vendors.
Table 3. Recommended Ceramic Capacitor Vendors
MANUFACTURER
AVX
WEBSITE
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT3971 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 4.7μF capacitor is capable of this task, but only if it is
placed close to the LT3971 (see the PCB Layout section).
Asecondprecautionregardingtheceramicinputcapacitor
concernsthemaximuminputvoltageratingoftheLT3971.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank
circuit. If the LT3971 circuit is plugged into a live supply,
the input voltage can ring to twice its nominal value, pos-
sibly exceeding the LT3971’s voltage rating. This situation
is easily avoided (see the Hot Plugging Safely section).
www.avxcorp.com
www.murata.com
www.t-yuden.com
www.vishay.com
www.tdk.com
Murata
Taiyo Yuden
Vishay Siliconix
TDK
Catch Diode Selection
The catch diode (D1 from Block Diagram) conducts cur-
rent only during switch off time. Average forward current
in normal operation can be calculated from:
V – VOUT
IN
ID(AVG) = IOUT
V
IN
where I
is the output load current. The only reason to
OUT
consideradiodewithalargercurrentratingthannecessary
3971fd
14
LT3971/LT3971-3.3/LT3971-5
APPLICATIONS INFORMATION
for nominal operation is for the worst-case condition of
shorted output. The diode current will then increase to the
typical peak switch current. Peak reverse voltage is equal
to the regulator input voltage. Use a diode with a reverse
voltage rating greater than the input voltage.
An additional consideration is reverse leakage current.
When the catch diode is reversed biased, any leakage
current will appear as load current. When operating under
light load conditions, the low supply current consumed
by the LT3971 will be optimized by using a catch diode
with minimum reverse leakage current. Low leakage
Schottky diodes often have larger forward voltage drops
at a given current, so a trade-off can exist between low
load and high load efficiency. Often Schottky diodes with
larger reverse bias ratings will have less leakage at a given
output voltage than a diode with a smaller reverse bias
rating. Therefore, superior leakage performance can be
achieved at the expense of diode size. Table 4 lists several
Schottky diodes and their manufacturers.
Table 4. Schottky Diodes. The Reverse Current Values Listed Are
Estimates Based Off of Typical Curves for Reverse Current
vs Reverse Voltage at 25°C.
I at V =
R
R
V
I
V at 1A
V at 2A
20V 25°C
(µA)
R
AVE
F
F
PART NUMBER (V)
(A)
(mV)
(mV)
On Semiconductor
MBR0520L
MBR0540
MBRM120E
MBRM140
Diodes Inc.
B0530W
B0540W
B120
20
40
20
40
0.5
0.5
1
30
0.4
0.5
20
620
530
550
595
1
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
whenusedwiththeLT3971duetotheirpiezoelectricnature.
When in Burst Mode operation, the LT3971’s switching
frequency depends on the load current, and at very light
loads the LT3971 can excite the ceramic capacitor at audio
frequencies, generating audible noise. Since the LT3971
operates at a lower current limit during Burst Mode op-
eration, the noise is typically very quiet to a casual ear. If
this is unacceptable, use a high performance tantalum or
electrolytic capacitor at the output.
30
40
20
30
40
50
20
30
40
40
40
60
100
40
0.5
0.5
1
15
1
620
500
500
500
700
1.1
1.1
1.1
0.4
20
0.6
1
B130
1
B140
1
B150
1
B220
2
500
500
B230
2
B140HB
DFLS240L
DFLS140
DFLS160
DFLS2100
B240
1
2
500
4
1.1
1
510
500
770
1
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT3971. As pre-
viously mentioned, a ceramic input capacitor combined
with trace or cable inductance forms a high quality (under
damped)tankcircuit. IftheLT3971circuitispluggedintoa
live supply, the input voltage can ring to twice its nominal
value,possiblyexceedingtheLT3971’srating.Thissituation
is easily avoided (see the Hot Plugging Safely section).
2.5
0.01
0.45
2
860
500
2
Central Semiconductor
CMSH1 - 40M
CMSH1 - 60M
40
60
1
1
1
2
2
2
2
2
500
700
400
CMSH1 - 40ML 40
CMSH2 - 40M
CMSH2 - 60M
CMSH2 - 40L
CMSH2 - 40
40
60
40
40
60
550
700
400
500
700
BOOST and BD Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see
the Block Diagram) are used to generate a boost volt-
age that is higher than the input voltage. In most cases
a 0.47μF capacitor will work well. Figure 3 shows three
ways to arrange the boost circuit. The BOOST pin must
be more than 2.3V above the SW pin for best efficiency.
CMSH2 - 60M
3971fd
15
LT3971/LT3971-3.3/LT3971-5
APPLICATIONS INFORMATION
The minimum operating voltage of an LT3971 application
is limited by the minimum input voltage (4.3V) and by
the maximum duty cycle as outlined in the Input Voltage
Range section. For proper start-up, the minimum input
voltage is also limited by the boost circuit. If the input
voltage is ramped slowly, the boost capacitor may not
be fully charged. Because the boost capacitor is charged
with the energy stored in the inductor, the circuit will rely
on some minimum load current to get the boost circuit
running properly. This minimum load will depend on input
and output voltages, and on the arrangement of the boost
circuit. The minimum load generally goes to zero once the
circuit has started. Figure 4 shows a plot of minimum load
to start and to run as a function of input voltage. In many
cases the discharged output capacitor will present a load
to the switcher, which will allow it to start. The plots show
Foroutputsof3Vandabove,thestandardcircuit(Figure 3a)
is best. For outputs between 2.8V and 3V, use a 1μF boost
capacitor. A 2.5V output presents a special case because it
is marginally adequate to support the boosted drive stage
whileusingtheinternalboostdiode.ForreliableBOOSTpin
operation with 2.5V outputs use a good external Schottky
diode (such as the ON Semi MBR0540), and a 1μF boost
capacitor (Figure 3b). For output voltages below 2.5V,
the boost diode can be tied to the input (Figure 3c), or to
another external supply greater than 2.8V. However, the
circuitinFigure3aismoreefficientbecausetheBOOSTpin
currentcomesfromalowervoltagesource. Youmustalso
be sure that the maximum voltage ratings of the BOOST
and BD pins are not exceeded.
BD
V
IN
V
BOOST
LT3971
theworst-casesituationwhereV isrampingveryslowly.
IN
IN
C3
5.0
4.8
4.6
SW
V
OUT
4.7µF
GND
4.4
TO START
4.2
(3a) For V
> 2.8V
OUT
4.0
TO RUN
3.8
3.6
3.4
3.2
3.0
D2
BD
V
IN
V
A
= 3.3V
OUT
V
BOOST
IN
T
= 25°C
LT3971
L = 4.7µH
f = 800kHz
C3
SW
V
OUT
4.7µF
10
100
1000
GND
LOAD CURRENT (mA)
6.4
6.2
6.0
5.8
5.6
5.4
5.2
5.0
(3b) For 2.5V < V
< 2.8V
OUT
TO START
TO RUN
BD
V
V
IN
BOOST
LT3971
IN
C3
V
A
= 5V
OUT
SW
V
4.7µF
OUT
T
= 25°C
GND
L = 4.7µH
f = 800kHz
10
100
1000
3971 FO3
LOAD CURRENT (mA)
3971 F04
(3c) For V
< 2.5V; V
= 27V
IN(MAX)
OUT
Figure 4. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
Figure 3. Three Circuits for Generating the Boost Voltage
3971fd
16
LT3971/LT3971-3.3/LT3971-5
APPLICATIONS INFORMATION
For lower start-up voltage, the boost diode can be tied to
Be aware that when the input voltage is below 4.3V, the
input current may rise to several hundred μA. And the part
V ; however, this restricts the input range to one-half of
IN
the absolute maximum rating of the BOOST pin.
may be able to switch at cold or for V
) thresholds less
IN(EN
than 7V. Figure 6 shows the magnitude of the increased
input current in a typical application with different pro-
At light loads, the inductor current becomes discontinu-
ous and this reduces the minimum input voltage to ap-
grammed V
.
IN(EN)
proximately 400mV above V . At higher load currents,
OUT
the inductor current is continuous and the duty cycle is
limitedbythemaximumdutycycleoftheLT3971,requiring
a higher input voltage to maintain regulation.
When operating in Burst Mode for light load currents, the
resistor network can easily be
current through the V
IN(EN)
greater than the supply current consumed by the LT3971.
Therefore,theV resistorsshouldbelargetominimize
IN(EN)
Enable Pin
their effect on efficiency at low loads.
The LT3971 is in shutdown when the EN pin is low and
active when the pin is high. The rising threshold of the
EN comparator is 1.01V, with 30mV of hysteresis. The EN
12V V
Input Current
IN(EN)
500
400
300
200
100
0
pin can be tied to V if the shutdown feature is not used.
IN
Adding a resistor divider from V to EN programs the
IN
LT3971 to regulate the output only when V is above a
IN
desired voltage (see Figure 5). Typically, this threshold,
V , is used in situations where the input supply is cur-
IN(EN)
rent limited, or has a relatively high source resistance. A
switchingregulatordrawsconstantpowerfromthesource,
so source current increases as source voltage drops. This
looks like a negative resistance load to the source and can
cause the source to current limit or latch low under low
0
1
2
3
4
5
6
7
8
9 10 11 12
INPUT VOLTAGE (V)
V
= 12V
IN(EN)
R3 = 11M
R4 = 1M
source voltage conditions. The V
threshold prevents
IN(EN)
the regulator from operating at source voltages where the
problems might occur. This threshold can be adjusted by
setting the values R3 and R4 such that they satisfy the
following equation:
6V V
Input Current
IN(EN)
500
400
300
200
100
0
R3
R4
V
=
+ 1
IN(EN)
where output regulation should not start until V is above
IN
V
. Due to the comparator’s hysteresis, switching will
IN(EN)
not stop until the input falls slightly below V
.
IN(EN)
LT3971
V
IN
0
1
2
3
4
5
6
INPUT VOLTAGE (V)
R3
3971 F06
V
= 6V
IN(EN)
1V
+
–
R3 = 5M
R4 = 1M
SHDN
EN
R4
Figure 6. Input Current vs Input Voltage
for a Programmed VIN(EN) of 6V and 12V
3971 F05
Figure 5. Programmed Enable Threshold
3971fd
17
LT3971/LT3971-3.3/LT3971-5
APPLICATIONS INFORMATION
The LT3971 will not enter Burst Mode operation at low
output loads while synchronized to an external clock, but
instead will pulse skip to maintain regulation.
Soft-Start
TheSSpincanbeusedtosoft-starttheLT3971bythrottling
themaximuminputcurrentduringstart-up.Aninternal1μA
current source charges an external capacitor generating a
voltagerampontheSSpin. TheSSpinclampstheinternal
The LT3971 may be synchronized over a 250kHz to 2MHz
range. The R resistor should be chosen to set the LT3971
T
switchingfrequency20%belowthelowestsynchronization
V node,whichslowlyrampsupthecurrentlimit.Maximum
C
input. For example, if the synchronization signal will be
current limit is reached when the SS pin is about 1.5V or
higher. By selecting a large enough capacitor, the output
can reach regulation without overshoot. For applications
with input voltages above 25V, a 100k resistor in series
with the soft-start capacitor is recommended. Figure 7
shows start-up waveforms for a typical application with
a 10nF capacitor on SS for a 3.3Ω load when the EN pin
is pulsed high for 13ms.
250kHz and higher, the R should be selected for 200kHz.
T
To assure reliable and safe operation the LT3971 will only
synchronize when the output voltage is near regulation as
indicatedbythePGflag.Itisthereforenecessarytochoose
alargeenoughinductorvaluetosupplytherequiredoutput
current at the frequency set by the R resistor (see the
T
InductorSelectionsection).Theslopecompensationisset
by the R value, while the minimum slope compensation
T
TheexternalSScapacitorisonlyactivelydischargedwhen
EN is low. With EN low, the external SS cap is discharged
through approximately 150Ω. The EN pin needs to be low
long enough for the external cap to completely discharge
through the 150Ω pull-down prior to start-up.
required to avoid subharmonic oscillations is established
by the inductor size, input voltage, and output voltage.
Since the synchronization frequency will not change the
slopes of the inductor current waveform, if the inductor
is large enough to avoid subharmonic oscillations at the
frequency set by R , than the slope compensation will be
T
sufficient for all synchronization frequencies.
V
SS
1V/DIV
Shorted and Reversed Input Protection
V
OUT
2V/DIV
If the inductor is chosen so that it won’t saturate exces-
sively, a LT3971 buck regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT3971 is absent. This may occur in battery charging ap-
plications or in battery backup systems where a battery
or some other supply is diode ORed with the LT3971’s
I
L
0.5A/DIV
3971 F07
2ms/DIV
Figure 7. Soft-Start Waveforms for Front-Page Application
with 10nF Capacitor on SS. EN is Pulsed High for About
13ms with a 3.3Ω Load Resistor
output. If the V pin is allowed to float and the EN pin
IN
is held high (either by a logic signal or because it is tied
Synchronization
to V ), then the LT3971’s internal circuitry will pull its
IN
quiescent current through its SW pin. This is fine if your
system can tolerate a few μA in this state. If you ground
the EN pin, the SW pin current will drop to essentially
To select low ripple Burst Mode operation, tie the SYNC
pin below 0.6V (this can be ground or a logic low output).
Synchronizing the LT3971 oscillator to an external fre-
quency can be done by connecting a square wave (with
20% to 80% duty cycle) to the SYNC pin. The square
wave amplitude should have valleys that are below 0.6V
and peaks above 1.0V (up to 6V).
zero. However, if the V pin is grounded while the output
IN
is held high, regardless of EN, parasitic diodes inside the
LT3971 can pull current from the output through the SW
pin and the V pin. Figure 8 shows a circuit that will run
IN
only when the input voltage is present and that protects
against a shorted or reversed input.
3971fd
18
LT3971/LT3971-3.3/LT3971-5
APPLICATIONS INFORMATION
D4
MBRS140
V
V
BOOST
SW
L1
IN
IN
C2
V
OUT
EN
V
OUT
LT3971
BD
FB
GND
+
BACKUP
GND
R
PG
R
T
C3
C4
3971 F07
Figure 8. Diode D4 Prevents a Shorted Input from Discharging a
Backup Battery Tied to the Output. It Also Protects the Circuit from
a Reversed Input. The LT3971 Runs Only When the Input is Present
R2
C5
R1
C1
D1
PCB Layout
GND
ForproperoperationandminimumEMI,caremustbetaken
during printed circuit board layout. Figure 9 shows the
recommended component placement with trace, ground
plane and via locations. Note that large, switched currents
3971 F09
VIAS TO V
VIAS TO LOCAL GROUND PLANE
VIAS TO V
VIAS TO RUN/SS
VIAS TO PG
IN
OUTLINE OF LOCAL
GROUND PLANE
VIAS TO SYNC
OUT
flow in the LT3971’s V and SW pins, the catch diode
IN
(D1), and the input capacitor (C1). The loop formed by
these components should be as small as possible. These
components,alongwiththeinductorandoutputcapacitor,
should be placed on the same side of the circuit board,
and their connections should be made on that layer. Place
a local, unbroken ground plane below these components.
The SW and BOOST nodes should be as small as possible.
Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation
the V pin of the LT3971 can ring to twice the nominal
IN
input voltage, possibly exceeding the LT3971’s rating and
damaging the part. If the input supply is poorly controlled
or the user will be plugging the LT3971 into an energized
supply, the input network should be designed to prevent
this overshoot. See Linear Technology Application Note
88 for a complete discussion.
Finally, keep the FB and R nodes small so that the ground
T
traces will shield them from the SW and BOOST nodes.
The Exposed Pad on the bottom of the package must be
soldered to ground so that the pad acts as a heat sink. To
keep thermal resistance low, extend the ground plane as
much as possible, and add thermal vias under and near
the LT3971 to additional ground planes within the circuit
board and on the bottom side.
High Temperature Considerations
For higher ambient temperatures, care should be taken in
the layout of the PCB to ensure good heat sinking of the
LT3971. The Exposed Pad on the bottom of the package
mustbesolderedtoagroundplane.Thisgroundshouldbe
tied to large copper layers below with thermal vias; these
layers will spread heat dissipated by the LT3971. Placing
additional vias can reduce thermal resistance further. The
maximum load current should be derated as the ambient
temperature approaches the maximum junction rating.
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3971 circuits. However, these ca-
pacitors can cause problems if the LT3971 is plugged into
a live supply. The low loss ceramic capacitor, combined
with stray inductance in series with the power source,
forms an under damped tank circuit, and the voltage at
Power dissipation within the LT3971 can be estimated by
calculatingthetotalpowerlossfromanefficiencymeasure-
ment and subtracting the catch diode loss and inductor
3971fd
19
LT3971/LT3971-3.3/LT3971-5
APPLICATIONS INFORMATION
loss. The die temperature is calculated by multiplying the
LT3971 power dissipation by the thermal resistance from
junction to ambient.
There are four items which require consideration in terms
of the application circuit to achieve fault tolerance: V -EN
IN
pin short, SYNC-GND pin short, SYNC-PG pin short, and
PG-RT pin short. If the EN pin is driven with a logic input,
then a series resistor is needed to protect the circuit gen-
Also keep in mind that the leakage current of the power
Schottky diode goes up exponentially with junction tem-
perature. When the power switch is closed, the power
Schottky diode is in parallel with the power converter’s
output filter stage. As a result, an increase in a diode’s
leakage current results in an effective increase in the load,
and a corresponding increase in input power. Therefore,
the catch Schottky diode must be selected with care to
avoid excessive increase in light load supply current at
high temperatures.
erating the logic input in the event of an EN-V pin short.
IN
If the SYNC pin is driven with a clock, a series resistor is
needed so that the clock source, which may be going to
other devices, is not pulled down in the event of a SYNC-
GND pin short. If the PG pull-up resistor is connected to a
voltage source higher than 6V, then the PG resistor needs
to be large enough such that the resistor divider formed
by a PG-RT pin short does not violate the RT pin absolute
maximum. Likewise, a SYNC resistor to GND is needed so
that the resistor divider formed by a PG-SYNC pin short
does not violate the SYNC pin absolute maximum. This
Fault Tolerance of MS16E Package
The MS16E package is designed to tolerate single fault
conditions. Shorting two adjacent pins together or leaving
one single pin floating does not raise the output voltage
or cause damage to the LT3971 regulator. However, the
applicationcircuitmustmeetafewrequirementsdiscussed
in this section in order to achieve fault tolerance.
means that typical applications where EN is tied to V ,
IN
SYNC is grounded, and PG is floating or connected to a
pull-up resistor to an output less than 6V are already set
up for fault tolerance. Figure 10, shows how fault toler-
ance can be achieved when PG, EN, and SYNC features
are used in a high output voltage application.
Tables 5 and 6 show the effects that result from shorting
adjacent pins or from a floating pin, respectively.
V
IN
15V TO 38V
V
IN
100k
BOOST
EN
OFF ON
10µF
0.47µF
10µH
SW
LT3971
150k
SS
RT
PG
BD
PGOOD
1M
V
OUT
1nF
49.9k
CLOCK IN
12V
FB
SYNC GND
49.9k
1.2A
10pF
110k
10µF
3971 F10
f
= 800kHz
SW
Figure 10. Fault Tolerant Application with EN, SYNC and PG Functions in Use when Using the MS16E Package
3971fd
20
LT3971/LT3971-3.3/LT3971-5
APPLICATIONS INFORMATION
Table 5: Effects of Pin Shorts
PINS
EFFECT
V -EN
IN
No effect. In most applications, EN is tied to V . If EN is driven with a logic signal, a series resistor is recommended to protect the circuit
IN
generating the logic signal from the full V voltage.
IN
SS-RT
RT-PG
V
may fall below regulation voltage. The switching frequency will be increased and the current limit will be reduced.
OUT
No effect if PG is floated.
V
will fall below regulation if PG is connected to the output with a resistor pull-up as long as the resister divider formed by the PG pin
OUT
pull-up and the R resistor prevents the RT pin absolute maximum from being violated. (see discussion in Fault Tolerance section)
T
In both cases, the switching frequency will be significantly increased if the output goes below regulation, which may cause the LT3975 to
go into pulse-skipping mode if the minimum on-time is violated.
PG-SYNC No effect if PG is floated.
No effect if PG is connected to the output with a resistor pull-up as long as there is a resistor to GND on the SYNC pin or the SYNC pin is
tied to GND. This is to ensure that the resistor divider formed by the PG pin pull-up and the SYNC pin resistor to GND prevents the SYNC
pin absolute maximum from being violated. (see discussion in Fault Tolerance section)
SYNC-GND No effect. If the SYNC pin is driven with a clock, a series resistor is recommended to prevent the clock source from getting shorted out.
Table 6: Effects of Floating Pins
PIN
SS
EFFECT
No effect; soft-start feature will not function.
BD
V
may fall below regulation voltage. With the BD pin disconnected, the boost capacitor cannot be charged and thus the power switch
OUT
cannot fully saturate, which increases power dissipation.
V may fall below regulation voltage. With the BOOST pin disconnected, the boost capacitor cannot be charged and thus the power switch
OUT
BOOST
SW
cannot fully saturate, which increases power dissipation.
V
V
V
will fall below regulation voltage.
OUT
OUT
OUT
V
will fall below regulation voltage.
IN
EN
may fall below regulation voltage. Part may work normally or be shutdown depending on how the application circuit couples to the
floating EN pin.
V may fall below regulation voltage.
OUT
RT
PG
No effect.
SYNC No effect. The LT3971 may be in Burst Mode operation or pulse-skipping mode depending on how the application circuit couples to the
floating SYNC pin.
FB
No effect; there are two FB pins.
GND
No effect; there are two GND connections. If exposed pad is floated, thermal performance will be degraded.
Other Linear Technology Publications
Application Notes 19, 35 and 44 contain more detailed
descriptions and design information for buck regulators
and other switching regulators. The LT1376 data sheet
has a more extensive discussion of output ripple, loop
compensation and stability testing. Design Note 318
shows how to generate a bipolar output supply using a
buck regulator.
3971fd
21
LT3971/LT3971-3.3/LT3971-5
TYPICAL APPLICATIONS
5V Step-Down Converter
V
IN
7V TO 38V
V
IN
EN
BOOST
SW
OFF ON
4.7µF
0.47µF
10pF
PG
SS
4.7µH
LT3971
RT
BD
FB
V
1M
22µF
49.9k
OUT
5V
SYNC GND
1.2A
309k
f = 800kHz
3971 TA02
3.3V Step Down Converter
No Load Supply Current
V
4.0
3.5
3.0
2.5
2.0
1.5
1.0
IN
4.5V TO 38V
V
IN
EN
BOOST
SW
OFF ON
0.47µF
4.7µH
10pF
V
3.3V
1.2A
PG
SS
OUT
LT3971
4.7µF
RT
BD
FB
1.78M
49.9k
SYNC GND
1M
22µF
0
10
20
30
40
3971 TA11
INPUT VOLTAGE (V)
3971 TA11b
5V Step-Down Converter
2.5V Step-Down Converter
V
V
IN
4.3V TO 38V
IN
7V TO 38V
V
IN
V
IN
EN
BOOST
SW
EN
BOOST
SW
OFF ON
4.7µF
OFF ON
4.7µF
0.47µF
1µF
PG
SS
PG
SS
4.7µH
4.7µH
LT3971-5
LT3971
RT
RT
BD
BD
FB
10pF
V
5V
1.2A
V
2.5V
1.2A
49.9k
1M
47µF
OUT
118k
OUT
V
OUT
SYNC GND
SYNC GND
22µF
909k
f = 800kHz
f = 400kHz
3971 TA03
3971 TA04
3971fd
22
LT3971/LT3971-3.3/LT3971-5
TYPICAL APPLICATIONS
1.8V Step-Down Converter
12V Step-Down Converter
V
V
IN
15V TO 38V
IN
4.3V TO 27V
V
IN
BD
BOOST
V
IN
EN
EN
BOOST
SW
OFF ON
4.7µF
OFF ON
10µF
0.47µF
10pF
0.47µF
10pF
PG
SS
PG
SS
4.7µH
SW
10µH
1M
LT3971
LT3971
RT
RT
BD
FB
V
1.8V
1.2A
118k
511k
V
OUT
OUT
FB
49.9k
f = 800kHz
12V
SYNC GND
SYNC GND
1.2A
1M
100µF
110k
10µF
f = 400kHz
3971 TA05
3971 TA06
3.3V Step-Down Converter with Undervoltage Lockout, Soft-Start, and Power Good
V
IN
6V TO 38V
5M
V
IN
BOOST
SW
EN
0.47µF
4.7µH
4.7µF
SS
RT
150k
LT3971
100k
1nF
PG
BD
PGOOD
1M
10pF
1M
V
3.3V
1.2A
OUT
49.9k
FB
SYNC GND
562k
22µF
f = 800kHz
3971 TA07
5V, 2MHz Step-Down Converter with Soft-Start
V
IN
9V TO 25V
V
IN
EN
BOOST
SW
OFF ON
0.47µF
10pF
PG
SS
2.2µH
LT3971
2.2µF
RT
BD
FB
1nF
11k
V
1M
22µF
OUT
5V
SYNC GND
1.2A
309k
f = 2MHz
3971 TA08
3971fd
23
LT3971/LT3971-3.3/LT3971-5
TYPICAL APPLICATIONS
4V Step-Down Converter with a High Impedance Input Source
+
–
11M
1M
V
IN
24V
EN
BOOST
SW
+
C
BULK
100µF
0.47µF
10pF
* AVERAGE OUTPUT POWER CANNOT
EXCEED THAT WHICH CAN BE PROVIDED
BY HIGH IMPEDANCE SOURCE.
NAMELY,
PG
SS
4.7µH
LT3971
4.7µF
2
RT
V
P
=
• η
OUT(MAX)
4R
BD
FB
1nF
WHERE V IS VOLTAGE OF SOURCE, R IS
INTERNAL SOURCE IMPEDANCE, AND η IS
LT3971 EFFICIENCY. MAXIMUM OUTPUT
CURRENT OF 1.2A CAN BE SUPPLIED FOR A
SHORT TIME BASED ON THE ENERGY
WHICH CAN BE SOURCED BY THE BULK
INPUT CAPACITANCE.
V
OUT
49.9k
1M
4V
1.2A*
SYNC GND
412k
100µF
f = 800kHz
3971 TA09a
Sourcing a Maximum Load Pulse
Start-Up from High Impedance Input Source
V
OUT
200mV/DIV
V
IN
1V/DIV
V
IN
5V/DIV
V
OUT
2V/DIV
I
I
L
L
1A/DIV
500mA/DIV
3971 TA09c
3971 TA09b
2ms/DIV
500µs/DIV
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
DD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1699 Rev C)
R = 0.125
TYP
6
0.40 ± 0.10
10
0.70 ±0.05
3.55 ±0.05
2.15 ±0.05 (2 SIDES)
1.65 ±0.05
3.00 ±0.10
(4 SIDES)
1.65 ± 0.10
(2 SIDES)
PIN 1 NOTCH
R = 0.20 OR
PIN 1
TOP MARK
(SEE NOTE 6)
0.35 × 45°
PACKAGE
OUTLINE
CHAMFER
(DD) DFN REV C 0310
5
1
0.25 ± 0.05
0.50 BSC
0.75 ±0.05
0.200 REF
0.25 ± 0.05
0.50
BSC
2.38 ±0.10
(2 SIDES)
2.38 ±0.05
(2 SIDES)
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
NOTE:
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2).
. 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
CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT 5. EXPOSED PAD SHALL BE SOLDER PLATED
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
3971fd
24
LT3971/LT3971-3.3/LT3971-5
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
MSE Package
10-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1664 Rev H)
BOTTOM VIEW OF
EXPOSED PAD OPTION
1.88
(.074)
1.88 ± 0.102
(.074 ± .004)
0.889 ± 0.127
(.035 ± .005)
1
0.29
REF
1.68
(.066)
0.05 REF
5.23
(.206)
MIN
1.68 ± 0.102 3.20 – 3.45
(.066 ± .004) (.126 – .136)
DETAIL “B”
CORNER TAIL IS PART OF
THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
DETAIL “B”
10
NO MEASUREMENT PURPOSE
0.50
(.0197)
BSC
0.305 ± 0.038
(.0120 ± .0015)
TYP
3.00 ± 0.102
(.118 ± .004)
(NOTE 3)
0.497 ± 0.076
(.0196 ± .003)
10 9
8
7 6
RECOMMENDED SOLDER PAD LAYOUT
REF
3.00 ± 0.102
(.118 ± .004)
(NOTE 4)
4.90 ± 0.152
(.193 ± .006)
DETAIL “A”
0.254
(.010)
0° – 6° TYP
1
2
3
4 5
GAUGE PLANE
0.53 ± 0.152
(.021 ± .006)
0.86
(.034)
REF
1.10
(.043)
MAX
DETAIL “A”
0.18
(.007)
SEATING
PLANE
0.17 – 0.27
(.007 – .011)
TYP
0.1016 ± 0.0508
(.004 ± .002)
0.50
(.0197)
BSC
MSOP (MSE) 0911 REV H
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.
3971fd
25
LT3971/LT3971-3.3/LT3971-5
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 E)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.845 ±0.102
(.112 ±.004)
2.845 ±0.102
(.112 ±.004)
0.889 ±0.127
(.035 ±.005)
1
8
0.35
REF
5.23
(.206)
MIN
1.651 ±0.102
(.065 ±.004)
1.651 ±0.102
(.065 ±.004)
3.20 – 3.45
(.126 – .136)
0.12 REF
DETAIL “B”
CORNER TAIL IS PART OF
THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
DETAIL “B”
16
9
0.305 ±0.038
0.50
(.0197)
BSC
NO MEASUREMENT PURPOSE
4.039 ±0.102
(.159 ±.004)
(NOTE 3)
(.0120 ±.0015)
TYP
0.280 ±0.076
(.011 ±.003)
RECOMMENDED SOLDER PAD LAYOUT
16151413121110
9
REF
DETAIL “A”
0.254
(.010)
3.00 ±0.102
(.118 ±.004)
(NOTE 4)
0° – 6° TYP
4.90 ±0.152
(.193 ±.006)
GAUGE PLANE
0.53 ±0.152
(.021 ±.006)
1 2 3 4 5 6 7 8
DETAIL “A”
0.86
(.034)
REF
1.10
(.043)
MAX
0.18
(.007)
SEATING
PLANE
0.17 – 0.27
(.007 – .011)
TYP
0.1016 ±0.0508
(.004 ±.002)
MSOP (MSE16) 0911 REV E
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.
3971fd
26
LT3971/LT3971-3.3/LT3971-5
REVISION HISTORY
REV
DATE
2/11
8/11
DESCRIPTION
PAGE NUMBER
A
Added fixed voltage options LT3971-3.3 and LT3971-5 reflected throughout data sheet
Added fixed voltage options LT3971-3.3 and LT3971-5 in DFN package
1 through 24
2
B
C
10/11 Modified Note 4
Add Start-Up and Dropout, Feedback Regulation curves to the Typical Performance Characteristics
4
7, 8
1, 2
8
D
7/12
Added MSOP-16E package option with enhanced pin-to-pin fault tolerance
Clarified pin function for MSOP-16E package option
Clarified saturation current at 3.8A
13
Clarified enhanced pin-to-pin fault tolerance
20, 21
3971fd
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 representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
27
LT3971/LT3971-3.3/LT3971-5
RELATED PARTS
PART NUMBER DESCRIPTION
COMMENTS
V : 4.2V to 40V, V
LT3970
LT3990
LT3991
LT3682
LT3689
40V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC
= 1.21V, I = 2.5µA,
Q
IN
OUT(MIN)
Converter with I = 2.5µA
I
<1µA, 3mm × 2mm DFN-10 and MSOP-10 Packages
Q
SD
62V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC
Converter with I = 2.5µA
V : 4.2V to 62V, V
= 1.21V, I = 2.5µA,
Q
IN
OUT(MIN)
I
<1µA, 3mm × 2mm DFN-10 and MSOP-10 Packages
SD
Q
55V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down DC/DC
Converter with I = 2.8µA
V : 4.3V to 38V, V
SD
= 1.2V, I = 2.8µA,
Q
IN
OUT(MIN)
I
<1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages
Q
36V, 60V
, 1A, 2.2MHz High Efficiency Micropower Step-Down
V : 3.6V to 36V, V
= 0.8V, I = 75µA, I <1µA,
Q SD
MAX
IN
OUT(MIN)
DC/DC Converter
3mm × 3mm DFN-12 Package
36V, 60V with Transient Protection 800mA, 2.2MHz, High Efficiency
Micropower Step-Down DC/DC Converter with POR Reset Watchdog
Timer
V : 3.6V to 36V, Transient to 60V, V
= 0.8V, I = 75µA,
Q
IN
SD
OUT(MIN)
I
<1µA, 3mm × 3mm QFN-16
LT3480
LT3980
36V with Transient Protection to 60V, 2A (I ), 2.4MHz, High Efficiency V : 3.6V to 36V, Transient to 60V, V
= 0.78V, I = 70µA,
Q
OUT
IN
OUT(MIN)
Step-Down DC/DC Converter with Burst Mode Operation
I
<1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages
SD
58V with Transient Protection to 80V, 2A (I ), 2.4MHz High Efficiency V : 3.6V to 58V, Transient to 80V, V
= 0.78V, I = 85µA,
Q
OUT
IN
OUT(MIN)
Step-Down DC/DC Converter with Burst Mode Operation
I
<1µA, MSOP-16E 3mm × 4mm DFN-16 Package and
SD
MSOP-16E Packages
3971fd
LT 0712 REV D • PRINTED IN USA
28 LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
●
●
LINEAR TECHNOLOGY CORPORATION 2009
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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