LT3971IMSE-3.3#TRPBF [Linear]
LT3971 - 38V, 1.2A, 2MHz Step-Down Regulator with 2.8µA Quiescent Current; Package: MSOP; Pins: 10; Temperature Range: -40°C to 85°C;型号: | LT3971IMSE-3.3#TRPBF |
厂家: | Linear |
描述: | LT3971 - 38V, 1.2A, 2MHz Step-Down Regulator with 2.8µA Quiescent Current; Package: MSOP; Pins: 10; Temperature Range: -40°C to 85°C 开关 光电二极管 |
文件: | 总24页 (文件大小:356K) |
中文: | 中文翻译 | 下载: | 下载PDF数据表文档文件 |
LT3971A/LT3971A-5
38V, 1.3A, 2MHz
Step-Down Regulator with
2.2µA Quiescent Current
DESCRIPTION
FEATURES
The LT®3971A 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 LT3971A, reducing the input supply current to
700nA. A capacitor on the SS pin provides a controlled
inrushcurrent(soft-start).Apowergoodflagsignalswhen
n
Ultralow Quiescent Current:
2.8μA I Regulating 12V to 3.3V
Q
IN
OUT
n
n
Fixed Output Voltages: 5V
2.2μA I Regulating 12V to 5V
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
1.3A 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
V
reaches91%oftheprogrammedoutputvoltage.The
OUT
Output Voltage: 1.19V to 30V
Small Thermally Enhanced 10-Lead MSOP Package
LT3971A is available in a small 10-lead MSOP package
with an exposed pad for low thermal resistance.
The LT3971A and LT3971A-5 have a more accurate ref-
erence voltage compared to the LT3971 and LT3971-5.
APPLICATIONS
The LT3971A-5 also has specified V
pin current and
OUT
n
USB VBUS Regulation
ABSMAX current. These characteristics are important for
USB applications.
L, LT, LTC, LTM, Linear Technology, Burst Mode and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
n
Automotive Battery Regulation
Power for Portable Products
n
n
Industrial Supplies
TYPICAL APPLICATION
No Load Supply Current
3.0
5.15V Step-Down Converter for USB Applications
OUTPUT IN REGULATION
V
IN
6.3V TO 38V
2.5
V
IN
LT3971A-5
EN
BOOST
SW
OFF ON
4.7μF
0.47μF
10μH
PG
SS
2.0
1.5
1.0
LT3971A-5
RT
BD
V
20k, 1%
OUT
5.15V
1.3A
V
OUT
118k
f = 400kHz
SYNC GND
715k
1%
47μF
5
10
15
20
25
30
35
INPUT VOLTAGE (V)
3971 TA01a
3971A TA01b
3971af
1
LT3971A/LT3971A-5
ABSOLUTE MAXIMUM RATINGS
(Note 1)
V , EN Voltage .........................................................38V
Operating Junction Temperature Range (Note 2)
LT3971AI/LT3971AI-5........................ –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, RT, SYNC, SS Voltage...........................................6V
PG, BD Voltage .........................................................30V
(MSE Only) .......................................................300°C
V
OUT
Pin Current....................................................–2mA
PIN CONFIGURATION
LT3971A
LT3971A-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
FB
9
8
7
6
PG
RT
SS
11
GND
11
GND
V
V
IN
EN
IN
EN
V
OUT
MSE PACKAGE
10-LEAD PLASTIC MSOP
MSE PACKAGE
10-LEAD PLASTIC MSOP
θ
= 45°C, θ = 10°C/W
θ
= 45°C, θ = 10°C/W
JA
JC
JA JC
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT3971AIMSE#PBF
LT3971AIMSE-5#PBF
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3971AIMSE#TRPBF
LTGFQ
10-Lead Plastic MSOP
10-Lead Plastic MSOP
–40°C to 125°C
–40°C to 125°C
LT3971AIMSE-5#TRPBF LTGFR
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
ꢀA
ꢀA
ꢀA
IN
EN
EN
EN
Low
Low
SYNC
SYNC
l
l
LT3971A FB Pin Current
V
= 1.19V
0.1
10
12
nA
FB
Internal Feedback Resistor Divider (LT3971A-5)
MΩ
3971af
2
LT3971A/LT3971A-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)
V
Pin Current
V
OUT
V
OUT
= 5V
= 5V
–0.65
–0.90
–0.5
–0.5
–0.38
–0.32
ꢀA
ꢀA
OUT
l
V
Pin Clamp Voltage
I
= –2mA
9
11
13
V
OUT
VOUT
Feedback Voltage
1.176
1.173
1.192
1.19
1.204
1.207
V
V
l
l
LT3971A-5 Output Voltage
4.94
4.93
5.01
5
5.06
5.07
V
V
FB Voltage Line Regulation
Switching Frequency
4.3V < V < 38V (Note 4)
0.0002
0.01
%/V
IN
R = 11k
1.8
0.8
160
2.2
1
200
2.6
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
2.5
330
0.02
770
0.02
1.4
20
ns
ns
A
150
3.2
2.0
Switch V
I
I
= 1A
mV
ꢀA
mV
ꢀA
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
SW
= 1A, V
= 15V
mA
V
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
LT3971A PG Threshold Offset from V
LT3971A PG Hysteresis
V
V
Rising
60
140
FB
FB
LT3971A-5 PG Threshold Offset from V
LT3971A-5 PG Hysteresis
PG Leakage
Rising
5.5
9
12.5
1
OUT
OUT
1.3
0.02
570
0.8
0.1
1
%
V
V
= 3V
μA
ꢀA
V
PG
PG
l
PG Sink Current
= 0.4V
300
0.6
SYNC Threshold
1.0
1.6
SYNC Pin Current
nA
ꢀA
SS Source Current
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.
Note 3: This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the switch.
Note 4: Minimum input voltage depends on application circuit.
Note 2: The LT3971AI is guaranteed over the full –40°C to 125°C operating
junction temperature range. High junction temperatures degrade operating
lifetimes. Operating lifetime is derated at junction temperatures greater
than 125°C.
3971af
3
LT3971A/LT3971A-5
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, unless otherwise noted.
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
100
V
= 5V
OUT
90
80
70
60
50
40
30
20
10
0
R1 = 1M
V
= 12V
IN
V
= 12V
R2 = 309k
IN
V
= 12V
IN
V
= 36V
IN
V
= 24V
V
= 24V
V
IN
= 36V
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
0.2
0.4
0.6
0.8
1
1.2
0.01
0.1
1
10
100
1000
LOAD CURRENT (A)
LOAD CURRENT (A)
LOAD CURRENT (mA)
3971A G01
3971A G02
3971A G03
Efficiency, VOUT = 3.3V
No Load Supply Current
No Load Supply Current
90
80
70
60
50
40
30
20
10
0
100
10
1
4.0
3.5
3.0
2.5
2.0
1.5
1.0
DIODES, INC.
DFLS2100
V
= 12V
IN
LT3971A
V
= 24V
V
= 3.3V
IN
OUT
V
= 36V
IN
LT3971A-5
0.01
0.1
1
10
100
1000
–55 –25
5
35
65
95 125 155
5
10
15
20
25
30
35
LOAD CURRENT (mA)
TEMPERATURE (°C)
INPUT VOLTAGE (V)
3971A G04
3971A G05
3971A G06
LT3971A Feedback Voltage
LT3971A-5 Output Voltage
Maximum Load Current
1.205
1.200
1.195
1.190
1.185
1.180
1.175
5.06
5.04
5.02
5.00
4.98
4.96
4.94
3.0
2.5
2.0
1.5
1.0
0.5
0
V
= 3.3V
OUT
TYPICAL
MINIMUM
–55 –25
5
35
65
95 125 155
–55 –25
5
35
65
95 125 155
5
10
15
20
25
30
35
40
TEMPERATURE (°C)
TEMPERATURE (°C)
INPUT VOLTAGE (V)
3971A G07
3971A G08
3971A G09
3971af
4
LT3971A/LT3971A-5
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
Switching Frequency
Maximum Load Current
Load Regulation
0.30
0.25
0.20
0.15
0.10
0.05
0
2.5
2.0
1.5
1.0
0.5
0
1000
950
900
850
800
750
700
650
600
V
= 5V
OUT
TYPICAL
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
–55 –25
5
35
65
95 125 155
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
TEMPERATURE (°C)
3971A G11
3971A G10
3971A G12
Switch VCESAT
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
600
500
400
300
200
100
0
DUTY CYCLE = 30%
0
20
40
60
80
100
–55 –25
5
35
65
95 125 155
0
250
500
750 1000 1250 1500
DUTY CYCLE (%)
TEMPERATURE (°C)
SWITCH CURRENT (mA)
3971A G13
3971A G14
3971A G15
Boost Pin Current
Frequency Foldback
LT3971A-5 Frequency Foldback
30
25
20
15
10
5
900
800
700
600
500
400
300
200
100
0
900
800
700
600
500
400
300
200
100
0
0
0
250
500
750 1000 1250 1500
0
0.2
0.4
0.6
0.8
1
1.2
0
20
40
(% OF REGULATION VOLTAGE)
3971A G18
60
80
100
SWITCH CURRENT (mA)
FB PIN VOLTAGE (V)
V
OUT
3971A G16
3971A G17
3971af
5
LT3971A/LT3971A-5
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
Minimum Switch On-Time/
Switch Off-Time
Minimum Input Voltage
Soft-Start
400
350
300
250
200
150
100
50
5.0
4.8
4.6
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
2.5
2.0
1.5
1.0
0.5
0
V
= 3.3V
OUT
MIN T
1A LOAD
OFF
TO START
MIN T
0.5A LOAD
OFF
TO RUN
MIN T
ON
0
–55 –25
5
35
65
95 125 155
0
200
400
600
800 1000 1200
0
0.25 0.5 0.75
1
1.25 1.5 1.75
2
TEMPERATURE (°C)
LOAD CURRENT (mA)
SS PIN VOLTAGE (V)
3971A G19
3971A G21
3971A G20
EN Threshold
Boost Diode Forward Voltage
Minimum Input Voltage
6.4
6.2
6.0
5.8
5.6
5.4
5.2
5.0
1.05
1.04
1.03
1.02
1.01
1.00
0.99
0.98
0.97
0.96
0.95
1.6
V
= 5V
OUT
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
TO START
RISING THRESHOLD
FALLING THRESHOLD
TO RUN
0
200
400
600
800 1000 1200
–55 –25
5
35
65
95 125 155
0
250
500
750 1000 1250 1500
LOAD CURRENT (mA)
TEMPERATURE (°C)
BOOST DIODE CURRENT (mA)
3971A G22
3971A G23
3971A G24
Transient Load Response,
Load Current Stepped from 25mA
(Burst Mode Operation) to 525mA
Transient Load Response,
Load Current Stepped from
0.5A to 1A
Power Good Threshold
95
94
93
92
91
90
89
88
87
86
85
V
OUT
V
OUT
100mV/
DIV
100mV/DIV
I
L
500mA/
DIV
I
L
500mA/DIV
3971A G26
3971A G27
–55 –25
5
35
65
95 125 155
10μs/DIV
10μs/DIV
TEMPERATURE (°C)
V
C
= 12V, V
OUT
= 3.3V
V
C
= 12V, V
OUT
= 3.3V
IN
OUT
IN
OUT
3971A G25
= 47μF
= 47μF
3971af
6
LT3971A/LT3971A-5
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.
Switching Waveforms;
Burst Mode Operation
Switching Waveforms; Full
Frequency Continuous Operation
3.3V Start-Up and Dropout
800kHz
3kΩ LOAD
V
V
SW
IN
V
SW
5V/DIV
5V/DIV
1V/DIV
I
L
I
L
500mA/DIV
500mA/DIV
V
OUT
V
V
OUT
OUT
20mV/DIV
20mV/DIV
3971A G28
3971A G29
3971A G30
5μs/DIV
= 3.3V
1μs/DIV
= 3.3V
0.5s/DIV
V
= 12V, V
V
= 12V, V
OUT
IN
OUT
IN
I
= 10mA
I
= 1A
LOAD
C
LOAD
C
= 22μF
= 22μF
OUT
OUT
3.3V Start-Up and Dropout
5V Start-Up and Dropout
5V Start-Up and Dropout
800kHz
6.7kΩ LOAD
800kHz
5kΩ LOAD
800kHz
10Ω LOAD
V
V
IN
IN
V
IN
1V/DIV
1V/DIV
1V/DIV
V
OUT
V
OUT
V
OUT
3971A G31
3971A G32
3971A G33
0.5s/DIV
0.5s/DIV
0.5s/DIV
Feedback Regulation Voltage
Minimum Input Voltage to Switch
1.6
1.2
0.8
0.4
0
5
4
3
2
1
2
2.5
3
3.5
4
4.5
5
–55 –25
5
35
65
95 125 155
INPUT VOLTAGE (V)
TEMPERATURE (°C)
3971A G34
3971A G35
3971af
7
LT3971A/LT3971A-5
PIN FUNCTIONS
BD (Pin 1): This pin connects to the anode of the boost
diode. The BD pin is normally connected to the output.
SS (Pin 7): A capacitor is tied between SS and ground to
slowly ramp up the peak current limit of the LT3971A on
start-up.Thesoft-startcapacitorisonlyactivelydischarged
when EN is low. The SS pin is released when the EN pin
goes high. Float this pin to disable soft-start. For applica-
tions with input voltages above 25V, add a 100k resistor
in series with the soft-start capacitor.
BOOST (Pin 2): This pin is used to provide a drive volt-
age, higher than the input voltage, to the internal bipolar
NPN power switch.
SW (Pin 3): The SW pin is the output of an internal power
switch. Connect this pin to the inductor, catch diode, and
boost capacitor.
RT (Pin 8): A resistor is tied between RT and ground to
set the switching frequency.
V (Pin 4): The V pin supplies current to the LT3971A’s
IN
IN
PG (Pin 9): 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
internal circuitry and to the internal power switch. This
pin must be locally bypassed.
EN (Pin 5): The part is in shutdown when this pin is low
and active when this pin is high. The hysteretic threshold
voltageis1.005Vgoingupand0.975Vgoingdown.TheEN
valid when the LT3971A is enabled and V is above 4.3V.
IN
SYNC (Pin 10): This is the external clock synchronization
input. Ground this pin for low ripple Burst Mode operation
at low output loads. Tie to a clock source for synchroni-
zation, which will include pulse-skipping at low output
loads. When in pulse-skipping mode, quiescent current
increases to 1.5mA.
threshold is only accurate when V is above 4.3V. If V is
IN
IN
lower than 4.2V, ground EN to place the part in shutdown.
Tie to V if shutdown feature is not used.
IN
FB (Pin 6, LT3971A Only): The LT3971A regulates the FB
pin to 1.19V. Connect the feedback resistor divider tap to
this pin. Also, connect a phase lead capacitor between FB
GND (Exposed Pad Pin 11): Ground. The exposed pad
must be soldered to PCB.
and V . Typically this capacitor is 10pF.
OUT
V
(Pin 6, LT3971A-5 Only): The LT3971A-5 regulates
OUT
OUT
the V
pin to 5V. This pin connects to the internal 10MΩ
feedback divider that programs the fixed output voltage.
3971af
8
LT3971A/LT3971A-5
BLOCK DIAGRAM
V
IN
C1
V
IN
–
+
INTERNAL 1.19V REF
BD
1V
+
–
SWITCH
LATCH
SLOPE COMP
Σ
SHDN
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
LT3971A-5
ONLY
R2
R1
C5
3971A BD
LT3971A-5: R1 = 7.62M, R2 = 2.38M
LT3971A
ONLY
OPERATION
The LT3971A 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 LT3971A 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 1.7ꢀA. In a typical application, 2.8ꢀA 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 LT3971A’s operating frequency
when the voltage at the FB pin is low. This frequency fold-
back 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 LT3971A 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 LT3971A
If the EN pin is low, the LT3971A 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
is enabled and V is above 4.3V.
IN
3971af
9
LT3971A/LT3971A-5
APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
leakage at room temperature. These two considerations
are reiterated in the FB Resistor Network and Catch Diode
Selection sections.
To enhance efficiency at light loads, the LT3971A operates
inlowrippleBurstMode, whichkeepstheoutputcapacitor
charged to the desired output voltage while minimizing It is important to note that another way to decrease the
the input quiescent current. In Burst Mode operation the pulse frequency is to increase the magnitude of each
LT3971A delivers single pulses of current to the output single current pulse. However, this increases the output
capacitorfollowedbysleepperiodswheretheoutputpower voltage ripple because each cycle delivers more power to
is supplied by the output capacitor. When in sleep mode the output capacitor. The magnitude of the current pulses
the LT3971A consumes 1.7ꢀA, but when it turns on all the was selected to ensure less than 15mV of output ripple in
circuitry to deliver a current pulse, the LT3971A consumes a typical application. See Figure 2.
1.5mA of input current in addition to the switch current.
Therefore, the total quiescent current will be greater than
1.7ꢀA when regulating.
V
SW
5V/DIV
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure 1) and the percentage
of time the LT3971A is in sleep mode increases, resulting
in much higher light load efficiency. By maximizing the
time between pulses, the converter quiescent current
gets closer to the 1.7ꢀA 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
feedback resistors and a low leakage Schottky catch diode
in applications utilizing the ultralow quiescent current
performance of the LT3971A. The feedback resistors
should preferably be on the order of MΩ and the Schottky
catch diode should have less than 1μA of typical reverse
I
L
500mA/DIV
V
OUT
20mV/DIV
3971A F02
5μs/DIV
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 22ꢀF output capacitor, the
output ripple is about 10mV, and with a 47ꢀF output ca-
pacitor the output ripple is about 5mV. The output voltage
ripple can continue to be decreased by increasing the
output capacitance.
1000
V
V
= 12V
IN
OUT
= 3.3V
800
600
400
200
0
At higher output loads (above 92mA for the front page
application) the LT3971A will be running at the frequency
programmed by the R resistor, and will be operating in
standardPWMmode.ThetransitionbetweenPWMandlow
ripple Burst Mode operation will exhibit slight frequency
jitter, but will not disturb the output voltage.
T
0
20
40
60
80
100
120
LOAD CURRENT (mA)
3971A F01
Figure 1. Switching Frequency in Burst Mode Operation
3971af
10
LT3971A/LT3971A-5
APPLICATIONS INFORMATION
To ensure proper Burst Mode operation, the SYNC pin
must be grounded. When synchronized with an external
clock, the LT3971A will pulse skip at light loads. The
quiescent 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.
connectors and cables, while keeping the VBUS voltage
withinspecificationatthedevice. ByusingtheLT3971A-5,
two external 1% resistors program the output to 5.15V
without degrading initial accuracy, which is determined
primarily by the LT3971A-5 output voltage specification.
The external resistor divider must be small compared
to the internal 10M to maintain output voltage accuracy.
The 0.5μA into the V
pin is process and temperature
OUT
dependent, so will degrade the accuracy of the output
voltage if it is not small compared to the total current in
the external resistor divider. Choose the resistor values
according to:
FB Resistor Network
The LT3971A output voltage is programmed with a resis-
tor divider between the output and the FB pin. Choose the
resistor values according to:
V
–5
OUT
R =
VOUT
1.19V
⎛
⎞
⎠
1
5
R1=R2
−1
⎜
⎝
⎟
+0.0005
R
2
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
R1 and R2 refer to the components in Figure 3 in kΩ.
1% resistors should be used to maintain output voltage
accuracy. It should also be noted that the smaller the
resistor values the more the input current will increase
during no load conditions.
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.
OUTPUT
LT3971A-5
R1
WhenusinglargeFBresistors,a10pFphaseleadcapacitor
V
OUT
should be connected from V
to FB.
OUT
R2
V
Pin
OUT
3971A F03
The LT3971A-5 contains an internal 10M feedback resis-
tor divider as well as an internal phase lead capacitor to
feedback the output voltage information to the internal
circuitry. The output will be regulated to 5V when con-
Figure 3. Resistor Divider Used to Increase the Output Voltage
Above 5V
nected directly to the V
pin.
OUT
The V
pin has an internal 11V clamp. Output voltage
OUT
transients above 11V can be tolerated as long as there is
enough series resistance to limit the current into the 11V
clamp to less than 2mA.
Anexternalresistordividercanbeaddedtoshifttheoutput
voltage higher than 5V. For example, a USB VBUS supply
programmed to 5.15V allows some voltage drop through
3971af
11
LT3971A/LT3971A-5
APPLICATIONS INFORMATION
Setting the Switching Frequency
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:
TheLT3971AusesaconstantfrequencyPWMarchitecture
that can be programmed to switch from 200kHz to 2MHz
by using a resistor tied from the RT pin to ground. A table
showing the necessary R value for a desired switching
T
frequency is in Table 1.
DCMIN = fSW ON(MIN)
t
Table 1. Switching Frequency vs RT Value
SWITCHING FREQUENCY (MHz)
DCMAX =1− fSW OFF(MIN)
t
R VALUE (kΩ)
T
where f is the switching frequency, the t
is the
ON(MIN)
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
255
118
SW
minimumswitchon-time,andthet
istheminimum
OFF(MIN)
71.5
49.9
35.7
28.0
22.1
17.4
14.0
11.0
switch off-time. These equations show that duty cycle
range increases when switching frequency is decreased.
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.
Input Voltage Range
Operating Frequency Tradeoffs
The minimum input voltage is determined by either the
LT3971A’s minimum operating voltage of 4.3V or by its
maximumdutycycle(seeequationinOperatingFrequency
Tradeoffs section). The minimum input voltage due to
duty cycle is:
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
highest acceptable switching frequency (f
) for a
SW(MAX)
1− fSW OFF(MIN)
t
given application can be calculated as follows:
VOUT + VD
tON(MIN)(VIN − VSW + VD)
where V
is the minimum input voltage, V
is
OUT
IN(MIN)
fSW(MAX)
=
the output voltage, V is the catch diode drop (~0.5V),
D
V
is the internal switch drop (~0.5V at max load), f
SW
SW
is
is the switching frequency (set by R ), and t
where V is the typical input voltage, V
is the output
OUT
T
OFF(MIN)
IN
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.
voltage, V is the catch diode drop (~0.5V), and V is
D
SW
theinternalswitchdrop(~0.5Vatmaxload).Thisequation
shows that slower switching frequency is necessary to
safely accommodate high V /V
ratio. Also, as shown
IN OUT
intheInputVoltageRangesection,lowerfrequencyallows
a lower dropout voltage. The input voltage range depends
on the switching frequency because the LT3971A switch
hasfiniteminimumonandofftimes. Theminimumswitch
on and off times are strong functions of temperature. Use
The maximum input voltage for LT3971A applications
depends on switching frequency, the Absolute Maximum
Ratings of the V and BOOST pins, and the operating
IN
mode. For a given application where the switching fre-
quency and the output voltage are already selected, the
3971af
12
LT3971A/LT3971A-5
APPLICATIONS INFORMATION
maximum input voltage (V
) that guarantees
IN(OP-MAX)
the saturation current should be above 3.5A. 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.
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)
Theinductorvaluemustbesufficienttosupplythedesired
maximum output current (I
), which is a function
OUT(MAX)
where t
is the minimum switch on-time. Note that
ON(MIN)
of the switch current limit (I ) and the ripple current.
LIM
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.
ΔIL
2
IOUT(MAX) =ILIM
–
The LT3971A limits its peak switch current in order to
protect itself and the system from overload faults. The
The circuit will tolerate inputs above the maximum op-
erating input voltage and up to the Absolute Maximum
LT3971A’sswitchcurrentlimit(I )istypically2.5Aatlow
LIM
Ratings of the V and BOOST pins, regardless of chosen
IN
duty cycles and decreases linearly to 1.75A at DC = 0.8.
switching frequency. However, during such transients
where V is higher than V
, the LT3971A will
IN(OP-MAX)
Table 2. Inductor Vendors
IN
enter pulse-skipping operation where some switching
pulses are skipped to maintain output regulation. The
output voltage ripple and inductor current ripple will be
higher than in typical operation. Do not overload when
VENDOR
Murata
TDK
URL
PART SERIES
TYPE
www.murata.com
LQH55D
Open
www.componenttdk.com SLF7045
SLF10145
Shielded
Shielded
V is greater than V
.
Toko
www.toko.com
D62CB
D63CB
D73C
Shielded
Shielded
Shielded
Open
IN
IN(OP-MAX)
Inductor Selection and Maximum Output Current
D75F
Coilcraft
Sumida
www.coilcraft.com
www.sumida.com
MSS7341
MSS1038
Shielded
Shielded
A good first choice for the inductor value is:
VOUT + VD
CR54
Open
L =
CDRH74
CDRH6D38
CR75
Shielded
Shielded
Open
fSW
where f is the switching frequency in MHz, V
is the
SW
OUT
output voltage, V is the catch diode drop (~0.5V) and L
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:
D
is the inductor value in ꢀH.
Theinductor’sRMScurrentratingmustbegreaterthanthe
maximumloadcurrentanditssaturationcurrentshouldbe
about 30% higher. For robust operation in fault conditions
(start-up or short-circuit) and high input voltage (>30V),
(1−DC) •(VOUT + VD)
ΔIL =
L • fSW
3971af
13
LT3971A/LT3971A-5
APPLICATIONS INFORMATION
Wheref istheswitchingfrequencyoftheLT3971A,DCis
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.
SW
the duty cycle and L is the value of the inductor. Therefore,
the maximum output current that the LT3971A 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
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 LT3971A and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 4.7ꢀF capacitor is capable of this task, but only if it is
placed close to the LT3971A (see the PCB Layout sec-
tion). A second precaution regarding the ceramic input
capacitor concerns the maximum input voltage rating of
the LT3971A. A ceramic input capacitor combined with
trace or cable inductance forms a high quality (under
damped) tank circuit. If the LT3971A circuit is plugged
into a live supply, the input voltage can ring to twice its
nominal value, possibly exceeding the LT3971A’s voltage
rating. This situation is easily avoided (see the Hot Plug-
ging Safely section).
not allow sufficient maximum output current (I
)
OUT(MAX)
giventheswitchingfrequency,andmaximuminputvoltage
used in the desired application.
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
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
with the inductor, it filters the square wave generated
by the LT3971A to produce the DC output. In this role it
determines 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 LT3971A’s control loop. Ceramic capacitors
have very low equivalent series resistance (ESR) and pro-
vide 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 LT3971A will
be able to deliver the required output current. Note again
that these equations assume that the inductor current is
100
continuous. Discontinuous operation occurs when I
OUT
COUT
=
is less than ∆I /2.
VOUT SW
f
L
Input Capacitor
wheref isinMHz, andC
istherecommendedoutput
OUT
SW
capacitance in ꢀF. 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 LT3971A 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 4.7ꢀF to 10ꢀF ceramic capacitor
is adequate to bypass the LT3971A and will easily handle
the ripple current. Note that larger input capacitance is
required when a lower switching frequency is used (due
3971af
14
LT3971A/LT3971A-5
APPLICATIONS INFORMATION
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 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
R
I
V at 1A
V at 2A
20V 25°C
(μA)
AVE
F
F
PART NUMBER (V)
(A)
(mV)
(mV)
On Semiconductor
Table 3. Recommended Ceramic Capacitor Vendors
MBR0520L
MBR0540
MBRM120E
MBRM140
Diodes Inc.
B0530W
B0540W
B120
20
40
20
40
0.5
0.5
1
30
0.4
0.5
20
MANUFACTURER
AVX
WEBSITE
620
530
550
www.avxcorp.com
www.murata.com
www.t-yuden.com
www.vishay.com
www.tdk.com
595
Murata
1
Taiyo Yuden
Vishay Siliconix
TDK
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
Catch Diode Selection
B130
1
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:
B140
1
B150
1
B220
2
500
500
V – VOUT
IN
B230
2
ID(AVG) =IOUT
V
B140HB
DFLS240L
DFLS140
DFLS160
DFLS2100
B240
1
IN
2
500
4
where I
is the output load current. The only reason to
OUT
1.1
1
510
500
770
1
consideradiodewithalargercurrentratingthannecessary
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.
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
CMSH2 - 60M
3971af
15
LT3971A/LT3971A-5
APPLICATIONS INFORMATION
Foroutputsof3Vandabove,thestandardcircuit(Figure 4a)
is best. For outputs between 2.8V and 3V, use a 1ꢀF 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 1ꢀF boost
capacitor (Figure 4b). For output voltages below 2.5V,
the boost diode can be tied to the input (Figure 4c), or to
another external supply greater than 2.8V. However, the
circuitinFigure4aismoreefficientbecausetheBOOSTpin
currentcomesfromalowervoltagesource. Youmustalso
be sure that the maximum voltage ratings of the BOOST
and BD pins are not exceeded.
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 LT3971A 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.
Ceramic Capacitors
BD
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3971A due to their piezoelectric
nature. When in Burst Mode operation, the LT3971A’s
switching frequency depends on the load current, and
at very light loads the LT3971A can excite the ceramic
capacitor at audio frequencies, generating audible noise.
Since the LT3971A operates at a lower current limit during
Burst Mode operation, 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.
V
V
IN
BOOST
LT3971A
IN
C3
SW
V
OUT
4.7μF
GND
(4a) For V
> 2.8V
OUT
D2
BD
V
IN
V
BOOST
IN
LT3971A
C3
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT3971A. As
previouslymentioned,aceramicinputcapacitorcombined
with trace or cable inductance forms a high quality (under
damped) tank circuit. If the LT3971A circuit is plugged
into a live supply, the input voltage can ring to twice its
nominal value, possibly exceeding the LT3971A’s rating.
This situation is easily avoided (see the Hot Plugging
Safely section).
SW
V
OUT
4.7μF
GND
(4b) For 2.5V < V
< 2.8V
OUT
BD
V
IN
V
BOOST
LT3971A
IN
C3
BOOST and BD Pin Considerations
SW
V
4.7μF
OUT
GND
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 0.47ꢀF capacitor will work well. Figure 4 shows three
ways to arrange the boost circuit. The BOOST pin must
be more than 2.3V above the SW pin for best efficiency.
3971A FO4
(4c) For V
< 2.5V; V
= 27V
OUT
IN(MAX)
Figure 4. Three Circuits for Generating the Boost Voltage
3971af
16
LT3971A/LT3971A-5
APPLICATIONS INFORMATION
TheminimumoperatingvoltageofanLT3971Aapplication
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 5 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
For lower start-up voltage, the boost diode can be tied to
IN
the absolute maximum rating of the BOOST pin.
V ; however, this restricts the input range to one-half of
Atlightloads,theinductorcurrentbecomesdiscontinuous
and this reduces the minimum input voltage to approxi-
mately 400mV above V . At higher load currents, the
OUT
inductorcurrentiscontinuousandthedutycycleislimited
by the maximum duty cycle of the LT3971A, requiring a
higher input voltage to maintain regulation.
Enable Pin
The LT3971A 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
pin can be tied to V if the shutdown feature is not used.
IN
theworst-casesituationwhereV isrampingveryslowly.
Adding a resistor divider from V to EN programs the
IN
IN
LT3971A to regulate the output only when V is above a
IN
5.0
4.8
4.6
desired voltage (see Figure 6). Typically, this threshold,
V
IN(EN)
, is used in situations where the input supply is cur-
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
4.4
TO START
4.2
4.0
TO RUN
3.8
3.6
3.4
3.2
3.0
V
A
= 3.3V
OUT
threshold prevents
source voltage conditions. The V
IN(EN)
T
= 25°C
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:
L = 4.7μH
f = 800kHz
10
100
1000
LOAD CURRENT (mA)
6.4
6.2
6.0
5.8
5.6
5.4
5.2
5.0
R3
R4
V
=
+1
IN(EN)
TO START
TO RUN
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)
V
A
= 5V
OUT
T
= 25°C
LT3971A
V
IN
L = 4.7μH
f = 800kHz
R3
R4
1V
+
–
10
100
1000
SHDN
EN
LOAD CURRENT (mA)
3971A F05
3971A F06
Figure 5. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
Figure 6. Programmed Enable Threshold
3971af
17
LT3971A/LT3971A-5
APPLICATIONS INFORMATION
Be aware that when the input voltage is below 4.3V, the
Soft-Start
input current may rise to several hundred ꢀA. And the part
The SS pin can be used to soft-start the LT3971A by
throttling the maximum input current during start-up. An
internal 1ꢀA current source charges an external capaci-
tor generating a voltage ramp on the SS pin. The SS pin
may be able to switch at cold or for V
) thresholds less
IN(EN
than 7V. Figure 7 shows the magnitude of the increased
input current in a typical application with different pro-
grammed V
.
IN(EN)
clamps the internal V node, which slowly ramps up the
C
When operating in Burst Mode for light load currents, the
current through the V resistor network can easily be
current limit. Maximum 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. Forapplicationswithinputvoltagesabove25V,
a 100k resistor in series with the soft-start capacitor is
recommended. Figure 8 shows start-up waveforms for a
typical application with a 10nF capacitor on SS for a 3.3ꢁ
load when the EN pin is pulsed high for 13ms.
IN(EN)
greaterthanthesupplycurrentconsumedbytheLT3971A.
Therefore,theV resistorsshouldbelargetominimize
IN(EN)
their effect on efficiency at low loads.
12V V
Input Current
IN(EN)
500
400
300
200
100
0
TheexternalSScapacitorisonlyactivelydischargedwhen
EN is low. With EN low, the external SS cap is discharged
through approximately 150ꢁ. The EN pin needs to be low
long enough for the external cap to completely discharge
through the 150ꢁ pull-down prior to start-up.
V
SS
1V/DIV
0
1
2
3
4
5
6
7
8
9 10 11 12
INPUT VOLTAGE (V)
V
= 12V
V
IN(EN)
OUT
R3 = 11M
R4 = 1M
2V/DIV
6V V
Input Current
I
IN(EN)
L
0.5A/DIV
500
400
300
200
100
0
3971A F08
2ms/DIV
Figure 8. Soft-Start Waveforms for Front-Page Application
with 10nF Capacitor on SS. EN is Pulsed High for About
13ms with a 3.3ꢀ Load Resistor
Synchronization
To select low ripple Burst Mode operation, tie the SYNC
pin below 0.6V (this can be ground or a logic low output).
0
1
2
3
4
5
6
Synchronizing the LT3971A 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).
INPUT VOLTAGE (V)
3971A F07
V
= 6V
IN(EN)
R3 = 5M
R4 = 1M
Figure 7. Input Current vs Input Voltage
for a Programmed VIN(EN) of 6V and 12V
3971af
18
LT3971A/LT3971A-5
APPLICATIONS INFORMATION
The LT3971A will not enter Burst Mode operation at low
output loads while synchronized to an external clock, but
instead will pulse skip to maintain regulation.
D4
MBRS140
V
V
BOOST
SW
IN
IN
EN
V
OUT
TheLT3971Amaybesynchronizedovera250kHzto2MHz
LT3971A
range.TheR resistorshouldbechosentosettheLT3971A
T
BD
FB
switchingfrequency20%belowthelowestsynchronization
GND
+
input. For example, if the synchronization signal will be
BACKUP
250kHz and higher, the R should be selected for 200kHz.
T
ToassurereliableandsafeoperationtheLT3971Awillonly
synchronize when the output voltage is near regulation as
indicatedbythePGflag.Itisthereforenecessarytochoose
alargeenoughinductorvaluetosupplytherequiredoutput
3971A F09
Figure 9. 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 LT3971A Runs Only When the Input Is Present
current at the frequency set by the R resistor (see the
T
InductorSelectionsection).Theslopecompensationisset
by the R value, while the minimum slope compensation
T
PCB Layout
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
ForproperoperationandminimumEMI,caremustbetaken
during printed circuit board layout. Figure 10 shows the
recommended component placement with trace, ground
plane and via locations. Note that large, switched currents
flow in the LT3971A’s V and SW pins, the catch diode
(D1), and the input capacitor (C1). The loop formed by
frequency set by R , than the slope compensation will be
IN
T
sufficient for all synchronization frequencies.
Shorted and Reversed Input Protection
L1
C2
If the inductor is chosen so that it won’t saturate exces-
sively, a LT3971A 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
LT3971A is absent. This may occur in battery charging
applications or in battery backup systems where a battery
or some other supply is diode ORed with the LT3971A’s
V
OUT
GND
R
PG
R
T
output. If the V pin is allowed to float and the EN pin
C3
IN
C4
is held high (either by a logic signal or because it is tied
to V ), then the LT3971A’s internal circuitry will pull its
IN
R2
C5
R1
quiescent current through its SW pin. This is fine if your
system can tolerate a few ꢀA in this state. If you ground
the EN pin, the SW pin current will drop to essentially
C1
D1
GND
zero. However, if the V pin is grounded while the output
IN
is held high, regardless of EN, parasitic diodes inside the
3971A F10
LT3971A can pull current from the output through the SW
VIAS TO V
VIAS TO LOCAL GROUND PLANE
VIAS TO RUN/SS
VIAS TO PG
IN
pin and the V pin. Figure 9 shows a circuit that will run
VIAS TO V
VIAS TO SYNC
OUT
OUTLINE OF LOCAL
GROUND PLANE
IN
only when the input voltage is present and that protects
against a shorted or reversed input.
Figure 10. A Good PCB Layout Ensures Proper, Low EMI Operation
3971af
19
LT3971A/LT3971A-5
APPLICATIONS INFORMATION
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.
mustbesolderedtoagroundplane.Thisgroundshouldbe
tied to large copper layers below with thermal vias; these
layers will spread heat dissipated by the LT3971A. Placing
additional vias can reduce thermal resistance further. The
maximum load current should be derated as the ambient
temperature approaches the maximum junction rating.
Finally, keep the FB and R nodes small so that the ground
T
PowerdissipationwithintheLT3971Acanbeestimatedby
calculatingthetotalpowerlossfromanefficiencymeasure-
ment and subtracting the catch diode loss and inductor
loss. The die temperature is calculated by multiplying the
LT3971Apowerdissipationbythethermalresistancefrom
junction to ambient.
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 LT3971A to additional ground planes within the circuit
board and on the bottom side.
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.
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3971A circuits. However, these
capacitors can cause problems if the LT3971A is plugged
intoalivesupply.Thelowlossceramiccapacitor,combined
with stray inductance in series with the power source,
forms an under damped tank circuit, and the voltage at
the V pin of the LT3971A can ring to twice the nominal
IN
Other Linear Technology Publications
inputvoltage,possiblyexceedingtheLT3971A’sratingand
damaging the part. If the input supply is poorly controlled
or the user will be plugging the LT3971A into an energized
supply, the input network should be designed to prevent
this overshoot. See Linear Technology Application Note
88 for a complete discussion.
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.
High Temperature Considerations
For higher ambient temperatures, care should be taken in
the layout of the PCB to ensure good heat sinking of the
LT3971A. The Exposed Pad on the bottom of the package
3971af
20
LT3971A/LT3971A-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
LT3971A
RT
BD
FB
V
1M
22μF
49.9k
OUT
5V
SYNC GND
1.3A
309k
f = 800kHz
3971A TA02
3.3V Step Down Converter
No Load Supply Current
V
IN
4.0
4.5V TO 38V
V
IN
3.5
3.0
2.5
2.0
1.5
1.0
EN
BOOST
SW
OFF ON
0.47μF
4.7μH
V
3.3V
1.3A
PG
SS
OUT
LT3971A
4.7μF
RT
BD
FB
10pF
1.78M
49.9k
f = 800kHz
SYNC GND
22μF
1M
0
10
20
30
40
3971A TA11
INPUT VOLTAGE (V)
3971A TA11b
5V Step-Down Converter
2.5V Step-Down Converter
V
V
IN
IN
4.3V TO 38V
7V TO 38V
V
V
IN
IN
EN
BOOST
SW
EN
BOOST
SW
OFF ON
OFF ON
4.7μF
0.47μF
1μF
PG
SS
PG
SS
4.7μH
4.7μH
4.7μF
LT3971A-5
LT3971A
RT
RT
BD
BD
FB
10pF
V
V
2.5V
1.3A
49.9k
1M
47μF
OUT
118k
OUT
5V
V
OUT
SYNC GND
SYNC GND
1.3A
22μF
909k
f = 800kHz
f = 400kHz
3971A TA03
3971A TA04
3971af
21
LT3971A/LT3971A-5
TYPICAL APPLICATIONS
1.8V Step-Down Converter
12V Step-Down Converter
V
V
IN
15V TO 38V
IN
4.3V TO 27V
V
V
IN
BD
BOOST
IN
EN
BOOST
SW
EN
OFF ON
10μF
OFF ON
4.7μF
0.47μF
10pF
0.47μF
10pF
PG
SS
PG
SS
4.7μH
SW
10μH
1M
LT3971A
LT3971A
RT
RT
BD
FB
V
V
1.8V
1.3A
118k
511k
OUT
OUT
49.9k
f = 800kHz
12V
FB
SYNC GND
SYNC GND
1.3A
110k
10μF
1M
100μF
f = 400kHz
3971A TA06
3971A TA05
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
LT3971A
100k
1nF
PG
BD
PGOOD
1M
10pF
1M
V
3.3V
1.3A
OUT
49.9k
FB
SYNC GND
562k
22μF
f = 800kHz
3971A TA07
5V, 2MHz Step-Down Converter with Soft-Start
V
IN
10V TO 25V
V
IN
EN
BOOST
SW
OFF ON
0.47μF
10pF
PG
SS
2.2μH
LT3971A
2.2μF
RT
BD
FB
1nF
11k
V
1M
22μF
OUT
5V
SYNC GND
1.3A
309k
f = 2MHz
3971A TA08
3971af
22
LT3971A/LT3971A-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 t 0.102
(.074 t .004)
0.889 t 0.127
(.035 t .005)
1
0.29
REF
1.68
(.066)
0.05 REF
5.23
(.206)
MIN
1.68 t 0.102 3.20 – 3.45
(.066 t .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 t 0.038
(.0120 t .0015)
TYP
3.00 t 0.102
(.118 t .004)
(NOTE 3)
0.497 t 0.076
(.0196 t .003)
10 9
8
7 6
RECOMMENDED SOLDER PAD LAYOUT
REF
3.00 t 0.102
(.118 t .004)
(NOTE 4)
4.90 t 0.152
(.193 t .006)
DETAIL “A”
0.254
(.010)
0s – 6s TYP
1
2
3
4 5
GAUGE PLANE
0.53 t 0.152
(.021 t .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 t 0.0508
(.004 t .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.
3971af
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.
23
LT3971A/LT3971A-5
TYPICAL APPLICATION
4V Step-Down Converter with a High Impedance Input Source
+
–
11M
1M
V
IN
24V
EN
BOOST
SW
+
C
BULK
0.47μF
10pF
* AVERAGE OUTPUT POWER CANNOT
EXCEED THAT WHICH CAN BE PROVIDED
BY HIGH IMPEDANCE SOURCE.
NAMELY,
PG
SS
4.7μH
100μF
LT3971A
4.7μF
2
RT
V
P
ꢀꢁꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀtꢀη
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
1.3A*
SYNC GND
412k
100μF
f = 800kHz
3971A TA09a
Sourcing a Maximum Load Pulse
Start-Up from High Impedance Input Source
V
OUT
V
IN
200mV/DIV
1V/DIV
V
IN
5V/DIV
V
OUT
2V/DIV
I
I
L
L
1A/DIV
500mA/DIV
3971A TA09b
3971A TA09c
500μs/DIV
2ms/DIV
RELATED PARTS
PART NUMBER DESCRIPTION
COMMENTS
V : 4.2V to 40V, V
LT3970
LT3971
LT3990
LT3991
LT3682
LT3689
40V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC
= 1.21V, I = 2.5μA,
Q
IN
OUT(MIN)
OUT(MIN)
Converter with I = 2.5μA
I
<1μA, 3mm × 2mm DFN-10 and MSOP-10 Packages
Q
SD
38V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down DC/DC
V : 4.3V to 38V, V
= 1.2V, I = 2.8μA, I < 1μA,
Q SD
IN
Converter with I = 2.8μA
3mm × 3mm DFN-10, MSOPE-10
Q
62V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC
V : 4.2V to 62V, V = 1.21V, I = 2.5μA,
IN
SD
OUT(MIN)
Q
Converter with I = 2.5μA
I
<1μA, 3mm × 2mm DFN-10 and MSOP-10 Packages
Q
55V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down DC/DC
V : 4.3V to 38V, V
SD
= 1.2V, I = 2.8μA,
Q
IN
OUT(MIN)
Converter with I = 2.8μA
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
3971af
LT 0212 • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
24
●
●
© LINEAR TECHNOLOGY CORPORATION 2012
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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