LT3971EMSETRPBF [Linear]
38V, 1.2A, 2MHz Step-Down Regulator with 2.8 Quiescent Current; 38V , 1.2A , 2MHz降压型稳压器具有2.8静态电流型号: | LT3971EMSETRPBF |
厂家: | Linear |
描述: | 38V, 1.2A, 2MHz Step-Down Regulator with 2.8 Quiescent Current |
文件: | 总24页 (文件大小:306K) |
中文: | 中文翻译 | 下载: | 下载PDF数据表文档文件 |
LT3971
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
Low Ripple Burst Mode® Operation:
n
Output Ripple < 15mV
P-P
n
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
Saturating Switch Design: 0.33ꢀ On-Resistance
Output Voltage: 1.19V to 30V
Small Thermally Enhanced 10-Pin MSOP Package
and (3mm × 3mm) DFN Packages
when V
reaches 91% of the programmed output volt-
OUT
age. The LT3971 is available in small 10-pin MSOP and
3mm × 3mm DFN packages with exposed pads for low
thermal resistance.
APPLICATIONS
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.
n
Automotive Battery Regulation
n
Power for Portable Products
n
Industrial Supplies
TYPICAL APPLICATION
3.3V Step Down Converter
No Load Supply Current
V
IN
4.0
4.5V TO 38V
V
IN
3.5
EN
BOOST
SW
OFF ON
0.47μF
4.7μH
10pF
V
PG
SS
OUT
3.0
3.3V
1.2A
2.5
LT3971
4.7μF
RT
BD
FB
2.0
1.5
1.78M
49.9k
SYNC GND
1M
22μF
1.0
0
10
20
30
40
3480 TA01
INPUT VOLTAGE (V)
3971 TA01b
3971f
1
LT3971
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, RT, SYNC, SS Voltage...........................................6V
PG, BD Voltage .........................................................30V
Boost Diode Current....................................................1A
(MSE Only) .......................................................300°C
PIN CONFIGURATION
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
FB
11
GND
11
GND
V
IN
EN
V
IN
EN
MSE PACKAGE
10-LEAD PLASTIC MSOP
= 45°C, θ = 10°C/W
JC
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
DD PACKAGE
θ
JA
10-LEAD (3mm s 3mm) PLASTIC DFN
= 45°C, θ = 10°C/W
JC
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
θ
JA
ORDER INFORMATION
LEAD FREE FINISH
LT3971EDD#PBF
LT3971IDD#PBF
LT3971EMSE#PBF
LT3971IMSE#PBF
TAPE AND REEL
PART MARKING*
LFJF
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3971EDD#TRPBF
LT3971IDD#TRPBF
LT3971EMSE#TRPBF
LT3971IMSE#TRPBF
–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
LFJF
LTFJG
LTFJG
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/
3971f
2
LT3971
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
4
4.3
V
V
V
V
Low
0.7
1.7
1.2
2.7
4.5
ꢁA
ꢁA
ꢁA
IN
EN
EN
EN
High, V
High, V
Low
Low
SYNC
SYNC
l
l
FB Pin Current
V
= 1.19V
0.1
12
nA
FB
Feedback Voltage
1.175
1.165
1.19
1.19
1.205
1.215
V
V
l
FB Voltage Line Regulation
Switching Frequency
4.3V < V < 40V
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
ꢁA
mV
ꢁA
V
CESAT
SW
SH
Switch Leakage Current
Boost Schottky Forward Voltage
Boost Schottky Reverse Leakage
Minimum Boost Voltage (Note 3)
BOOST Pin Current
1
= 100mA
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
60
1.01
30
1.07
EN Voltage Hysteresis
mV
nA
mV
mV
μA
ꢁA
V
EN Pin Current
0.2
100
20
20
PG Threshold Offset from V
PG Hysteresis
V
Rising
140
FB
FB
PG Leakage
V
V
= 3V
0.02
570
0.8
0.1
1
1
PG
PG
l
PG Sink Current
SYNC Threshold
SYNC Pin Current
SS Source Current
= 0.4V
300
0.6
1.0
1.6
nA
ꢁA
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 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
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
guarantee full saturation of the switch.
3971f
3
LT3971
TA = 25°C, unless otherwise noted.
Efficiency, VOUT = 5V
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency, VOUT = 5V
Efficiency, VOUT = 3.3V
100
90
80
70
60
50
40
30
20
100
100
90
80
70
60
50
40
30
20
FRONT PAGE APPLICATION
OUT
R1 = 1M
90
80
70
60
50
40
30
20
10
0
V
= 5V
V
= 12V
IN
V
= 12V
IN
R2 = 309k
V
= 12V
IN
V
= 36V
IN
V
= 24V
V
= 36V
V
= 24V
IN
IN
IN
V
= 24V
V
= 36V
IN
IN
FRONT PAGE APPLICATION
V
= 5V
OUT
R1 = 1M
FRONT PAGE APPLICATION
0.2 0.4 0.6
LOAD CURRENT (A)
R2 = 309k
0
0.2
0.4
0.6
0.8
1
1.2
0.01
0.1
1
10
100
1000
0
0.8
1
1.2
LOAD CURRENT (A)
LOAD CURRENT (mA)
3971 G01
3971 G03
3971 G02
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
FRONT PAGE APPLICATION
FRONT PAGE APPLICATION
= 3.3V
V
OUT
V
= 12V
IN
V
= 24V
IN
V
= 36V
IN
0.01
0.1
1
10
100
1000
–55 –25
5
35
65
95 125 155
0
10
20
INPUT VOLTAGE (V)
30
40
LOAD CURRENT (mA)
TEMPERATURE (°C)
3971 G04
3971 G05
3971 G06
Feedback Voltage
Maximum Load Current
Maximum Load Current
1.205
1.200
1.195
1.190
1.185
1.180
1.175
3.0
2.5
2.0
1.5
1.0
0.5
0
2.5
2.0
1.5
1.0
0.5
0
FRONT PAGE APPLICATION
FRONT PAGE APPLICATION
V
OUT
= 3.3V
V
= 5V
OUT
TYPICAL
TYPICAL
MINIMUM
MINIMUM
–55 –25
5
35
65
95 125 155
5
10
15
20
25
30
35
40
5
10
15
20
25
30
35
40
TEMPERATURE (°C)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
3971 G07
3971 G08
3971 G09
3971f
4
LT3971
TA = 25°C, unless otherwise noted.
TYPICAL PERFORMANCE CHARACTERISTICS
Load Regulation
Switching Frequency
Switch Current Limit
0.30
0.25
0.20
0.15
0.10
0.05
0
3.0
1000
950
900
850
800
750
700
650
600
2.5
2.0
1.5
1.0
0.5
0
–0.05
–0.10
–0.15
–0.20
–0.25
–0.30
FRONT PAGE APPLICATION
REFERENCED FROM V
AT 0.5A LOAD
OUT
0
200
400
600
800 1000 1200
0
20
40
60
80
100
–55 –25
5
35
65
95 125 155
LOAD CURRENT (mA)
DUTY CYCLE (%)
TEMPERATURE (°C)
3971 G10
3971 G12
3971 G11
Boost Pin Current
Switch Current Limit
Switch VCESAT
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
30
25
20
15
10
5
DUTY CYCLE = 30%
0
–55 –25
5
35
65
95 125 155
0
250
500
750 1000 1250 1500
0
250
500
750 1000 1250 1500
TEMPERATURE (°C)
SWITCH CURRENT (mA)
SWITCH CURRENT (mA)
3971 G13
3971 G14
3971 G15
Minimum Switch On-Time/
Switch Off-Time
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
0
0.25 0.5 0.75
1
1.25 1.5 1.75
2
0
0.2
0.4
0.6
0.8
1
1.2
–55 –25
5
35
65
95 125 155
SS PIN VOLTAGE (V)
FB PIN VOLTAGE (V)
TEMPERATURE (°C)
3971 G18
3971 G16
3971 G17
3971f
5
LT3971
TA = 25°C, unless otherwise noted.
TYPICAL PERFORMANCE CHARACTERISTICS
Minimum Input Voltage
Minimum Input Voltage
EN Threshold
5.0
4.8
4.6
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
6.4
6.2
6.0
5.8
5.6
5.4
5.2
5.0
1.05
FRONT PAGE APPLICATION
FRONT PAGE APPLICATION
1.04
1.03
1.02
V
= 3.3V
V
= 5V
OUT
OUT
TO START
RISING THRESHOLD
1.01
TO START
1.00
0.99
TO RUN
FALLING THRESHOLD
0.98
0.97
0.96
0.95
TO RUN
0
200
400
600
800 1000 1200
0
200
400
600
800 1000 1200
–55 –25
5
35
65
95 125 155
LOAD CURRENT (mA)
LOAD CURRENT (mA)
TEMPERATURE (°C)
3971 G19
3971 G20
3971 G21
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)
FRONT PAGE APPLICATION
TEMPERATURE (°C)
3971 G22
3971 G23
V
C
= 12V, V
= 3.3V
IN
OUT
OUT
= 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 G25
3971 G26
1μs/DIV
10μs/DIV
5μs/DIV
FRONT PAGE APPLICATION
FRONT PAGE APPLICATION
FRONT PAGE APPLICATION
V
I
= 12V, V
= 1A
= 3.3V
V
C
= 12V, V
= 3.3V
V
I
= 12V, V
= 3.3V
IN
OUT
IN
OUT
OUT
= 47μF
IN
OUT
= 10mA
LOAD
LOAD
C
OUT
= 22μF
C
OUT
= 22μF
3971f
6
LT3971
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 LT3971 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 LT3971’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 valid
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
when the LT3971 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): The LT3971 regulates the FB pin to 1.19V.
Connect the feedback resistor divider tap to this pin. Also,
connect a phase lead capacitor between FB and V
Typically this capacitor is 10pF.
GND (Exposed Pad Pin 11): Ground. The exposed pad
must be soldered to PCB.
.
OUT
3971f
7
LT3971
BLOCK DIAGRAM
V
IN
V
IN
–
+
C1
INTERNAL 1.19V REF
SHDN
BD
1V
+
–
SWITCH
LATCH
SLOPE COMP
3
BOOST
EN
RT
R
C3
L1
OSCILLATOR
200kHz TO 2MHz
Q
S
R
T
V
OUT
SW
Burst Mode
DETECT
SYNC
PG
D1
C2
ERROR AMP
V
CLAMP
C
V
C
+
–
+
–
1.09V
1μA
SS
R3
C4
SHDN
GND
FB
R2
R1
3991 BD
C5
3971f
8
LT3971
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
currentto1.7ꢁA.Inatypicalapplication,2.8ꢁAwillbecon-
sumed 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 4.2V.
IN
APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
1000
FRONT PAGE APPLICATION
V
V
= 12V
IN
OUT
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 1.7ꢁA, 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
1.7ꢁA when regulating.
= 3.3V
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
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
gets closer to the 1.7ꢁA 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
3971f
9
LT3971
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.
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:
VOUT
1.19V
⎛
⎞
⎠
R1=R2
−1
⎟
⎜
⎝
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
V
SW
5V/DIV
I
L
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.
500mA/DIV
V
OUT
20mV/DIV
3971 F02
5μs/DIV
FRONT PAGE APPLICATION
WhenusinglargeFBresistors,a10pFphaseleadcapacitor
V
V
= 12V
IN
OUT
= 3.3V
= 10mA
should be connected from V
to FB.
OUT
I
LOAD
Figure 2. Burst Mode Operation
Setting the Switching Frequency
While in Burst Mode operation, the burst frequency and
the charge delivered with each pulse will not change with
outputcapacitance.Therefore,theoutputvoltageripplewill
be inversely proportional to the output capacitance. In a
typicalapplicationwitha22ꢁFoutputcapacitor, theoutput
ripple is about 10mV, and with a 47ꢁF output capacitor
the output ripple is about 5mV. The output voltage ripple
can continue to be decreased by increasing the output
capacitance.
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
showing the necessary R value for a desired switching
T
frequency is in Table 1.
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
71.5
49.9
35.7
28.0
22.1
17.4
14.0
11.0
At higher output loads (above 92mA for the front page
application) the LT3971 will be running at the frequency
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.
3971f
10
LT3971
APPLICATIONS INFORMATION
Operating Frequency Tradeoffs
Input Voltage Range
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
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:
VOUT + VD
highest acceptable switching frequency (f
) for a
VIN(MIN)
=
− VD+ VSW
SW(MAX)
1− fSW OFF(MIN)
t
given application can be calculated as follows:
VOUT + VD
ON(MIN)(VIN − VSW + VD)
where V
is the minimum input voltage, V
is
OUT
IN(MIN)
fSW(MAX)
=
t
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
where V is the typical input voltage, V
is the output
OUT
is the switching frequency (set by R ), and t
IN
T
OFF(MIN)
voltage, V is the catch diode drop (~0.5V), and V is
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.
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
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:
The maximum input voltage for LT3971 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
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)
DCMIN = fSW ON(MIN)
t
DCMAX = 1− fSW OFF(MIN)
t
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.
where f is the switching frequency, the t
is the
ON(MIN)
SW
minimumswitchon-time,andthet
istheminimum
OFF(MIN)
switchoff-time.Theseequationsshowthatdutycyclerange
increases when switching frequency is decreased.
The circuit will tolerate inputs above the maximum op-
erating input voltage and up to the Absolute Maximum
A good choice of switching frequency should allow ad-
equate input voltage range (see Input Voltage Range sec-
tion) and keep the inductor and capacitor values small.
Ratings of the V and BOOST pins, regardless of chosen
IN
switching frequency. However, during such transients
3971f
11
LT3971
APPLICATIONS INFORMATION
Table 2. Inductor Vendors
where V is higher than V
, the LT3971 will enter
IN(OP-MAX)
IN
VENDOR
Murata
TDK
URL
PART SERIES
TYPE
pulse-skippingoperationwheresomeswitchingpulsesare
skipped to maintain output regulation. The output voltage
ripple and inductor current ripple will be higher than in
www.murata.com
LQH55D
Open
www.componenttdk.com SLF7045
SLF10145
Shielded
Shielded
typical operation. Do not overload when V is greater
IN
Toko
www.toko.com
D62CB
D63CB
D73C
Shielded
Shielded
Shielded
Open
than V
.
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
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:
output voltage, V is the catch diode drop (~0.5V) and L
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),
the saturation current should be above 2.8A. 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.
(1−DC)•(VOUT + VD)
ΔIL =
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
Theinductorvaluemustbesufficienttosupplythedesired
not allow sufficient maximum output current (I
)
OUT(MAX)
maximum output current (I
), which is a function
OUT(MAX)
giventheswitchingfrequency,andmaximuminputvoltage
of the switch current limit (I ) and the ripple current.
LIM
used in the desired application.
ΔIL
2
IOUT(MAX) =ILIM
–
The optimum inductor for a given application may differ
fromtheoneindicatedbythissimpledesignguide.Alarger
valueinductorprovidesahighermaximumloadcurrentand
reducestheoutputvoltageripple.Ifyourloadislowerthan
the maximum load current, than you can relax the value of
the inductor and operate with higher ripple current. This
allowsyoutouseaphysicallysmallerinductor, oronewith
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,lowinductancemayresultindiscontinuousmode
operation, which further reduces maximum load current.
TheLT3971limitsitspeakswitchcurrentinordertoprotect
itself and the system from overload faults. The LT3971’s
switchcurrentlimit(I )isatleast2.4Aatlowdutycycles
LIM
and decreases linearly to 1.75A at DC = 0.8.
3971f
12
LT3971
APPLICATIONS INFORMATION
For details of maximum output current and discontinuous
operation, see Linear Technology’s Application Note 44.
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:
Finally, for duty cycles greater than 50% (V /V >0.5),
OUT IN
a minimum inductance is required to avoid sub-harmonic
oscillations. See Application Note 19.
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
VOUT SW
f
OUT
is less than ΔI /2.
L
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.
Input Capacitor
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 4.7ꢁF to 10ꢁF 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 4.7ꢁF 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 cir-
cuit. If the LT3971 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possibly
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
3971f
13
LT3971
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 0.47ꢁF 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
3971f
14
LT3971
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 1ꢁF 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 1ꢁF 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
V
IN
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
V
A
= 3.3V
OUT
V
IN
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
3971f
15
LT3971
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 ꢁA. 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
current through the V resistor network can easily be
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 pin
12V V
Input Current
IN(EN)
500
400
300
200
100
0
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
R4
3971 F06
V
= 6V
1V
+
–
IN(EN)
SHDN
R3 = 5M
R4 = 1M
EN
3971 F05
Figure 6. Input Current vs Input Voltage
for a Programmed VIN(EN) of 6V and 12V
Figure 5. Programmed Enable Threshold
3971f
16
LT3971
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.Aninternal1ꢁA
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 3.3ꢀ 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 150ꢀ. The EN pin needs to be low
long enough for the external cap to completely discharge
through the 150ꢀ 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 3.3ꢀ 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 ꢁA in this state. If you ground
the EN pin, the SW pin current will drop to essentially
ToselectlowrippleBurstModeoperation,tietheSYNCpin
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.
3971f
17
LT3971
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
3971f
18
LT3971
APPLICATIONS INFORMATION
loss. The die temperature is calculated by multiplying the
LT3971 power dissipation by the thermal resistance from
junction to ambient.
avoid excessive increase in light load supply current at
high temperatures.
Other Linear Technology Publications
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
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.
TYPICAL APPLICATIONS
5V Step-Down Converter
V
IN
7V TO 38V
V
IN
EN
BOOST
OFF ON
4.7μF
0.47μF
4.7μH
PG
SS
SW
LT3971
RT
BD
10pF
V
1M
22μF
49.9k
OUT
5V
FB
SYNC GND
1.2A
309k
f = 800kHz
3971 TA02
3.3V Step-Down Converter
V
IN
4.3V TO 38V
V
IN
EN
BOOST
SW
OFF ON
0.47μF
10pF
PG
SS
4.7μH
4.7μF
LT3971
RT
BD
FB
V
3.3V
1.2A
71.5k
1M
OUT
SYNC GND
562k
22μF
f = 600kHz
3971 TA03
3971f
19
LT3971
TYPICAL APPLICATIONS
1.8V Step-Down Converter
2.5V Step-Down Converter
V
V
IN
IN
4.3V TO 38V
4.3V TO 27V
V
V
BD
BOOST
IN
IN
EN
BOOST
SW
EN
OFF ON
4.7μF
OFF ON
4.7μF
1μF
0.47μF
10pF
PG
SS
PG
SS
4.7μH
4.7μH
SW
LT3971
LT3971
RT
RT
BD
FB
10pF
V
2.5V
1.2A
V
1.8V
1.2A
1M
47μF
118k
511k
118k
OUT
OUT
FB
SYNC GND
SYNC GND
909k
1M
100μF
f = 400kHz
f = 400kHz
3971 TA04
3971 TA05
12V Step-Down Converter
V
IN
15V TO 38V
V
IN
EN
BOOST
SW
OFF ON
10μF
0.47μF
10μH
PG
SS
LT3971
RT
BD
FB
10pF
V
1M
10μF
OUT
49.9k
f = 800kHz
12V
SYNC GND
1.2A
110k
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
3971f
20
LT3971
TYPICAL APPLICATIONS
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
4V Step-Down Converter with a High Impedance Input Source
+
–
11M
V
IN
24V
EN
BOOST
SW
+
C
BULK
0.47μF
10pF
1M
* AVERAGE OUTPUT POWER CANNOT
EXCEED THAT WHICH CAN BE PROVIDED
BY HIGH IMPEDANCE SOURCE.
NAMELY,
PG
SS
4.7μH
100μF
LT3971
4.7μF
2
RT
V
P
=
• H
OUT(MAX)
4R
BD
FB
1nF
WHERE V IS VOLTAGE OF SOURCE, R IS
INTERNAL SOURCE IMPEDANCE, AND N 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
V
IN
200mV/DIV
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
3971f
21
LT3971
PACKAGE DESCRIPTION
DD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1699)
R = 0.115
TYP
6
0.38 0.10
10
0.675 0.05
3.50 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
PACKAGE
OUTLINE
TOP MARK
(SEE NOTE 6)
(DD) DFN 1103
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).
CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
3971f
22
LT3971
PACKAGE DESCRIPTION
MSE Package
10-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1664 Rev C)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.06 p 0.102
(.081 p .004)
1.83 p 0.102
(.072 p .004)
2.794 p 0.102
(.110 p .004)
0.889 p 0.127
(.035 p .005)
1
0.29
REF
0.05 REF
5.23
(.206)
MIN
2.083 p 0.102 3.20 – 3.45
(.082 p .004) (.126 – .136)
DETAIL “B”
CORNER TAIL IS PART OF
THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
NO MEASUREMENT PURPOSE
DETAIL “B”
10
0.50
(.0197)
BSC
0.305 p 0.038
(.0120 p .0015)
TYP
3.00 p 0.102
(.118 p .004)
(NOTE 3)
0.497 p 0.076
(.0196 p .003)
10 9
8
7 6
RECOMMENDED SOLDER PAD LAYOUT
REF
3.00 p 0.102
(.118 p .004)
(NOTE 4)
4.90 p 0.152
(.193 p .006)
DETAIL “A”
0.254
(.010)
0o – 6o TYP
1
2
3
4 5
GAUGE PLANE
0.53 p 0.152
(.021 p .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 p 0.0508
(.004 p .002)
0.50
(.0197)
BSC
MSOP (MSE) 0908 REV C
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
3971f
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
LT3971
RELATED PARTS
PART
DESCRIPTION
COMMENTS
= 3.6V, V
LT3980
58V, 80V Transient Protection, 2A, 2.4MHz High Efficiency Micropower
Step-Down DC/DC Converter
V
= 58V, Transient to 80V, V
= 0.79V,
IN(MIN)
IN(MAX)
OUT(MIN)
I = 75μA, I <1μA, MSOP-16E 3mm × 4mm DFN-16 Package
Q
SD
LT3970
LT3695
LT3689
40V, 350mA High Efficiency Micropower Step-Down DC/DC Converter
V
= 4.2V, V
= 40V, V
= 1.21V, I = 2.2μA,
IN(MIN)
IN(MAX)
OUT(MIN) Q
with I = 2.5μA
I
<1μA, MSOP-10 3mm × 2mm DFN-10 Package
SD
Q
36V, 60V Transient Protection, 1A, 2.2MHz High Efficiency Micropower
Step-Down DC/DC Converter with 1A Fault Tolerance
V
Q
= 3.6V, V
= 36V, Transient to 60V, V
= 0.8V,
= 0.8V,
IN(MIN)
IN(MAX)
OUT(MIN)
I = 75μA, I <1μA, MSOP-16E Package
SD
36V, 60V Transient Protection, 800mA, 2.2MHz High Efficiency
Micropower Step-Down DC/DC Converter with POR Reset and
Watchdog Timer
V
Q
= 3.6V, V
= 36V, Transient to 60V, V
IN(MAX)
IN(MIN)
OUT(MIN)
I = 75μA, I <1μA, 3mm × 3mm QFN-16 Package
SD
LT3682
LT3480
LT3685
LT3481
LT3684
LT3508
LT3505
LT3500
LT3507
LT3437
36V, 60V
, 1A, 2.2MHz High Efficiency Micropower Step-Down
V
Q
= 3.6V, V
= 36V, V
= 0.8V,
MAX
IN(MIN)
IN(MAX)
OUT(MIN)
DC/DC Converter
I = 75μA, I <1μA, 3mm × 3mm DFN-12 Package
SD
36V with Transient Protection to 60V, 2A (I ), 2.4MHz, High
Efficiency Step-Down DC/DC Converter with Burst Mode Operation
V
Q
= 3.6V, V
= 38V, V
= 0.78V,
OUT
IN(MIN)
IN(MAX)
OUT(MIN)
I = 70μA, I <1μA, 3mm × 3mm DFN-10, MSOP-10E Package
SD
36V with Transient Protection to 60V, 2A (I ), 2.4MHz,
High Efficiency Step-Down DC/DC Converter
V
= 3.6V, V
= 38V, V
= 0.78V, I = 70μA,
OUT
IN(MIN)
IN(MAX)
OUT(MIN) Q
I
<1μA, 3mm × 3mm DFN-10, MSOP-10E Package
SD
34V with Transient Protection to 36V, 2A (I ), 2.8MHz, High
Efficiency Step-Down DC/DC Converter with Burst Mode Operation
V
= 3.6V, V
= 34V, V
= 1.26V, I = 50μA,
OUT
IN(MIN)
IN(MAX)
OUT(MIN) Q
I
<1μA 3mm × 3mm DFN-10, MSOP-10E Package
SD
34V with Transient Protection to 36V, 2A (I ), 2.8MHz,
High Efficiency Step-Down DC/DC Converter
V
= 3.6V, V
= 34V, V
= 1.26V, I = 850μA,
OUT(MIN) Q
OUT
IN(MIN)
IN(MAX)
I
<1μA, 3mm × 3mm DFN-10, MSOP-10E Package
SD
36V with Transient Protection to 40V, Dual 1.4A (I ), 3MHz,
High Efficiency Step-Down DC/DC Converter
V
= 3.7V, V
= 37V, V
= 0.8V, I = 4.6mA,
OUT
IN(MIN)
IN(MAX)
OUT(MIN) Q
I
= 1μA, 4mm × 4mm QFN-24, TSSOP-16E Package
SD
36V with Transient Protection to 40V, 1.4A (I ), 3MHz,
High Efficiency Step-Down DC/DC Converter
V
= 3.6V, V
= 34V, V
= 0.78V, I = 2mA,
OUT
IN(MIN)
IN(MAX)
OUT(MIN) Q
I
= 2μA, 3mm × 3mm DFN-8, MSOP-8E Package
SD
36V, 40V
and LDO Controller
, 2A, 2.5MHz High Efficiency Step-Down DC/DC Converter
MAX
V
= 3.6V, V
= 36V, V
= 0.8V, I = 2.5mA,
IN(MIN)
IN(MAX)
OUT(MIN) Q
I
<10μA, 3mm × 3mm DFN-10 Package
SD
36V 2.5MHz, Triple (2.4A + 1.5A + 1.5A (I )) with LDO Controller
High Efficiency Step-Down DC/DC Converter
V
= 4.0V, V
= 36V, V
= 0.8V, I = 7mA,
OUT
IN(MIN)
IN(MAX)
OUT(MIN) Q
I
= 1μA, 5mm × 7mm QFN-38 Package
SD
60V, 400mA (I ), Micropower Step-Down DC/DC Converter with
Burst Mode Operation
V
= 3.3V, V
= 60V, V
= 1.25V, I = 100μA,
OUT
IN(MIN)
IN(MAX)
OUT(MIN) Q
I
<1μA, 3mm × 3mm DFN-10, TSSOP-16E Package
SD
LT1976/ 60V, 1.2A (I ), 200kHz/500kHz, High Efficiency Step-Down DC/DC
V
= 3.3V, V
= 60V, V
= 1.20V, I = 100μA,
Q
OUT
IN(MIN)
IN(MAX)
OUT(MIN)
OUT(MIN)
OUT(MIN)
OUT(MIN)
LT1977
Converter with Burst Mode Operation
I
<1μA, TSSOP16E Package
SD
LT3434/ 60V, 2.4A (I ), 200kHz/500kHz, High Efficiency Step-Down DC/DC
V
= 3.3V, V
= 60V, V
= 1.20V, I = 100μA,
Q
OUT
IN(MIN)
IN(MAX)
LT3435
LT1936
Converter with Burst Mode Operation
I
<1μA, TSSOP16E Package
SD
36V, 1.4A(I ) , 500kHz High Efficiency Step-Down DC/DC Converter
V
= 3.6V, V
<1μA, MS8E Package
= 36V, V
= 1.2V, I = 1.9mA,
Q
OUT
IN(MIN)
IN(MAX)
I
SD
LT3493
LT1766
36V, 1.4A(I ), 750kHz High Efficiency Step-Down DC/DC Converter
V
= 3.6V, V
= 36V, V
= 0.8V, I = 1.9mA,
Q
OUT
IN(MIN)
IN(MAX)
I
<1μA, 2mm × 3mm DFN-6 Package
SD
60V, 1.2A (I ), 200kHz, High Efficiency Step-Down DC/DC Converter
V
= 5.5V, V
= 25μA, TSSOP16E Package
= 60V, V
= 1.20V, I = 2.5mA,
OUT
IN(MIN)
IN(MAX)
OUT(MIN) Q
I
SD
3971f
LT 1109 • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
24
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●
© LINEAR TECHNOLOGY CORPORATION 2009
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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