LT3971A/LT3971A-5
38V, 1.3A, 2MHz
Step-Down Regulator with
2.2µA Quiescent Current
DESCRIPTION
FEATURES
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Ultralow Quiescent Current:
2.8μA IQ Regulating 12VIN to 3.3VOUT
Fixed Output Voltages: 5V
2.2μA IQ Regulating 12VIN to 5VOUT
Low Ripple Burst Mode® Operation:
Output Ripple < 15mVP-P
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: IQ = 700nA
Power Good Flag
Soft-Start Capability
Internal Compensation
Output Voltage: 1.19V to 30V
Small Thermally Enhanced 10-Lead MSOP Package
The LT®3971A is an adjustable frequency monolithic buck
switching regulator that accepts a wide input voltage range
up to 38V. Low quiescent current design consumes only
2.8μA of supply current while regulating with no load. 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
inrush current (soft-start). A power good flag signals when
VOUT reaches 91% of the programmed output voltage. The
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 reference voltage compared to the LT3971 and LT3971-5.
The LT3971A-5 also has specified VOUT pin current and
ABSMAX current. These characteristics are important for
USB applications.
APPLICATIONS
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USB VBUS Regulation
Automotive Battery Regulation
Power for Portable Products
Industrial Supplies
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.
TYPICAL APPLICATION
No Load Supply Current
5.15V Step-Down Converter for USB Applications
3.0
OUTPUT IN REGULATION
OFF ON
EN
VIN
BOOST
0.47μF
PG
SS
4.7μF
INPUT CURRENT (μA)
VIN
6.3V TO 38V
10μH
SW
LT3971A-5
RT
BD
20k, 1%
118k
f = 400kHz
SYNC
GND
VOUT
715k
1%
47μF
VOUT
5.15V
1.3A
2.5
LT3971A-5
2.0
1.5
1.0
5
3971 TA01a
10
15
20
25
30
INPUT VOLTAGE (V)
35
3971A TA01b
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LT3971A/LT3971A-5
ABSOLUTE MAXIMUM RATINGS
(Note 1)
VIN, EN Voltage .........................................................38V
BOOST Pin Voltage ...................................................55V
BOOST Pin Above SW Pin.........................................30V
FB, RT, SYNC, SS Voltage ...........................................6V
PG, BD Voltage .........................................................30V
VOUT Pin Current ....................................................–2mA
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)
(MSE Only) ....................................................... 300°C
PIN CONFIGURATION
LT3971A
LT3971A-5
TOP VIEW
TOP VIEW
BD
BOOST
SW
VIN
EN
1
2
3
4
5
11
GND
10
9
8
7
6
SYNC
PG
RT
SS
FB
BD
BOOST
SW
VIN
EN
1
2
3
4
5
11
GND
10
9
8
7
6
SYNC
PG
RT
SS
VOUT
MSE PACKAGE
10-LEAD PLASTIC MSOP
MSE PACKAGE
10-LEAD PLASTIC MSOP
θJA = 45°C, θJC = 10°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
θJA = 45°C, θJC = 10°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3971AIMSE#PBF
LT3971AIMSE#TRPBF
LTGFQ
10-Lead Plastic MSOP
–40°C to 125°C
LT3971AIMSE-5#PBF
LT3971AIMSE-5#TRPBF
LTGFR
10-Lead Plastic MSOP
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
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
Minimum Input Voltage
Quiescent Current from VIN
LT3971A FB Pin Current
Internal Feedback Resistor Divider (LT3971A-5)
CONDITIONS
(Note 4)
VEN Low
VEN High, VSYNC Low
VEN High, VSYNC Low
VFB = 1.19V
MIN
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TYP
4
0.7
1.7
0.1
10
MAX
4.3
1.2
2.7
4.5
12
UNITS
V
μA
μA
μA
nA
MΩ
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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)
VOUT Pin Current
VOUT Pin Clamp Voltage
Feedback Voltage
VOUT = 5V
VOUT = 5V
IVOUT = –2mA
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LT3971A-5 Output Voltage
FB Voltage Line Regulation
Switching Frequency
Minimum Switch On Time
Minimum Switch Off Time
Switch Current Limit
Switch VCESAT
Switch Leakage Current
Boost Schottky Forward Voltage
Boost Schottky Reverse Leakage
Minimum Boost Voltage (Note 3)
BOOST Pin Current
EN Voltage Threshold
EN Voltage Hysteresis
EN Pin Current
LT3971A PG Threshold Offset from VFB
LT3971A PG Hysteresis
LT3971A-5 PG Threshold Offset from VOUT
LT3971A-5 PG Hysteresis
PG Leakage
PG Sink Current
SYNC Threshold
SYNC Pin Current
SS Source Current
l
4.3V < VIN < 38V (Note 4)
RT = 11k
RT = 35.7k
RT = 255k
–0.65
–0.90
9
1.176
1.173
4.94
4.93
1.8
0.8
160
2.0
ISW = 1A
ISH = 100mA
VREVERSE = 12V
VIN = 5V
ISW = 1A, VBOOST = 15V
EN Rising
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0.95
VFB Rising
60
VOUT Rising
5.5
VPG = 3V
VPG = 0.4V
VSS = 1V
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 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.
l
300
0.6
0.6
–0.5
–0.5
11
1.192
1.19
5.01
5
0.0002
2.2
1
200
80
110
2.5
330
0.02
770
0.02
1.4
20
1.01
30
0.2
100
20
9
1.3
0.02
570
0.8
0.1
1
–0.38
–0.32
13
1.204
1.207
5.06
5.07
0.01
2.6
1.2
240
150
3.2
1
1
1.8
28
1.07
20
140
12.5
1
1.0
1.6
μA
μA
V
V
V
V
V
%/V
MHz
MHz
kHz
ns
ns
A
mV
μA
mV
μA
V
mA
V
mV
nA
mV
mV
%
%
μA
μA
V
nA
μA
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.
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LT3971A/LT3971A-5
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency, VOUT = 5V
Efficiency, VOUT = 3.3V
100
VIN = 12V
VOUT = 5V
90 R1 = 1M
R2 = 309k
80
VIN = 12V
70
VIN = 36V
VIN = 24V
60
50
40
70
VIN = 36V
60
EFFICIENCY (%)
80
EFFICIENCY (%)
EFFICIENCY (%)
100
90
80
20
Efficiency, VOUT = 5V
100
90
30
TA = 25°C, unless otherwise noted.
VIN = 24V
50
0
0.2
0.4
0.6
0.8
LOAD CURRENT (A)
1
1.2
40
10
0
0.2
0.4
0.6
0.8
LOAD CURRENT (A)
3971A G01
1
0
0.01
1.2
0.1
1
10
100
LOAD CURRENT (mA)
3971A G02
Efficiency, VOUT = 3.3V
100
1000
3971A G03
No Load Supply Current
90
80
VIN = 36V
VIN = 24V
50
20
30
20
60
30
40
VOUT = 5V
R1 = 1M
R2 = 309k
VIN = 12V
70
No Load Supply Current
4.0
DIODES, INC.
DFLS2100
VIN = 12V
3.5
VIN = 24V
50
VIN = 36V
40
30
INPUT CURRENT (μA)
60
INPUT CURRENT (μA)
EFFICIENCY (%)
70
10
20
LT3971A
VOUT = 3.3V
3.0
2.5
LT3971A-5
2.0
1.5
10
0
0.01
0.1
1
10
100
LOAD CURRENT (mA)
1
–55
1000
1.0
–25
5
35
65
95
TEMPERATURE (°C)
3971A G04
155
5
LT3971A-5 Output Voltage
5.04
2.5
1.180
1.175
–55
LOAD CURRENT (A)
1.200
OUTPUT VOLTAGE (V)
3.0
1.185
5.02
5.00
4.98
4.96
–25
5
35
65
95
TEMPERATURE (°C)
125
155
3971A G07
4.94
–55
35
Maximum Load Current
5.06
1.190
15
20
25
30
INPUT VOLTAGE (V)
3971A G06
1.205
1.195
10
3971A G05
LT3971A Feedback Voltage
FEEDBACK VOLTAGE (V)
125
VOUT = 3.3V
TYPICAL
2.0
MINIMUM
1.5
1.0
0.5
–25
5
35
65
95
TEMPERATURE (°C)
125
155
3971A G08
0
5
10
15
20
25
30
INPUT VOLTAGE (V)
35
40
3971A G09
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LT3971A/LT3971A-5
TYPICAL PERFORMANCE CHARACTERISTICS
0.30
VOUT = 5V
950
0.20
1.5
MINIMUM
1.0
0.5
0.15
900
FREQUENCY (kHz)
TYPICAL
LOAD REGULATION (%)
LOAD CURRENT (A)
1000
0.25
2.0
0.10
0.05
0
–0.05
–0.10
–0.15
–0.30
5
25
30
15
20
INPUT VOLTAGE (V)
10
35
40
850
800
750
700
–0.20
650
–0.25
0
Switching Frequency
Load Regulation
Maximum Load Current
2.5
TA = 25°C, unless otherwise noted.
REFERENCED FROM VOUT AT 0.5A LOAD
0
200
400
600
800 1000
LOAD CURRENT (mA)
600
–55
1200
–25
5
35
65
95
TEMPERATURE (°C)
3971A G11
3971A G10
155
3971A G12
Switch VCESAT
Switch Current Limit
Switch Current Limit
3.0
125
2.5
600
2.0
1.5
1.0
0.5
500
2.3
2.2
400
VCESAT (mV)
SWITCH CURRENT LIMIT (A)
SWITCH CURRENT LIMIT (A)
2.4
2.5
2.1
2.0
1.9
300
200
1.8
1.7
100
1.6
0
0
20
40
60
DUTY CYCLE (%)
80
DUTY CYCLE = 30%
1.5
–55 –25
5
35
65
95
TEMPERATURE (°C)
100
125
3971A G13
Boost Pin Current
15
10
5
900
800
800
700
600
500
400
300
200
100
0
250
500
750 1000 1250
SWITCH CURRENT (mA)
1500
3971A G16
0
500
750 1000 1250
SWITCH CURRENT (mA)
1500
LT3971A-5 Frequency Foldback
900
SWITCHING FREQUENCY (kHz)
SWITCHING FREQUENCY (kHz)
BOOST PIN CURRENT (mA)
20
250
3971A G15
Frequency Foldback
25
0
3971A G14
30
0
0
155
700
600
500
400
300
200
100
0
0.2
0.6
0.4
0.8
FB PIN VOLTAGE (V)
1
1.2
3971A G17
0
0
20
60
40
80
VOUT (% OF REGULATION VOLTAGE)
100
3971A G18
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LT3971A/LT3971A-5
TYPICAL PERFORMANCE CHARACTERISTICS
Minimum Switch On-Time/
Switch Off-Time
TA = 25°C, unless otherwise noted.
Minimum Input Voltage
Soft-Start
400
5.0
2.5
MIN TOFF 1A LOAD
250
200
MIN TOFF 0.5A LOAD
150
100
MIN TON
4.6
2.0
INPUT VOLTAGE (V)
SWITCH CURRENT LIMIT (A)
SWITCH ON/OFF TIME (ns)
300
1.5
1.0
4.2
TO START
4.0
3.8
TO RUN
3.6
3.2
–25
35
95
5
65
TEMPERATURE (°C)
125
0
155
3.0
0
0.25 0.5 0.75 1 1.25 1.5 1.75
SS PIN VOLTAGE (V)
3971A G19
VOUT = 5V
THRESHOLD VOLTAGE (V)
6.0
TO START
5.8
5.6
5.4
1.6
1.04
1.4
1.03
1.02
RISING THRESHOLD
1.01
1.00
0.99
FALLING THRESHOLD
0.98
200
400
800 1000
600
LOAD CURRENT (mA)
1200
1.2
1.0
0.8
0.6
0.2
0.95
–55
–25
5
35
65
95
TEMPERATURE (°C)
3971A G22
125
155
0
0
250
500
750 1000 1250
BOOST DIODE CURRENT (mA)
3971A G23
1500
3971A G24
Transient Load Response,
Load Current Stepped from 25mA
(Burst Mode Operation) to 525mA
Power Good Threshold
1200
0.4
0.96
0
600
400
800 1000
LOAD CURRENT (mA)
Boost Diode Forward Voltage
1.05
0.97
TO RUN
200
3971A G21
EN Threshold
6.2
5.2
0
3971A G20
Minimum Input Voltage
6.4
2
BOOST DIODE VF (V)
0
–55
INPUT VOLTAGE (V)
4.4
3.4
0.5
50
5.0
VOUT = 3.3V
4.8
350
Transient Load Response,
Load Current Stepped from
0.5A to 1A
95
THRESHOLD VOLTAGE (%)
94
93
VOUT
100mV/
DIV
VOUT
100mV/DIV
92
91
90
89
IL
500mA/
DIV
88
87
IL
500mA/DIV
86
85
–55
–25
5
35
65
95
TEMPERATURE (°C)
125
155
3971A G25
10μs/DIV
VIN = 12V, VOUT = 3.3V
COUT = 47μF
3971A G26
10μs/DIV
VIN = 12V, VOUT = 3.3V
COUT = 47μF
3971A G27
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LT3971A/LT3971A-5
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, unless otherwise noted.
Switching Waveforms; Full
Frequency Continuous Operation
Switching Waveforms;
Burst Mode Operation
3.3V Start-Up and Dropout
800kHz
3kΩ LOAD
VSW
5V/DIV
VSW
5V/DIV
VIN
1V/DIV
IL
500mA/DIV
IL
500mA/DIV
VOUT
20mV/DIV
VOUT
20mV/DIV
VOUT
3971A G28
5μs/DIV
VIN = 12V, VOUT = 3.3V
ILOAD = 10mA
COUT = 22μF
3971A G29
1μs/DIV
VIN = 12V, VOUT = 3.3V
ILOAD = 1A
COUT = 22μF
3.3V Start-Up and Dropout
5V Start-Up and Dropout
800kHz
6.7kΩ LOAD
800kHz
5kΩ LOAD
3971A G30
0.5s/DIV
5V Start-Up and Dropout
800kHz
10Ω LOAD
VIN
VIN
VIN
1V/DIV
1V/DIV
1V/DIV
VOUT
VOUT
VOUT
3971A G31
0.5s/DIV
3971A G32
0.5s/DIV
5
1.2
4
0.8
0.4
0
3971A G33
Minimum Input Voltage to Switch
1.6
INPUT VOLTAGE (V)
FEEDBACK VOLTAGE (V)
Feedback Regulation Voltage
0.5s/DIV
3
2
2
2.5
3
3.5
4
INPUT VOLTAGE (V)
4.5
5
3971A G34
1
–55
–25
5
35
65
95
TEMPERATURE (°C)
125
155
3971A G35
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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.
BOOST (Pin 2): This pin is used to provide a drive voltage, 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.
VIN (Pin 4): The VIN pin supplies current to the LT3971A’s
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
voltage is 1.005V going up and 0.975V going down. The EN
threshold is only accurate when VIN is above 4.3V. If VIN is
lower than 4.2V, ground EN to place the part in shutdown.
Tie to VIN if shutdown feature is not used.
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
and VOUT. Typically this capacitor is 10pF.
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. The soft-start capacitor is only actively discharged
when EN is low. The SS pin is released when the EN pin
goes high. Float this pin to disable soft-start. For applications with input voltages above 25V, add a 100k resistor
in series with the soft-start capacitor.
RT (Pin 8): A resistor is tied between RT and ground to
set the switching frequency.
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 when the LT3971A is enabled and VIN is above 4.3V.
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 synchronization, which will include pulse-skipping at low output
loads. When in pulse-skipping mode, quiescent current
increases to 1.5mA.
GND (Exposed Pad Pin 11): Ground. The exposed pad
must be soldered to PCB.
VOUT (Pin 6, LT3971A-5 Only): The LT3971A-5 regulates
the VOUT pin to 5V. This pin connects to the internal 10MΩ
feedback divider that programs the fixed output voltage.
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LT3971A/LT3971A-5
BLOCK DIAGRAM
VIN
C1
INTERNAL 1.19V REF
1V
EN
RT
–
+
VIN
+
–
Σ
SHDN
BD
SWITCH
LATCH
SLOPE COMP
BOOST
R
OSCILLATOR
200kHz TO 2MHz
RT
C3
Q
S
L1
VOUT
SW
Burst Mode
DETECT
SYNC
PG
ERROR AMP
+
–
+
–
1.09V
D1
VC CLAMP
VC
1μA
SS
C5
SHDN
R2
GND
R3
C4
R1
VOUT
FB
R2
C2
R1
LT3971A-5
ONLY
3971A BD
LT3971A
ONLY
LT3971A-5: R1 = 7.62M, R2 = 2.38M
C5
OPERATION
The LT3971A is a constant frequency, current mode stepdown 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
between the VIN and SW pins, turning the switch off when
this current reaches a level determined by the voltage at
VC (see Block Diagram). An error amplifier measures the
output voltage through an external resistor divider tied to
the FB pin and servos the VC node. If the error amplifier’s
output increases, more current is delivered to the output;
if it decreases, less current is delivered. An active clamp
on the VC node provides current limit. The VC node is
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.
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 VIN or from the
BOOST pin. An external capacitor is used to generate a
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 1.7μA. In a typical application, 2.8μA will be
consumed from the supply when regulating with no load.
The oscillator reduces the LT3971A’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.
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
is enabled and VIN is above 4.3V.
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LT3971A/LT3971A-5
APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT3971A operates
in low ripple Burst Mode, which keeps the output capacitor
charged to the desired output voltage while minimizing
the input quiescent current. In Burst Mode operation the
LT3971A delivers single pulses of current to the output
capacitor followed by sleep periods where the output power
is supplied by the output capacitor. When in sleep mode
the LT3971A consumes 1.7μA, but when it turns on all the
circuitry to deliver a current pulse, the LT3971A consumes
1.5mA of input current in addition to the switch current.
Therefore, the total quiescent current will be greater than
1.7μA when regulating.
As the output load decreases, the frequency of single current 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 1.7μA ideal. Therefore, to optimize the
quiescent current performance at light loads, the current
in the feedback resistor divider and the reverse current
in the catch diode must be minimized, as these appear
to the output as load currents. Use the largest possible
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
SWITCHING FREQUENCY (kHz)
1000
VIN = 12V
VOUT = 3.3V
800
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.
VSW
5V/DIV
IL
500mA/DIV
VOUT
20mV/DIV
5μs/DIV
3971A F02
VIN = 12V
VOUT = 3.3V
ILOAD = 10mA
Figure 2. Burst Mode Operation
While in Burst Mode operation, the burst frequency and
the charge delivered with each pulse will not change with
output capacitance. Therefore, the output voltage ripple
will be inversely proportional to the output capacitance.
In a typical application with a 22μF output capacitor, the
output ripple is about 10mV, and with a 47μF output capacitor the output ripple is about 5mV. The output voltage
ripple can continue to be decreased by increasing the
output capacitance.
At higher output loads (above 92mA for the front page
application) the LT3971A will be running at the frequency
programmed by the RT resistor, and will be operating in
standard PWM mode. The transition between PWM and low
ripple Burst Mode operation will exhibit slight frequency
jitter, but will not disturb the output voltage.
600
400
200
0
leakage at room temperature. These two considerations
are reiterated in the FB Resistor Network and Catch Diode
Selection sections.
0
20
40
60
80
LOAD CURRENT (mA)
100
120
3971A F01
Figure 1. Switching Frequency in Burst Mode Operation
3971af
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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.
FB Resistor Network
The LT3971A output voltage is programmed with a resistor divider between the output and the FB pin. Choose the
resistor values according to:
⎛ V
⎞
R1= R2 ⎜ OUT − 1⎟
⎝ 1.19V ⎠
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
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.
When using large FB resistors, a 10pF phase lead capacitor
should be connected from VOUT to FB.
VOUT Pin
The LT3971A-5 contains an internal 10M feedback resistor 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 connected directly to the VOUT pin.
An external resistor divider can be added to shift the output
voltage higher than 5V. For example, a USB VBUS supply
programmed to 5.15V allows some voltage drop through
connectors and cables, while keeping the VBUS voltage
within specification at the device. By using the LT3971A-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 VOUT pin is process and temperature
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:
R1 =
VOUT – 5
5
+ 0.0005
R2
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.
OUTPUT
LT3971A-5
R1
VOUT
R2
3971A F03
Figure 3. Resistor Divider Used to Increase the Output Voltage
Above 5V
The VOUT pin has an internal 11V clamp. Output voltage
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.
3971af
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LT3971A/LT3971A-5
APPLICATIONS INFORMATION
Setting the Switching Frequency
The LT3971A 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 RT value for a desired switching
frequency is in Table 1.
the typical minimum on and off curves to design for an
application’s maximum temperature, while adding about
30% for part-to-part variation. The minimum and maximum
duty cycles that can be achieved taking minimum on and
off times into account are:
DCMIN = fSW tON(MIN)
DCMAX = 1− fSW tOFF(MIN)
Table 1. Switching Frequency vs RT Value
SWITCHING FREQUENCY (MHz)
RT VALUE (kΩ)
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
where fSW is the switching frequency, the tON(MIN) is the
minimum switch on-time, and the tOFF(MIN) is the minimum
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.
Operating Frequency Tradeoffs
Input Voltage Range
Selection of the operating frequency is a tradeoff between
efficiency, component size, minimum dropout voltage, and
maximum input voltage. The advantage of high frequency
operation is that smaller inductor and capacitor values may
be used. The disadvantages are lower efficiency, lower
maximum input voltage, and higher dropout voltage. The
highest acceptable switching frequency (fSW(MAX)) for a
given application can be calculated as follows:
The minimum input voltage is determined by either the
LT3971A’s minimum operating voltage of 4.3V or by its
maximum duty cycle (see equation in Operating Frequency
Tradeoffs section). The minimum input voltage due to
duty cycle is:
fSW(MAX) =
VOUT + VD
tON(MIN)(VIN − VSW + VD )
where VIN is the typical input voltage, VOUT is the output
voltage, VD is the catch diode drop (~0.5V), and VSW is
the internal switch drop (~0.5V at max load). This equation
shows that slower switching frequency is necessary to
safely accommodate high VIN/VOUT ratio. Also, as shown
in the Input Voltage Range section, lower frequency allows
a lower dropout voltage. The input voltage range depends
on the switching frequency because the LT3971A switch
has finite minimum on and off times. The minimum switch
on and off times are strong functions of temperature. Use
VIN(MIN) =
VOUT + VD
− VD + VSW
1− fSW tOFF(MIN)
where VIN(MIN) is the minimum input voltage, VOUT is
the output voltage, VD is the catch diode drop (~0.5V),
VSW is the internal switch drop (~0.5V at max load), fSW
is the switching frequency (set by RT), and tOFF(MIN) is
the minimum switch off-time. Note that higher switching frequency will increase the minimum input voltage.
If a lower dropout voltage is desired, a lower switching
frequency should be used.
The maximum input voltage for LT3971A applications
depends on switching frequency, the Absolute Maximum
Ratings of the VIN and BOOST pins, and the operating
mode. For a given application where the switching frequency and the output voltage are already selected, the
3971af
12
LT3971A/LT3971A-5
APPLICATIONS INFORMATION
maximum input voltage (VIN(OP-MAX)) that guarantees
optimum output voltage ripple for that application can be
found by applying the following equation:
VIN(OP-MAX) =
VOUT + VD
–V +V
fSW • tON(MIN) D SW
where tON(MIN) is the minimum switch on-time. Note that
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.
The circuit will tolerate inputs above the maximum operating input voltage and up to the Absolute Maximum
Ratings of the VIN and BOOST pins, regardless of chosen
switching frequency. However, during such transients
where VIN is higher than VIN(OP-MAX), the LT3971A will
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
VIN is greater than VIN(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.
The inductor value must be sufficient to supply the desired
maximum output current (IOUT(MAX)), which is a function
of the switch current limit (ILIM) and the ripple current.
IOUT(MAX) =ILIM –
ΔIL
2
The LT3971A limits its peak switch current in order to
protect itself and the system from overload faults. The
LT3971A’s switch current limit (ILIM) is typically 2.5A at low
duty cycles and decreases linearly to 1.75A at DC = 0.8.
Table 2. Inductor Vendors
VENDOR
URL
PART SERIES
TYPE
Murata
www.murata.com
LQH55D
Open
TDK
www.componenttdk.com SLF7045
SLF10145
Shielded
Shielded
Toko
www.toko.com
D62CB
D63CB
D73C
D75F
Shielded
Shielded
Shielded
Open
Coilcraft
www.coilcraft.com
MSS7341
MSS1038
Shielded
Shielded
Sumida
www.sumida.com
CR54
CDRH74
CDRH6D38
CR75
Open
Shielded
Shielded
Open
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
L=
VOUT + VD
fSW
where fSW is the switching frequency in MHz, VOUT is the
output voltage, VD is the catch diode drop (~0.5V) and L
is the inductor value in μH.
The inductor’s RMS current rating must be greater than the
maximum load current and its saturation current should be
about 30% higher. For robust operation in fault conditions
(start-up or short-circuit) and high input voltage (>30V),
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:
ΔIL =
(1− DC) • (VOUT + VD )
L • fSW
3971af
13
LT3971A/LT3971A-5
APPLICATIONS INFORMATION
Where fSW is the switching frequency of the LT3971A, DC is
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
not allow sufficient maximum output current (IOUT(MAX))
given the switching frequency, and maximum input voltage
used in the desired application.
The optimum inductor for a given application may differ
from the one indicated by this simple design guide. A larger
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 current. 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
maximum load current. For details of maximum output current and discontinuous operation, see Linear Technology’s
Application Note 44. Finally, for duty cycles greater than
50% (VOUT/VIN>0.5), 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 inductors, 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
continuous. Discontinuous operation occurs when IOUT
is less than ∆IL/2.
Input Capacitor
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 4.7μF to 10μF 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
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.
Step-down regulators draw current from the input supply 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 4.7μF capacitor is capable of this task, but only if it is
placed close to the LT3971A (see the PCB Layout section). 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 Plugging Safely section).
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 provide the best ripple performance. A good starting value is:
COUT =
100
VOUT fSW
where fSW is in MHz, and COUT is the recommended output
capacitance in μF. Use X5R or X7R types. This choice will
provide low output ripple and good transient response.
Transient performance can be improved with a higher
value capacitor. Increasing the output capacitance will
also decrease the output voltage ripple. A lower value of
output capacitor can be used to save space and cost but
transient performance will suffer.
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
physically larger capacitor or one with a higher voltage rating
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.
Table 3. Recommended Ceramic Capacitor Vendors
On Semiconductor
PART NUMBER
VR
(V)
IAVE
(A)
MANUFACTURER
WEBSITE
MBR0520L
20
0.5
VF at 1A
(mV)
VF at 2A
(mV)
IR at VR =
20V 25°C
(μA)
30
AVX
www.avxcorp.com
MBR0540
40
0.5
620
Murata
www.murata.com
MBRM120E
20
1
530
Taiyo Yuden
www.t-yuden.com
MBRM140
40
1
550
Vishay Siliconix
www.vishay.com
Diodes Inc.
TDK
www.tdk.com
30
0.5
B0540W
40
0.5
620
1
Catch Diode Selection
B120
20
1
500
1.1
The catch diode (D1 from Block Diagram) conducts current only during switch off time. Average forward current
in normal operation can be calculated from:
B130
30
1
500
1.1
B140
40
1
500
1.1
B150
50
1
700
0.4
B220
20
2
500
20
B230
30
2
500
0.6
B140HB
40
1
V –V
ID(AVG) =IOUT IN OUT
VIN
where IOUT is the output load current. The only reason to
consider a diode with a larger current rating than necessary
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.
B0530W
0.4
595
0.5
20
15
1
DFLS240L
40
2
DFLS140
40
1.1
510
500
DFLS160
60
1
500
DFLS2100
100
2
770
B240
40
2
4
1
2.5
860
0.01
500
0.45
Central Semiconductor
CMSH1 - 40M
40
1
500
CMSH1 - 60M
60
1
700
CMSH1 - 40ML
40
1
400
CMSH2 - 40M
40
2
550
CMSH2 - 60M
60
2
700
CMSH2 - 40L
40
2
400
CMSH2 - 40
40
2
500
CMSH2 - 60M
60
2
700
3971af
15
LT3971A/LT3971A-5
APPLICATIONS INFORMATION
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.
For outputs of 3V and above, the standard circuit (Figure 4a)
is best. For outputs between 2.8V and 3V, use a 1μF boost
capacitor. A 2.5V output presents a special case because it
is marginally adequate to support the boosted drive stage
while using the internal boost diode. For reliable BOOST pin
operation with 2.5V outputs use a good external Schottky
diode (such as the ON Semi MBR0540), and a 1μF 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
circuit in Figure 4a is more efficient because the BOOST pin
current comes from a lower voltage source. You must also
be sure that the maximum voltage ratings of the BOOST
and BD pins are not exceeded.
Ceramic Capacitors
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.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT3971A. As
previously mentioned, 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 rating.
This situation is easily avoided (see the Hot Plugging
Safely section).
BD
VIN
BOOST
LT3971A
4.7μF
GND
C3
SW
VOUT
(4a) For VOUT > 2.8V
D2
BD
VIN
VIN
BOOST
LT3971A
4.7μF
GND
C3
SW
VOUT
(4b) For 2.5V < VOUT < 2.8V
BD
VIN
VIN
BOOST
LT3971A
BOOST and BD Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see
the Block Diagram) are used to generate a boost voltage that is higher than the input voltage. In most cases
a 0.47μF 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.
VIN
4.7μF
GND
C3
VOUT
SW
3971A FO4
(4c) For VOUT < 2.5V; VIN(MAX) = 27V
Figure 4. Three Circuits for Generating the Boost Voltage
3971af
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LT3971A/LT3971A-5
APPLICATIONS INFORMATION
The minimum operating voltage of an LT3971A 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 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
the worst-case situation where VIN is ramping very slowly.
5.0
4.8
INPUT VOLTAGE (V)
4.6
4.4
TO START
4.2
4.0
TO RUN
3.8
3.6
3.4 VOUT = 3.3V
TA = 25°C
3.2 L = 4.7μH
f = 800kHz
3.0
10
100
LOAD CURRENT (mA)
1000
6.4
INPUT VOLTAGE (V)
TO START
6.0
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 VIN if the shutdown feature is not used.
Adding a resistor divider from VIN to EN programs the
LT3971A to regulate the output only when VIN is above a
desired voltage (see Figure 6). Typically, this threshold,
VIN(EN), is used in situations where the input supply is current limited, or has a relatively high source resistance. A
switching regulator draws constant power from the source,
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
source voltage conditions. The VIN(EN) threshold prevents
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:
R3
+1
R4
where output regulation should not start until VIN is above
VIN(EN). Due to the comparator’s hysteresis, switching will
not stop until the input falls slightly below VIN(EN).
5.8
5.6
TO RUN
5.2
At light loads, the inductor current becomes discontinuous
and this reduces the minimum input voltage to approximately 400mV above VOUT. At higher load currents, the
inductor current is continuous and the duty cycle is limited
by the maximum duty cycle of the LT3971A, requiring a
higher input voltage to maintain regulation.
VIN(EN) =
6.2
5.4
For lower start-up voltage, the boost diode can be tied to
VIN; however, this restricts the input range to one-half of
the absolute maximum rating of the BOOST pin.
VOUT = 5V
TA = 25°C
L = 4.7μH
f = 800kHz
5.0
R3
10
LT3971A
VIN
100
LOAD CURRENT (mA)
1V
1000
EN
3971A F05
+
–
SHDN
R4
Figure 5. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
3971A F06
Figure 6. Programmed Enable Threshold
3971af
17
LT3971A/LT3971A-5
APPLICATIONS INFORMATION
Be aware that when the input voltage is below 4.3V, the
input current may rise to several hundred μA. And the part
may be able to switch at cold or for VIN(EN) thresholds less
than 7V. Figure 7 shows the magnitude of the increased
input current in a typical application with different programmed VIN(EN).
When operating in Burst Mode for light load currents, the
current through the VIN(EN) resistor network can easily be
greater than the supply current consumed by the LT3971A.
Therefore, the VIN(EN) resistors should be large to minimize
their effect on efficiency at low loads.
12V VIN(EN) Input Current
500
The SS pin can be used to soft-start the LT3971A by
throttling the maximum input current during start-up. An
internal 1μA current source charges an external capacitor generating a voltage ramp on the SS pin. The SS pin
clamps the internal VC node, which slowly ramps up the
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. For applications with input voltages above 25V,
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 3.3Ω
load when the EN pin is pulsed high for 13ms.
The external SS capacitor is only actively discharged when
EN is low. With EN low, the external SS cap is discharged
through approximately 150Ω. The EN pin needs to be low
long enough for the external cap to completely discharge
through the 150Ω pull-down prior to start-up.
400
INPUT CURRENT (μA)
Soft-Start
300
200
100
0
0
1
2
3
4 5 6 7 8 9 10 11 12
INPUT VOLTAGE (V)
VIN(EN) = 12V
R3 = 11M
R4 = 1M
VSS
1V/DIV
VOUT
2V/DIV
6V VIN(EN) Input Current
IL
0.5A/DIV
500
2ms/DIV
INPUT CURRENT (μA)
400
3971A F08
Figure 8. Soft-Start Waveforms for Front-Page Application
with 10nF Capacitor on SS. EN is Pulsed High for About
13ms with a 3.3Ω Load Resistor
300
200
Synchronization
100
To select low ripple Burst Mode operation, tie the SYNC
pin below 0.6V (this can be ground or a logic low output).
0
0
1
VIN(EN) = 6V
R3 = 5M
R4 = 1M
2
3
4
INPUT VOLTAGE (V)
5
6
3971A F07
Figure 7. Input Current vs Input Voltage
for a Programmed VIN(EN) of 6V and 12V
Synchronizing the LT3971A oscillator to an external frequency 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).
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.
The LT3971A may be synchronized over a 250kHz to 2MHz
range. The RT resistor should be chosen to set the LT3971A
switching frequency 20% below the lowest synchronization
input. For example, if the synchronization signal will be
250kHz and higher, the RT should be selected for 200kHz.
To assure reliable and safe operation the LT3971A will only
synchronize when the output voltage is near regulation as
indicated by the PG flag. It is therefore necessary to choose
a large enough inductor value to supply the required output
current at the frequency set by the RT resistor (see the
Inductor Selection section). The slope compensation is set
by the RT value, while the minimum slope compensation
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 RT, than the slope compensation will be
sufficient for all synchronization frequencies.
D4
MBRS140
VIN
VIN
BOOST
EN
SW
VOUT
LT3971A
GND
BD
FB
+
BACKUP
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
PCB Layout
For proper operation and minimum EMI, care must be taken
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 VIN and SW pins, the catch diode
(D1), and the input capacitor (C1). The loop formed by
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate excessively, 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
output. If the VIN pin is allowed to float and the EN pin
is held high (either by a logic signal or because it is tied
to VIN), then the LT3971A’s internal circuitry will pull its
quiescent current through its SW pin. This is fine if your
system can tolerate a few μA in this state. If you ground
the EN pin, the SW pin current will drop to essentially
zero. However, if the VIN pin is grounded while the output
is held high, regardless of EN, parasitic diodes inside the
LT3971A can pull current from the output through the SW
pin and the VIN pin. Figure 9 shows a circuit that will run
only when the input voltage is present and that protects
against a shorted or reversed input.
L1
C2
VOUT
RPG
GND
RT
C3
C4
C5
D1
R2
R1
C1
GND
3971A F10
VIAS TO LOCAL GROUND PLANE
VIAS TO VOUT
VIAS TO SYNC
VIAS TO RUN/SS
VIAS TO PG
VIAS TO VIN
OUTLINE OF LOCAL
GROUND PLANE
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, along with the inductor and output capacitor,
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.
Finally, keep the FB and RT nodes small so that the ground
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.
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
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
the VIN pin of the LT3971A can ring to twice the nominal
input voltage, possibly exceeding the LT3971A’s rating and
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.
High Temperature Considerations
must be soldered to a ground plane. This ground should be
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.
Power dissipation within the LT3971A can be estimated by
calculating the total power loss from an efficiency measurement and subtracting the catch diode loss and inductor
loss. The die temperature is calculated by multiplying the
LT3971A power dissipation by the thermal resistance from
junction to ambient.
Also keep in mind that the leakage current of the power
Schottky diode goes up exponentially with junction temperature. 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.
Other Linear Technology Publications
Application Notes 19, 35 and 44 contain more detailed
descriptions and design information for buck regulators
and other switching regulators. The LT1376 data sheet
has a more extensive discussion of output ripple, loop
compensation and stability testing. Design Note 318
shows how to generate a bipolar output supply using a
buck regulator.
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
VIN
7V TO 38V
VIN
EN
OFF ON
BOOST
0.47μF
PG
4.7μF
4.7μH
SW
SS
LT3971A
RT
BD
10pF
VOUT
5V
1.3A
1M
49.9k
SYNC
FB
GND
22μF
309k
f = 800kHz
3971A TA02
3.3V Step Down Converter
No Load Supply Current
VIN
4.5V TO 38V
4.0
VIN
3.5
BOOST
0.47μF
PG
SS
4.7μF
4.7μH
VOUT
3.3V
1.3A
SW
LT3971A
RT
BD
10pF
1.78M
49.9k
SYNC
GND
INPUT CURRENT (μA)
EN
OFF ON
3.0
2.5
2.0
1.5
FB
22μF
1M
f = 800kHz
1.0
10
0
3971A TA11
20
30
INPUT VOLTAGE (V)
40
3971A TA11b
5V Step-Down Converter
2.5V Step-Down Converter
VIN
7V TO 38V
VIN
4.3V TO 38V
VIN
EN
OFF ON
VIN
BOOST
0.47μF
PG
4.7μF
BOOST
1μF
PG
4.7μH
SW
SS
EN
OFF ON
4.7μF
LT3971A-5
SS
RT
4.7μH
SW
LT3971A
RT
BD
BD
49.9k
SYNC
f = 800kHz
GND
VOUT
22μF
3971A TA03
VOUT
5V
1.3A
10pF
1M
118k
SYNC
f = 400kHz
GND
FB
909k
47μF
VOUT
2.5V
1.3A
3971A TA04
3971af
21
LT3971A/LT3971A-5
TYPICAL APPLICATIONS
1.8V Step-Down Converter
12V Step-Down Converter
VIN
15V TO 38V
VIN
4.3V TO 27V
VIN
VIN
BD
EN
OFF ON
0.47μF
PG
BOOST
0.47μF
PG
4.7μH
SW
SS
EN
OFF ON
BOOST
10μF
LT3971A
SW
SS
4.7μF
10μH
LT3971A
RT
RT
10pF
BD
118k
SYNC
GND
100μF
1M
f = 400kHz
1M
VOUT
1.8V
1.3A
511k
FB
10pF
49.9k
GND
SYNC
FB
110k
f = 800kHz
10μF
VOUT
12V
1.3A
3971A TA06
3971A TA05
3.3V Step-Down Converter with Undervoltage Lockout, Soft-Start, and Power Good
VIN
6V TO 38V
5M
VIN
BOOST
EN
0.47μF
4.7μH
SW
4.7μF
SS
100k
150k
LT3971A
RT
PG
BD
1M
PGOOD
10pF
1nF
49.9k
1M
SYNC
VOUT
3.3V
1.3A
FB
GND
562k
f = 800kHz
22μF
3971A TA07
5V, 2MHz Step-Down Converter with Soft-Start
VIN
10V TO 25V
VIN
EN
OFF ON
BOOST
0.47μF
PG
SS
2.2μH
SW
LT3971A
2.2μF
RT
BD
1nF
10pF
11k
1M
SYNC
f = 2MHz
GND
FB
309k
22μF
VOUT
5V
1.3A
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 t 0.102
(.074 t .004)
5.23
(.206)
MIN
1
0.889 t 0.127
(.035 t .005)
0.05 REF
10
DETAIL “B”
CORNER TAIL IS PART OF
DETAIL “B” THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
NO MEASUREMENT PURPOSE
3.00 t 0.102
(.118 t .004)
(NOTE 3)
10 9 8 7 6
DETAIL “A”
0s – 6s TYP
1 2 3 4 5
GAUGE PLANE
0.53 t 0.152
(.021 t .006)
DETAIL “A”
0.18
(.007)
0.497 t 0.076
(.0196 t .003)
REF
3.00 t 0.102
(.118 t .004)
(NOTE 4)
4.90 t 0.152
(.193 t .006)
0.254
(.010)
0.29
REF
1.68
(.066)
1.68 t 0.102 3.20 – 3.45
(.066 t .004) (.126 – .136)
0.50
0.305 t 0.038
(.0197)
(.0120 t .0015)
BSC
TYP
RECOMMENDED SOLDER PAD LAYOUT
1.88
(.074)
SEATING
PLANE
0.86
(.034)
REF
1.10
(.043)
MAX
0.17 – 0.27
(.007 – .011)
TYP
0.50
(.0197)
BSC
0.1016 t 0.0508
(.004 t .002)
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 representation 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
24V
–
+
VIN
EN
CBULK
100μF
1M
BOOST
0.47μF
PG
* AVERAGE OUTPUT POWER CANNOT
EXCEED THAT WHICH CAN BE PROVIDED
BY HIGH IMPEDANCE SOURCE.
NAMELY,
V2
POUT(MAX) tη
4R
4.7μH
SW
SS
LT3971A
4.7μF
RT
BD
1nF
10pF
49.9k
1M
SYNC
GND
FB
f = 800kHz
412k
VOUT
4V
1.3A*
100μF
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.
3971A TA09a
Sourcing a Maximum Load Pulse
VOUT
200mV/DIV
Start-Up from High Impedance Input Source
VIN
1V/DIV
VIN
5V/DIV
VOUT
2V/DIV
IL
1A/DIV
IL
500mA/DIV
3971A TA09b
500μs/DIV
2ms/DIV
3971A TA09c
RELATED PARTS
PART NUMBER DESCRIPTION
COMMENTS
LT3970
40V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC
Converter with IQ = 2.5μA
VIN: 4.2V to 40V, VOUT(MIN) = 1.21V, IQ = 2.5μA,
ISD
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