LT3972EMSE-TRPBF [Linear]
33V, 3.5A, 2.4MHz Step-Down Switching Regulator with 75μA Quiescent Current; 33V ， 3.5A ， 2.4MHz是降压型开关稳压器75μA静态电流
| 型号： | LT3972EMSE-TRPBF |
| 厂家： | 
Linear |
| 描述： | 33V, 3.5A, 2.4MHz Step-Down Switching Regulator with 75μA Quiescent Current |
| 文件： | 总24页 (文件大小：286K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT3972
33V, 3.5A, 2.4MHz
Step-Down Switching Regulator
with 75µA Quiescent Current
FEATURES
DESCRIPTION
TheLT®3972isanadjustablefrequency(200kHzto2.4MHz)
monolithic buck switching regulator that accepts input
voltages up to 33V (62V Maximum). A high efficiency
95mΩ switch is included on the die along with a boost
Schottky diode and the necessary oscillator, control, and
■
Wide Input Range:
Operation from 3.6V to 33V
Over-Voltage Lockout Protects Circuits
Through 62V Transients
■
3.5A Maximum Output Current
Low Ripple (<15mV ) Burst Mode® Operation:
logic circuitry. Current mode topology is used for fast
transient response and good loop stability. Low ripple
Burst Mode operation maintains high efficiency at low
output currents while keeping output ripple below 15mV
in a typical application. In addition, the LT3972 can fur-
ther enhance low output current efficiency by drawing
■
P-P
IN
I = 75μA at 12V to 3.3V
Q
OUT
■
■
■
■
■
■
■
■
■
■
Adjustable Switching Frequency: 200kHz to 2.4MHz
Low Shutdown Current: I < 1μA
Integrated Boost Diode
Synchronizable Between 250kHz to 2MHz
Power Good Flag
Q
bias current from the output when V
is above 3V.
OUT
Saturating Switch Design: 95mΩ On-Resistance
Output Voltage: 0.79V to 30V
Thermal Protection
Soft-Start Capability
Small 10-Pin Thermally Enhanced MSOP and
(3mm × 3mm) DFN Packages
Shutdown reduces input supply current to less than 1μA
while a resistor and capacitor on the RUN/SS pin provide a
controlled output voltage ramp (soft-start). A power good
flag signals when V
reaches 91% of the programmed
OUT
output voltage. The LT3972 is available in 10-Pin MSOP
and 3mm × 3mm DFN packages with exposed pads for
low thermal resistance.
APPLICATIONS
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
■
Automotive Battery Regulation
■
Distributed Supply Regulation
■
Industrial Supplies
Wall Transformer Regulation
■
TYPICAL APPLICATION
5V Step-Down Converter
Efficiency
100
V
OUT
V
IN
5V
6.3V TO 33V
TRANSIENT
TO 62V
V
= 12V
3.5A
IN
90
80
70
60
50
V
BD
IN
V
= 30V
IN
RUN/SS
BOOST
OFF ON
V
= 24V
IN
0.47μF
4.7μH
536k
15k
V
C
SW
LT3972
GND
10μF
RT
680pF
PG
V
= 5V
OUT
L = 4.7μH
63.4k
SYNC
FB
f = 600kHz
100k
47μF
0
0.5
1
1.5
2
2.5
3.5
3
OUTPUT CURRENT (A)
3680 TA01a
3680 TA01
3972f
1
LT3972
ABSOLUTE MAXIMUM RATINGS (Note 1)
Operating Junction Temperature Range (Note 2)
LT3972E.............................................–40°C to 125°C
LT3972I..............................................–40°C to 125°C
Storage Temperature Range...................–65°C to 150°C
Lead Temperature (Soldering, 10 sec)
V , RUN/SS Voltage (Note 5)...................................62V
IN
BOOST Pin Voltage ...................................................56V
BOOST Pin Above SW Pin.........................................30V
FB, RT, V Voltage.......................................................5V
C
PG, BD, SYNC Voltage ..............................................30V
(MSE Only) ....................................................... 300°C
PIN CONFIGURATION
TOP VIEW
TOP VIEW
BD
BOOST
SW
1
2
3
4
5
10 RT
BD
BOOST
SW
1
2
3
4
5
10 RT
9
8
7
6
V
C
9
8
7
6
V
C
11
11
FB
FB
V
PG
SYNC
IN
V
IN
PG
RUN/SS
RUN/SS
SYNC
MSE PACKAGE
10-LEAD PLASTIC MSOP
DD PACKAGE
θ
= 45°C/W, θ = 10°C/W
JA
JC
10-LEAD (3mm s 3mm) PLASTIC DFN
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
θ
= 45°C/W, θ = 10°C/W
JC
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
JA
ORDER INFORMATION
LEAD FREE FINISH
LT3972EDD#PBF
LT3972IDD#PBF
LT3972EMSE#PBF
LT3972IMSE#PBF
TAPE AND REEL
PART MARKING*
LDXR
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3972EDD#TRPBF
LT3972IDD#TRPBF
LT3972EMSE#TRPBF
LT3972IMSE#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
LDXR
LTDXS
LTDXS
10-Lead Plastic MSOP
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VRUN/SS = 10V, VBOOST = 15V, VBD = 3.3V unless otherwise
noted. (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
3
MAX
3.6
37
UNITS
V
●
●
Minimum Input Voltage
V
IN
Overvoltage Lockout
33
35
V
Quiescent Current from V
V
V
V
V
= 0.2V
0.01
30
0.5
65
μA
IN
RUN/SS
●
= 3V, Not Switching
= 0, Not Switching
μA
BD
120
0.01
160
0.5
μA
BD
Quiescent Current from BD
= 0.2V
μA
RUN/SS
3972f
2
LT3972
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VRUN/SS = 10V, VBOOST = 15V, VBD = 3.3V unless otherwise
noted. (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
90
1
MAX
130
5
UNITS
μA
●
V
BD
BD
= 3V, Not Switching
= 0, Not Switching
V
μA
Minimum Bias Voltage (BD Pin)
Feedback Voltage
2.7
3
V
780
775
790
790
800
805
mV
mV
●
●
FB Pin Bias Current (Note 3)
FB Voltage Line Regulation
V
= 0.8V, V = 1.2V
10
0.002
500
2000
60
40
nA
%/V
FB
C
4V < V < 33V
0.01
IN
Error Amp g
μMho
m
Error Amp Gain
V Source Current
μA
μA
A/V
V
C
V Sink Current
C
60
V Pin to Switch Current Gain
C
5.3
V Clamp Voltage
C
2
Switching Frequency
R = 8.66k
2.2
1.0
200
2.45
1.1
230
2.7
1.25
260
MHz
MHz
kHz
T
R = 29.4k
T
R = 187k
T
●
●
Minimum Switch Off-Time
Switch Current Limit
60
5.4
335
0.02
1.5
35
150
6.2
nS
A
Duty Cycle = 5%
4.6
Switch V
I
= 3.5A
= 0V
mV
μA
V
CESAT
SW
Boost Schottky Reverse Leakage
Minimum Boost Voltage (Note 4)
BOOST Pin Current
V
2
2
BD
I
= 1A
50
8
mA
μA
V
SW
RUN/SS Pin Current
V
= 2.5V
5
RUN/SS
RUN/SS Input Voltage High
RUN/SS Input Voltage Low
PG Threshold Offset from Feedback Voltage
PG Hysteresis
2.5
0.2
V
V
FB
Rising
65
10
mV
mV
μA
μA
V
PG Leakage
V
V
= 5V
0.1
800
1
PG
●
PG Sink Current
= 0.4V
200
0.5
PG
SYNC Low Threshold
SYNC High Threshold
SYNC Pin Bias Current
0.7
V
V
SYNC
= 0V
0.1
μA
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 3: Bias current flows out of the FB pin.
Note 4: This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the switch.
Note 5: Absolute Maximum at V and RUN/SS Pins is 62V for non-
IN
Note 2: The LT3972E is guaranteed to meet performance specifications
from 0°C to 125°C. Specifications over the –40°C to 125°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. The LT3972I specifications are
guaranteed over the –40°C to 125°C temperature range.
repetitive 1 minute transients, and 40V for continuous operation.
3972f
3
LT3972
TA = 25°C unless otherwise noted.
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency
Efficiency
Efficiency
100
90
80
70
60
50
100
90
80
70
60
50
100
3.0
2.5
2.0
1.5
1.0
0.5
V
= 12V
IN
V
= 12V
IN
90
80
70
V
= 30V
IN
V
= 30V
IN
V
= 24V
IN
V
= 24V
IN
V
V
= 12V
= 5V
OUT
IN
60
50
V
= 5V
V
= 3.3V
OUT
OUT
L = 4.7μH
L = 4.7μH
f = 600kHz
L = 3.3μH
f = 600kHz
f = 600kHz
0
0.5
1
1.5
2
2.5
3
3.5
2
OUTPUT CURRENT (A)
3.5
2
3.5
0
0.5
1
1.5
2.5
3
0
0.5
1
1.5
2.5
3
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
3680 G01
3680 G02
3680 G03
No Load Supply Current
No Load Supply Current
Maximum Load Current
130
110
90
5.5
5.0
4.5
400
V
= 3.3V
OUT
CATCH DIODE: DIODES, INC. PDS360
TYPICAL
350
300
V
V
= 12V
IN
OUT
= 3.3V
INCREASED SUPPLY
250
200
150
100
50
CURRENT DUE TO CATCH
DIODE LEAKAGE AT
MINIMUM
70
4.0
3.5
HIGH TEMPERATURE
50
V
T
= 3.3V
OUT
A
= 25°C
30
3.0
2.5
L = 4.7μH
f = 600kHz
10
0
0
5
10
15
20
25
30
35
5
10
15
20
25
30
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3680 G05
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
3680 G04
3680 G06
Switch Current Limit
Maximum Load Current
Switch Current Limit
5.5
5.0
4.5
4.0
3.5
3.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
6.0
5.5
5.0
TYPICAL
DUTY CYCLE = 10 %
DUTY CYCLE = 90 %
MINIMUM
4.5
4.0
V
A
= 5V
OUT
T
= 25°C
3.5
3.0
L = 4.7μH
f = 600kHz
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3680 G09
10
20
INPUT VOLTAGE (V)
25
30
5
15
20
60
40
DUTY CYCLE (%)
80
100
0
3680 G07
3680 G08
3972f
4
LT3972
TA = 25°C unless otherwise noted.
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Voltage Drop
Boost Pin Current
Feedback Voltage
120
105
90
75
60
45
30
15
0
840
700
600
820
800
780
760
500
400
300
200
100
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3680 G12
0
1
2
3
4
5
0
1
2
3
4
5
SWITCH CURRENT (A)
SWITCH CURRENT (A)
3680 G10
3680 G11
Switching Frequency
Frequency Foldback
Minimum Switch On-Time
1200
1000
800
1.20
1.15
1.10
1.05
1.00
0.95
0.90
0.85
0.80
140
120
100
80
60
40
20
600
400
200
0
0
700 800 900
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3680 G13
0
100 200 300 400 500 600
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3680 G15
FB PIN VOLTAGE (mV)
3680 G14
Soft-Start
RUN/SS Pin Current
Boost Diode
7
6
5
4
3
2
1
0
12
10
8
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
6
4
2
0
0.5
1
2
2.5
3
3.5
20
RUN/SS PIN VOLTAGE (V)
30
35
0
1.5
0
5
10
15
25
0
0.5
1.0
1.5
2.0
RUN/SS PIN VOLTAGE (V)
BOOST DIODE CURRENT (A)
3680 G16
3680 G17
3680 G18
3972f
5
LT3972
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C unless otherwise noted.
Error Amp Output Current
Minimum Input Voltage
Minimum Input Voltage
50
40
5.0
4.5
4.0
3.5
3.0
2.5
2.0
6.5
30
6.0
5.5
5.0
20
10
0
–10
–20
–30
–40
–50
V
A
= 5V
OUT
V
A
= 3.3V
OUT
4.5
4.0
T
= 25 °C
T
= 25°C
L = 4.7μH
f = 800kHz
L = 4.7μH
f = 800kHz
1
10
100
1000
10000
1
10
100
1000
10000
–200
–100
0
100
200
FB PIN ERROR VOLTAGE (mV)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
3680 G20
3680 G21
3680 G19
VC Voltages
Power Good Threshold
Switching Waveforms; Burst Mode
2.50
95
90
85
80
75
2.00
1.50
V
SW
5V/DIV
CURRENT LIMIT CLAMP
SWITCHING THRESHOLD
I
L
0.2A/DIV
1.00
0.50
0
V
OUT
10mV/DIV
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3680 G22
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3680 G23
3680 G24
V
V
I
= 12V
5μs/DIV
IN
= 3.3V
OUT
= 10mA
LOAD
Switching Waveforms; Transition
from Burst Mode to Full Frequency
Switching Waveforms; Full
Frequency Continuous Operation
V
SW
5V/DIV
V
SW
5V/DIV
I
I
L
L
0.5A/DIV
0.2A/DIV
V
OUT
V
OUT
10mV/DIV
10mV/DIV
3680 G25
3680 G26
V
V
I
= 12V
1μs/DIV
1μs/DIV
V
V
I
= 12V
IN
OUT
IN
OUT
= 3.3V
= 3.3V
= 1A
= 110mA
LOAD
LOAD
3972f
6
LT3972
PIN FUNCTIONS
BD (Pin 1): This pin connects to the anode of the boost
Schottky diode. BD also supplies current to the internal
regulator.
SYNC (Pin 6): This is the external clock synchronization
input.GroundthispinforlowrippleBurstModeoperationat
lowoutputloads.Tietoaclocksourceforsynchronization.
Clockedgesshouldhaveriseandfalltimesfasterthan1μs.
Tie Pin to GND if not used. See Synchronization section
in Applications Information.
BOOST (Pin 2): This pin is used to provide a drive
voltage,higherthantheinputvoltage,totheinternalbipolar
NPN power switch.
PG (Pin 7): The PG pin is the open collector output of an
internal comparator. PG remains low until the FB pin is
within9%ofthefinalregulationvoltage. PGoutputisvalid
SW (Pin 3): The SW pin is the output of the internal power
switch. Connect this pin to the inductor, catch diode and
boost capacitor.
when V is above 3.6V and RUN/SS is high.
IN
V (Pin 4): The V pin supplies current to the LT3972’s
IN
IN
FB (Pin 8): The LT3972 regulates the FB pin to 0.790V.
Connect the feedback resistor divider tap to this pin.
internal regulator and to the internal power switch. This
pin must be locally bypassed.
V (Pin 9): The V pin is the output of the internal error
C
C
RUN/SS (Pin 5): The RUN/SS pin is used to put the
LT3972 in shutdown mode. Tie to ground to shut down
the LT3972. Tie to 2.5V or more for normal operation. If
amplifier. The voltage on this pin controls the peak switch
current. Tie an RC network from this pin to ground to
compensate the control loop.
the shutdown feature is not used, tie this pin to the V
IN
pin. RUN/SS also provides a soft-start function; see the
RT(Pin10):OscillatorResistorInput.Connectingaresistor
to ground from this pin sets the switching frequency.
Applications Information section.
Exposed Pad (Pin 11): Ground. The Exposed Pad must
be soldered to PCB.
BLOCK DIAGRAM
V
IN
V
4
IN
–
+
C1
BD
1
INTERNAL 0.79V REF
RUN/SS
5
SLOPE COMP
3
SWITCH
BOOST
2
3
LATCH
C3
R
RT
OSCILLATOR
200kHzTO2.4MHz
Q
10
6
S
L1
R
SW
T
V
OUT
DISABLE
SYNC
C2
D1
BurstMode
DETECT
SOFT-START
PG
7
V
CLAMP
ERROR AMP
C
+ 0.73V
–
+
–
V
C
9
C
C
C
F
R
GND
11
FB
C
8
R2
R1
3680 BD
3972f
7
LT3972
OPERATION
The LT3972 is a constant frequency, current mode step-
down regulator. An oscillator, with frequency set by RT,
enables an RS flip-flop, turning on the internal power
switch. An amplifier and comparator monitor the current
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 opera-
tion.
flowing between the V and SW pins, turning the switch
IN
To further optimize efficiency, the LT3972 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 75μA in a typical application.
off when this current reaches a level determined by the
voltage at V . An error amplifier measures the output
C
voltage through an external resistor divider tied to the FB
pin and servos the V pin. If the error amplifier’s output
C
increases, more current is delivered to the output; if it
The oscillator reduces the LT3972’s operating frequency
when the voltage at the FB pin is low. This frequency
foldbackhelpstocontroltheoutputcurrentduringstartup
and overload.
decreases,lesscurrentisdelivered.Anactiveclamponthe
V pinprovidescurrentlimit. TheV pinisalsoclampedto
C
C
the voltage on the RUN/SS pin; soft-start is implemented
by generating a voltage ramp at the RUN/SS pin using an
external resistor and capacitor.
TheLT3972containsapowergoodcomparatorwhichtrips
when the FB pin is at 91% of its regulated value. The PG
output is an open-collector 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 LT3972 is
Aninternalregulatorprovidespowertothecontrolcircuitry.
The bias regulator normally draws power from the V pin,
IN
but if the BD pin is connected to an external voltage higher
than 3V bias power will be drawn from the external source
(typically the regulated output voltage). This improves
efficiency. The RUN/SS pin is used to place the LT3972
in shutdown, disconnecting the output and reducing the
input current to less than 0.5μA.
enabled and V is above 3.6V.
IN
The LT3972 has an overvoltage protection feature which
disables switching action when the V goes above 35V
IN
typical (33V minimum). When switching is disabled, the
LT3972 can safely sustain input voltages up to 62V.
The switch driver operates from either the input or from
the BOOST pin. An external capacitor and diode are used
3972f
8
LT3972
APPLICATIONS INFORMATION
FB Resistor Network
where V is the typical input voltage, V
is the output
OUT
IN
voltage, V is the catch diode drop (~0.5V) and V is the
D
SW
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
internal switch drop (~0.5V at max load). This equation
shows that slower switching frequency is necessary to
safely accommodate high V /V
ratio. Also, as shown
IN OUT
VOUT
0.79V
⎛
⎞
⎠
inthenextsection,lowerfrequencyallowsalowerdropout
voltage. The reason input voltage range depends on the
switchingfrequencyisbecausetheLT3972switchhasfinite
minimum on and off times. The switch can turn on for a
minimumof~150nsandturnoffforaminimumof~150ns.
Typical minimum on time at 25°C is 80ns. This means
that the minimum and maximum duty cycles are:
R1=R2
–1
⎜
⎝
⎟
Reference designators refer to the Block Diagram.
Setting the Switching Frequency
The LT3972 uses a constant frequency PWM architecture
thatcanbeprogrammedtoswitchfrom200kHzto2.4MHz
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 Figure 1.
DCMIN = fSWtON MIN
(
)
DCMAX =1– fSWtOFF MIN
(
)
where f is the switching frequency, the t
is the
ON(MIN)
SW
SWITCHING FREQUENCY (MHz)
R VALUE (kΩ)
T
minimum switch on time (~150ns), and the t
is
OFF(MIN)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
215
140
100
78.7
63.4
53.6
45.3
39.2
34
26.7
22.1
18.2
15
the minimum switch off time (~150ns). These equations
show that duty cycle range increases when switching
frequency is decreased.
A good choice of switching frequency should allow ad-
equate input voltage range (see next section) and keep
the inductor and capacitor values small.
Input Voltage Range
The maximum input voltage for LT3972 applications de-
pendsonswitchingfrequency,AbsoluteMaximumRatings
12.7
10.7
9.09
of the V and BOOST pins, and the operating mode.
IN
Figure 1. Switching Frequency vs. RT Value
TheLT3972canoperatefrominputvoltagesofupto33V,
and withstand voltages up to 62V. Note that while V is
IN
Operating Frequency Tradeoffs
above 35V typical (33V minimum and 37V maximum)
the part will keep the switch off and the output will not
be in regulation.
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
To safely allow inputs of up to 62V, be sure to choose a
Schottky diode, inductor size, and switching frequency
to allow safe operation at 37V according to the following
discussions.
highest acceptable switching frequency (f
) for a
SW(MAX)
While the output is in start-up, short-circuit, or other
overload conditions, the switching frequency should be
chosen according to the following equation:
given application can be calculated as follows:
VD + VOUT
fSW MAX
=
(
)
tON MIN V + V – V
(
)
D
IN
SW
(
)
3972f
9
LT3972
APPLICATIONS INFORMATION
the ripple current is:
VOUT + VD
V
=
) – VD + VSW
IN MAX
(
)
ΔI = 0.4(I
)
fSWtON MIN
L
OUT(MAX)
(
where I
is the maximum output load current. To
OUT(MAX)
where V
OUT
is the maximum operating input voltage,
IN(MAX)
guarantee sufficient output current, peak inductor current
V
is the output voltage, V is the catch diode drop
D
mustbelowerthantheLT3972’sswitchcurrentlimit(I ).
The peak inductor current is:
LIM
(~0.5V), V is the internal switch drop (~0.5V at max
SW
load), f is the switching frequency (set by R ), and
SW
ON(MIN)
T
I
= I
+ ΔI /2
L(PEAK)
OUT(MAX) L
t
istheminimumswitchontime(~100ns).Notethat
a higher switching frequency will depress the maximum
operating input voltage. Conversely, a lower switching
frequency will be necessary to achieve safe operation at
high input voltages.
where I
is the peak inductor current, I
is
L(PEAK)
OUT(MAX)
the maximum output load current, and ΔI is the inductor
L
ripple current. The LT3972’s switch current limit (I ) is
LIM
5.5A at low duty cycles and decreases linearly to 4.5A at
DC = 0.8. The maximum output current is a function of
the inductor ripple current:
If the output is in regulation and no short-circuit, start-
up, or overload events are expected, then input voltage
transients of up to 33V are acceptable regardless of the
switching frequency. In this mode, the LT3972 may enter
pulse skipping operation where some switching pulses
are skipped to maintain output regulation. In this mode
the output voltage ripple and inductor current ripple will
be higher than in normal operation.
I
= I – ΔI /2
LIM L
OUT(MAX)
Be sure to pick an inductor ripple current that provides
sufficient maximum output current (I ).
OUT(MAX)
The largest inductor ripple current occurs at the highest
V . To guarantee that the ripple current stays below the
IN
specified maximum, the inductor value should be chosen
The minimum input voltage is determined by either the
LT3972’s minimum operating voltage of ~3.6V or by its
maximum duty cycle (see equation in previous section).
The minimum input voltage due to duty cycle is:
according to the following equation:
⎛
⎞
⎛
⎜
⎝
⎞
⎟
⎠
VOUT + VD
fSWΔIL
VOUT + VD
⎜
⎜
⎝
⎟
⎟
⎠
L=
1–
V
IN(MAX)
VOUT + VD
V
=
) – VD + VSW
IN MIN
(
)
where V is the voltage drop of the catch diode (~0.4V),
1– fSWtOFF MIN
D
(
V
is the maximum input voltage, V
is the output
IN(MAX)
OUT
voltage, f is the switching frequency (set by RT), and
whereV
istheminimuminputvoltage,andt
SW
IN(MIN)
OFF(MIN)
L is in the inductor value.
is the minimum switch off time (150ns). 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.
Theinductor’sRMScurrentratingmustbegreaterthanthe
maximumloadcurrentanditssaturationcurrentshouldbe
about30%higher. Forrobustoperationinfaultconditions
(start-up or short circuit) and high input voltage (>30V),
the saturation current should be above 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 1 lists several vendors
and suitable types.
Inductor Selection
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The ripple current ΔI increases with higher V or V
L
IN
OUT
and decreases with higher inductance and faster switch-
ing frequency. A reasonable starting point for selecting
3972f
10
LT3972
APPLICATIONS INFORMATION
Table 1. Inductor Vendors
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 LT3972 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 10μF capacitor is capable of this task, but only if it is
placed close to the LT3972 and the catch diode (see the
PCB Layout section). A second precaution regarding the
ceramic input capacitor concerns the maximum input
voltage rating of the LT3972. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT3972 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3972’s
voltage rating. This situation is easily avoided (see the Hot
Plugging Safety section).
VENDOR URL
PART SERIES
TYPE
Murata
TDK
www.murata.com
LQH55D
Open
www.componenttdk.com SLF10145
Shielded
Toko
www.toko.com
D75C
D75F
Shielded
Open
Sumida
NEC
www.sumida.com
CDRH74
CR75
Shielded
Open
CDRH8D43
Shielded
www.nec.com
MPLC073
MPBI0755
Shielded
Shielded
Of course, such a simple design guide will not always re-
sult in the optimum inductor for your application. A larger
value inductor provides a slightly higher maximum load
current and will reduce the output voltage ripple. If your
load is lower than 3.5A, then you can decrease the value
oftheinductorandoperatewithhigherripplecurrent. This
allows you to use a physically smaller inductor, or one
with a lower DCR resulting in higher efficiency. There are
several graphs in the Typical Performance Characteristics
section of this data sheet that show the maximum load
current as a function of input voltage and inductor value
for several popular output voltages. Low inductance may
result in discontinuous mode operation, which is okay
but further reduces maximum load current. For details of
maximum output current and discontinuous mode opera-
tion, see Linear Technology Application Note 44. Finally,
For space sensitive applications, a 4.7μF ceramic capaci-
tor can be used for local bypassing of the LT3972 input.
However, the lower input capacitance will result in in-
creased input current ripple and input voltage ripple, and
may couple noise into other circuitry. Also, the increased
voltage ripple will raise the minimum operating voltage
of the LT3972 to ~3.7V.
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
withtheinductor,itfiltersthesquarewavegeneratedbythe
LT3972toproducetheDCoutput. Inthisroleitdetermines
the output ripple, and low impedance at the switching
frequency is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
LT3972’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
for duty cycles greater than 50% (V /V > 0.5), there
OUT IN
is a minimum inductance required to avoid subharmonic
oscillations. See AN19.
Input Capacitor
Bypass the input of the LT3972 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 10μF to 22μF ceramic capacitor is
adequate to bypass the LT3972 and will easily handle the
ripplecurrent.Notethatlargerinputcapacitanceisrequired
when a lower switching frequency is used. 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 lower performance electrolytic capacitor.
100
COUT
=
VOUT SW
f
where f
is in MHz, and C is the recommended
OUT
SW
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 if the compensation network is
also adjusted to maintain the loop bandwidth. A lower
3972f
11
LT3972
APPLICATIONS INFORMATION
Table 2. Capacitor Vendors
VENDOR
PHONE
URL
PART SERIES
COMMANDS
Panasonic
(714) 373-7366
www.panasonic.com
Ceramic,
Polymer,
Tantalum
EEF Series
Kemet
Sanyo
(864) 963-6300
(408) 749-9714
www.kemet.com
Ceramic,
Tantalum
T494, T495
POSCAP
www.sanyovideo.com
Ceramic,
Polymer,
Tantalum
Murata
AVX
(408) 436-1300
www.murata.com
www.avxcorp.com
Ceramic
Ceramic,
Tantalum
TPS Series
Taiyo Yuden
(864) 963-6300
www.taiyo-yuden.com
Ceramic
value of output capacitor can be used to save space and
cost but transient performance will suffer. See the Fre-
quency Compensation section to choose an appropriate
compensation network.
typical peak switch current. Peak reverse voltage is equal
to the regulator input voltage. Use a Schottky diode with a
reverse voltage rating greater than the input voltage. The
overvoltage protection feature in the LT3972 will keep the
switch off when V > 35V which allows the use of 40V
IN
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. High performance tantalum or
electrolyticcapacitorscanbeusedfortheoutputcapacitor.
Low ESR is important, so choose one that is intended for
use in switching regulators. The ESR should be specified
by the supplier, and should be 0.05Ω or less. Such a
capacitor will be larger than a ceramic capacitor and will
have a larger capacitance, because the capacitor must be
large to achieve low ESR. Table 2 lists several capacitor
vendors.
rated Schottky even when V ranges up to 62V. Table 3
IN
lists several Schottky diodes and their manufacturers.
Table 3. Diode Vendors
V
I
V AT 3A
R
AVE
F
PART NUMBER
(V)
(A)
(mV)
On Semiconductor
MBRA340
40
3
500
Diodes Inc.
PDS340
B340A
40
40
40
3
3
3
500
500
450
B340LA
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3972 due to their piezoelectric
nature. When in Burst Mode operation, the LT3972’s
switching frequency depends on the load current, and at
very light loads the LT3972 can excite the ceramic capaci-
tor at audio frequencies, generating audible noise. Since
the LT3972 operates at a lower current limit during Burst
Mode operation, the noise is nearly silent to a casual ear.
If this is unacceptable, use a high performance tantalum
or electrolytic capacitor at the output.
Catch Diode
The catch diode conducts current only during switch off
time. Average forward current in normal operation can
be calculated from:
I
= I
(V – V )/V
OUT IN OUT IN
D(AVG)
where I
is the output load current. The only reason to
OUT
consideradiodewithalargercurrentratingthannecessary
for nominal operation is for the worst-case condition of
shorted output. The diode current will then increase to the
3972f
12
LT3972
APPLICATIONS INFORMATION
Frequency Compensation
well as long as the value of the inductor is not too high
and the loop crossover frequency is much lower than the
The LT3972 uses current mode control to regulate the
output. This simplifies loop compensation. In particular,
theLT3972doesnotrequiretheESRoftheoutputcapacitor
for stability, so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size. Frequency
compensation is provided by the components tied to the
switching frequency. A phase lead capacitor (C ) across
PL
the feedback divider may improve the transient response.
Figure 3 shows the transient response when the load cur-
rent is stepped from 1A to 3A and back to 1A.
LT3972
V pin, as shown in Figure 2. Generally a capacitor (C )
C
C
and a resistor (R ) in series to ground are used. In addi-
CURRENT MODE
SW
C
OUTPUT
POWER STAGE
tion, there may be lower value capacitor in parallel. This
ERROR
g
= 5.3mho
m
C
PL
R1
AMPLIFIER
capacitor (C ) is not part of the loop compensation but
F
FB
–
is used to filter noise at the switching frequency, and is
required only if a phase-lead capacitor is used or if the
output capacitor has high ESR.
g
=
m
500μmho
ESR
+
0.8V
C1
+
3M
C1
Loop compensation determines the stability and transient
performance.Designingthecompensationnetworkisabit
complicatedandthebestvaluesdependontheapplication
and in particular the type of output capacitor. A practical
approach is to start with one of the circuits in this data
sheet that is similar to your application and tune the com-
pensation network to optimize the performance. Stability
should then be checked across all operating conditions,
includingloadcurrent, inputvoltageandtemperature. The
LT1375datasheetcontainsamorethoroughdiscussionof
loop compensation and describes how to test the stabil-
ity using a transient load. Figure 2 shows an equivalent
circuit for the LT3972 control loop. The error amplifier is a
transconductance amplifier with finite output impedance.
The power section, consisting of the modulator, power
switch and inductor, is modeled as a transconductance
amplifier generating an output current proportional to
POLYMER
OR
CERAMIC
V
GND
C
TANTALUM
R
C
F
R2
C
C
C
3680 F02
Figure 2. Model for Loop Response
V
OUT
100mV/DIV
I
L
1A/DIV
the voltage at the V pin. Note that the output capacitor
C
integratesthiscurrent, andthatthecapacitorontheV pin
C
V
V
= 12V
10μs/DIV
IN
OUT
3680 F03
(C )integratestheerroramplifieroutputcurrent,resulting
= 3.3V
C
in two poles in the loop. In most cases a zero is required
and comes from either the output capacitor ESR or from
a resistor R in series with C . This simple model works
Figure 3. Transient Load Response of the LT3972 Front Page
Application as the Load Current is Stepped from 1A to 3A.
VOUT = 5V
C
C
3972f
13
LT3972
APPLICATIONS INFORMATION
Low-Ripple Burst Mode and Pulse-Skip Mode
that the LT3972 will enter full frequency standard PWM
operationataloweroutputloadcurrentthanwheninBurst
Mode. The front page application circuit will switch at full
frequency at output loads higher than about 60mA.
The LT3972 is capable of operating in either Low-Ripple
Burst Mode or Pulse-Skip Mode which are selected us-
ing the SYNC pin. See the Synchronization section for
details.
BOOST and BIAS Pin Considerations
To enhance efficiency at light loads, the LT3972 can be
operatedinLow-RippleBurstModeoperationwhichkeeps
the output capacitor charged to the proper voltage while
minimizingtheinputquiescentcurrent.DuringBurstMode
operation,theLT3972deliverssinglecycleburstsofcurrent
to the output capacitor followed by sleep periods where
the output power is delivered to the load by the output
capacitor.BecausetheLT3972deliverspowertotheoutput
with single, low current pulses, the output ripple is kept
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 0.22μF capacitor will work well. Figure 2 shows three
ways to arrange the boost circuit. The BOOST pin must be
more than 2.3V above the SW pin for best efficiency. For
outputs of 3V and above, the standard circuit (Figure 5a)
is best. For outputs between 2.8V and 3V, use a 1μF boost
capacitor. A 2.5V output presents a special case because it
is marginally adequate to support the boosted drive stage
whileusingtheinternalboostdiode.ForreliableBOOSTpin
operation with 2.5V outputs use a good external Schottky
diode (such as the ON Semi MBR0540), and a 1μF boost
capacitor (see Figure 5b). For lower output voltages the
boost diode can be tied to the input (Figure 5c), or to
below 15mV for a typical application. In addition, V and
IN
BD quiescent currents are reduced to typically 30μA and
80μA respectively during the sleep time. As the load cur-
rentdecreasestowardsanoloadcondition,thepercentage
of time that the LT3972 operates in sleep mode increases
and the average input current is greatly reduced resulting
in high efficiency even at very low loads. See Figure 4.
At higher output loads (above 140mA for the front page
application) the LT3972 will be running at the frequency
another supply greater than 2.8V. Tying BD to V reduces
IN
the maximum input voltage to 28V. The circuit in Figure 5a
is more efficient because the BOOST pin current and BD
pin quiescent 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.
programmed by the R resistor, and will be operating in
T
standard PWM mode. The transition between PWM and
Low-Ripple Burst Mode is seamless, and will not disturb
the output voltage.
The minimum operating voltage of an LT3972 application
is limited by the minimum input voltage (3.6V) and by the
maximum duty cycle as outlined in a previous section. For
proper startup, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
or the LT3972 is turned on with its RUN/SS pin when the
output is already in regulation, then 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 6 shows a plot
of minimum load to start and to run as a function of input
voltage. In many cases the discharged output capacitor
If low quiescent current is not required the LT3972 can
operate in Pulse-Skip mode. The benefit of this mode is
V
SW
5V/DIV
I
L
0.2A/DIV
V
OUT
10mV/DIV
3680 F04
5μs/DIV
V
V
LOAD
= 12V
IN
= 3.3V
OUT
I
= 10mA
Figure 4. Burst Mode Operation
will present a load to the switcher, which will allow it to
3972f
14
LT3972
APPLICATIONS INFORMATION
V
6.0
5.5
5.0
4.5
4.0
3.5
3.0
OUT
BD
TO START
(WORST CASE)
BOOST
V
V
IN
LT3972
GND
IN
C3
SW
4.7μF
TO RUN
V
A
= 3.3V
OUT
T
= 25°C
(5a) For V
> 2.8V
OUT
2.5
2.0
L = 8.2μH
f = 700kHz
V
1
10
100
1000
10000
OUT
LOAD CURRENT (mA)
D2
BD
BOOST
8.0
7.0
6.0
5.0
4.0
3.0
2.0
V
V
IN
LT3972
IN
C3
TO START
(WORST CASE)
SW
GND
4.7μF
TO RUN
(5b) For 2.5V < V
< 2.8V
OUT
V
T
= 5V
OUT
A
V
OUT
= 25°C
L = 8.2μH
f = 700kHz
BD
BOOST
V
1
10
100
1000
10000
V
IN
LT3972
IN
LOAD CURRENT (mA)
C3
3680 F06
SW
GND
4.7μF
Figure 6. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
3680 FO5
Soft-Start
(5c) For V
< 2.5V; V
= 30V
IN(MAX)
OUT
The RUN/SS pin can be used to soft-start the LT3972,
reducing the maximum input current during start-up.
The RUN/SS pin is driven through an external RC filter to
create a voltage ramp at this pin. Figure 7 shows the start-
up and shut-down waveforms with the soft-start circuit.
By choosing a large RC time constant, the peak start-up
current can be reduced to the current that is required to
regulate the output, with no overshoot. Choose the value
oftheresistorsothatitcansupply20μAwhentheRUN/SS
pin reaches 2.5V.
Figure 5. Three Circuits For Generating The Boost Voltage
start. The plots show the worst-case situation where V
IN
is ramping very slowly. For lower start-up voltage, the
boost diode can be tied to V ; however, this restricts the
IN
input range to one-half of the absolute maximum rating
of the BOOST pin.
At light loads, the inductor current becomes discontinu-
ous and the effective duty cycle can be very high. This
reduces the minimum input voltage to approximately
Synchronization
300mV above V . At higher load currents, the inductor
OUT
current is continuous and the duty cycle is limited by the
maximum duty cycle of the LT3972, requiring a higher
input voltage to maintain regulation.
To select Low-Ripple Burst Mode operation, tie the SYNC
pin below 0.5V (this can be ground or a logic output).
3972f
15
LT3972
APPLICATIONS INFORMATION
plications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT3972’s
I
L
output. If the V pin is allowed to float and the RUN/SS
RUN
15k
1A/DIV
IN
pin is held high (either by a logic signal or because it is
RUN/SS
GND
V
RUN/SS
2V/DIV
tied to V ), then the LT3972’s internal circuitry will pull
IN
0.22μF
its quiescent current through its SW pin. This is fine if
your system can tolerate a few mA in this state. If you
ground the RUN/SS pin, the SW pin current will drop to
V
OUT
2V/DIV
3680 F07
2ms/DIV
essentially zero. However, if the V pin is grounded while
IN
the output is held high, then parasitic diodes inside the
Figure 7. To Soft-Start the LT3972, Add a Resisitor
and Capacitor to the RUN/SS Pin
LT3972 can pull large currents from the output through
the SW pin and the V pin. Figure 8 shows a circuit that
IN
Synchronizing the LT3972 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 0.3V
and peaks that are above 0.8V (up to 6V).
will run only when the input voltage is present and that
protects against a shorted or reversed input.
D4
MBRS340
V
IN
V
BOOST
SW
IN
The LT3972 will not enter Burst Mode at low output loads
while synchronized to an external clock, but instead will
skip pulses to maintain regulation.
LT3972
V
OUT
RUN/SS
V
C
GND FB
The LT3972 may be synchronized over a 250kHz to 2MHz
BACKUP
range. The R resistor should be chosen to set the LT3972
T
switchingfrequency20%belowthelowestsynchronization
input. For example, if the synchronization signal will be
3680 F08
250kHz and higher, the R should be chosen for 200kHz.
T
Figure 8. Diode D4 Prevents a Shorted Input from
To assure reliable and safe operation the LT3972 will only
synchronize when the output voltage is near regulation
as indicated by the PG flag. It is therefore necessary to
choosealargeenoughinductorvaluetosupplytherequired
Discharging a Backup Battery Tied to the Output. It Also
Protects the Circuit from a Reversed Input. The LT3972
Runs Only When the Input is Present
PCB Layout
output current at the frequency set by the R resistor. See
T
Inductor Selection section. It is also important to note that
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figure 9 shows
the recommended component placement with trace,
ground plane and via locations. Note that large, switched
slope compensation is set by the R value: When the sync
T
frequency is much higher than the one set by R , the slope
T
compensation will be significantly reduced which may
require a larger inductor value to prevent subharmonic
oscillation.
currents flow in the LT3972’s V and SW pins, the catch
IN
diode (D1) and the input capacitor (C1). The loop formed
bythesecomponentsshouldbeassmallaspossible.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.
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT3972 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
LT3972 is absent. This may occur in battery charging ap-
Finally, keep the FB and V nodes small so that the ground
C
3972f
16
LT3972
APPLICATIONS INFORMATION
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 LT3972 to additional ground planes within the circuit
board and on the bottom side.
L1
C2
V
OUT
C
C
R
RT
R
C
Hot Plugging Safely
R2
R1
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypasscapacitorofLT3972circuits.However,thesecapaci-
tors can cause problems if the LT3972 is plugged into a
live supply (see Linear Technology Application Note 88 for
a complete discussion). The low loss ceramic capacitor,
combined with stray inductance in series with the power
source, forms an under damped tank circuit, and the
C1
D1
R
GND
PG
3680 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
Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation
CLOSING SWITCH
DANGER
SIMULATES HOT PLUG
V
IN
20V/DIV
I
IN
V
IN
RINGING V MAY EXCEED
IN
ABSOLUTE MAXIMUM RATING
LT3972
4.7μF
+
I
IN
10A/DIV
LOW
STRAY
IMPEDANCE
ENERGIZED
24V SUPPLY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
20μs/DIV
(10a)
0.7W
V
IN
20V/DIV
LT3972
4.7μF
+
0.1μF
I
IN
10A/DIV
20μs/DIV
(10b)
V
IN
20V/DIV
LT3972
4.7μF
+
+
22μF
35V
AI.EI.
I
IN
10A/DIV
3680 F10
20μs/DIV
(10c)
Figure 10. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation when the LT3972 is Connected to a Live Supply
3972f
17
LT3972
APPLICATIONS INFORMATION
voltage at the V pin of the LT3972 can ring to twice the
to ambient can be reduced to
= 35°C/W or less. With
JA
IN
nominal input voltage, possibly exceeding the LT3972’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LT3972 into an
energized supply, the input network should be designed
topreventthisovershoot. Figure10showsthewaveforms
that result when an LT3972 circuit is connected to a 24V
supply through six feet of 24-gauge twisted pair. The
first plot is the response with a 4.7μF ceramic capacitor
at the input. The input voltage rings as high as 50V and
the input current peaks at 26A. A good solution is shown
in Figure 10b. A 0.7Ω resistor is added in series with the
input to eliminate the voltage overshoot (it also reduces
the peak input current). A 0.1μF capacitor improves high
frequency filtering. For high input voltages its impact on
efficiency is minor, reducing efficiency by 1.5 percent for
a 5V output at full load operating from 24V.
100 LFPM airflow, this resistance can fall by another 25%.
Further increases in airflow will lead to lower thermal re-
sistance. Because of the large output current capability of
the LT3972, it is possible to dissipate enough heat to raise
thejunctiontemperaturebeyondtheabsolutemaximumof
125°C. When operating at high ambient temperatures, the
maximum load current should be derated as the ambient
temperature approaches 125°C.
Power dissipation within the LT3972 can be estimated by
calculatingthetotalpowerlossfromanefficiencymeasure-
ment and subtracting the catch diode loss and inductor
loss. The die temperature is calculated by multiplying the
LT3972 power dissipation by the thermal resistance from
junction to ambient.
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 100
shows how to generate a bipolar output supply using a
buck regulator.
High Temperature Considerations
The PCB must provide heat sinking to keep the LT3972
cool. The Exposed Pad on the bottom of the package must
be soldered to a ground plane. This ground should be tied
to large copper layers below with thermal vias; these lay-
ers will spread the heat dissipated by the LT3972. Place
additional vias can reduce thermal resistance further. With
these steps, the thermal resistance from die (or junction)
TYPICAL APPLICATIONS
5V Step-Down Converter
V
OUT
V
IN
5V
6.3V TO 33V
TRANSIENT
TO 62V
3.5A
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
D
4.7μH
V
SW
C
LT3972
GND
10μF
RT
15k
PG
536k
SYNC
FB
63.4k
680pF
47μF
100k
f = 600kHz
3680 TA02
D: ON SEMI MBRA340
L: NEC MPLC0730L4R7
3972f
18
LT3972
TYPICAL APPLICATIONS
3.3V Step-Down Converter
V
3.3V
3.5A
OUT
V
IN
4.4V TO 33V
TRANSIENT
TO 62V
V
IN
BD
RUN/SS
BOOST
ON OFF
L
0.47μF
D
3.3μH
V
SW
C
LT3972
GND
4.7μF
RT
19k
PG
316k
SYNC
63.4k
FB
680pF
22μF
100k
f = 600kHz
3680 TA03
D: ON SEMI MBRA340
L: NEC MPLC0730L3R3
2.5V Step-Down Converter
V
2.5V
3.5A
OUT
V
IN
4V TO 33V
TRANSIENT
TO 62V
V
BD
D2
IN
RUN/SS
BOOST
ON OFF
L
1μF
D1
3.3μH
V
SW
C
4.7μF
LT3972
GND
RT
15.4k
PG
215k
SYNC
FB
63.4k
680pF
47μF
100k
f = 600kHz
3680 TA04
D1: ON SEMI MBRA340
D2: MBR0540
L: NEC MPLC0730L3R3
3972f
19
LT3972
TYPICAL APPLICATIONS
5V, 2MHz Step-Down Converter
V
V
OUT
IN
5V
8.6V TO 22V
TRANSIENT
TO 62V
2.5A
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
D
2.2μH
V
SW
C
LT3972
GND
4.7μF
RT
15k
PG
536k
SYNC
FB
12.7k
680pF
22μF
100k
f = 2MHz
3680 TA05
D: ON SEMI MBRA340
L: NEC MPLC0730L2R2
12V Step-Down Converter
V
OUT
V
IN
12V
15V TO 33V
TRANSIENT
TO 62V
3.5A
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
D
8.2μH
V
SW
C
LT3972
GND
10μF
RT
17.4k
PG
715k
SYNC
FB
63.4k
680pF
47μF
50k
f = 600kHz
3680 TA06
D: ON SEMI MBRA340
L: NEC MBP107558R2P
3972f
20
LT3972
TYPICAL APPLICATIONS
1.8V Step-Down Converter
V
1.8V
3.5A
OUT
V
IN
3.5V TO 27V
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
D
3.3μH
V
SW
C
LT3972
GND
4.7μF
RT
16.9k
PG
127k
SYNC
FB
78.7k
680pF
47μF
100k
f = 500kHz
3680 TA08
D: ON SEMI MBRA340
L: NEC MPLC0730L3R3
3972f
21
LT3972
PACKAGE DESCRIPTION
DD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1699)
0.675 p 0.05
3.50 p 0.05
2.15 p 0.05 (2 SIDES)
1.65 p 0.05
PACKAGE
OUTLINE
0.25 p 0.05
0.50
BSC
2.38 p 0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
R = 0.115
0.38 p 0.10
TYP
6
10
3.00 p 0.10 1.65 p 0.10
(4 SIDES)
(2 SIDES)
PIN 1
TOP MARK
(SEE NOTE 6)
(DD) DFN 1103
5
1
0.25 p 0.05
0.50 BSC
0.75 p 0.05
0.200 REF
2.38 p 0.10
(2 SIDES)
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
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
3972f
22
LT3972
PACKAGE DESCRIPTION
MSE Package
10-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1664 Rev B)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.06 p 0.102
2.794 p 0.102
(.110 p .004)
0.889 p 0.127
(.035 p .005)
(.081 p .004)
1
1.83 p 0.102
(.072 p .004)
5.23
(.206)
MIN
2.083 p 0.102 3.20 – 3.45
(.082 p .004) (.126 – .136)
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)
REF
10 9
8
7 6
RECOMMENDED SOLDER PAD LAYOUT
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) 0307 REV B
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
3972f
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
LT3972
TYPICAL APPLICATION
1.2V Step-Down Converter
V
1.2V
3.5A
OUT
V
IN
3.6V TO 27V
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
D
3.3μH
V
SW
C
LT3972
GND
4.7μF
RT
17k
PG
52.3k
SYNC
FB
78.7k
470pF
100k
100μF
f = 500kHz
3680 TA09
D: ON SEMI MBRA340
L: NEC MPLC0730L3R3
RELATED PARTS
PART NUMBER DESCRIPTION
COMMENTS
V : 3.6V to 36V, V
LT1933
LT1936
LT1940
500mA (I ), 500kHz Step-Down Switching Regulator in SOT-23
= 1.2V, I = 1.6mA, I < 1μA,
OUT(MIN) Q SD
OUT
IN
ThinSOTTM Package
36V, 1.4A (I ), 500kHz, High Efficiency Step-Down DC/DC
V : 3.6V to 36V, V
= 1.2V, I = 1.9mA, I < 1μA,
Q SD
OUT
IN
OUT(MIN)
Converter
MS8E Package
Dual 25V, 1.4A (I ), 1.1MHz, High Efficiency Step-Down DC/DC
Converter
V : 3.6V to 25V, V
= 1.2V, I = 3.8mA, I < 30μA,
Q SD
OUT
IN
OUT(MIN)
TSSOP16E Package
LT1976/LT1967 60V, 1.2A (I ), 200kHz/500kHz, High Efficiency Step-Down
V : 3.3V to 60V, V
= 1.2V, I = 100μA, I < 1μA,
Q SD
OUT
IN
OUT(MIN)
DC/DC Converters with Burst Mode Operation
TSSOP16E Package
LT3434/LT3435 60V, 2.4A (I ), 200kHz/500kHz, High Efficiency Step-Down
V : 3.3V to 60V, V
= 1.2V, I = 100μA, I < 1μA,
Q SD
OUT
IN
OUT(MIN)
DC/DC Converters with Burst Mode Operation
TSSOP16 Package
LT3437
LT3480
LT3481
LT3493
LT3505
LT3508
LT3680
LT3684
LT3685
LT3693
60V, 400mA (I ), Micropower Step-Down DC/DC Converter with V : 3.3V to 60V, V
= 1.25V, I = 100μA, I < 1μA,
Q SD
OUT
IN
OUT(MIN)
Burst Mode Operation
3mm
×
3mm DFN10 and TSSOP16E Packages
V : 3.6V to 38V, V = 0.78V, I = 70μA, I < 1μA,
IN OUT(MIN)
36V with Transient Protection to 60V, 2A (I ), 2.4MHz, High
OUT
Q
SD
Efficiency Step-Down DC/DC Converter with Burst Mode Operation
3mm
×
3mm DFN10 and MSOP10E Packages
34V with Transient Protection to 36V, 2A (I ), 2.8MHz, High
V : 3.6V to 34V, V
= 1.26V, I = 50μA, I < 1μA,
OUT
IN
OUT(MIN)
Q
SD
Efficiency Step-Down DC/DC Converter with Burst Mode Operation
3mm
×
3mm DFN10 and MSOP10E Packages
36V, 1.4A (I ), 750kHz High Efficiency Step-Down
V : 3.6V to 36V, V
= 0.8V, I = 1.9mA, I < 1μA,
OUT
IN
OUT(MIN)
Q
SD
DC/DC Converter
2mm
×
3mm DFN6 Package
36V with Transient Protection to 40V, 1.4A (I ), 3MHz,
V : 3.6V to 34V, V
= 0.78V, I = 2mA, I = 2μA,
OUT
IN
OUT(MIN)
Q
SD
High Efficiency Step-Down DC/DC Converter
3mm
×
3mm DFN8 and MSOP8E Packages
36V with Transient Protection to 40V, Dual 1.4A (I ), 3MHz,
V : 3.7V to 37V, V
= 0.8V, I = 4.6mA, I = 1μA,
OUT
IN
OUT(MIN)
Q
SD
High Efficiency Step-Down DC/DC Converter
4mm
×
4mm QFN24 and TSSOP16E Packages
36V, 3.5A, 2.4MHz, Low Quiescent Current (<75μA) Step-Down
DC/DC Converter
V : 3.6V to 36V, V
= 0.8V, I = 75μA, I < 1μA, 3mm ×
IN
OUT(MIN)
Q
SD
3mm DFN10, MS10E Package
34V with Transient Protection to 36V, 2A (I ), 2.8MHz,
V : 3.6V to 34V, V = 1.26V, I = 850μA, I < 1μA,
OUT
IN
OUT(MIN)
Q
SD
High Efficiency Step-Down DC/DC Converter
3mm
×
3mm DFN10 and MSOP10E Packages
36V with Transient Protection to 60V, Dual 2A (I ), 2.4MHz,
V : 3.6V to 38V, V
= 0.78V, I = 70μA, I < 1μA,
OUT
IN
OUT(MIN)
Q
SD
High Efficiency Step-Down DC/DC Converter
3mm
×
3mm DFN10 and MSOP10E Packages
36V, 3.5A, 2.4MHz, Step-Down DC/DC Converter
V : 3.6V to 36V, V
= 0.8V, I = 1.3mA, I < 1μA, 3mm ×
IN
OUT(MIN)
Q
SD
3mm DFN10, MS10E Package
3972f
LT 0908 • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
24
●
●
© LINEAR TECHNOLOGY CORPORATION 2008
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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