LT3973/LT3973-3.3/LT3973-5
42V, 750mA Step-Down
Regulator with 2.5µA
Quiescent Current and
Integrated Diodes
FEATURES
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DESCRIPTION
The
LT
®
3973
is an adjustable frequency monolithic buck
switching regulator that accepts a wide input voltage range
up to 42V, and consumes only 2.5µA of quiescent current.
A high efficiency switch is included on the die along with
the catch diode, boost diode, and the necessary oscillator,
control and logic circuitry. Low ripple Burst Mode operation
maintains high efficiency at low output currents while keep-
ing the output ripple below 10mV in a typical application.
A minimum dropout voltage of 530mV is maintained when
the input voltage drops below the programmed output volt-
age, such as during automotive cold crank. Current mode
topology is used for fast transient response and good loop
stability. A catch diode current limit provides protection
against shorted outputs and overvoltage conditions, with
thermal shutdown providing additional fault protection. An
accurate programmable undervoltage lockout feature is
available, producing a low shutdown current of 0.75µA.
A power good flag signals when V
OUT
reaches 90% of
the programmed output voltage. The LT3973 is available
in small, thermally enhanced 10-lead MSOP and 3mm ×
3mm DFN packages.
L,
LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
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Ultralow Quiescent Current
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2.5µA I at 12V to 3.3V
Q
IN
OUT
®
Low Ripple Burst Mode Operation
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Output Ripple < 10mV
P-P
Wide Input Voltage Range: 4.2V to 42V Operating
Adjustable Switching Frequency: 200kHz to 2.2MHz
Integrated Boost and Catch Diodes
750mA Output Current
Excellent Start-Up and Dropout Performance
Fixed Output Voltages: 3.3V, 5V
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1.9µA I at 12V
Q
IN
Accurate Programmable Undervoltage Lockout
Low Shutdown Current: I
Q
= 0.75µA
Internal Catch Diode Current Limit
Power Good Flag
Thermal Shutdown
Small, Thermally Enhanced 10-Lead MSOP and
(3mm × 3mm) DFN Packages
Automotive Battery Regulation
Power for Portable Products
Industrial Supplies
Gate Drive Bias
APPLICATIONS
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TYPICAL APPLICATION
5V Step-Down Converter
V
IN
5.6V TO 42V
V
IN
OFF ON
BOOST
LT3973
EN/UVLO
PG
RT
215k
f = 600kHz
SW
BD
OUT
FB
316k
3973 TA01a
Efficiency
90
V
IN
= 12V
1000
EFFICIENCY (%)
C3
0.47µF
L1
15µH
V
OUT
5V
750mA
1M
C2
22µF
80
100
POWER LOSS (mW)
70
10
60
1
C1
4.7µF
15pF
GND
50
0.1
40
0.01
0.1
1
100
10
LOAD CURRENT (mA)
3973 TA01b
0.01
3973fb
For more information
www.linear.com/LT3973
1
LT3973/LT3973-3.3/LT3973-5
ABSOLUTE MAXIMUM RATINGS
(Note 1)
V
IN
, EN/UVLO Voltage ...............................................42V
BOOST Pin Voltage ...................................................55V
BOOST Pin Above SW Pin.........................................25V
FB/V
OUT
, RT, PG Voltage .............................................6V
BD Voltage ................................................................25V
OUT Voltage ..............................................................14V
Operating Junction Temperature Range (Note 2)
LT3973E/LT3973E-X .......................... –40°C to 125°C
LT3973I/LT3973I-X ............................ –40°C to 125°C
LT3973H/LT3973H-X ......................... –40°C to 150°C
Storage Temperature Range .................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
MSE Only .......................................................... 300°C
PIN CONFIGURATION
TOP VIEW
*FB/V
OUT
OUT
EN/UVLO
V
IN
GND
1
2
3
4
5
11
GND
10 RT
9 PG
8 BD
7 BOOST
6 SW
TOP VIEW
*FB/V
OUT
OUT
EN/UVLO
V
IN
GND
1
2
3
4
5
11
GND
10
9
8
7
6
RT
PG
BD
BOOST
SW
DD PACKAGE
10-LEAD (3mm
×
3mm) PLASTIC DFN
MSE PACKAGE
10-LEAD PLASTIC MSOP
θ
JA
= 40°C/W,
θ
JC
= 5°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
θ
JA
= 45°C/W,
θ
JC
= 10°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
* FB for LT3973, V
OUT
for LT3973-3.3, LT3973-5.
ORDER INFORMATION
LEAD FREE FINISH
LT3973EDD#PBF
LT3973IDD#PBF
LT3973HDD#PBF
LT3973EMSE#PBF
LT3973IMSE#PBF
LT3973HMSE#PBF
LT3973EMSE-3.3#PBF
LT3973IMSE-3.3#PBF
LT3973HMSE-3.3#PBF
LT3973EDD-3.3#PBF
LT3973IDD-3.3#PBF
LT3973HDD-3.3#PBF
LT3973EMSE-5#PBF
LT3973IMSE-5#PBF
LT3973HMSE-5#PBF
TAPE AND REEL
LT3973EDD#TRPBF
LT3973IDD#TRPBF
LT3973HDD#TRPBF
LT3973EMSE#TRPBF
LT3973IMSE#TRPBF
LT3973HMSE#TRPBF
LT3973IMSE-3.3#TRPBF
LT3973EDD-3.3#TRPBF
LT3973IDD-3.3#TRPBF
LT3973HDD-3.3#TRPBF
LT3973EMSE-5#TRPBF
LT3973IMSE-5#TRPBF
LT3973HMSE-5#TRPBF
PART MARKING*
LGCH
LGCH
LGCH
LTFYS
LTFYS
LTFYS
LTGGB
LGGC
LGGC
LGGC
LTGGD
LTGGD
LTGGD
PACKAGE DESCRIPTION
10-Lead (3mm × 3mm) Plastic DFN
10-Lead (3mm × 3mm) Plastic DFN
10-Lead (3mm × 3mm) Plastic DFN
10-Lead Plastic MSOP
10-Lead Plastic MSOP
10-Lead Plastic MSOP
10-Lead Plastic MSOP
10-Lead Plastic MSOP
10-Lead Plastic MSOP
10-Lead (3mm × 3mm) Plastic DFN
10-Lead (3mm × 3mm) Plastic DFN
10-Lead (3mm × 3mm) Plastic DFN
10-Lead Plastic MSOP
10-Lead Plastic MSOP
10-Lead Plastic MSOP
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
–40°C to 150°C
–40°C to 125°C
–40°C to 125°C
–40°C to 150°C
–40°C to 125°C
–40°C to 125°C
–40°C to 150°C
–40°C to 125°C
–40°C to 125°C
–40°C to 150°C
–40°C to 125°C
–40°C to 125°C
–40°C to 150°C
LT3973EMSE-3.3#TRPBF LTGGB
LT3973HMSE-3.3#TRPBF LTGGB
2
3973fb
For more information
www.linear.com/LT3973
LT3973/LT3973-3.3/LT3973-5
ORDER INFORMATION
LEAD FREE FINISH
LT3973EDD-5#PBF
LT3973IDD-5#PBF
LT3973HDD-5#PBF
TAPE AND REEL
LT3973EDD-5#TRPBF
LT3973IDD-5#TRPBF
LT3973HDD-5#TRPBF
PART MARKING*
LGGF
LGGF
LGGF
PACKAGE DESCRIPTION
10-Lead (3mm × 3mm) Plastic DFN
10-Lead (3mm × 3mm) Plastic DFN
10-Lead (3mm × 3mm) Plastic DFN
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
–40°C to 150°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/
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 12V, V
BD
= 3.3V unless otherwise noted. (Note 2)
PARAMETER
Minimum Input Voltage (Note 3)
Quiescent Current from V
IN
V
EN/UVLO
Low
V
EN/UVLO
High
V
EN/UVLO
High, –40°C to 125°C
V
EN/UVLO
High, –40°C to 150°C
CONDITIONS
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ELECTRICAL CHARACTERISTICS
MIN
TYP
3.8
0.75
1.8
MAX
4.2
1.3
2.8
6
12
1.225
1.235
3.34
3.366
5.06
5.1
20
0.01
2.58
948
234
1.98
1.44
2
2
2
1.8
13
580
UNITS
V
µA
µA
µA
µA
V
V
V
V
V
V
nA
%/V
MHz
kHz
kHz
A
A
mV
µA
mV
µA
mV
µA
V
mA
mV
mV
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LT3973 Feedback Voltage
LT3973-3.3 Output Voltage
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1.195
1.185
3.26
3.234
4.94
4.9
1.21
1.21
3.3
3.3
5
5
0.1
0.0002
LT3973-5 Output Voltage
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LT3973 FB Pin Bias Current
FB/Output Voltage Line Regulation
Switching Frequency
V
FB
= 1.21V
4.2V < V
IN
< 40V
R
T
= 41.2k, V
IN
= 6V
R
T
= 158k, V
IN
= 6V
R
T
= 768k, V
IN
= 6V
V
IN
= 5V, V
FB
= 0V
V
IN
= 5V
I
SW
= 500mA
I
SCH
= 200mA, V
IN
= V
BD
= NC
V
SW
= 12V
I
SCH
= 50mA, V
IN
= NC, V
BOOST
= 0V
V
REVERSE
= 12V
V
IN
= 5V
I
SW
= 500mA, V
BOOST
= 15V
(V
IN
- OUT) Falling, V
IN
= 5V
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1.72
632
156
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2.15
790
195
1.65
1.15
250
0.05
550
0.05
820
0.02
Switch Current Limit
Catch Schottky Current Limit
Switch V
CESAT
Switch Leakage Current
Catch Schottky Forward Voltage
Catch Schottky Reverse Leakage
Boost Schottky Forward Voltage
Boost Schottky Reverse Leakage
Minimum Boost Voltage (Note 4)
BOOST Pin Current
Dropout Comparator Threshold
Dropout Comparator Hysteresis
1.237
0.92
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1.4
10
400
490
40
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3973fb
For more information
www.linear.com/LT3973
3
LT3973/LT3973-3.3/LT3973-5
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 12V, V
BD
= 3.3V unless otherwise noted. (Note 2)
EN/UVLO Pin Current
EN/UVLO Voltage Threshold
EN/UVLO Voltage Threshold
EN/UVLO Voltage Hysteresis
PG Threshold Offset from Feedback Voltage
PG Hysteresis as % of Output Voltage
PG Leakage
PG Sink Current
Minimum Switch On-Time
Minimum Switch Off-Time (Note 5)
V
IN
= 10V
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 LT3973E is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the –40°C
to 125°C operating junction temperature range are assured by design,
characterization, and correlation with statistical process controls. The
LT3973I is guaranteed over the full –40°C to 125°C operating junction
temperature range. The LT3973H is guaranteed over the full –40°C to
150°C operating junction temperature range. High junction temperatures
degrade operating lifetimes. Operating lifetime is derated at junction
temperatures greater than 125°C. The junction temperature (T
J
, in °C) is
calculated from the ambient temperature (T
A
in °C) and power dissipation
(PD, in Watts) according to the formula:
T
J
= T
A
+ (PD •
θ
JA
)
where
θ
JA
(in °C/W) is the package thermal impedance.
V
PG
= 3V
V
PG
= 0.4V
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ELECTRICAL CHARACTERISTICS
V
EN/UVLO
= 12V
EN/UVLO Falling, V
IN
≥ 4.2V
EN/UVLO Rising, V
IN
≥ 4.2V
V
FB
Rising
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1
1.09
1.12
6.5
1.16
1.19
30
10
0.8
0.01
220
350
70
130
30
1.23
1.28
45
13.5
1
nA
V
V
mV
%
%
µA
µA
ns
180
ns
Note 3:
This is the minimum input voltage for operation with accurate FB
reference voltage.
Note 4:
This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the switch.
Note 5:
The LT3973 contains circuitry that extends the maximum duty
cycle if there is sufficient voltage across the boost capacitor. See the
Application Information section for more details.
Note 6:
This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed the maximum operating junction temperature
when overtemperature protection is active. Continuous operation above
the specified maximum operating junction temperature may impair device
reliability or permanently damage the device.
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3973fb
For more information
www.linear.com/LT3973
LT3973/LT3973-3.3/LT3973-5
TYPICAL PERFORMANCE CHARACTERISTICS
T
A
= 25°C, unless otherwise noted.
90
80
70
EFFICIENCY (%)
60
50
40
30
20
Efficiency, V
OUT
= 3.3V
V
IN
= 12V
90
80
EFFICIENCY (%)
Efficiency, V
OUT
= 5V
V
IN
= 12V
Efficiency, V
OUT
= 3.3V
90
80
V
IN
= 12V
EFFICIENCY (%)
V
IN
= 24V
V
IN
= 36V
70
60
50
40
30
V
IN
= 24V
V
IN
= 36V
70
60
50
40
30
V
IN
= 24V
V
IN
= 36V
FRONT PAGE APPLICATION
V
OUT
= 3.3V
R1 = 1M
R2 = 576k
0
0.1
0.2 0.3 0.4 0.5
LOAD CURRENT (A)
0.6
0.7
3973 G01
FRONT PAGE APPLICATION
0
0.1
0.2 0.3 0.4 0.5
LOAD CURRENT (A)
0.6
0.7
3973 G02
20
0.01
FRONT PAGE APPLICATION
V
OUT
= 3.3V
R1 = 1M
R2 = 576k
0.1
1
10
100
LOAD CURRENT (mA)
3973 G03
Efficiency, V
OUT
= 5V
90
80
EFFICIENCY (%)
70
60
50
40
30
0.01
FRONT PAGE APPLICATION
0.1
1
10
100
LOAD CURRENT (mA)
3973 G04
LT3973 Feedback Voltage
1.220
3.32
LT3973-3.3 Output Voltage
V
IN
= 12V
1.215
FEEDBACK VOLTAGE (V)
OUTPUT VOLTAGE (V)
25 50
75 100 125 150
TEMPERATURE (°C)
3973 G05
3.31
V
IN
= 24V
V
IN
= 36V
1.210
3.30
1.205
3.29
1.200
3.28
1.195
–50 –25
0
3.27
–50 –25
0
25 50
75 100 125 150
TEMPERATURE (°C)
3973 G06
LT3973-5 Output Voltage
5.04
4.0
3.5
SUPPLY CURRENT (µA)
3.0
2.5
2.0
1.5
No-Load Supply Current
FRONT PAGE APPLICATION
V
OUT
= 3.3V
R1 = 1M
R2 = 576k
LT3973-3.3
35
30
SUPPLY CURRENT (µA)
25
20
15
10
5
No-Load Supply Current
FRONT PAGE APPLICATION
V
IN
= 12V
V
OUT
= 3.3V
R1 = 1M
R2 = 576k
5.02
OUTPUT VOLTAGE (V)
5.00
4.98
4.96
4.94
–50 –25
0
25 50
75 100 125 150
TEMPERATURE (°C)
3973 G07
5
10
30
15
25
20
INPUT VOLTAGE (V)
35
40
0
–50 –25
0
25 50
75 100 125 150
TEMPERATURE (°C)
3973 G09
3973 G08
3973fb
For more information
www.linear.com/LT3973
5
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