LT3970 Series
1
3970fb
Typical applicaTion
DescripTion
40V, 350mA Step-Down
Regulator with 2.5µA
Quiescent Current and
Integrated Diodes
The LT
®
3970 is an adjustable frequency monolithic buck
switching regulator that accepts a wide input voltage
range up to 40V, 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 neces-
sary oscillator, control and logic circuitry. Low ripple Burst
Mode operation maintains high efficiency at low output
currents while keeping the output ripple below 5mV in a
typical application. 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. An enable pin with accurate
threshold is available, producing a low shutdown current
of 0.7µA. A power good flag signals when VOUT reaches
90% of the programmed output voltage. The LT3970 is
available in small 10-pin MSOP and 3mm × 2mm DFN
packages.
L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
5V Step-Down Converter
FeaTures
applicaTions
n Low Ripple Burst Mode
®
Operation
2.5µA IQ at 12VIN to 3.3VOUT
Output Ripple < 5mVP-P
n Wide Input Voltage Range: 4.2V to 40V Operating
n Adjustable Switching Frequency: 200kHz to 2.2MHz
n Integrated Boost and Catch Diodes
n 350mA Output Current
n Fixed Output Voltages: 3.3V, 3.42V, 5V
1.8µA IQ at 12VIN
n Accurate 1V Enable Pin Threshold
n Low Shutdown Current: IQ = 0.7µA
n Internal Sense Limits Catch Diode Current
n Power Good Flag
n Output Voltage: 1.21V to 25V
n Internal Compensation
n Small 10-Pin MSOP and (3mm × 2mm) DFN Packages
n Automotive Battery Regulation
n Power for Portable Products
n Industrial Supplies
VIN BOOST
LT3970
SWEN
PG
RT
0.22µF
22pF
22µF
2.2µF
VIN
6V TO 40V
VOUT
5V
350mA
1M
316k
226k
f = 600kHz
22µH
BD
FB
GND
OFF ON
3490 TA01a
Efficiency
LOAD CURRENT (mA)
50
EFFICIENCY (%)
POWER LOSS (mW)
70
90
40
60
80
0.01 1 10 100
3970 TA01b
1
10
100
1000
0.01
0.1
0.1
VIN = 12V
LT3970 Series
2
3970fb
VIN, EN Voltage .........................................................40V
BOOST Pin Voltage ...................................................55V
BOOST Pin Above SW Pin .........................................30V
FB/VOUT, RT Voltage .................................................... 6V
PG, BD Voltage .........................................................30V
(Note 1)
orDer inFormaTion
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT3970EDDB#PBF LT3970EDDB#TRPBF LFCZ 10-Lead (3mm × 2mm) Plastic DFN –40°C to 125°C
LT3970IDDB#PBF LT3970IDDB#TRPBF LFCZ 10-Lead (3mm × 2mm) Plastic DFN –40°C to 125°C
LT3970EMS#PBF LT3970EMS#TRPBF LTFDB 10-Lead Plastic MSOP –40°C to 125°C
LT3970IMS#PBF LT3970IMS#TRPBF LTFDB 10-Lead Plastic MSOP –40°C to 125°C
LT3970EDDB-3.3#PBF LT3970EDDB-3.3#TRPBF LFQH 10-Lead (3mm × 2mm) Plastic DFN –40°C to 125°C
LT3970IDDB-3.3#PBF LT3970IDDB-3.3#TRPBF LFQH 10-Lead (3mm × 2mm) Plastic DFN –40°C to 125°C
LT3970EMS-3.3#PBF LT3970EMS-3.3#TRPBF LTFQG 10-Lead Plastic MSOP –40°C to 125°C
LT3970IMS-3.3#PBF LT3970IMS-3.3#TRPBF LTFQG 10-Lead Plastic MSOP –40°C to 125°C
LT3970HMS-3.3#PBF LT3970HMS-3.3#TRPBF LTFQG 10-Lead Plastic MSOP –40°C to 150°C
LT3970EDDB-3.42#PBF LT3970EDDB-3.42#TRPBF LGGG 10-Lead (3mm × 2mm) Plastic DFN –40°C to 125°C
LT3970EDDB-5#PBF LT3970EDDB-5#TRPBF LFQF 10-Lead (3mm × 2mm) Plastic DFN –40°C to 125°C
LT3970IDDB-5#PBF LT3970IDDB-5#TRPBF LFQF 10-Lead (3mm × 2mm) Plastic DFN –40°C to 125°C
LT3970EMS-5#PBF LT3970EMS-5#TRPBF LTFQD 10-Lead Plastic MSOP –40°C to 125°C
LT3970IMS-5#PBF LT3970IMS-5#TRPBF LTFQD 10-Lead Plastic MSOP –40°C to 125°C
LT3970HMS-5#PBF LT3970HMS-5#TRPBF LTFQD 10-Lead Plastic MSOP –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/
absoluTe maximum raTings
TOP VIEW
11
GND
DDB PACKAGE
10-LEAD (3mm
×
2mm) PLASTIC DFN
*FB/VOUT
EN
VIN
GND
GND
RT
PG
BD
BOOST
SW
6
8
7
9
10
5
4
2
3
1
θJA = 76°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
1
2
3
4
5
*FB/VOUT
EN
VIN
GND
GND
10
9
8
7
6
RT
PG
BD
BOOST
SW
TOP VIEW
MS PACKAGE
10-LEAD PLASTIC MSOP
θJA = 100°C/W
*FB for LT3970, VOUT for LT3970-3.3, LT3970-3.42, LT3970-5.
pin conFiguraTion
Operating Junction Temperature Range (Note 2)
E-, I-grade .......................................... –40°C to 125°C
H-grade .............................................. –40°C to 150°C
Storage Temperature Range ................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
MS Only ............................................................ 300°C
LT3970 Series
3
3970fb
elecTrical characTerisTics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VBD = 3.3V unless otherwise noted. (Note 2)
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Input Voltage l4 4.2 V
Quiescent Current from VIN VEN Low
VEN High
VEN High, –40°C to 125°C
VEN High, –40°C to 150°C
l
l
0.7
1.7
1.2
2.7
3.5
4
µA
µA
µA
µA
LT3970 Feedback Voltage
l
1.195
1.185
1.21
1.21
1.225
1.235
V
V
LT3970-3.3 Output Voltage
–40°C to 125°C
–40°C to 150°C
l
l
3.26
3.234
3.217
3.3
3.3
3.3
3.34
3.366
3.366
V
V
V
LT3970-3.42 Output Voltage
–40°C to 125°C
l
3.379
3.352
3.42
3.42
3.461
3.488
V
V
LT3970-5 Output Voltage
–40°C to 125°C
–40°C to 150°C
l
l
4.94
4.9
4.875
5
5
5
5.06
5.1
5.1
V
V
V
LT3970 FB Pin Bias Current (Note 3) VFB = 1.21V l0.1 20 nA
FB/Output Voltage Line Regulation 4.2V < VIN < 40V 0.0002 0.01 %/V
Switching Frequency RT = 41.2k, VIN = 6V
RT = 158k, VIN = 6V
RT = 768k, VIN = 6V
1.76
640
160
2.25
800
200
2.64
960
240
MHz
kHz
kHz
Switch Current Limit VIN = 5V, VFB = 0V 535 700 865 mA
Catch Schottky Current Limit VIN = 5V 350 400 500 mA
Switch VCESAT ISW = 200mA 175 mV
Switch Leakage Current 0.05 2 µA
Catch Schottky Forward Voltage ISCH = 100mA, VIN = VBD = NC 650 mV
Catch Schottky Reverse Leakage VSW = 12V 0.05 2 µA
Boost Schottky Forward Voltage ISCH = 50mA, VIN = NC, VBOOST = 0V 875 mV
Boost Schottky Reverse Leakage VREVERSE = 12V 0.02 2 µA
Minimum Boost Voltage (Note 4) VIN = 5V l1.4 1.8 V
BOOST Pin Current ISW = 200mA, VBOOST = 15V 7 10 mA
EN Pin Current VEN = 12V 1 30 nA
LT3970 EN Voltage Threshold EN Rising, VIN ≥ 4.2V l0.94 1 1.06 V
LT3970-X EN Voltage Threshold EN Rising, VIN ≥ 4.2V l0.93 1 1.07 V
EN Voltage Hysteresis 30 mV
LT3970 PG Threshold Offset from Feedback Voltage VFB Rising 80 120 160 mV
LT3970 PG Hysteresis 12 mV
LT3970-X PG Threshold Offset from Output Voltage VFB Rising 6.5 10 13.5 %
LT3970-X PG Hysteresis as % of Output Voltage 1.0 %
PG Leakage VPG = 3V 0.01 1 µA
PG Sink Current VPG = 0.4V l30 80 µA
Minimum Switch On-Time 90 ns
Minimum Switch Off-Time VIN = 10V l100 160 ns
LT3970 Series
4
3970fb
Typical perFormance characTerisTics
Efficiency, VOUT = 3.3V Efficiency, VOUT = 5V LT3970 Feedback Voltage
TA = 25°C, unless otherwise noted.
LOAD CURRENT (mA)
50
EFFICIENCY (%)
70
90
40
60
80
0.01 1 10 100
3970 G01
30
20 0.1
VIN = 12V
FRONT PAGE APPLICATION
VOUT = 3.3V
R1 = 1M
R2 = 576k
VIN = 36V
VIN = 24V
LOAD CURRENT (mA)
50
EFFICIENCY (%)
70
90
40
60
80
0.01 1 10 100
3970 G02
30
0.1
VIN = 12V
FRONT PAGE APPLICATION
VIN = 36V
VIN = 24V
TEMPERATURE (°C)
–50
FEEDBACK VOLTAGE (V)
1.210
1.215
1.220
25 75 150
3970 G03
1.205
1.200
1.195
–25 0 50 100 125
elecTrical characTerisTics
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 LT3970E 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
LT3970I is guaranteed over the full –40°C to 125°C operating junction
temperature range. The LT3970H 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.
Note 3: Bias current flows into the FB pin.
Note 4: This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the switch.
LT3970-5 Output VoltageLT3970-3.3 Output Voltage LT3970-3.42 Output Voltage
TEMPERATURE (°C)
–50
OUTPUT VOLTAGE (V)
3.30
3.31
3.32
25 75 150
3970 G04
3.29
3.28
3.27
–25 0 50 100 125
TEMPERATURE (°C)
–50
OUTPUT VOLTAGE (V)
3.42
3.43
3.44
25 75 125
3970 G05
3.41
3.40
3.39
–25 0 50 100
TEMPERATURE (°C)
–50
OUTPUT VOLTAGE (V)
5.00
5.02
5.04
25 75 150
3970 G06
4.98
4.96
4.94
–25 0 50 100 125
LT3970 Series
5
3970fb
No-Load Supply Current No-Load Supply Current Maximum Load Current
Maximum Load Current Maximum Load Current Load Regulation
Typical perFormance characTerisTics
Switch Current Limit Switch Current Limit Switching Frequency
TA = 25°C, unless otherwise noted.
INPUT VOLTAGE (V)
SUPPLY CURRENT (µA)
1.5
2.0
2.5
3.0
10 20 30 40
3970 G07
3.5
4.0
5 15 25 35
FRONT PAGE APPLICATION
VOUT = 3.3V
R1 = 1MΩ
R2 = 576k
LT3970-5
LT3970-3.3
TEMPERATURE (°C)
–50
SUPPLY CURRENT (µA)
9
12
15
25 75 150
3970 G08
6
3
0
–25 0 50 100 125
FRONT PAGE APPLICATION
VIN = 12V
VOUT = 3.3V
R1 = 1M
R2 = 576k
INPUT VOLTAGE (V)
5
350
LOAD CURRENT (mA)
400
450
500
550
10 15 20 25
3870 G09
30 35 40
FRONT PAGE APPLICATION
VOUT = 3.3V
TYPICAL
MINIMUM
INPUT VOLTAGE (V)
5
350
LOAD CURRENT (mA)
400
450
500
600
550
10 15 20 25
3870 G10
30 35 40
FRONT PAGE APPLICATION
VOUT = 5V
TYPICAL
MINIMUM
TEMPERATURE (°C)
–50
LOAD CURRENT (mA)
300
400
500
600
25 75
3970 G11
200
100
0
–25 0 50 100 150125
LIMITED BY MAXIMUM
JUNCTION TEMPERATURE;
θJA = 76°C/W
FRONT PAGE APPLICATION
VIN = 12V
VOUT = 5V
LIMITED BY CURRENT LIMIT H GRADE
LOAD CURRENT (mA)
0
LOAD REGULATION (%)
0.15
150
3970 G12
0
–0.10
50 100 200
–0.15
–0.20
0.20
0.10
0.05
–0.05
250 300 350
FRONT PAGE APPLICATION
REFERENCED FROM VOUT AT 100mA LOAD
DUTY CYCLE (%)
0
200
SWITCH CURRENT LIMIT (mA)
300
400
500
600
700
800
20 40 60 80
3970 G13
100
SWITCH PEAK
CURRENT LIMIT
CATCH DIODE VALLEY CURRENT LIMIT
TEMPERATURE (°C)
–50
200
SWITCH CURRENT LIMIT (mA)
300
400
500
600
0 50 100 150
3970 G14
700
800
–25 25 75 125
SWITCH PEAK CURRENT LIMIT
CATCH DIODE VALLEY CURRENT LIMIT
TEMPERATURE (°C)
–50
0
FREQUENCY (MHz)
0.4
0.8
1.2
1.6
2.4
–25 0 25 50 75
3970 G15
100 125 150
2.0
0.2
0.6
1.0
1.4
2.2
1.8
LT3970 Series
6
3970fb
Typical perFormance characTerisTics
Boost Diode Forward Voltage Catch Diode Forward Voltage Catch Diode Leakage
TA = 25°C, unless otherwise noted.
BOOST Pin Current
Minimum Input Voltage,
VOUT = 3.3V
Minimum Input Voltage,
VOUT = 5V
Switch VCESAT (ISW = 200mA)
vs Temperature Switch VCESAT
Minimum
Switch On-Time/Switch Off-Time
TEMPERATURE (°C)
–50
0
SWITCH ON-TIME/SWITCH OFF-TIME (ns)
20
60
80
100
200
140
050 75
3970 G16
40
160
180
120
–25 25 100 125 150
MINIMUM OFF-TIME
LOAD CURRENT = 175mA
MINIMUM ON-TIME
TEMPERATURE (°C)
–50
100
SWITCH V
CESAT
(mV)
150
200
250
–25 0 25 50
3970 G17
75 100 125 150
SWITCH CURRENT (mA)
0
SWITCH V
CESAT
(mV)
300
400
400
3970 G18
200
100
0100 200 300 500
SWITCH CURRENT (mA)
0
10
12
14
16
400
3970 G19
8
6
100 200 300 500
4
2
BOOST PIN CURRENT (mA)
LOAD CURRENT (mA)
0 50
2.5
INPUT VOLTAGE (V)
3.5
5.0
100 200 250
3970 G20
3.0
4.5
4.0
150 300 350
FRONT PAGE APPLICATION
VOUT = 3.3V
TO START/RUN
LOAD CURRENT (mA)
0 50
4.0
INPUT VOLTAGE (V)
5.0
6.5
100 200 250
3970 G21
4.5
6.0
5.5
150 300 350
FRONT PAGE APPLICATION
VOUT = 5V
TO START
TO RUN
BOOST DIODE CURRENT (mA)
0
0
BOOST DIODE VF (V)
0.2
0.4
0.6
0.8
1.0
1.2
50 100 150 200
3970 G22
–50°C
25°C
125°C
150°C
CATCH DIODE CURRENT (mA)
0
F
0.6
0.8
400
3970 G23
0.4
0.2
0100 200 300
–50°C
25°C
125°C
150°C
TEMPERATURE (°C)
–50
CATCH DIODE LEAKAGE (µA)
12
16
20
25 75 150
3970 G24
8
4
0
–25 0 50 100 125
VR = 12V
LT3970 Series
7
3970fb
Power Good Threshold EN Threshold
Transient Load Response; Load
Current is Stepped from 100mA
to 200mA
Switching Waveforms,
Burst Mode Operation
Transient Load Response; Load
Current is Stepped from 10mA
(Burst Mode Operation) to 110mA
Switching Waveforms, Full
Frequency Continuous Operation
Typical perFormance characTerisTics
TA = 25°C, unless otherwise noted.
TEMPERATURE (°C)
–50
88
THRESHOLD (%)
89
90
91
92
–25 0 25 50
3970 G25
75 100 125 150
TEMPERATURE (°C)
–50
0.950
THRESHOLD VOLTAGE (V)
0.975
1.000
1.025
1.050
–25 0 25 50
3970 G26
75 100 125 150
VOUT
100mV/DIV
IL
100mA/DIV
100µs/DIV
FRONT PAGE APPLICATION
VIN = 12V
VOUT = 5V
3970 G27
VOUT
100mV/DIV
IL
100mA/DIV
100µs/DIV
FRONT PAGE APPLICATION
VIN = 12V
VOUT = 5V
3970 G28
VOUT
5mV/DIV
VSW
5V/DIV
IL
100mA/DIV
2µs/DIV
FRONT PAGE APPLICATION
VIN = 12V
VOUT = 5V
ILOAD = 10mA
3970 G29
VOUT
5mV/DIV
VSW
5V/DIV
IL
200mA/DIV
1µs/DIV
FRONT PAGE APPLICATION
VIN = 12V
VOUT = 5V
ILOAD = 350mA
3970 G30
LT3970 Series
8
3970fb
pin FuncTions
FB (Pin 1, LT3970 Only): The LT3970 Regulates the FB
Pin to 1.21V. Connect the feedback resistor divider tap to
this pin.
VOUT (Pin 1, LT3970-X Only): The LT3970-3.3, LT3970-3.42
and LT3970-5 regulate the VOUT Pin to 3.3V, 3.42V and 5V
respectively. This pin connects to the internal feedback
divider that programs the fixed output voltage.
EN (Pin 2): The part is in shutdown when this pin is low
and active when this pin is high. The hysteretic threshold
voltage is 1V going up and 0.97V going down. Tie to VIN
if shutdown feature is not used. The EN threshold is ac-
curate only when VIN is above 4.2V. If VIN is lower than
4.2V, ground EN to place the part in shutdown.
VIN (Pin 3): The VIN pin supplies current to the LT3970’s
internal circuitry and to the internal power switch. This
pin must be locally bypassed.
GND (Pins 4, 5, Exposed Pad (Pin 11, DFN Only)): Ground.
Must be soldered to PCB.
SW (Pin 6): The SW pin is the output of an internal power
switch. Connect this pin to the inductor.
BOOST (Pin 7): This pin is used to provide a drive volt-
age, higher than the input voltage, to the internal bipolar
NPN power switch.
BD (Pin 8): This pin connects to the anode of the boost
diode. This pin also supplies current to the LT3970’s internal
regulator when BD is above 3.2V.
PG (Pin 9): The PG pin is the open-drain output of an
internal comparator. PG remains low until the FB pin is
within 10% of the final regulation voltage. PG is valid when
VIN is above 4.2V and EN is high.
RT (Pin 10): A resistor is tied between RT and ground to
set the switching frequency.
block Diagram
R
SWITCH LATCH
DBOOST
DCATCH
BOOST
OSCILLATOR
200kHz TO 2.2MHz
SLOPE COMP
S
Q
–
+
–
+
–
+
Burst Mode
DETECT
ERROR
AMP
1.09V
SHDN
EN
1V
C1
VIN
INTERNAL 1.21V REF
–
+
2
RT
RT
10
PG
9
FB
LT3970 ONLY
* LT3970-3.3: R1 = 12.65M, R2 = 7.35M
LT3970-3.42: R1 = 12.65M, R2 = 6.93M
LT3970-5: R1 = 15.15M, R2 = 4.85M
R2 R1
R2 R1
VC
1
GND
(4, 5)
VIN
3
7
BD 8
SW VOUT
6
C2
C3
3990 BD
L1
VOUT
LT3970-X
ONLY*
1
LT3970 Series
9
3970fb
operaTion
The LT3970 is a constant frequency, current mode step-
down regulator. An oscillator, with frequency set by RT,
sets an RS flip-flop, turning on the internal power switch.
An amplifier and comparator monitor the current flowing
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.
Another comparator monitors the current flowing through
the catch diode and reduces the operating frequency when
the current exceeds the 400mA bottom current limit. This
foldback in frequency helps to control the output current
in fault conditions such as a shorted output with high
input voltage. Maximum deliverable current to the output
is therefore limited by both switch current limit and catch
diode current limit.
An internal regulator provides power to the control cir-
cuitry. The bias regulator normally draws power from
the VIN pin, but if the BD pin is connected to an external
voltage higher than 3.2V, bias power will be drawn from
the external source (typically the regulated output voltage).
This improves efficiency.
If the EN pin is low, the LT3970 is shut down and draws
0.7µA 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 LT3970 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.
The LT3970 contains a power good comparator which
trips when the FB pin is at 90% 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 LT3970 is
enabled and VIN is above 4.2V.
LT3970 Series
10
3970fb
applicaTions inFormaTion
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
R1=R2 VOUT
1.21 – 1
Reference designators refer to the Block Diagram. Note
that choosing larger resistors will decrease the quiescent
current of the application circuit.
Setting the Switching Frequency
The LT3970 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 2.2MHz
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.
Table 1. Switching Frequency vs RT Value
SWITCHING FREQUENCY (MHz) RT VALUE (kΩ)
0.2
0.3
0.4
0.5
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
768
499
357
280
226
158
124
100
80.6
68.1
57.6
49.9
42.2
Operating Frequency Trade-Offs
Selection of the operating frequency is a trade-off 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:
fSW(MAX) =
V
OUT
+V
D
tON(MIN) VIN – VSW +VD
( )
where VIN is the typical input voltage, VOUT is the output
voltage, VD is the integrated catch diode drop (~0.7V),
and VSW is the internal switch drop (~0.5V at max load).
This equation shows that slower switching frequency is
necessary to accommodate high VIN/VOUT ratio.
Lower frequency also allows a lower dropout voltage. The
input voltage range depends on the switching frequency
because the LT3970 switch has finite minimum on and off
times. The switch can turn on for a minimum of ~150ns and
turn off for a minimum of ~160ns (note that the minimum
on-time is a strong function of temperature). This means
that the minimum and maximum duty cycles are:
DCMIN = fSW • tON(MIN)
DCMAX = 1 – fSW • tOFF(MIN)
where fSW is the switching frequency, the tON(MIN) is the
minimum switch on-time (~150ns), and the tOFF(MIN) is
the minimum switch off-time (~160ns). 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 minimum input voltage is determined by either the
LT3970’s minimum operating voltage of 4.2V or by its
maximum duty cycle (as explained in previous section).
The minimum input voltage due to duty cycle is:
VIN(MIN) =
V
OUT
+V
D
1– fSW •tOFF(MIN)
– VD+VSW
where VIN(MIN) is the minimum input voltage, VOUT is the
output voltage, VD is the catch diode drop (~0.7V), 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 (160ns). Note that higher switch-
ing frequency will increase the minimum input voltage.
If a lower dropout voltage is desired, a lower switching
frequency should be used.
LT3970 Series
11
3970fb
applicaTions inFormaTion
The highest allowed VIN during normal operation
(VIN(OP-MAX)) is limited by minimum duty cycle and can
be calculated by the following equation:
VIN(OP-MAX) =
V
OUT
+V
D
fSW •tON(MIN)
– VD+VSW
where tON(MIN) is the minimum switch on-time (~150ns).
However, the circuit will tolerate inputs up to the absolute
maximum ratings of the VIN and BOOST pins, regardless of
chosen switching frequency. During such transients where
VIN is higher than VIN(OP-MAX), the switching frequency will
be reduced below the programmed frequency to prevent
damage to the part. The output voltage ripple and inductor
current ripple may also be higher than in typical operation,
however the output will still be in regulation.
Inductor Selection
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The ripple current increases with higher VIN or VOUT and
decreases with higher inductance and faster switching
frequency. A good starting point for selecting the induc-
tor value is:
L=3
V
OUT
+V
D
fSW
where VD is the voltage drop of the catch diode (~0.7V),
L is in µH and fSW is in MHz. 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), the saturation current
should be above 500mA. 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 ap-
plications. Table 2 lists several vendors and suitable types.
This simple design guide will not always result in the
optimum inductor selection for a given application. As a
general rule, lower output voltages and higher switching
frequency will require smaller inductor values. If the ap-
plication requires less than 350mA load current, then a
lesser inductor value may be acceptable. This allows use
of 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 for several popular output voltages. Low
inductance may result in discontinuous mode operation,
which is acceptable but reduces maximum load current.
For details of maximum output current and discontinu-
ous mode operation, see Linear Technology Application
Note 44. Finally, for duty cycles greater than 50% (VOUT/
VIN > 0.5), there is a minimum inductance required to
avoid subharmonic oscillations. See Application Note 19.
Input Capacitor
Bypass the input of the LT3970 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 1µF to 4.7µF ceramic capacitor is
adequate to bypass the LT3970 and will easily handle the
ripple current. Note that larger input capacitance is required
Table 2. Inductor Vendors
VENDOR URL
Coilcraft www.coilcraft.com
Sumida www.sumida.com
Toko www.tokoam.com
Würth Elektronik www.we-online.com
Coiltronics www.cooperet.com
Murata www.murata.com
LT3970 Series
12
3970fb
applicaTions inFormaTion
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 sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT3970 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 1µF capacitor is capable of this task, but only if it is
placed close to the LT3970 (see the PCB Layout section).
A second precaution regarding the ceramic input capacitor
concerns the maximum input voltage rating of the LT3970.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank
circuit. If the LT3970 circuit is plugged into a live supply,
the input voltage can ring to twice its nominal value, pos-
sibly exceeding the LT3970’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. It stores
energy in order to satisfy transient loads and stabilize the
LT3970’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
COUT =
50
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 if combined with a phase lead capacitor (typically
22pF) between the output and the feedback pin. A lower
value of output capacitor can be used to save space and
cost but transient performance will suffer.
The second function is that the output capacitor, along
with the inductor, filters the square wave generated by the
LT3970 to produce the DC output. In this role it determines
the output ripple, so low impedance (at the switching
frequency) is important. The output ripple decreases with
increasing output capacitance, down to approximately
1mV. See Figure 1. Note that a larger phase lead capacitor
should be used with a large output capacitor.
COUT (µF)
0
0
WORST-CASE OUTPUT RIPPLE (mV)
2
6
8
10
40 80 100
18
3970 F01
4
20 60
12
14
16
FRONT PAGE APPLICATION
CLEAD = 47pF FOR COUT ≥ 47µF
VIN = 24V
VIN = 12V
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 3. Recommended Ceramic Capacitor Vendors
MANUFACTURER WEBSITE
AVX www.avxcorp.com
Murata www.murata.com
Taiyo Yuden www.t-yuden.com
Vishay Siliconix www.vishay.com
TDK www.tdk.com
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3970 due to their piezoelectric nature.
When in Burst Mode operation, the LT3970’s switching
frequency depends on the load current, and at very light
loads the LT3970 can excite the ceramic capacitor at audio
frequencies, generating audible noise. Since the LT3970
operates at a lower current limit during Burst Mode
Figure 1. Worst-Case Output Ripple Across Full Load Range
LT3970 Series
13
3970fb
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 LT3970. As pre-
viously mentioned, a ceramic input capacitor combined
with trace or cable inductance forms a high quality (under
damped) tank circuit. If the LT3970 circuit is plugged into a
live supply, the input voltage can ring to twice its nominal
value, possibly exceeding the LT3970’s rating. This situation
is easily avoided (see the Hot Plugging Safely section).
Low Ripple Burst Mode Operation
To enhance efficiency at light loads, the LT3970 operates
in low ripple Burst Mode operation which keeps the output
capacitor charged to the proper voltage while minimizing
the input quiescent current. During Burst Mode opera-
tion, the LT3970 delivers single cycle bursts of current to
the output capacitor followed by sleep periods where the
output power is delivered to the load by the output capaci-
tor. Because the LT3970 delivers power to the output with
single, low current pulses, the output ripple is kept below
5mV for a typical application. See Figure 2.
As the load current decreases towards a no load condition,
the percentage of time that the LT3970 operates in sleep
mode increases and the average input current is greatly
reduced resulting in high efficiency even at very low loads.
Note that during Burst Mode operation, the switching
frequency will be lower than the programmed switching
frequency. See Figure 3.
At higher output loads (above ~45mA for the front page
application) the LT3970 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 is seamless, and will not disturb
the output voltage.
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 0.22µF
capacitor will work well. Figure 4 shows two ways to ar-
range the boost circuit. The BOOST pin must be more than
1.9V above the SW pin for best efficiency. For outputs of
2.2V and above, the standard circuit (Figure 4a) is best.
For outputs between 2.2V and 2.5V, use a 0.47µF boost
capacitor. For output voltages below 2.2V, the boost diode
can be tied to the input (Figure 4b), or to another external
supply greater than 2.2V. However, the circuit in Figure 4a
is more efficient because the BOOST pin current and BD
pin quiescent current come from a lower voltage source.
Also, be sure that the maximum voltage ratings of the
BOOST and BD pins are not exceeded.
The minimum operating voltage of an LT3970 application
is limited by the minimum input voltage (4.2V) and by the
maximum duty cycle as outlined in a previous section. For
output voltages greater than 3.4V, the minimum input volt-
age is also limited by the boost circuit for proper start-up.
applicaTions inFormaTion
Figure 2. Burst Mode Operation
Figure 3. Switching Frequency in Burst Mode Operation
VOUT
5mV/DIV
VSW
5V/DIV
IL
100mA/DIV
2µs/DIV
FRONT PAGE APPLICATION
VIN = 12V
VOUT = 5V
ILOAD = 10mA
3970 F02
LOAD CURRENT (mA)
0
400
500
700
150 250
3070 F03
300
200
50 100 200 300 350
100
0
600
SWITCHING FREQUENCY (kHz)
FRONT PAGE APPLICATION
LT3970 Series
14
3970fb
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. 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.
Enable Pin
The LT3970 is in shutdown when the EN pin is low and
active when the pin is high. The rising threshold of the EN
comparator is 1V, with a 30mV hysteresis. This threshold
is accurate when VIN is above 4.2V. If VIN is lower than
4.2V, tie EN pin to GND to place the part in shutdown.
applicaTions inFormaTion
Figure 4. Two Circuits for Generating the Boost Voltage
Figure 5. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
Adding a resistor divider from VIN to EN programs the
LT3970 to regulate the output only when VIN is above a
desired voltage (see Figure 6). This threshold voltage,
VIN(EN), can be adjusted by setting the values R3 and R4
such that they satisfy the following equation:
VIN(EN) =
R3+R4
R4
•1V
where output regulation should not start until VIN is above
VIN(EN). Note that due to the comparator’s hysteresis,
regulation will not stop until the input falls slightly below
VIN(EN).
BD
LT3970
(4a) For VOUT ≥ 2.2V
BOOSTVIN
VIN
C3
VOUT
SW
GND
BD
LT3970
(4b) For VOUT < 2.2V; VIN < 27V
BOOSTVIN
VIN
C3
3970 F04
VOUT
SW
GND
LOAD CURRENT (mA)
0 50
2.5
INPUT VOLTAGE (V)
3.5
5.0
100 200 250
3.0
4.5
4.0
150 300 350
FRONT PAGE APPLICATION
VOUT = 3.3V
TO START/RUN
LOAD CURRENT (mA)
0 50
4.0
INPUT VOLTAGE (V)
5.0
6.5
100 200 250
3970 F05
4.5
6.0
5.5
150 300 350
FRONT PAGE APPLICATION
VOUT = 5V
TO START
TO RUN
LT3970 Series
15
3970fb
applicaTions inFormaTion
Be aware that while VIN is below 4.2V, the input current
may rise up to several hundred µA and the part may begin
to switch while the internal circuitry starts up. Figure 7
shows the startup behavior of a typical application with
different programmed VIN(EN).
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, a LT3970 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
LT3970 is absent. This may occur in battery charging ap-
plications or in battery backup systems where a battery
or some other supply is diode ORed with the LT3970’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 LT3970’s internal circuitry will pull its
quiescent current through its SW pin. This is fine if the
system can tolerate a few µA in this state. If the EN pin is
grounded, the SW pin current will drop to 0.7µA. However,
if the VIN pin is grounded while the output is held high,
regardless of EN, parasitic diodes inside the LT3970 can
pull current from the output through the SW pin and the
VIN pin. Figure 8 shows a circuit that will run only when
the input voltage is present and that protects against a
shorted or reversed input.
Figure 8. Diode D4 Prevents a Shorted Input from Discharging a
Backup Battery Tied to the Output. It Also Protects the Circuit from
a Reversed Input. The LT3970 Runs Only when the Input is Present
160
120
INPUT CURRENT (µA)
80
40
0
4
INPUT VOLTAGE (V)
3
OUTPUT VOLTAGE (V)
2
1
0
160
120
INPUT CURRENT (µA)
80
40
0
4
3
OUTPUT VOLTAGE (V)
2
1
0
3970 F07
VIN(EN) = 6V
R3 = 5M
R4 = 1M
VIN(EN) = 12V
R3 = 11M
R4 = 1M
012345678
INPUT VOLTAGE (V)
0 2 4 6 8 10 12 14
Figure 7. VIN Start-Up of Front Page Application with VOUT = 3.3V,
No-Load Current, and VIN(EN) programmed as in Figure 6.
BD
LT3970
BOOSTVIN
EN
VIN
VOUT
BACKUP
3970 F08
SW
D4
MBRS140
FBGND +
Figure 6. Enable
+
–
1V SHDN
3970 F06
LT3970
EN
VIN
VIN
R3
R4
LT3970 Series
16
3970fb
applicaTions inFormaTion
PCB Layout
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
currents flow in the LT3970’s VIN and SW pins, the internal
catch diode and the input capacitor. The loop formed by
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 nodes small so that the ground traces
will shield them from the SW and BOOST nodes. The
Exposed Pad on the bottom of the DFN 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 LT3970 to additional ground planes within the circuit
board and on the bottom side.
Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation
with stray inductance in series with the power source,
forms an under damped tank circuit, and the voltage at
the VIN pin of the LT3970 can ring to twice the nominal
input voltage, possibly exceeding the LT3970’s rating and
damaging the part. If the input supply is poorly controlled
or the user will be plugging the LT3970 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
For higher ambient temperatures, care should be taken
in the layout of the PCB to ensure good heat sinking
of the LT3970. The Exposed Pad on the bottom of the
DFN package must be soldered to a ground plane. This
ground should be tied to large copper layers below with
thermal vias; these layers will spread the heat dissipated
by the LT3970. Placing additional vias can reduce thermal
resistance further. In the MSOP package, the copper lead
frame is fused to GND (Pin 5) so place thermal vias near
this pin. The maximum load current should be derated
as the ambient temperature approaches the maximum
junction rating.
Power dissipation within the LT3970 can be estimated by
calculating the total power loss from an efficiency measure-
ment and subtracting inductor loss. The die temperature
is calculated by multiplying the LT3970 power dissipation
by the thermal resistance from junction to ambient.
Finally, be aware that at high ambient temperatures the
internal Schottky diode will have significant leakage current
(see Typical Performance Characteristics) increasing the
quiescent current of the LT3970 converter.
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.
6
8
7
9
10
5
4
2
3
1
VIAS TO LOCAL GROUND PLANE
VIAS TO VOUT
EN
GND
GND
PG
VOUT
GND
VIN
3970 F09
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3970 circuits. However, these ca-
pacitors can cause problems if the LT3970 is plugged into
a live supply. The low loss ceramic capacitor, combined
LT3970 Series
17
3970fb
Typical applicaTions
3.3V Step-Down Converter
VIN BOOST
LT3970
SWEN
PG
RT
C3
0.22µF
22pF
C2
22µF
C1
2.2µF
VIN
4.2V TO 40V
VOUT
3.3V
350mA
R1
1M
R2
576k
226k
f = 600kHz
L1
22µH
BD
FB
GND
OFF ON
3490 TA02
5V Step-Down Converter
2.5V Step-Down Converter
VIN BOOST
LT3970
SWEN
PG
RT
C3
0.22µF
22pF
226k
f = 600kHz
C2
22µF
C1
2.2µF
VIN
6V TO 40V
VOUT
5V
350mA
R1
1M
R2
316k
L1
22µH
BD
FB
GND
OFF ON
3490 TA03
VIN BOOST
LT3970
SWEN
PG
RT
C3
0.47µF
47pF
226k
f = 600kHz
C2
47µF
C1
2.2µF
VIN
4.2V TO 40V
VOUT
2.5V
350mA
R1
1M
R2
931k
L1
15µH
BD
FB
GND
OFF ON
3490 TA04
1.8V Step-Down Converter
VIN BOOST
LT3970
SWEN
BD
PG
RT
C3
0.22µF
47pF
226k
f = 600kHz
C2
47µF
C1
2.2µF
VIN
4.2V TO 27V
VOUT
1.8V
350mA
R1
487k
R2
1M
L1
10µH
FB
GND
OFF ON
3490 TA05
3.3V Step-Down Converter 5V Step-Down Converter
VIN BOOST
LT3970-3.3
SWEN
PG
RT
0.22µF
226k
f = 600kHz
22µF
2.2µF
VIN
4.2V TO 40V
VOUT
3.3V
350mA
22µH
BD
VOUT
GND
OFF ON
3490 TA03a
VIN BOOST
LT3970-5
SWEN
PG
RT
0.22µF
226k
f = 600kHz
22µF
2.2µF
VIN
6V TO 40V
VOUT
5V
350mA
22µH
BD
VOUT
GND
OFF ON
3490 TA03b
LT3970 Series
18
3970fb
Typical applicaTions
12V Step-Down Converter 5V, 2MHz Step-Down Converter
VIN BOOST
LT3970
SWEN
PG
RT
C3
0.1µF
22pF
226k
f = 600kHz
C2
22µF
C1
2.2µF
VIN
14V TO 40V
VOUT
12V
350mA
R1
1M
R2
113k
L1
33µH
BD
FB
GND
OFF ON
3490 TA06
VIN BOOST
LT3970
SWEN
PG
RT
49.9k
f = 2MHz
C3
0.1µF
22pF
C2
10µF
C1
1µF
VIN
8.5V TO 16V
TRANSIENTS
TO 40V
VOUT
5V
350mA
R1
1M
R2
316k
L1
10µH
BD
FB
GND
OFF ON
3490 TA07
5V Step-Down Converter with Reduced Input Current During Start-Up
VIN BOOST
LT3970
SWEN
PG
RT
0.22µF
22pF
22µF
2.2µF
VIN
6V TO 40V
VOUT
5V
350mA
1M
316k
226k
5M
kΩ
f = 600kHz
22µH
BD
FB
GND
3490 TA08a
1M
+
–
Input Current During Start-Up
INPUT VOLTAGE (V)
0
–0.5
INPUT CURRENT (mA)
0.5
1.5
2.5
246 8
3970 TA08b
10
3.5
4.5
0
1.0
2.0
3.0
4.0
12
INPUT CURRENT
DROPOUT
CONDITIONS
FRONT PAGE
APPLICATION
FRONT PAGE
APPLICATION
WITH EN
PROGRAMMED
TO 6V
Start-Up from High Impedance Input Source
VOUT
2V/DIV
VIN
5V/DIV
5ms/DIV
FRONT PAGE APPLICATION
VOUT = 5V
1k INPUT SOURCE RESISTANCE
2.5mA LOAD
3970 TA08c
EN PROGRAMMED TO 6V
LT3970 Series
19
3970fb
package DescripTion
DDB Package
10-Lead Plastic DFN (3mm × 2mm)
(Reference LTC DWG # 05-08-1722 Rev Ø)
2.00 ±0.10
(2 SIDES)
NOTE:
1. DRAWING CONFORMS TO VERSION (WECD-1) IN JEDEC PACKAGE OUTLINE M0-229
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
0.40 ± 0.10
BOTTOM VIEW—EXPOSED PAD
0.64 ± 0.05
(2 SIDES)
0.75 ±0.05
R = 0.115
TYP
R = 0.05
TYP
2.39 ±0.05
(2 SIDES)
3.00 ±0.10
(2 SIDES)
15
106
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
0.200 REF
0 – 0.05
(DDB10) DFN 0905 REV Ø
0.25 ± 0.05
2.39 ±0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
0.64 ±0.05
(2 SIDES)
1.15 ±0.05
0.70 ±0.05
2.55 ±0.05
PACKAGE
OUTLINE
0.25 ± 0.05
0.50 BSC
PIN 1
R = 0.20 OR
0.25 × 45°
CHAMFER
0.50 BSC
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
LT3970 Series
20
3970fb
package DescripTion
MSOP (MS) 0307 REV E
0.53 ± 0.152
(.021 ± .006)
SEATING
PLANE
0.18
(.007)
1.10
(.043)
MAX
0.17 –0.27
(.007 – .011)
TYP
0.86
(.034)
REF
0.50
(.0197)
BSC
1234 5
4.90 ± 0.152
(.193 ± .006)
0.497 ± 0.076
(.0196 ± .003)
REF
8910 76
3.00 ± 0.102
(.118 ± .004)
(NOTE 3)
3.00 ± 0.102
(.118 ± .004)
(NOTE 4)
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
0.254
(.010) 0° – 6° TYP
DETAIL “A”
DETAIL “A”
GAUGE PLANE
5.23
(.206)
MIN
3.20 – 3.45
(.126 – .136)
0.889 ± 0.127
(.035 ± .005)
RECOMMENDED SOLDER PAD LAYOUT
0.305 ± 0.038
(.0120 ± .0015)
TYP
0.50
(.0197)
BSC
0.1016 ± 0.0508
(.004 ± .002)
MS Package
10-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1661 Rev E)
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
LT3970 Series
21
3970fb
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.
revision hisTory
REV DATE DESCRIPTION PAGE NUMBER
A 5/10 Added LT3970-3.3 and LT3970-5 1 - 22
B 3/12 Title and Features clarified to add 3.42V fixed output version.
Clarified the Absolute Maximum Ratings section, added 3.42V output option in the Order Information section.
Added 3.42V output option in the Electrical Characteristics table.
Added 3.42V Output Voltage vs Temperature graph.
Clarified VOUT Pin Function and Block Diagram.
1
2
3
4
8
LT3970 Series
22
3970fb
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com
LINEAR TECHNOLOGY CORPORATION 2009
LT 0312 REV B • PRINTED IN USA
relaTeD parTs
PART NUMBER DESCRIPTION COMMENTS
LT3971 38V, 1.2A (IOUT), IQ = 2.8µA High Efficiency Step-Down DC/DC
Converter
VIN = 4.3V to 38V, VOUT(MIN) = 1.2V, IQ = 2.8µA, ISD < 1µA,
MSOP-10E, 3mm × 3mm DFN-10
LT3991 55V, 1.2A (IOUT), IQ = 2.8µA High Efficiency Step-Down DC/DC
Converter
VIN = 4.3V to 55V, VOUT(MIN) = 1.2V, IQ = 2.8µA, ISD < 1µA,
MSOP-10E, 3mm × 3mm DFN-10
LT3689 36V, 60V Transient Protection, 800mA, 2.2MHz High Efficiency
Micropower Step-Down DC/DC Converter with POR Reset and
Watchdog Timer
VIN = 3.6V to 36V, Transient to 60V, VOUT(MIN) = 0.8V, IQ = 75µA,
ISD < 1µA, 3mm × 3mm QFN-16
LT3682 36V, 60VMAX, 1A, 2.2MHz High Efficiency Micropower Step-Down
DC/DC Converter
VIN = 3.6V to 36V, VOUT(MIN) = 0.8V, IQ = 75µA, ISD < 1µA,
3mm × 3mm DFN-12
LT3480 36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High
Efficiency Step-Down DC/DC Converter with Burst Mode
®
Operation
VIN = 3.6V to 38V, VOUT(MIN) = 0.78V, IQ = 70µA, ISD < 1µA,
3mm × 3mm DFN-10, MSOP-10E
LT3685 36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High
Efficiency Step-Down DC/DC Converter
VIN = 3.6V to 38V, VOUT(MIN) = 0.78V, IQ = 70µA, ISD < 1µA,
3mm × 3mm DFN-10, MSOP-10E
LT3481 34V with Transient Protection to 36V, 2A (IOUT), 2.8MHz, High
Efficiency Step-Down DC/DC Converter with Burst Mode Operation
VIN = 3.6V to 34V, VOUT(MIN) = 1.26V, IQ = 50µA, ISD < 1µA,
3mm × 3mm DFN-10, MSOP-10E
LT1976/LT1977 60V, 1.2A (IOUT), 200/500kHz, High Efficiency Step-Down DC/DC
Converter with Burst Mode Operation
VIN = 3.3V to 60V, VOUT(MIN) = 1.20V, IQ = 100µA, ISD < 1µA,
TSSOP-16E