LT3470ETS8#TRMPBF [Linear]
LT3470 - Micropower Buck Regulator with Integrated Boost and Catch Diodes; Package: SOT; Pins: 8; Temperature Range: -40°C to 85°C;型号: | LT3470ETS8#TRMPBF |
厂家: | Linear |
描述: | LT3470 - Micropower Buck Regulator with Integrated Boost and Catch Diodes; Package: SOT; Pins: 8; Temperature Range: -40°C to 85°C 稳压器 开关式稳压器或控制器 调节器 电源电路 二极管 开关式控制器 光电二极管 |
文件: | 总20页 (文件大小:240K) |
中文: | 中文翻译 | 下载: | 下载PDF数据表文档文件 |
LT3470
Micropower Buck Regulator
with Integrated Boost and
Catch Diodes
FeaTures
DescripTion
The LT®3470 is a micropower step-down DC/DC con-
verter that integrates a 300mA power switch, catch diode
and boost diode into low profile 3mm × 2mm DD and
ThinSOT™ packages. The LT3470 combines Burst Mode
and continuous operation to allow the use of tiny induc-
tor and capacitors while providing a low ripple output to
loads of up to 200mA.
n
Low Quiescent Current: 26µA at 12V to 3.3V
IN
OUT
n
Integrated Boost and Catch Diodes
n
Input Range: 4V to 40V
n
Low Output Ripple: <10mV
n
<1µA in Shutdown Mode
n
Output Voltage: 1.25V to 16V
n
200mA Output Current
n
Hysteretic Mode Control
With its wide input range of 4V to 40V, the LT3470 can
regulateawidevarietyofpowersources,from2-cellLi-Ion
batteries to unregulated wall transformers and lead-acid
batteries. Quiescent current in regulation is just 26µA in
a typical application while a zero current shutdown mode
disconnects the load from the input source, simplifying
power management in battery-powered systems. Fast
currentlimitingandhystereticcontrolprotectstheLT3470
and external components against shorted outputs, even
at 40V input.
– Low Ripple Burst Mode® Operation at Light Loads
– Continuous Operation at Higher Loads
2
n
n
Solution Size as Small as 50mm
Low Profile (0.75mm) 3mm × 2mm Thermally
Enhanced 8-Lead DD and 1mm ThinSOT Packages
applicaTions
n
Automotive Battery Regulation
n
Power for Portable Products
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks
and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
n
Distributed Supply Regulation
n
Industrial Supplies
n
Wall Transformer Regulation
Typical applicaTion
Efficiency and Power Loss vs Load Current
90
80
70
60
50
40
30
20
10
1000
100
10
V
IN
V
= 12V
IN
7V TO 40V
0.22µF
33µH
V
BOOST
IN
LT3470
SHDN
V
OUT
5V
OFF ON
2.2µF
SW
200mA
BIAS
604k
1%
22pF
FB
22µF
GND
200k
1%
1
3470 TA01a
0.1
0.1
1
10
100
LOAD CURRENT (mA)
3470 TA02
3470fd
1
LT3470
absoluTe MaxiMuM raTings (Note 1)
V , SHDN Voltage ................................................... 40V
Operating Temperature Range (Note 2)
IN
BOOST Pin Voltage .................................................. 47V
BOOST Pin Above SW Pin........................................ 25V
FB Voltage.................................................................. 5V
BIAS Voltage.............................................................25V
LT3470E...............................................–40°C to 85°C
LT3470I ............................................. –40°C to 125°C
LT3470H ............................................ –40°C to 150°C
Storage Temperature Range .................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec) ..................300°C
SW Voltage ................................................................V
Maximum Junction Temperature
IN
LT3470E, LT3470I............................................. 125°C
LT3470H ........................................................... 150°C
pin conFiguraTion
TOP VIEW
TOP VIEW
FB
BIAS
1
2
3
4
8
7
6
5
SHDN
SHDN 1
NC 2
8 FB
NC
7 BIAS
6 BOOST
5 SW
9
BOOST
SW
V
IN
V
IN
3
GND
GND 4
TS8 PACKAGE
8-LEAD PLASTIC TSOT-23
= 140°C/W
DDB8 PACKAGE
8-LEAD (3mm × 2mm) PLASTIC DFN
= 180°C/W
θ
JA
θ
JA
EXPOSED PAD (PIN 9) IS GROUND (MUST BE SOLDERED TO PCB)
orDer inForMaTion
LEAD FREE FINISH
LT3470EDDB#PBF
LT3470IDDB#PBF
LT3470HDDB#PBF
TAPE AND REEL
PART MARKING
LBPN
PACKAGE DESCRIPTION
TEMPERATURE RANGE
–40°C to 85°C
LT3470EDDB#TRPBF
LT3470IDDB#TRPBF
LT3470HDDB#TRPBF
8-Lead (3mm × 2mm) Plastic DFN
8-Lead (3mm × 2mm) Plastic DFN
8-Lead (3mm × 2mm) Plastic DFN
8-Lead Plastic TSOT-23
LBPP
–40°C to 125°C
–40°C to 150°C
–40°C to 85°C
LCNR
LT3470ETS8#PBF
LT3470ITS8#PBF
LT3470HTS8#PBF
LEAD BASED FINISH
LT3470EDDB
LT3470ETS8#TRPBF
LT3470ITS8#TRPBF
LT3470HTS8#TRPBF
TAPE AND REEL
LTBDM
LTBPW
8-Lead Plastic TSOT-23
–40°C to 125°C
–40°C to 150°C
TEMPERATURE RANGE
–40°C to 85°C
LTCNQ
8-Lead Plastic TSOT-23
PART MARKING
LBPN
PACKAGE DESCRIPTION
LT3470EDDB#TR
8-Lead (3mm × 2mm) Plastic DFN
8-Lead (3mm × 2mm) Plastic DFN
8-Lead (3mm × 2mm) Plastic DFN
8-Lead Plastic TSOT-23
LT3470IDDB
LT3470HDDB
LT3470ETS8
LT3470ITS8
LT3470HTS8
LT3470IDDB#TR
LT3470HDDB#TR
LT3470ETS8#TR
LT3470ITS8#TR
LT3470HTS8#TR
LBPP
–40°C to 125°C
–40°C to 150°C
–40°C to 85°C
LCNR
LTBDM
LTBPW
LTCNQ
8-Lead Plastic TSOT-23
–40°C to 125°C
–40°C to 150°C
8-Lead Plastic TSOT-23
Consult LTC Marketing for parts specified with wider operating temperature ranges.
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/
3470fd
2
LT3470
The l denotes the specifications which apply over the full operating
elecTrical characTerisTics
temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VSHDN = 10V, VBOOST = 15V, VBIAS = 3V unless otherwise specified.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Minimum Input Voltage
Quiescent Current from V
●
●
4
V
V
V
V
= 0.2V
0.1
10
35
0.5
18
50
µA
µA
µA
IN
SHDN
= 3V, Not Switching
= 0V, Not Switching
BIAS
BIAS
Quiescent Current from Bias
V
V
V
= 0.2V
0.1
25
0.5
60
µA
µA
µA
SHDN
= 3V, Not Switching
= 0V, Not Switching
●
●
BIAS
BIAS
0.1
1.5
FB Comparator Trip Voltage
FB Pin Bias Current (Note 3)
V
FB
V
FB
Falling
1.228
1.250
1.265
V
= 1V, E- and I-Grade
35
35
80
150
nA
nA
●
●
H-Grade
4V < V < 40V
35
0.0006
500
225
nA
%/V
ns
FB Voltage Line Regulation
Minimum Switch Off-Time (Note 5)
Switch Leakage Current
0.01
IN
0.7
1.5
µA
Switch V
I
SW
I
SW
= 100mA (TS8 Package)
= 100mA (DD8 Package)
215
215
300
mV
mV
CESAT
Switch Top Current Limit
Switch Bottom Current Limit
Catch Schottky Drop
V
V
= 0V
= 0V
250
325
225
435
775
mA
mA
FB
FB
I
I
= 100mA (TS8 Package)
= 100mA (DD8 Package)
630
630
mV
mV
SH
SH
Catch Schottky Reverse Leakage
Boost Schottky Drop
V
= 10V
0.2
650
0.2
1.7
7
2
775
2
µA
mV
µA
V
SW
I
SH
= 30mA
Boost Schottky Reverse Leakage
Minimum Boost Voltage (Note 4)
BOOST Pin Current
V
= 10V, V
= 0V
BIAS
SW
●
2.2
12
5
I
SW
= 100mA
mA
µA
V
SHDN Pin Current
V
= 2.5V
1
SHDN
SHDN Input Voltage High
SHDN Input Voltage Low
2.5
0.2
V
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 LT3470E is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and
correlation with statistical process controls. The LT3470I specifications
are guaranteed over the –40°C to 125°C temperature range. LT3470H
specifications are guaranteed over –40°C to 150°C temperature range.
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: This parameter is assured by design and correlation with statistical
process controls.
3470fd
3
LT3470
Typical perForMance characTerisTics
Efficiency, VOUT = 3.3V
Efficiency, VOUT = 5V
VFB vs Temperature
90
90
1.260
1.255
1.250
1.245
L = TOKO D52LC 47µH
L = TOKO D52LC 47µH
T
= 25°C
T = 25°C
A
V
= 7V
A
IN
V
= 12V
IN
80
70
80
70
V
= 12V
= 24V
IN
V
= 36V
IN
V
IN
V
= 36V
V
= 24V
IN
IN
60
50
60
50
40
30
40
30
1.240
0.1
1
10
100
0.1
1
10
100
–50 –25
0
25
50
75 100 125
LOAD CURRENT (mA)
LOAD CURRENT (mA)
TEMPERATURE (°C)
3470 G01
3470 G02
3470 G03
Top and Bottom Switch Current
Limits (VFB = 0V) vs Temperature
VIN Quiescent Current
vs Temperature
BIAS Quiescent Current
(Bias > 3V) vs Temperature
30
25
20
15
50
40
30
20
10
0
400
350
300
BIAS < 3V
BIAS > 3V
250
200
150
100
50
10
5
0
0
50
100 125 150
–50 –25
0
25
75
–25
0
50 75 100 125 150
25
TEMPERATURE (°C)
–50
50
125 150
–50 –25
0
25
75 100
TEMPERATURE (°C)
TEMPERATURE (°C)
3470 G06
3470 G04
3470 G05
SHDN Bias Current
vs Temperature
FB Bias Current (VFB = 1V)
vs Temperature
60
50
40
30
20
10
0
9
8
7
6
5
4
3
2
1
V
= 36V
SHDN
V
= 2.5V
SHDN
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
–50 –25
0
25
125 150
50 75 100
TEMPERATURE (°C)
3470 G08
3470 G07
3470fd
4
LT3470
Typical perForMance characTerisTics
Boost Diode VF (IF = 50mA)
vs Temperature
FB Bias Current (VFB = 0V)
vs Temperature
Switch VCESAT (ISW = 100mA)
vs Temperature
120
100
80
300
250
200
150
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
60
40
20
0
100
50
0
0
50
100 125 150
50
100 125 150
–25
0
50 75 100 125 150
25
TEMPERATURE (°C)
–50 –25
0
25
75
–50 –25
0
25
75
–50
TEMPERATURE (°C)
TEMPERATURE (°C)
3470 G09
3470 G10
3470 G11
Catch Diode VF (IF = 100mA)
vs Temperature
Diode Leakage (VR = 36V)
vs Temperature
Switch VCESAT
60
55
50
45
40
35
30
25
20
15
10
5
0.7
0.6
700
600
500
CATCH
BOOST
0.5
0.4
0.3
0.2
0.1
400
300
200
100
0
0
0
50
100 125 150
100
200
400
–50 –25
0
25
75
50
TEMPERATURE (°C)
0
300
–50 –25
0
25
75 100
125 150
TEMPERATURE (°C)
SWITCH CURRENT (mA)
3470 G12
3470 G14
3470 G13
BOOST Pin Current
Catch Diode Forward Voltage
14
12
10
1.0
0.8
0.6
0.4
0.2
0
8
6
4
2
0
100
200
400
0
300
0
100
200
300
400
SWITCH CURRENT (mA)
CATCH DIODE CURRENT (mA)
3470 G15
3470 G16
3470fd
5
LT3470
Typical perForMance characTerisTics
Boost Diode Forward Voltage
Minimum Input Voltage, VOUT = 3.3V
Minimum Input Voltage, VOUT = 5V
8
7
6
5
4
6.0
5.5
900
800
700
600
500
400
300
200
100
0
T
= 25°C
T
A
= 25°C
A
V
TO START
IN
V
TO START
IN
5.0
4.5
V
TO RUN
IN
4.0
3.5
3.0
V
TO RUN
100
IN
100
150
0
200
100
BOOST DIODE CURRENT (mA)
0
50
150
200
50
0
50
150
200
LOAD CURRENT (mA)
LOAD CURRENT (mA)
3470 G19
3470 G18
3470 G17
(ThinSOT/DD)
pin FuncTions
SHDN (Pin 1/Pin 8): The SHDN pin is used to put the
LT3470 in shutdown mode. Tie to ground to shut down
the LT3470. Apply 2V or more for normal operation. If the
BOOST (Pin 6/Pin 3): The BOOST pin is used to provide
a drive voltage, which is higher than the input voltage, to
the internal bipolar NPN power switch.
shutdown feature is not used, tie this pin to the V pin.
IN
BIAS (Pin 7/Pin 2): The BIAS pin connects to the internal
boost Schottky diode and to the internal regulator. Tie to
NC (Pin 2/Pin 7): This pin can be left floating or connected
to V .
V
when V
> 2V or to V otherwise. When V
>
IN
OUT
OUT
IN
BIAS
3VtheBIASpinwillsupplycurrenttotheinternalregulator.
V
(Pin 3/Pin 6): The V pin supplies current to the
IN
IN
LT3470’s internal regulator and to the internal power
FB (Pin 8/Pin 1): The LT3470 regulates its feedback pin
to 1.25V. Connect the feedback resistor divider tap to this
pin. Set the output voltage according to VOUT = 1.25V
(1 + R1/R2) or R1 = R2 (VOUT/1.25 – 1).
switch. This pin must be locally bypassed.
GND (Pin 4/Pin 5): Tie the GND pin to a local ground plane
below the LT3470 and the circuit components. Return the
feedback divider to this pin.
Exposed Pad (DD, Pin 9): Ground. Must be soldered to
PCB.
SW (Pin 5/Pin 4): The SW pin is the output of the internal
powerswitch. Connectthispintotheinductor, catchdiode
and boost capacitor.
3470fd
6
LT3470
block DiagraM
V
IN
BIAS
V
IN
C1
+
–
BOOST
500ns
ONE SHOT
R
S
Q′
C3
Q
L1
C2
–
+
SW
V
OUT
ENABLE
Burst Mode
DETECT
NC
SHDN
V
REF
1.25V
g
m
GND
FB
R2
R1
3470 BD
3470fd
7
LT3470
operaTion
TheLT3470usesahystereticcontrolschemeinconjunction
with Burst Mode operation to provide low output ripple
and low quiescent current while using a tiny inductor and
capacitors.
comparator trips and resets the latch causing the switch
to turn off. While the switch is off, the inductor current
ramps down through the catch diode. When both the bot-
tom current comparator trips and the minimum off-time
one-shot expires, the latch turns the switch back on thus
completingafullcycle.Thehystereticactionofthiscontrol
scheme results in a switching frequency that depends
on inductor value, input and output voltage. Since the
switch only turns on when the catch diode current falls
below threshold, the part will automatically switch slower
to keep inductor current under control during start-up or
short-circuit conditions.
Operation can best be understood by studying the Block
Diagram. An error amplifier measures the output voltage
through an external resistor divider tied to the FB pin. If
the FB voltage is higher than VREF, the error amplifier will
shut off all the high power circuitry, leaving the LT3470
in its micropower state. As the FB voltage falls, the error
amplifier will enable the power section, causing the chip
to begin switching, thus delivering charge to the output
capacitor. If the load is light the part will alternate between
micropower and switching states to keep the output in
regulation (See Figure 1a). At higher loads the part will
switch continuously while the error amp servos the top
and bottom current limits to regulate the FB pin voltage
to 1.25V (See Figure 1b).
The switch driver operates from either the input or from
the BOOST pin. An external capacitor and internal diode
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.
The switching action is controlled by an RS latch and two
current comparators as follows: The switch turns on,
and the current through it ramps up until the top current
If the SHDN pin is grounded, all internal circuits are turned
off and V current reduces to the device leakage current,
IN
typically a few nA.
NO LOAD
200mA LOAD
V
V
OUT
20mV/DIV
OUT
20mV/DIV
I
L
100mA/DIV
I
L
100mA/DIV
1ms/DIV
1µs/DIV
10mA LOAD
150mA LOAD
V
V
OUT
20mV/DIV
OUT
20mV/DIV
I
I
L
L
100mA/DIV
100mA/DIV
3470 F01a
3470 F1b
5µs/DIV
1µs/DIV
(1a) Burst Mode Operation
(1b) Continuous Operation
Figure 1. Operating Waveforms of the LT3470 Converting 12V to 5V Using a 33µH Inductor and 10µF Output Capacitor
3470fd
8
LT3470
applicaTions inForMaTion
Input Voltage Range
where V
is the maximum input voltage for the ap-
IN(MAX)
plication,t
is~150nsandI
isthemaximum
ON-TIME(MIN)
MAX
The minimum input voltage required to generate a par-
ticular output voltage in an LT3470 application is limited
by either its 4V undervoltage lockout or by its maximum
duty cycle. The duty cycle is the fraction of time that the
internal switch is on and is determined by the input and
output voltages:
allowable increase in switch current during a minimum
switch on-time (150mA). While this equation provides a
safe inductor value, the resulting application circuit may
switch at too high a frequency to yield good efficiency.
It is advised that switching frequency be below 1.2MHz
during normal operation:
VOUT + VD
DC=
1–DC V + V
(
)
(
)
D
OUT
V – VSW + VD
f =
IN
L • ∆IL
where V is the forward voltage drop of the catch diode
D
(~0.6V) and V is the voltage drop of the internal switch
wherefistheswitchingfrequency, ∆I istheripplecurrent
SW
L
at maximum load (~0.4V). Given DC
to a minimum input voltage of:
= 0.90, this leads
in the inductor (~150mA), V is the forward voltage drop
MAX
D
of the catch diode, and V
is the desired output voltage.
OUT
If the application circuit is intended to operate at high duty
VOUT + VD
DCMAX
V
=
+ VSW – VD
IN(MIN)
cycles (V close to V ), it is important to look at the
IN
OUT
calculated value of the switch off-time:
Thisanalysisassumestheparthasstartedupsuchthatthe
capacitor tied between the BOOST and SW pins is charged
to more than 2V. For proper start-up, the minimum input
voltage is limited by the boost circuit as detailed in the
section BOOST Pin Considerations.
1–DC
tOFF-TIME
=
f
The calculated t
should be more than LT3470’s
OFF-TIME
minimum t
(See Electrical Characteristics), so
OFF-TIME
the application circuit is capable of delivering full rated
The maximum input voltage is limited by the absolute
output current. If the full output current of 200mA is not
maximum V rating of 40V, provided an inductor of suf-
IN
required, the calculated t
can be made less than
OFF-TIME
ficient value is used.
minimum t
possibly allowing the use of a smaller
OFF-TIME
inductor. See Table 1 for an inductor value selection guide.
Inductor Selection
Table 1. Recommended Inductors for Loads up to 200mA
The switching action of the LT3470 during continuous
operation produces a square wave at the SW pin that
results in a triangle wave of current in the inductor. The
hysteretic mode control regulates the top and bottom
current limits (see Electrical Characteristics) such that
the average inductor current equals the load current. For
safe operation, it must be noted that the LT3470 cannot
turn the switch on for less than ~150ns. If the inductor is
smallandtheinputvoltageishigh, thecurrentthroughthe
switch may exceed safe operating limit before the LT3470
is able to turn off. To prevent this from happening, the
following equation provides a minimum inductor value:
V
V
UP TO 16V
10µH
V
IN
UP TO 40V
33µH
OUT
IN
2.5V
3.3V
5V
10µH
33µH
15µH
33µH
12V
33µH
47µH
Chooseaninductorthatisintendedforpowerapplications.
Table 2 lists several manufacturers and inductor series.
For robust output short-circuit protection at high V (up
IN
to 40V) use at least a 33µH inductor with a minimum
450mA saturation current. If short-circuit performance is
not required, inductors with I of 300mA or more may
SAT
VIN(MAX) • tON-TIME(MIN)
LMIN
=
IMAX
3470fd
9
LT3470
applicaTions inForMaTion
Table 2. Inductor Vendors
VENDOR
URL
PART SERIES
INDUCTANCE RANGE (µH)
SIZE (mm)
Coilcraft
www.coilcraft.com
DO1605
ME3220
DO3314
10 to 47
10 to 47
10 to 47
1.8 × 5.4 × 4.2
2.0 × 3.2 × 2.5
1.4 × 3.3 × 3.3
Sumida
www.sumida.com
CR32
10 to 47
10 to 33
10 to 47
10 to 15
3.0 × 3.8 × 4.1
1.8 × 4.0 × 4.0
3.0 × 4.0 × 4.0
2.0 × 3.2 × 3.2
CDRH3D16/HP
CDRH3D28
CDRH2D18/HP
Toko
www.tokoam.com
DB320C
D52LC
10 to 27
10 to 47
2.0 × 3.8 × 3.8
2.0 × 5.0 × 5.0
Würth Elektronik
www.we-online.com
WE-PD2 Typ S
WE-TPC Typ S
10 to 47
10 to 22
3.2 × 4.0 × 4.5
1.6 × 3.8 × 3.8
Coiltronics
Murata
www.cooperet.com
www.murata.com
SD10
10 to 47
1.0 × 5.0 × 5.0
LQH43C
LQH32C
10 to 47
10 to 15
2.6 × 3.2 × 4.5
1.6 × 2.5 × 3.2
be used. It is important to note that inductor saturation
current is reduced at high temperatures—see inductor
vendors for more information.
LT3470’s switching frequency. The capacitor’s equivalent
seriesresistance(ESR)determinesthisimpedance.Choose
onewithlowESRintendedforuseinswitchingregulators.
The contribution to ripple voltage due to the ESR is ap-
Input Capacitor
proximately I • ESR. ESR should be less than ~150mΩ.
LIM
The value of the output capacitor must be large enough to
accept the energy stored in the inductor without a large
change in output voltage. Setting this voltage step equal
to 1% of the output voltage, the output capacitor must be:
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 V pin of the LT3470 and to force this switching
IN
2
current into a tight local loop, minimizing EMI. The input
capacitor must have low impedance at the switching
frequency to do this effectively. A 1µF to 2.2µF ceramic
capacitor satisfies these requirements.
ILIM
COUT > 50 •L •
V
OUT
where I is the top current limit with V = 0V (see Elec-
LIM
FB
trical Characteristics). For example, an LT3470 producing
3.3V with L = 33µH requires 22µF. The calculated value
can be relaxed if small circuit size is more important than
low output ripple.
If the input source impedance is high, a larger value ca-
pacitor may be required to keep input ripple low. In this
case, an electrolytic of 10µF or more in parallel with a 1µF
ceramic is a good combination. Be aware that the input
capacitor is subject to large surge currents if the LT3470
circuit is connected to a low impedance supply, and that
some electrolytic capacitors (in particular tantalum) must
be specified for such use.
Sanyo’s POSCAP series in B-case and provides very good
performance in a small package for the LT3470. Similar
performance in traditional tantalum capacitors requires
a larger package (C-case). With a high quality capacitor
filtering the ripple current from the inductor, the output
voltage ripple is determined by the delay in the LT3470’s
feedback comparator. This ripple can be reduced further
by adding a small (typically 22pF) phase lead capacitor
between the output and the feedback pin.
Output Capacitor and Output Ripple
The output capacitor filters the inductor’s ripple current
and stores energy to satisfy the load current when the
LT3470 is quiescent. In order to keep output voltage ripple
low, the impedance of the capacitor must be low at the
3470fd
10
LT3470
applicaTions inForMaTion
Ceramic Capacitors
BOOST and BIAS Pin Considerations
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3470. Not all ceramic capacitors are
suitable. X5R and X7R types are stable over temperature
and applied voltage and give dependable service. Other
types, including Y5V and Z5U have very large temperature
and voltage coefficients of capacitance. In an application
circuit they may have only a small fraction of their nominal
capacitanceresultinginmuchhigheroutputvoltageripple
than expected.
Capacitor C3 and the internal boost Schottky diode (see
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 2 shows two ways to ar-
range the boost circuit. The BOOST pin must be more than
2.5V above the SW pin for best efficiency. For outputs of
3.3V and above, the standard circuit (Figure 2a) is best.
For outputs between 2.5V and 3V, use a 0.47µF. For lower
output voltages the boost diode can be tied to the input
V
IN
C3
Ceramiccapacitorsarepiezoelectric.TheLT3470’sswitch-
ing frequency depends on the load current, and at light
loads the LT3470 can excite the ceramic capacitor at audio
frequencies, generating audible noise. Since the LT3470
operates at a lower current limit during Burst Mode opera-
tion, the noise is typically very quiet to a casual ear. If this
audible noise is unacceptable, use a high performance
electrolytic capacitor at the output. The input capacitor
can be a parallel combination of a 2.2µF ceramic capacitor
and a low cost electrolytic capacitor.
0.22µF
V
BOOST
LT3470
IN
V
SW
OUT
BIAS
GND
V
– V
BOOST
≅ V
SW OUT
BOOST
MAX V
≅ V + V
IN OUT
(2a)
V
IN
C3
0.22µF
V
BOOST
SW
IN
LT3470
A final precaution regarding ceramic capacitors concerns
themaximuminputvoltageratingoftheLT3470.Aceramic
input capacitor combined with trace or cable inductance
forms a high quality (under damped) tank circuit. If the
LT3470 circuit is plugged into a live supply, the input volt-
agecanringtotwiceitsnominalvalue, possiblyexceeding
the LT3470’s rating. This situation is easily avoided; see
the Hot-Plugging Safely section.
V
BIAS
OUT
GND
3470 F02
V
– V
BOOST
≅ V
SW IN
BOOST
MAX V
≅ 2V
IN
(2b)
Figure 2. Two Circuits for Generating the Boost Voltage
Table 3. Capacitor Vendors
VENDOR
PHONE
URL
PART SERIES
COMMENTS
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
(404) 436-1300
(864) 963-6300
www.murata.com
www.avxcorp.com
Ceramic
Ceramic,
Tantalum
TPS Series
Taiyo Yuden
www.taiyo-yuden.com Ceramic
3470fd
11
LT3470
applicaTions inForMaTion
(Figure 2b). The circuit in Figure 2a is more efficient
because the BOOST pin current and BIAS pin quiescent
currentcomesfromalowervoltagesource. Youmustalso
be sure that the maximum voltage ratings of the BOOST
and BIAS pins are not exceeded.
V to start and to run. At light loads, the inductor current
IN
becomes discontinuous and the effective duty cycle can
be very high. This reduces the minimum input voltage to
approximately300mVaboveV .Athigherloadcurrents,
OUT
the inductor current is continuous and the duty cycle is
limitedbythemaximumdutycycleoftheLT3470,requiring
a higher input voltage to maintain regulation.
The minimum operating voltage of an LT3470 application
is limited by the undervoltage lockout (4V) and by the
maximum duty cycle as outlined in a previous section. For
proper start-up, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
or the LT3470 is turned on with its SHDN pin when the
outputisalreadyinregulation,thentheboostcapacitormay
not be fully charged. The plots in Figure 3 show minimum
Shorted Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively at the top switch current limit maximum of 450mA,
an LT3470 buck regulator will tolerate a shorted output
even if V = 40V. There is another situation to consider
IN
in systems where the output will be held high when the
input to the LT3470 is absent. This may occur in battery
charging applications or in battery backup systems where
a battery or some other supply is diode OR-ed with the
Minimum Input Voltage, VOUT = 3.3V
6.0
T
= 25°C
A
V
TO START
IN
5.5
LT3470’s output. If the V pin is allowed to float and the
IN
SHDN pin is held high (either by a logic signal or because
5.0
4.5
it is tied to V ), then the LT3470’s internal circuitry will
IN
pull 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 SHDN pin, the SW pin current will drop to es-
4.0
3.5
3.0
V
TO RUN
IN
sentially zero. However, if the V pin is grounded while
IN
the output is held high, then parasitic diodes inside the
LT3470 can pull large currents from the output through
0
50
100
150
200
LOAD CURRENT (mA)
the SW pin and the V pin. Figure 4 shows a circuit that
IN
3470 G18
will run only when the input voltage is present and that
protects against a shorted or reversed input.
Minimum Input Voltage, VOUT = 5V
8
7
6
5
4
T
A
= 25°C
D1
V
TO START
IN
V
IN
V
BOOST
IN
LT3470 SOT-23
100k
1M
V
SHDN
SW
OUT
BIAS
V
TO RUN
IN
FB
GND
BACKUP
3470 F04
100
150
0
200
50
LOAD CURRENT (mA)
Figure 4. Diode D1 Prevents a Shorted Input from Discharging a
Backup Battery Tied to the Output; It Also Protects the Circuit
from a Reversed Input. The LT3470 Runs Only When the Input Is
Present Hot-Plugging Safely
3470 G19
Figure 3. The Minimum Input Voltage Depends on Output
Voltage, Load Current and Boost Circuit
3470fd
12
LT3470
applicaTions inForMaTion
PCB Layout
ideally at the ground terminal of the output capacitor C2.
Additionally, the SW and BOOST nodes should be kept as
small as possible. Unshielded inductors can induce noise
in the feedback path resulting in instability and increased
output ripple. To avoid this problem, use vias to route the
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Note that large,
switched currents flow in the power switch, the internal
catch diode and the input capacitor. The loop formed by
thesecomponentsshouldbeassmallaspossible.Further-
more, the system ground should be tied to the regulator
ground in only one place; this prevents the switched cur-
rent from injecting noise into the system ground. 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,
andtiethisgroundplanetosystemgroundatonelocation,
V
trace under the ground plane to the feedback divider
OUT
(as shown in Figure 5). Finally, keep the FB node as small
as possible so that the ground pin and ground traces
will shield it from the SW and BOOST nodes. Figure 5
shows component placement with trace, ground plane
and via locations. Include vias near the GND pin, or pad,
of the LT3470 to help remove heat from the LT3470 to
the ground plane.
SHDN
SHDN
V
IN
V
IN
C1
GND
GND
C2
V
OUT
V
OUT
3470 F05
VIAS TO FEEDBACK DIVIDER
VIAS TO LOCAL GROUND PLANE
OUTLINE OF LOCAL GROUND PLANE
(5a)
(5b)
Figure 5. A Good PCB Layout Ensures Proper, Low EMI Operation
3470fd
13
LT3470
applicaTions inForMaTion
Hot-Plugging Safely
(it also reduces the peak input current). A 0.1µF capacitor
improves high frequency filtering. This solution is smaller
and less expensive than the electrolytic capacitor. For high
input voltages its impact on efficiency is minor, reducing
efficiency less than one half percent for a 5V output at full
load operating from 24V.
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypasscapacitorofLT3470.However,thesecapacitorscan
cause problems if the LT3470 is plugged into a live supply
(seeLinearTechnologyApplicationNote88foracomplete
discussion).Thelowlossceramiccapacitorcombinedwith
stray inductance in series with the power source forms an
High Temperature Considerations
under damped tank circuit, and the voltage at the V pin
The die junction temperature of the LT3470 must be
lower than the maximum rating of 125°C (150°C for the
H-grade). Thisisgenerallynotaconcernunlesstheambi-
ent temperature is above 85°C. For higher temperatures,
care should be taken in the layout of the circuit to ensure
good heat sinking of the LT3470. The maximum load
current should be derated as the ambient temperature
approaches the maximum junction rating. The die tem-
perature is calculated by multiplying the LT3470 power
dissipation by the thermal resistance from junction to
ambient. Power dissipation within the LT3470 can be
estimated by calculating the total power loss from an
efficiency measurement. Thermal resistance depends
on the layout of the circuit board and choice of package.
The DD package with the exposed pad has a thermal
resistance of approximately 80°C/W while the ThinSOT
is approximately 150°C/W. Finally, be aware that at high
ambient temperatures the internal Schottky diode will
havesignificantleakagecurrent(seeTypicalPerformance
Characteristics) increasing the quiescent current of the
LT3470 converter.
IN
of the LT3470 can ring to twice the nominal input voltage,
possibly exceeding the LT3470’s rating and damaging the
part. If the input supply is poorly controlled or the user will
be plugging the LT3470 into an energized supply, the input
network should be designed to prevent this overshoot.
Figure 6 shows the waveforms that result when an LT3470
circuit is connected to a 24V supply through six feet of
24-gauge twisted pair. The first plot is the response with
a 2.2µF ceramic capacitor at the input. The input voltage
rings as high as 35V and the input current peaks at 20A.
One method of damping the tank circuit is to add another
capacitor with a series resistor to the circuit. In Figure 6b
an aluminum electrolytic capacitor has been added. This
capacitor’s high equivalent series resistance damps the
circuit and eliminates the voltage overshoot. The extra
capacitor improves low frequency ripple filtering and can
slightly improve the efficiency of the circuit, though it is
likelytobethelargestcomponentinthecircuit. Analterna-
tive solution is shown in Figure 6c. A 1Ω resistor is added
in series with the input to eliminate the voltage overshoot
3470fd
14
LT3470
applicaTions inForMaTion
CLOSING SWITCH
SIMULATES HOT PLUG
I
IN
V
IN
LT3470
2.2µF
V
IN
10V/DIV
+
I
IN
10A/DIV
LOW
STRAY
IMPEDANCE
ENERGIZED
24V SUPPLY
INDUCTANCE
10µs/DIV
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
(6a)
LT3470
2.2µF
+
10µF
35V
AI.EI.
(6b)
1Ω
LT3470
2.2µF
0.1µF
3470 F06
(6c)
Figure 6. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT3470 Is Connected to a Live Supply
3470fd
15
LT3470
Typical applicaTions
3.3V Step-Down Converter
5V Step-Down Converter
V
V
IN
IN
C3
C3
5.5V TO 40V
7V TO 40V
0.22µF, 6.3V
0.22µF, 6.3V
V
BOOST
V
BOOST
IN
IN
L1
33µH
L1
33µH
LT3470
SHDN
LT3470
SHDN
V
V
OUT
OUT
3.3V
200mA
5V
OFF ON
SW
OFF ON
SW
200mA
BIAS
BIAS
R1
R1
22pF
22pF
324k
604k
C1
1µF
C2
22µF
C1
1µF
C2
FB
FB
22µF
GND
GND
R2
200k
R2
200k
3470 TA03
3470 TA04
C1: TDK C3216JB1H105M
C2: CE JMK316 BJ226ML-T
L1: TOKO A914BYW-330M=P3
C1: TDK C3216JB1H105M
C2: CE JMK316 BJ226ML-T
L1: TOKO A993AS-270M=P3
2.5V Step-Down Converter
1.8V Step-Down Converter
V
V
IN
IN
C3
C3
4V TO 23V
4.7V TO 40V
0.22µF, 25V
0.47µF, 6.3V
V
BOOST
SW
V
BOOST
LT3470
SHDN
IN
IN
L1
22µH
L1
33µH
LT3470
V
V
OUT
OUT
1.8V
2.5V
OFF ON
SHDN
OFF ON
SW
200mA
200mA
BIAS
R1
BIAS
R1
22pF
22pF
147k
200k
C1
1µF
C2
C1
1µF
C2
22µF
FB
FB
22µF
GND
GND
R2
332k
R2
200k
3470 TA07
3470 TA05
C1: TDK C3216JB1H105M
C2: TDK C2012JB0J226M
L1: MURATA LQH32CN150K53
C1: TDK C3216JB1H105M
C2: TDK C2012JB0J226M
L1: SUMIDA CDRH3D28
12V Step-Down Converter
V
IN
C3
15V TO 34V
0.22µF, 16V
L1
33µH
V
BOOST
IN
LT3470
SHDN
V
OUT
12V
OFF ON
SW
200mA
BIAS
R1
22pF
866k
C1
1µF
C2
10µF
FB
GND
R2
100k
3470 TA06
C1: TDK C3216JB1H105M
C2: TDK C3216JB1C106M
L1: MURATA LQH32CN150K53
3470fd
16
LT3470
package DescripTion
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
TS8 Package
8-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1637 Rev A)
2.90 BSC
(NOTE 4)
0.40
MAX
0.65
REF
1.22 REF
1.4 MIN
1.50 – 1.75
(NOTE 4)
2.80 BSC
3.85 MAX 2.62 REF
PIN ONE ID
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
0.22 – 0.36
8 PLCS (NOTE 3)
0.65 BSC
0.80 – 0.90
0.20 BSC
DATUM ‘A’
0.01 – 0.10
1.00 MAX
0.30 – 0.50 REF
1.95 BSC
TS8 TSOT-23 0710 REV A
0.09 – 0.20
(NOTE 3)
NOTE:
1. DIMENSIONS ARE IN MILLIMETERS
2. DRAWING NOT TO SCALE
3. DIMENSIONS ARE INCLUSIVE OF PLATING
4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR
5. MOLD FLASH SHALL NOT EXCEED 0.254mm
6. JEDEC PACKAGE REFERENCE IS MO-193
3470fd
17
LT3470
package DescripTion
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
DDB Package
8-Lead Plastic DFN (3mm × 2mm)
(Reference LTC DWG # 05-08-1702 Rev B)
0.61 0.05
(2 SIDES)
R = 0.115
0.40 ± 0.10
3.00 0.10
TYP
5
R = 0.05
(2 SIDES)
TYP
8
0.70 0.05
2.55 0.05
2.00 ±0.10
(2 SIDES)
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
1.15 0.05
PIN 1
R = 0.20 OR
0.25 × 45°
PACKAGE
OUTLINE
0.56 ± 0.05
(2 SIDES)
CHAMFER
4
1
(DDB8) DFN 0905 REV B
0.25 0.05
0.25 0.05
0.75 ±0.05
0.200 REF
0.50 BSC
2.20 0.05
(2 SIDES)
0.50 BSC
2.15 ±0.05
(2 SIDES)
0 – 0.05
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
BOTTOM VIEW—EXPOSED PAD
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
3470fd
18
LT3470
revision hisTory (Revision history begins at Rev D)
REV
DATE
DESCRIPTION
PAGE NUMBER
D
09/11 Corrected lead-based tape and reel part numbers in the Order Information section.
2
3470fd
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.
19
LT3470
relaTeD parTs
PART NUMBER
DESCRIPTION
COMMENTS
= 3.6V to 25V, V
LT1616
25V, 500mA (I ), 1.4MHz, High Efficiency
Step-Down DC/DC Converter
V
= 1.25V, I = 1.9mA, I < 1µA,
Q SD
OUT
IN
OUT
OUT
ThinSOT Package
LT1676
60V, 440mA (I ), 100kHz, High Efficiency
V
= 7.4V to 60V, V
= 1.24V, I = 3.2mA, I = 2.5µA,
Q SD
OUT
IN
Step-Down DC/DC Converter
S8 Package
LT1765
25V, 2.75A (I ), 1.25MHz, High Efficiency
V
= 3V to 25V, V
= 1.2V, I = 1mA, I = 15µA,
OUT Q SD
OUT
IN
Step-Down DC/DC Converter
S8, TSSOP16E Packages
V = 5.5V to 60V, V = 1.2V, I = 2.5mA, I = 25µA,
IN
LT1766
60V, 1.2A (I ), 200kHz, High Efficiency
OUT
OUT
Q
SD
Step-Down DC/DC Converter
TSSOP16/E Package
= 3V to 25V; V = 1.2V, I = 1mA, I = 6µA,
OUT Q SD
LT1767
25V, 1.2A (I ), 1.25MHz, High Efficiency
V
OUT
IN
Step-Down DC/DC Converter
MS8/E Packages
LT1776
40V, 550mA (I ), 200kHz, High Efficiency
V
= 7.4V to 40V; V
= 1.24V, I = 3.2mA, I = 30µA,
OUT Q SD
OUT
IN
Step-Down DC/DC Converter
N8, S8 Packages
LTC®1877
LTC1879
LT1933
600mA (I ), 550kHz, Synchronous
V
= 2.7V to 10V; V
= 0.8V, I = 10µA, I ≤ 1µA,
Q SD
OUT
IN
OUT
OUT
OUT
OUT
OUT
Step-Down DC/DC Converter
MS8 Package
1.2A (I ), 550kHz, Synchronous
V
= 2.7V to 10V; V
= 0.8V, I = 15µA, I ≤ 1µA,
Q SD
OUT
IN
Step-Down DC/DC Converter
TSSOP16 Package
36V, 600mA, 500kHz, High Efficiency
Step-Down DC/DC Converter
V
= 3.6V to 36V; V
= 1.25V, I = 2.5µA, I ≤ 1µA,
Q SD
IN
??? Package
LT1934
34V, 250mA (I ), Micropower, Step-Down
V
= 3.2V to 34V; V
= 1.25V, I = 12µA, I ≤ 1µA,
Q SD
OUT
IN
DC/DC Converter
??? Package
LT1956
60V, 1.2A (I ), 500kHz, High Efficiency
V
= 5.5V to 60V, V
= 1.2V, I = 2.5mA, I = 25µA,
Q SD
OUT
IN
Step-Down DC/DC Converter
TSSOP16/E Package
= 2.7V to 6V, V = 0.8V, I = 20µA, I ≤ 1µA,
OUT Q SD
LTC3405/LTC3405A
LTC3406/LTC3406B
LTC3411
LTC3412
LTC3430
300mA (I ), 1.5MHz, Synchronous
V
OUT
IN
Step-Down DC/DC Converter
ThinSOT Package
600mA (I ), 1.5MHz, Synchronous
V
= 2.5V to 5.5V, V
= 0.6V, I = 20µA, I ≤ 1µA,
OUT Q SD
OUT
IN
Step-Down DC/DC Converter
ThinSOT Package
1.25A (I ), 4MHz, Synchronous
V
= 2.5V to 5.5V, V
= 0.8V, I = 60µA, I ≤ 1µA,
Q SD
OUT
IN
OUT
OUT
OUT
Step-Down DC/DC Converter
MS Package
2.5A (I ), 4MHz, Synchronous
V
= 2.5V to 5.5V, V
= 0.8V, I = 60µA, I ≤ 1µA,
Q SD
OUT
IN
Step-Down DC/DC Converter
TSSOP16E Package
60V, 2.75A (I ), 200kHz, High Efficiency
V
= 5.5V to 60V, V
= 1.2V, I = 2.5mA, I = 30µA,
Q SD
OUT
IN
Step-Down DC/DC Converter
TSSOP16E Package
3470fd
LT 0911 REV D • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
20
●
●
LINEAR TECHNOLOGY CORPORATION 2004
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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LT3470HTS8#PBF
LT3470 - Micropower Buck Regulator with Integrated Boost and Catch Diodes; Package: SOT; Pins: 8; Temperature Range: -40°C to 125°C
Linear
LT3470HTS8#TR
IC SWITCHING REGULATOR, PDSO8, 1 MM HEIGHT, PLASTIC, MO-193, SOT-23, 8 PIN, Switching Regulator or Controller
Linear
LT3470HTS8#TRMPBF
LT3470 - Micropower Buck Regulator with Integrated Boost and Catch Diodes; Package: SOT; Pins: 8; Temperature Range: -40°C to 125°C
Linear
LT3470HTS8#TRPBF
LT3470 - Micropower Buck Regulator with Integrated Boost and Catch Diodes; Package: SOT; Pins: 8; Temperature Range: -40°C to 125°C
Linear
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