LT3470 [Linear Systems]
Micropower Buck Regulator with Integrated Boost and Catch Diodes; 微功率降压型调节器，集成的升压和钳位二极管
| 型号： | LT3470 |
| 厂家： | 
Linear Systems |
| 描述： | Micropower Buck Regulator with Integrated Boost and Catch Diodes |
| 文件： | 总16页 (文件大小：200K) |
| 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
U
FEATURES
DESCRIPTIO
TheLT®3470isamicropowerstep-downDC/DCconverter
that integrates a 300mA power switch, catch diode and
boost diode into a low profile (1mm) ThinSOTTM package.
The LT3470 combines Burst Mode and continuous opera-
tion to allow the use of tiny inductor and capacitors while
providing a low ripple output to loads of up to 200mA.
■
Low Quiescent Current: 26µA at 12VIN to 3.3VOUT
Integrated Boost and Catch Diodes
Input Range: 4V to 40V
Low Output Ripple: <10mV
<1µA in Shutdown Mode
■
■
■
■
■
■
■
Output Voltage: 1.25V to 16V
200mA Output Current
Hysteretic Mode Control
With its wide input range of 4V to 40V, the LT3470 can
regulate a wide variety of power sources, from 2-cell
Li-Ionbatteriestounregulatedwalltransformersandlead-
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, simpli-
fying power management in battery-powered systems.
Fast current limiting and hysteretic control protects the
LT3470 and external components against shorted out-
puts, even at 40V input.
– Low Ripple Burst Mode® Operation at Light Loads
– Continuous Operation at Higher Loads
Solution Size as Small as 50mm2
Low Profile (1mm) ThinSOT Package
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■
■
APPLICATIO S
■
Automotive Battery Regulation
■
Power for Portable Products
■
Distributed Supply Regulation
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Burst Mode is a registered trademark of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
■
Industrial Supplies
■
Wall Transformer Regulation
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TYPICAL APPLICATIO
Efficiency and Power Loss vs Load Current
90
80
70
60
50
40
30
20
10
1000
100
10
V
V
= 12V
IN
IN
7V TO 40V
0.22µF
33µH
V
BOOST
IN
LT3470
SHDN
V
OUT
5V
OFF ON
SW
200mA
BIAS
604k
1%
22pF
FB
22µF
2.2µF
GND
200k
1%
1
0.1
0.1
1
10
100
LOAD CURRENT (mA)
3470 TA02
3470f
1
LT3470
W W
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ABSOLUTE AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
(Note 1)
VIN, SHDN Voltage .................................................. 40V
BOOST Pin Voltage ................................................. 47V
BOOST Pin Above SW Pin ...................................... 25V
FB Voltage ................................................................ 5V
BIAS Voltage............................................................ 25V
SW Voltage ................................................................VIN
Maximum Junction Temperature ......................... 125°C
Operating Temperature Range (Note 2) .. –40°C to 85°C
Storage Temperature Range ................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
TOP VIEW
SHDN 1
NC 2
8 FB
LT3470ETS8
LT3470ITS8
7 BIAS
6 BOOST
5 SW
V
3
IN
GND 4
TS8 PART MARKING
TS8 PACKAGE
8-LEAD PLASTIC TSOT-23
TJMAX = 125°C, θJA = 140°C/ W
LTBDM
LTBPW
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are 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
SHDN
0.1
10
35
0.5
18
50
µA
µA
µA
IN
= 3V, Not Switching
= 0V, Not Switching
BIAS
BIAS
Quiescent Current from Bias
V
V
V
= 0.2V
= 3V, Not Switching
= 0V, Not Switching
0.1
25
0.1
0.5
60
1.5
µA
µA
µA
SHDN
●
●
●
BIAS
BIAS
FB Comparator Trip Voltage
FB Pin Bias Current (Note 3)
V
V
Falling
1.228
1.250
1.265
V
FB
FB
= 1V
35
35
80
150
nA
nA
FB Voltage Line Regulation
Minimum Switch Off-Time (Note 5)
Switch Leakage Current
4V < V < 40V
0.0006
500
0.01
%/V
ns
IN
0.7
1.5
300
435
µA
Switch V
I
= 100mA
= 0V
215
mV
mA
mA
CESAT
SW
Switch Top Current Limit
V
V
250
325
FB
FB
Switch Bottom Current Limit
= 0V
225
3470f
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LT3470
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.
VIN = 10V, VSHDN = 10V, VBOOST = 15V, VBIAS = 3V unless otherwise specified.
PARAMETER
CONDITIONS
= 100mA
MIN
TYP
630
0.2
650
0.2
1.7
7
MAX
775
2
UNITS
mV
µA
mV
µA
V
Catch Schottky Drop
I
SH
Catch Schottky Reverse Leakage
Boost Schottky Drop
V
= 10V
SW
I
= 30mA
775
2
SH
Boost Schottky Reverse Leakage
Minimum Boost Voltage (Note 4)
BOOST Pin Current
V
= 10V, V
= 0V
BIAS
SW
●
2.2
12
5
I
= 100mA
mA
µA
V
SW
SHDN Pin Current
V
= 2.5V
1
SHDN
SHDN Input Voltage High
SHDN Input Voltage Low
2.5
0.2
V
Note 1: Absolute Maximum Ratings are those values beyond which the life
Note 3: Bias current flows out of the FB pin.
of a device may be impaired.
Note 4: This is the minimum voltage across the boost capacitor needed to
Note 2: The LT3470E is guaranteed to meet performance specifications
from 0°C to 70°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.
guarantee full saturation of the switch.
Note 5: This parameter is assured by design and correlation with statistical
process controls.
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TYPICAL PERFOR A CE 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
A
= 25°C
T
= 25°C
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
–50 –25
0
25
50
75 100 125
0.1
1
10
100
LOAD CURRENT (mA)
TEMPERATURE (°C)
LOAD CURRENT (mA)
3470 G02
3470 G01
3470 G03
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LT3470
TYPICAL PERFOR A CE CHARACTERISTICS
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BIAS Quiescent Current
(Bias > 3V) vs Temperature
Top and Bottom Switch Current
Limits (VFB = 0V) vs Temperature
VIN Quiescent Current
vs Temperature
50
40
30
20
10
0
400
30
25
20
15
350
300
BIAS < 3V
BIAS > 3V
250
200
150
100
50
10
5
0
0
–50 –25
0
25
50
75 100 125
–25
0
50
75 100 125
50
TEMPERATURE (°C)
100 125
–50
25
–50 –25
0
25
75
TEMPERATURE (°C)
TEMPERATURE (°C)
3470 G05
3470 G04
3470 G06
SHDN Bias Current
vs Temperature
FB Bias Current (VFB = 1V)
vs Temperature
FB Bias Current (VFB = 0V)
vs Temperature
9
8
7
6
5
4
3
2
1
50
40
30
20
10
0
120
100
80
V
= 36V
SHDN
60
40
20
0
V
0
= 2.5V
SHDN
0
–50 –25
25
125
–50 –25
0
25
50
75 100 125
–50 –25
0
25
50
75
100 125
50
75 100
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
3470 G07
3470 G08
3470 G09
Switch VCESAT (ISW = 100mA)
vs Temperature
Boost Diode VF (IF = 50mA)
vs Temperature
Catch Diode VF (IF = 100mA)
vs Temperature
300
250
200
150
0.8
0.7
0.6
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.5
0.4
0.3
0.2
0.1
100
50
0
0
0
50
TEMPERATURE (°C)
100 125
–25
0
50
75 100 125
50
TEMPERATURE (°C)
100 125
3470 G12
–50 –25
0
25
75
–50
25
–50 –25
0
25
75
TEMPERATURE (°C)
3470 G10
3470 G11
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LT3470
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TYPICAL PERFOR A CE CHARACTERISTICS
Diode Leakage (VR = 36V)
vs Temperature
Switch VCESAT
BOOST Pin Current
25
20
15
10
5
14
12
10
700
600
500
BOOST
CATCH
8
6
4
2
0
400
300
200
100
0
0
100
200
400
0
300
–50 –25
0
25
50
75 100 125
0
100
200
300
400
TEMPERATURE (°C)
SWITCH CURRENT (mA)
SWITCH CURRENT (mA)
3470 G15
3470 G13
3470 G14
Catch Diode Forward Voltage
Boost Diode Forward Voltage
1.0
0.8
0.6
0.4
0.2
0
900
800
700
600
500
400
300
200
100
0
0
100
200
300
0
50
100
BOOST DIODE CURRENT (mA)
150
200
400
CATCH DIODE CURRENT (mA)
3470 G16
3470 G17
Minimum Input Voltage, VOUT = 3.3V
Minimum Input Voltage, VOUT = 5V
6.0
5.5
8
7
6
5
4
T
A
= 25°C
T
= 25°C
A
V
IN
TO START
V
TO START
IN
5.0
4.5
V
TO RUN
IN
4.0
3.5
3.0
V
IN
TO RUN
100
0
50
150
200
100
LOAD CURRENT (mA)
0
150
200
50
LOAD CURRENT (mA)
3470 G18
3470 G19
3470f
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LT3470
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PI FU CTIO S
SHDN (Pin 1): 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 shutdown
feature is not used, tie this pin to the VIN pin.
BOOST (Pin 6): The BOOST pin is used to provide a drive
voltage, which is higher than the input voltage, to the
internal bipolar NPN power switch.
BIAS (Pin 7): The BIAS pin connects to the internal boost
Schottky diode and to the internal regulator. Tie to VOUT
when VOUT > 2V or to VIN otherwise. When VBIAS > 3V the
BIAS pin will supply current to the internal regulator.
NC(Pin2):ThispincanbeleftfloatingorconnectedtoVIN.
VIN (Pin 3): The VIN pin supplies current to the LT3470’s
internal regulator and to the internal power switch. This
pin must be locally bypassed.
FB (Pin 8): 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).
GND(Pin4):TietheGNDpintoalocalgroundplanebelow
the LT3470 and the circuit components. Return the feed-
back divider to this pin.
SW (Pin 5): The SW pin is the output of the internal power
switch. Connect this pin to the inductor, catch diode and
boost capacitor.
W
BLOCK DIAGRA
V
IN
BIAS
7
V
3
IN
C1
+
–
BOOST
6
500ns
ONE SHOT
R
S
Q′
C3
L1
Q
–
+
SW
V
5
OUT
C2
ENABLE
BURST MODE
DETECT
2
1
NC
SHDN
V
REF
1.25V
g
m
GND
FB
8
4
R2
R1
3470 BD
3470f
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LT3470
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OPERATIO
The LT3470 uses a hysteretic control scheme in conjunc-
tion with Burst Mode operation to provide low output
ripple and low quiescent current while using a tiny induc-
tor and capacitors.
comparatortripsandresetsthelatchcausingtheswitchto
turnoff. Whiletheswitchisoff, theinductorcurrentramps
down through the catch diode. When both the bottom
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,thepartwillautomaticallyswitchslowertokeep
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. Iftheloadislightthepartwillalternatebetween
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
saturatetheinternalbipolarNPNpowerswitchforefficient
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 VIN current reduces to the device leakage current,
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
3470f
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LT3470
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APPLICATIO S I FOR ATIO
Input Voltage Range
where VIN(MAX) is the maximum input voltage for the
application, tON-TIME(MIN) is ~150ns and IMAX is the maxi-
mum allowable increase in switch current during a mini-
mumswitchon-time(150mA).Whilethisequationprovides
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:
Theminimuminputvoltagerequiredtogenerateaparticu-
lar output voltage in an LT3470 application is limited by
either its 4V undervoltage lockout or by its maximum duty
cycle. Thedutycycleisthefractionoftimethattheinternal
switch is on and is determined by the input and output
voltages:
V
OUT + VD
DC =
1–DC V + V
(
)
D
OUT
V – VSW + VD
IN
f =
L • ∆IL
where VD is the forward voltage drop of the catch diode
(~0.6V) and VSW is the voltage drop of the internal switch
at maximum load (~0.4V). Given DCMAX = 0.90, this leads
to a minimum input voltage of:
wherefistheswitchingfrequency, ∆IL istheripplecurrent
in the inductor (~150mA), VD is the forward voltage drop
of the catch diode, and VOUT is the desired output voltage.
If the application circuit is intended to operate at high duty
cycles (VIN close to VOUT), it is important to look at the
calculated value of the switch off-time:
V
OUT + VD
V
=
IN(MIN)
DCMAX – VD + VSW
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 tOFF-TIME should be more than LT3470’s
minimum tOFF-TIME (See Electrical Characteristics), so the
application circuit is capable of delivering full rated output
current. If the full output current of 200mA is not required,
the calculated tOFF-TIME can be made less than minimum
tOFF-TIME possibly allowing the use of a smaller inductor.
See Table 1 for an inductor value selection guide.
The maximum input voltage is limited by the absolute
maximum VIN rating of 40V, provided an inductor of
sufficient value is used.
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,itmustbenotedthattheLT3470cannotturnthe
switch on for less than ~150ns. If the inductor is small and
the input voltage is high, the current through the 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
Choose an inductor that is intended for power applica-
tions. Table 2 lists several manufacturers and inductor
series.
For robust output short-circuit protection at high VIN (up
to 40V) use at least a 33µH inductor with a minimum
450mA saturation current. If short-circuit performance is
not required, inductors with ISAT of 300mA or more may
be used. It is important to note that inductor saturation
VIN(MAX) • tON-TIME(MIN)
LMIN
=
IMAX
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LT3470
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APPLICATIO S I FOR ATIO
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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
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
approximatelyILIM•ESR.ESRshouldbelessthan~150mΩ.
The value of the output capacitor must be large enough to
accept the energy stored in the inductor without a large
changeinoutputvoltage. Settingthisvoltagestepequalto
1% of the output voltage, the output capacitor must be:
Input Capacitor
Step-down regulators draw current from the input supply
in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage ripple
at the VIN pin of the LT3470 and to force this switching
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.
2
⎛ ILIM
⎝ VOUT
⎞
COUT > 50 •L •
⎜
⎟
⎠
If the input source impedance is high, a larger value
capacitor 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.
Where ILIM is the top current limit with VFB = 0V (see
Electrical Characteristics). For example, an LT3470 pro-
ducing 3.3V with L = 33µH requires 22µF. The calculated
value can be relaxed if small circuit size is more important
than low output ripple.
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
LT3470isquiescent. Inordertokeepoutputvoltageripple
low, the impedance of the capacitor must be low at the
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LT3470
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APPLICATIO S I FOR ATIO
BOOST and BIAS Pin Considerations
Ceramic Capacitors
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
arrange the boost circuit. The BOOST pin must be more
than2.5VabovetheSWpinforbestefficiency. Foroutputs
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
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
whenusedwiththeLT3470. Notallceramiccapacitorsare
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.
V
IN
Ceramiccapacitorsarepiezoelectric.TheLT3470’sswitch-
ing frequency depends on the load current, and at light
loadstheLT3470canexcitetheceramiccapacitorataudio
frequencies, generating audible noise. Since the LT3470
operates at a lower current limit during BurstMode opera-
tion, the noise is typically very quiet to a casual ear. If this
audible noise is unacceptable, use a high performance
electrolyticcapacitorattheoutput.Theinputcapacitorcan
be a parallel combination of a 2.2µF ceramic capacitor and
a low cost electrolytic capacitor.
C3
3
6
0.22µF
V
IN
BOOST
LT3470
5
7
V
SW
OUT
BIAS
GND
4
V
– V
≅ V
BOOST
MAX V
SW OUT
≅ V + V
IN OUT
BOOST
(2a)
V
IN
C3
0.22µF
3
6
V
IN
BOOST
LT3470
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT3470. A ce-
ramic input capacitor combined with trace or cable induc-
tance forms a high quality (under damped) tank circuit. If
the LT3470 circuit is plugged into a live supply, the input
voltage can ring to twice its nominal value, possibly
exceeding the LT3470’s rating. This situation is easily
avoided; see the Hot Plugging Safely section.
7
5
V
BIAS
SW
OUT
GND
4
3470 F02
V
– V
BOOST
≅ V
BOOST
MAX V
SW IN
≅ 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
www.murata.com
www.avxcorp.com
Ceramic
Ceramic,
Tantalum
TPS Series
Taiyo Yuden (864) 963-6300
www.taiyo-yuden.com Ceramic
3470f
10
LT3470
W U U
APPLICATIO S I FOR ATIO
U
(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.
minimum VIN to start and to run. At light loads, the
inductor current becomes discontinuous and the effective
duty cycle can be very high. This reduces the minimum
input voltage to approximately 300mV above VOUT. At
higher load currents, the inductor current is continuous
and the duty cycle is limited by the maximum duty cycle of
the LT3470, 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
output is already in regulation, then the boost capacitor
may not be fully charged. The plots in Figure 3 show
Shorted Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively at the top switch current limit maximum of 450µA,
an LT3470 buck regulator will tolerate a shorted output
even if VIN = 40V. There is another situation to consider in
systems where the output will be held high when the input
to the LT3470 is absent. This may occur in battery charg-
ing applications or in battery backup systems where a
battery or some other supply is diode OR-ed with the
LT3470’s output. If the VIN pin is allowed to float and the
SHDN pin is held high (either by a logic signal or because
it is tied to VIN), then the LT3470’s internal circuitry will
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
essentially zero. However, if the VIN pin is grounded while
the output is held high, then parasitic diodes inside the
LT3470 can pull large currents from the output through
the SW pin and the VIN pin. Figure 4 shows a circuit that
will run only when the input voltage is present and that
protects against a shorted or reversed input.
Minimum Input Voltage, VOUT = 3.3V
6.0
T
= 25°C
A
V
TO START
IN
5.5
5.0
4.5
4.0
3.5
3.0
V
TO RUN
100
IN
0
50
150
200
LOAD CURRENT (mA)
3470 G18
Minimum Input Voltage, VOUT = 5V
8
T
= 25°C
A
V
TO START
IN
D1
7
6
5
4
V
IN
3
6
V
BOOST
IN
LT3470 SOT-23
100k
1M
1
5
7
V
SHDN
SW
OUT
V
TO RUN
IN
BIAS
8
FB
GND
4
BACKUP
3470 F04
100
150
0
200
50
LOAD CURRENT (mA)
3470 G19
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
Figure 3. The Minimum Input Voltage Depends on Output
Voltage, Load Current and Boost Circuit
3470f
11
LT3470
W U U
U
APPLICATIO S I FOR ATIO
PCB Layout
Hot Plugging Safely
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
these components should be as small as possible. Fur-
thermore, the system ground should be tied to the regu-
lator ground in only one place; this prevents the switched
current from injecting noise into the system ground.
These components, along with the inductor and output
capacitor, should be placed on the same side of the circuit
board,andtheirconnectionsshouldbemadeonthatlayer.
Place a local, unbroken ground plane below these compo-
nents, and tie this ground plane to system ground at one
location, 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 instabil-
ity and increased output ripple. To avoid this problem, use
vias to route the VOUT trace under the ground plane to the
feedback divider (as shown in Figure 5). Finally, keep the
FB node as small as possible so that the ground pin and
groundtraceswillshielditfromtheSWandBOOSTnodes.
Figure 5 shows component placement with trace, ground
plane and via locations. Include vias near the GND pin of
the LT3470 to help remove heat from the LT3470 to the
ground plane.
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3470. However, these capacitors
can cause problems if the LT3470 is plugged into a live
supply (see Linear Technology Application Note 88 for a
complete discussion). The low loss ceramic capacitor
combined with stray inductance in series with the power
source forms an under damped tank circuit, and the
voltage at the VIN pin 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
energizedsupply, theinputnetworkshouldbedesignedto
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 likely to be the largest component
SHDN
V
IN
C1
GND
C2
V
OUT
3470 F05
VIAS TO FEEDBACK DIVIDER
VIAS TO LOCAL GROUND PLANE
OUTLINE OF LOCAL GROUND PLANE
Figure 5. A Good PCB Layout Ensures Proper, Low EMI Operation
3470f
12
LT3470
W U U
U
APPLICATIO S I FOR ATIO
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
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
10µs/DIV
(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
in the circuit. An alternative solution is shown in Figure 6c.
A 1Ω resistor is added in series with the input to eliminate
the voltage overshoot (it also reduces the peak input
current).A0.1µFcapacitorimproveshighfrequencyfilter-
ing. This solution is smaller and less expensive than the
electrolyticcapacitor.Forhighinputvoltagesitsimpacton
efficiency is minor, reducing efficiency less than one half
percent for a 5V output at full load operating from 24V.
temperature approaches 125°C. The die temperature 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 mea-
surement. Thermal resistance depends on the layout of
the circuit board, but a value of 150°C/W is typical. The
temperature rise for an LT3470 producing 5V at 200mA
is approximately 30°C, allowing it to deliver full load to
100°C ambient. Above this temperature the load current
should be reduced. For 3.3V at 200mA the temperature
rise is 20°C. Finally, be aware that at high ambient
temperaturestheinternalSchottkydiodewillhavesignifi-
cant leakage current (See Typical Performance Charac-
teristics) increasing the quiescent current of the LT3470
converter.
High Temperature Considerations
ThedietemperatureoftheLT3470mustbelowerthanthe
maximum rating of 125°C. This is generally not a concern
unlesstheambienttemperatureisabove85°C. Forhigher
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
3470f
13
LT3470
U
TYPICAL APPLICATIO S
5V Step-Down Converter
3.3V Step-Down Converter
V
V
IN
5.5V TO 40V
IN
7V TO 40V
C3
C3
3
6
3
6
0.22µF
0.22µF
V
BOOST
V
BOOST
IN
IN
L1
33µH
L1
33µH
LT3470
SHDN
LT3470
SHDN
V
V
OUT
OUT
1
1
5
7
5
7
3.3V
5V
OFF ON
SW
OFF ON
SW
200mA
200mA
BIAS
BIAS
R1
R1
22pF
22pF
324k
604k
8
8
C1
1µF
C2
22µF
C1
1µF
C2
22µF
FB
FB
GND
GND
R2
200k
R2
200k
4
4
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
V
IN
4.7V TO 40V
C3
3
6
0.47µF
V
BOOST
IN
L1
33µH
LT3470
SHDN
V
OUT
1
5
7
2.5V
OFF ON
SW
200mA
BIAS
R1
200k
22pF
8
C1
1µF
C2
22µF
FB
GND
R2
200k
4
3470 TA07
C1: TDK C3216JB1H105M
C2: TDK C2012JB0J226M
L1: SUMIDA CDRH3D28
12V Step-Down Converter
1.8V Step-Down Converter
V
V
IN
IN
15V TO 35V
C3
C3
4V TO 25V
3
6
3
6
0.22µF
0.22µF
V
BOOST
V
BOOST
IN
IN
L1
22µH
L1
33µH
LT3470
LT3470
SHDN
V
V
OUT
OUT
1
7
1
5
8
5
7
1.8V
12V
OFF ON
SHDN
SW
FB
OFF ON
SW
200mA
200mA
BIAS
R1
BIAS
R1
22pF
22pF
147k
866k
8
C1
1µF
C2
22µF
C1
1µF
C2
10µF
FB
GND
GND
R2
332k
R2
100k
4
4
3470 TA04
3470 TA06
C1: TDK C3216JB1H105M
C2: TDK C3216JB1C106M
L1: MURATA LQH32CN150K53
C1: TDK C3216JB1H105M
C2: TDK C2012JB0J226M
L1: MURATA LQH32CN150K53
3470f
14
LT3470
U
PACKAGE DESCRIPTIO
TS8 Package
8-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1637)
2.90 BSC
(NOTE 4)
0.52
MAX
0.65
REF
1.22 REF
1.50 – 1.75
(NOTE 4)
2.80 BSC
1.4 MIN
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
0.09 – 0.20
(NOTE 3)
TS8 TSOT-23 0802
NOTE:
1. DIMENSIONS ARE IN MILLIMETERS
2. DRAWING NOT TO SCALE
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
3. DIMENSIONS ARE INCLUSIVE OF PLATING
3470f
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 represen-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
15
LT3470
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1613
550mA (I ), 1.4MHz, High Efficiency Step-Up
DC/DC Converter
V : 0.9V to 10V, V
ThinSOT Package
= 34V, I = 3mA, I < 1µA,
Q SD
SW
IN
OUT(MAX)
OUT(MAX)
OUT(MAX)
OUT(MAX)
LT1615/LT1615-1
300mA/80mA (I ), Constant Off-Time,
V : 1.2V to 15V, V
= 34V, I = 20µA, I < 1µA,
Q SD
SW
IN
High Efficiency Step-Up DC/DC Converters
ThinSOT Package
LT1944/LT1944-1 (Dual) Dual Output 350mA/100mA (I ), Constant Off-Time,
V : 1.2V to 15V, V
= 34V, I = 20µA, I < 1µA,
Q SD
SW
IN
High Efficiency Step-Up DC/DC Converters
MS Package
LT1945 (Dual)
LT1961
Dual Output, Pos/Neg, 350mA (I ), Constant Off-Time, V : 1.2V to 15V, V
= ±34V, I = 20µA, I < 1µA,
Q SD
SW
IN
High Efficiency Step-Up DC/DC Converter
MS Package
1.5A (I ), 1.25MHz, High Efficiency Step-Up
V : 3V to 25V, V
= 35V, I = 0.9mA, I < 6µA,
SW
IN
OUT(MAX) Q SD
DC/DC Converter
MS8E Package
LTC®3400/LTC3400B
LTC3401
600mA (I ), 1.2MHz, Synchronous Step-Up
V : 0.85V to 5V, V
= 5V, I = 19µA/300µA, I < 1µA,
OUT(MAX) Q SD
SW
IN
DC/DC Converter
ThinSOT Package
1A (I ), 3MHz, Synchronous Step-Up
V : 0.5V to 5V, V
IN
= 6V, I = 38µA, I < 1µA,
OUT(MAX) Q SD
SW
DC/DC Converter
MS Package
LT3460
0.32A (I ), 1.3MHz, High Efficiency Step-Up
DC/DC Converter
V : 2.5V to 16V, V
MS8E Package
= 36V, I = 2mA, I < 1µA,
Q SD
SW
IN
OUT(MAX)
LT3461/LT3461A
LT3464
0.3A (I ), 1.3MHz/3MHz, High Efficiency Step-Up
DC/DC Converters
V : 2.5V to 16V, V
SC70, ThinSOT Packages
= 38V, I = 2.8mA, I < 1µA,
Q SD
SW
IN
OUT(MAX)
0.08A (I ), High Efficiency Step-Up DC/DC Converter
V : 2.3V to 10V, V
= 34V, I = 25µA, I < 1µA,
SW
IN
OUT(MAX) Q SD
with Integrated Schottky, Output Disconnect
ThinSOT Package
3470f
LT/TP 1104 1K • PRINTED IN THE USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
16
●
●
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
©LINEAR TECHNOLOGY CORPORATION 2004
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