LDPR [Linear]
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： | LDPR |
| 厂家： | 
Linear |
| 描述： | Micropower Buck Regulator with Integrated Boost and Catch Diodes |
| 文件： | 总20页 (文件大小：208K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT3470A
Micropower Buck Regulator
with Integrated Boost and
Catch Diodes
FEATURES
DESCRIPTION
The LT®3470A is a micropower step-down DC/DC con-
verter that integrates a 440mA power switch, catch diode
and boost diode into low profile 2mm × 3mm DFN
package. The LT3470A combines Burst Mode and
continuous operation to allow the use of tiny inductor
and capacitors while providing a low ripple output to
loads of up to 250mA.
n
Low Quiescent Current: 35μA at 12V to 3.3V
IN
OUT
n
Integrated Boost and Catch Diodes
n
Input Range: 4V to 40V
n
3.3V at 250mA from 4V to 40V Input
n
5V at 250mA from 5.7V to 40V Input
n
Low Output Ripple: <10mV
n
<1μA in Shutdown Mode
n
Output Voltage: 1.25V to 16V
With its wide input range of 4V to 40V, the LT3470A can
regulateawidevarietyofpowersources,from2-cellLi-Ion
batteries to unregulated wall transformers and lead-acid
batteries. Quiescent current in regulation is just 35μA in
a typical application while a zero current shutdown mode
disconnects the load from the input source, simplifying
powermanagementinbattery-poweredsystems.Fastcur-
rent limiting and hysteretic control protects the LT3470A
and external components against shorted outputs, even
at 40V input. The LT3470A has higher output current and
improved start-up and dropout performance compared
to the LT3470.
n
Hysteretic Mode Control
– 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) 2mm × 3mm Thermally
Enhanced 8-Lead DFN Package
APPLICATIONS
n
Automotive Battery Regulation
n
Power for Portable Products
n
Distributed Supply Regulation
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Burst Mode
is a registered trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
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
= 12V
IN
V
IN
5.7V TO 40V
0.22μF
33μH
V
BOOST
IN
LT3470A
SHDN
V
OUT
5V
OFF ON
2.2μF
SW
250mA
BIAS
604k
1%
22pF
FB
22μF
GND
200k
1%
1
3470a TA01
0.1
0.1
1
10
100
300
LOAD CURRENT (mA)
3470a TA02
3470afa
1
LT3470A
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
TOP VIEW
V , SHDN Voltage ................................................... 40V
IN
BOOST Pin Voltage .................................................. 47V
BOOST Pin Above SW Pin........................................ 25V
FB Voltage.................................................................. 5V
BIAS Voltage.............................................................15V
FB
BIAS
1
2
3
4
8
7
6
5
SHDN
NC
9
BOOST
SW
V
IN
GND
SW Voltage ................................................................V
Maximum Junction Temperature
LT3470AE, LT3470AI......................................... 125°C
Operating Temperature Range (Note 2)
IN
DDB8 PACKAGE
8-LEAD (3mm × 2mm) PLASTIC DFN
= 80°C/W
θ
JA
EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB
LT3470AE.............................................–40°C to 85°C
LT3470AI............................................ –40°C to 125°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec) ................. 300°C
ORDER INFORMATION
LEAD FREE FINISH
LT3470AEDDB#PBF
LT3470AIDDB#PBF
TAPE AND REEL
PART MARKING*
LDPR
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3470AEDDB#TRPBF
LT3470AIDDB#TRPBF
–40°C to 85°C
–40°C to 125°C
8-Lead (3mm × 2mm) Plastic DFN
8-Lead (3mm × 2mm) Plastic DFN
LDPR
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/
3470afa
2
LT3470A
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating 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
SHDN
BIAS
BIAS
0.1
10
40
0.5
18
55
μA
μA
μA
IN
= 3V, Not Switching
= 0V, Not Switching
Quiescent Current from Bias
V
V
V
= 0.2V
0.1
30
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
35
35
80
150
nA
nA
FB Voltage Line Regulation
Minimum Switch Off-Time (Note 5)
Maximum Duty Cycle
4V < V < 40V
0.0006
500
95
0.02
%/V
ns
IN
●
90
%
Switch Leakage Current
0.7
150
0.9
440
280
600
0.2
690
0.2
1.7
2.3
50
1.5
μA
mV
V
Switch V
Switch V
I
= 100mA
SW
CESAT
CESAT
Without Boost
V
V
V
= V
1.2
BOOST
SW
Switch Top Current Limit
Switch Bottom Current Limit
Catch Schottky Drop
= 0V
320
560
mA
mA
mV
μA
mV
μA
V
FB
FB
= 0V
I
= 100mA
= 10V
SW
Catch Schottky Reverse Leakage
Boost Schottky Drop
V
2
775
2
SW
I
= 50mA
= 10V, V
BIAS
Boost Schottky Reverse Leakage
Minimum Boost Voltage (Note 4)
BOOST Pin Current
V
= 0V
BIAS
SW
●
2.2
5
I
= 100mA
mA
mA
μA
V
SW
Bias Pin Preload
V
V
= 10V
BOOST
SHDN
SHDN Pin Current
= 2.5V
1
5
SHDN Input Voltage High
SHDN Input Voltage Low
2
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 LT3470AE 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 LT3470AI specifications are
guaranteed over the –40°C to 125°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.
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3
LT3470A
TA = 25°C unless otherwise noted.
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency, VOUT = 3.3V
Efficiency, VOUT = 5V
VFB vs Temperature
90
90
1.260
L = TOKO D52LC 47μH
L = TOKO D52LC 47μH
V = 12V
IN
V
= 7V
IN
T
= 25°C
T
= 25°C
A
A
V
= 12V
IN
80
70
80
70
1.255
V
= 36V
IN
V
= 24V
IN
V
= 24V
= 36V
V
IN
IN
1.250
1.245
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)
3470a G02
3470a G01
3470a G03
Top and Bottom Switch Current
Limits (VFB = 0V) vs Temperature
VIN Quiescent Current
vs Temperature
600
50
40
30
20
10
0
550
500
BIAS < 3V
BIAS > 3V
450
400
350
300
250
200
–25
0
50
75 100 125
–50
25
–50 –25
0
25
50
75 100 125
TEMPERATURE (°C)
TEMPERATURE (°C)
3470a G04
3470a G05
BIAS Quiescent Current
(Bias > 3V) vs Temperature
SHDN Bias Current
vs Temperature
30
25
20
15
9
8
7
6
5
4
3
2
1
V
= 36V
SHDN
10
5
V
= 2.5V
SHDN
0
0
50
TEMPERATURE (°C)
100 125
–50 –25
0
25
75
–50 –25
0
25
125
50
75 100
TEMPERATURE (°C)
3470a G06
3470a G07
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4
LT3470A
TA = 25°C unless otherwise noted.
TYPICAL PERFORMANCE CHARACTERISTICS
FB Bias Current (VFB = 1V)
vs Temperature
FB Bias Current (VFB = 0V)
vs Temperature
60
50
40
30
20
10
0
120
100
80
60
40
20
0
–50 –25
0
25
50
75 100 125
50
TEMPERATURE (°C)
100 125
–50 –25
0
25
75
TEMPERATURE (°C)
3470a G08
3470a G09
Boost Diode VF (IF = 50mA)
vs Temperature
Switch VCESAT (ISW = 100mA)
vs Temperature
300
250
200
150
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
100
50
0
0
50
TEMPERATURE (°C)
100 125
–50 –25
0
25
75
–25
0
50
75 100 125
–50
25
TEMPERATURE (°C)
3470a G10
3470a G11
Catch Diode VF (IF = 100mA)
vs Temperature
Diode Leakage (VR = 36V)
vs Temperature
60
55
50
45
40
35
30
25
20
15
10
5
0.7
0.6
CATCH
BOOST
0.5
0.4
0.3
0.2
0.1
0
0
50
TEMPERATURE (°C)
100 125
–50 –25
0
25
75
50
TEMPERATURE (°C)
125
–50 –25
0
25
75 100
3470a G12
3470a G13
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5
LT3470A
TA = 25°C unless otherwise noted.
TYPICAL PERFORMANCE CHARACTERISTICS
Switch VCESAT
BOOST Pin Current
700
600
500
14
12
10
400
300
200
100
0
8
6
4
2
0
100
200
SWITCH CURRENT (mA)
500
100
200
SWITCH CURRENT (mA)
500
0
300
400
0
300
400
3470a G14
3470a G15
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
100
200
300
100
BOOST DIODE CURRENT (mA)
0
0
200
400
50
150
CATCH DIODE CURRENT (mA)
3470a G16
3470a G17
Minimum Input Voltage, VOUT = 3.3V
Minimum Input Voltage, VOUT = 5V
6.0
5.5
8
7
6
5
4
T
= 25°C
A
T
A
= 25°C
5.0
4.5
V
TO RUN/START
IN
4.0
3.5
3.0
V
TO RUN/START
IN
0
50
100
150
200
250
100
150
200
0
250
50
LOAD CURRENT (mA)
LOAD CURRENT (mA)
3470a G18
3470a G19
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6
LT3470A
PIN FUNCTIONS
SHDN (Pin 8): The SHDN pin is used to put the LT3470A in
shutdown mode. Tie to ground to shut down the LT3470A.
Apply 2V or more for normal operation. If the shutdown
BOOST (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.
feature is not used, tie this pin to the V pin.
IN
BIAS (Pin 2): The BIAS pin connects to the internal boost
NC (Pin 7): This pin can be left floating, connected to V ,
or tied to GND.
Schottky diode and to the internal regulator. Tie to V
IN
OUT
>3Vthe
whenV >2.5VortoV otherwise.WhenV
OUT
IN
BIAS
BIAS pin will supply current to the internal regulator.
V (Pin 6): The V pin supplies current to the LT3470A’s
IN
IN
internal regulator and to the internal power switch. This
FB (Pin 1): The LT3470A 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).
pin must be locally bypassed.
GND (Pin 5): Tie the GND pin to a local ground plane
below the LT3470A and the circuit components. Return
the feedback divider to this pin.
Exposed Pad (Pin 9): Ground. Must be soldered to PCB.
SW (Pin 4): The SW pin is the output of the internal power
switch. Connect this pin to the inductor, catch diode and
boost capacitor.
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7
LT3470A
BLOCK DIAGRAM
V
IN
BIAS
V
IN
C1
+
–
BOOST
500ns
ONE SHOT
R
S
Qʹ
C3
Q
L1
C2
–
+
SW
V
OUT
ENABLE
BURST MODE
DETECT
NC
SHDN
V
REF
1.25V
g
m
GND
FB
R2
R1
3470a BD
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8
LT3470A
OPERATION
TheLT3470Ausesahystereticcontrolschemeinconjunc-
tionwithBurstModeoperationtoprovidelowoutputripple
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 LT3470A
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 100nA.
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
3470a F01a
3470a F1b
5μs/DIV
1μs/DIV
(1a) Burst Mode Operation
(1b) Continuous Operation
Figure 1. Operating Waveforms of the LT3470A Converting 12V to 5V Using a 33μH Inductor and 10μF Output Capacitor
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9
LT3470A
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 LT3470A 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
IN
f =
L • ΔIL
where V is the forward voltage drop of the catch diode
D
(~0.6V) and V is the voltage drop of the internal switch
where f is the switching frequency, ΔI is the ripple cur-
rent in the inductor (~200mA), V is the forward voltage
drop of the catch diode, and V
voltage.
SW
L
at maximum load (~0.4V). Given DC
to a minimum input voltage of:
= 0.90, this leads
MAX
D
OUT
is the desired output
ꢀ
D ꢃ
OUT + V
V
V
=
+ VSW – VD
IN(MIN)
ꢂ
ꢅ
If the application circuit is intended to operate at high duty
DCMAX
ꢁ
ꢄ
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
minimum t
should be more than LT3470A’s
OFF-TIME
(See Electrical Characteristics), so the
OFF-TIME
The maximum input voltage is limited by the absolute
application circuit is capable of delivering full rated output
maximum V rating of 40V, provided an inductor of suf-
IN
current. If the full output current of 250mA is not required,
ficient value is used.
the calculated t
can be made less than minimum
OFF-TIME
t
possibly allowing the use of a smaller inductor.
OFF-TIME
Inductor Selection
See Table 1 for an inductor value selection guide.
The switching action of the LT3470A during continuous
operationproducesasquarewaveattheSWpinthatresults
in a triangle wave of current in the inductor. The hysteretic
mode control regulates the top and bottom current limits
(seeElectricalCharacteristics)suchthattheaverageinduc-
tor current equals the load current. For safe operation, it
must be noted that the LT3470A cannot turn the 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 LT3470A is able to
turn off. To prevent this from happening, the following
equation provides a minimum inductor value:
Table 1. Recommended Inductors for Loads up to 250mA
V
V
IN
Up to 16V
10μH
V
IN
Up to 40V
33μH
OUT
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
VIN(MAX) • tON-TIME(MIN)
not required, inductors with I of 300mA or more may
SAT
LMIN
=
IMAX
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10
LT3470A
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.
at the LT3470A’s switching frequency. The capacitor’s
equivalent series resistance (ESR) determines this im-
pedance. Choose one with low ESR intended for use in
switching regulators. The contribution to ripple voltage
Input Capacitor
due to the ESR is approximately I
• ESR. ESR should
LIM
be less than ~150mΩ. 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 LT3470A and to force this switching
IN
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.
ꢀ
ꢃ2
ILIM
COUT >50 •L •
ꢂ
ꢅ
V
ꢁ
OUT ꢄ
WhereI isthetopcurrentlimitwithV =0V(seeElectri-
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 LT3470A
circuit is connected to a low impedance supply, and that
some electrolytic capacitors (in particular tantalum) must
be specified for such use.
LIM
FB
cal Characteristics). For example, an LT3470A 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.
Sanyo’s POSCAP series in B-case and provides very good
performance in a small package for the LT3470A. 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 LT3470A’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
LT3470A is quiescent. In order to keep output voltage
ripple low, the impedance of the capacitor must be low
3470afa
11
LT3470A
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
whenusedwiththeLT3470A.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.
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
0.22μF
Ceramic capacitors are piezoelectric. The LT3470A’s
switching frequency depends on the load current, and at
light loads the LT3470A can excite the ceramic capacitor
at audio frequencies, generating audible noise. Since the
LT3470A operates at a lower current limit during Burst
Mode operation, the noise is typically very quiet to a ca-
sual ear. If this audible noise is unacceptable, use a high
performanceelectrolyticcapacitorattheoutput. Theinput
capacitor can be a parallel combination of a 2.2μF ceramic
capacitor and a low cost electrolytic capacitor.
V
BOOST
LT3470A
IN
V
SW
OUT
BIAS
GND
V
– V
BOOST
≅ V
OUT
BOOST
SW
MAX V
≅ V + V
IN OUT
(2a)
V
IN
C3
0.22μF
V
BOOST
SW
IN
LT3470A
V
BIAS
OUT
A final precaution regarding ceramic capacitors concerns
themaximuminputvoltageratingoftheLT3470A.Aceramic
input capacitor combined with trace or cable inductance
forms a high quality (under damped) tank circuit. If the
LT3470Acircuitispluggedintoalivesupply,theinputvolt-
agecanringtotwiceitsnominalvalue, possiblyexceeding
the LT3470A’s rating. This situation is easily avoided; see
the Hot-Plugging Safely section.
GND
3470a F02
V
– V
BOOST
≅ V
SW
BOOST
IN
IN
MAX V
≅ 2•V
(2b)
Figure 2. Two Circuits for Generating the Boost Voltage
Table 2. 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
3470afa
12
LT3470A
APPLICATIONS INFORMATION
the boost capacitor. When the boost capacitor voltage is
above 1.8V (typical) the current source turns off, and the
part may enter BurstMode. This cycle will repeat anytime
there is an undervoltage condition on the boost capaci-
tor. See Figure 3 for minimum input voltage for outputs
of 3.3V and 5V.
(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.
The LT3470A monitors the boost capacitor for sufficient
voltage such that the switch is allowed to fully saturate.
When boost voltage falls below adequate levels (1.8V
typical) the switch will operate with about 1V of drop, and
an internal current source will begin to pull 50mA (typi-
cal) from the BIAS pin which is typically connected to the
output. This current forces the LT3470A to switch more
often and with more inductor current, which recharges
Shorted Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively at the top switch current limit maximum of 525mA,
an LT3470A 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 LT3470A 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
LT3470A’s output. If the V pin is allowed to float and the
IN
5.5
SHDN pin is held high (either by a logic signal or because
it is tied to V ), then the LT3470A’s internal circuitry will
5.0
4.5
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/START
IN
sentially zero. However, if the V pin is grounded while
IN
the output is held high, then parasitic diodes inside the
LT3470A can pull large currents from the output through
0
50
100
150
200
250
the SW pin and the V pin. Figure 4 shows a circuit that
IN
LOAD CURRENT (mA)
will run only when the input voltage is present and that
3470a F03a
protects against a shorted or reversed input.
Minimum Input Voltage, VOUT = 5V
D1
8
7
6
5
4
T
A
= 25°C
V
IN
V
BOOST
IN
LT3470A
SHDN
100k
1M
V
SW
OUT
BIAS
FB
V
TO RUN/START
IN
GND
BACKUP
3470a F04
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 LT3470A Runs Only When the Input is
Present Hot-Plugging Safely
100
150
200
0
250
50
LOAD CURRENT (mA)
3470a F03b
Figure 3. The Minimum Input Voltage Depends on Output
Voltage, Load Current and Boost Circuit
3470afa
13
LT3470A
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
LT3470A to help remove heat from the LT3470A to the
ground plane.
SHDN
V
IN
GND
V
OUT
3470a F05
Figure 5. A Good PCB Layout Ensures Proper, Low EMI Operation
3470afa
14
LT3470A
APPLICATIONS INFORMATION
Hot-Plugging Safely
voltage overshoot (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
bypass capacitor of LT3470A. However, these capacitors
can cause problems if the LT3470A 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 volt-
High Temperature Considerations
The die junction temperature of the LT3470A must be
lowerthanthemaximumratingof125°C.Thisisgenerally
not a concern unless the ambient 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
LT3470A. The maximum load current should be derated
as the ambient temperature approaches the maximum
junction rating. The die temperature is calculated by
multiplyingtheLT3470Apowerdissipationbythethermal
resistance from junction to ambient. Power dissipation
within the LT3470A can be estimated by calculating the
totalpowerlossfromanefficiencymeasurement.Thermal
resistance depends on the layout of the circuit board and
choice of package. The DFN package with the exposed
pad has a thermal resistance of approximately 80°C/W.
Finally, be aware that at high ambient temperatures the
internalSchottkydiodewillhavesignificantleakagecurrent
(see Typical Performance Characteristics) increasing the
quiescent current of the LT3470A converter.
age at the V pin of the LT3470A can ring to twice the
IN
nominal input voltage, possibly exceeding the LT3470A’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LT3470A into an
energized supply, the input network should be designed
to prevent this overshoot. Figure 6 shows the waveforms
that result when an LT3470A 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
ripplefilteringandcanslightlyimprovetheefficiencyofthe
circuit,thoughitislikelytobethelargestcomponentinthe
circuit. An alternative solution is shown in Figure 6c. A 1Ω
resistor is added in series with the input to eliminate the
3470afa
15
LT3470A
APPLICATIONS INFORMATION
CLOSING SWITCH
SIMULATES HOT PLUG
I
IN
V
IN
LT3470A
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)
V
LT3470A
2.2μF
IN
10V/DIV
+
10μF
35V
AI.EI.
I
IN
10A/DIV
10μs/DIV
(6b)
1ꢀ
V
LT3470A
2.2μF
IN
10V/DIV
0.1μF
I
IN
10A/DIV
3470a F06
10μs/DIV
(6c)
Figure 6: A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT3470A is Connected to a Live Supply
3470afa
16
LT3470A
APPLICATIONS INFORMATION
3.3V Step-Down Converter
V
IN
C3
4V TO 40V
0.22μF, 6.3V
L1
33μH
V
BOOST
IN
LT3470A
SHDN
V
OUT
3.3V
OFF ON
SW
250mA
BIAS
R1
22pF
324k
C1
1μF
C2
22μF
FB
GND
R2
200k
3470a TA03
C1: TDK C3216JB1H105M
C2: CE JMK316 BJ226ML-T
L1: TOKO A993AS-270M=P3
5V Step-Down Converter
V
IN
C3
5.7V TO 40V
0.22μF, 6.3V
L1
33μH
V
BOOST
IN
LT3470A
SHDN
V
OUT
5V
OFF ON
SW
250mA
BIAS
R1
22pF
604k
C1
1μF
C2
22μF
FB
GND
R2
200k
3470a TA04
C1: TDK C3216JB1H105M
C2: CE JMK316 BJ226ML-T
L1: TOKO A914BYW-330M=P3
2.5V Step-Down Converter
V
IN
C3
4V TO 40V
0.47μF, 6.3V
L1
33μH
V
BOOST
IN
LT3470A
SHDN
V
OUT
2.5V
OFF ON
SW
250mA
BIAS
R1
22pF
200k
C1
1μF
C2
22μF
FB
GND
R2
200k
3470a TA07
C1: TDK C3216JB1H105M
C2: TDK C2012JB0J226M
L1: SUMIDA CDRH3D28
3470afa
17
LT3470A
TYPICAL APPLICATIONS
1.8V Step-Down Converter
V
IN
C3
4V TO 23V
0.22μF, 25V
L1
22μH
V
BOOST
SW
IN
LT3470A
V
OUT
1.8V
OFF ON
SHDN
250mA
R1
BIAS
22pF
147k
C1
1μF
C2
22μF
FB
GND
R2
332k
3470a TA05
C1: TDK C3216JB1H105M
C2: TDK C2012JB0J226M
L1: MURATA LQH32CN150K53
12V Step-Down Converter
V
IN
C3
15V TO 34V
0.22μF, 16V
L1
33μH
V
BOOST
IN
LT3470A
SHDN
V
OUT
12V
OFF ON
SW
250mA
BIAS
R1
22pF
866k
C1
1μF
C2
10μF
FB
GND
R2
100k
3470a TA06
C1: TDK C3216JB1H105M
C2: TDK C3216JB1C106M
L1: MURATA LQH32CN150K53
3470afa
18
LT3470A
PACKAGE DESCRIPTION
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
8
3.00 0.10
(2 SIDES)
TYP
5
R = 0.05
TYP
0.70 0.05
2.55 0.05
1.15 0.05
2.00 0.10
PIN 1 BAR
TOP MARK
PIN 1
(2 SIDES)
R = 0.20 OR
0.25 × 45°
CHAMFER
(SEE NOTE 6)
PACKAGE
OUTLINE
0.56 0.05
(2 SIDES)
4
1
(DDB8) DFN 0905 REV B
0.25 0.05
0.50 BSC
0.25 0.05
0.75 0.05
0.200 REF
0.50 BSC
2.20 0.05
(2 SIDES)
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
3470afa
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
LT3470A
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
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60V, 1.2A (I ), 200kHz, High Efficiency
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IN
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V
OUT
IN
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LT1776
40V, 550mA (I ), 200kHz, High Efficiency
V
= 7.4V to 40V; V
= 1.24V, I = 3.2mA, I = 30μA,
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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
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
ThinSOT and 2mm × 3mm DFN-6 Package
V = 3.2V to 34V; V = 1.25V, I = 12μA, I = <1μA,
IN
LT1934
34V, 250mA (I ), Micropower, Step-Down
OUT
OUT
Q
SD
DC/DC Converter
ThinSOT and 2mm × 3mm DFN-6 Package
LT1956
60V, 1.2A (I ), 500kHz, High Efficiency
V
IN
= 5.5V to 60V, V = 1.2V, I = 2.5mA, I = 25μA,
OUT
OUT
Q
SD
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
LT3430
300mA (I ), 1.5MHz, Synchronous
V
IN
OUT
Step-Down DC/DC Converter
ThinSOT Package
600mA (I ), 1.5MHz, Synchronous
V
IN
= 2.5V to 5.5V, V = 0.6V, I = 20μA, I = <1μA,
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ThinSOT Package
1.25A (I ), 4MHz, Synchronous
V
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Q SD
OUT
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MS Package
2.5A (I ), 4MHz, Synchronous
V
= 2.5V to 5.5V, V
= 0.8V, I = 60μA, I = <1μA,
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OUT
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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,
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OUT
IN
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TSSOP16E Package
= 4V to 40V, V = 1.25V, I = 35μA, I = <1μA, DFN-8,
OUT Q SD
LT3470
40V, 200mA, Micropower Step-Down DC/DC Converter
V
IN
ThinSOT Packages
3470afa
LT 1108 REV A • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
20
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