LT3470 Datasheet by Analog Devices Inc.

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LTLII‘IEAQ mm TECHNOLOGY OFF ON— _| L7 LJUW 1
LT3470
1
3470fd
Typical applicaTion
FeaTures
applicaTions
DescripTion
Micropower Buck Regulator
with Integrated Boost and
Catch Diodes
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.
With its wide input range of 4V to 40V, the LT3470 can
regulate a wide variety of power sources, from 2-cell Li-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
current limiting and hysteretic control protects the LT3470
and external components against shorted outputs, even
at 40V input.
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.
Efficiency and Power Loss vs Load Current
n Low Quiescent Current: 26µA at 12VIN to 3.3VOUT
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
– Low Ripple Burst Mode
®
Operation at Light Loads
– Continuous Operation at Higher Loads
n
Solution Size as Small as 50mm2
n Low Profile (0.75mm) 3mm × 2mm Thermally
Enhanced 8-Lead DD and 1mm ThinSOT Packages
n Automotive Battery Regulation
n
Power for Portable Products
n
Distributed Supply Regulation
n
Industrial Supplies
n Wall Transformer Regulation
LOAD CURRENT (mA)
30
EFFICIENCY (%)
POWER LOSS (mW)
40
60
80
90
0.1 10 100
3470 TA02
20
1
70
50
10
1
1000
100
10
0.1
VIN = 12V
VIN BOOST
LT3470
SWSHDN
0.22µF
22pF
22µF
2.2µF
3470 TA01a
VIN
7V TO 40V
VOUT
5V
200mA
604k
1%
200k
1%
33µH
BIAS
FB
GND
OFF ON
[I347O :33: :::: L7LJCUEN2
LT3470
2
3470fd
absoluTe MaxiMuM raTings
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
LT3470E, LT3470I ............................................. 125°C
LT3470H ........................................................... 150°C
(Note 1)
orDer inForMaTion
Operating Temperature Range (Note 2)
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
TOP VIEW
9
DDB8 PACKAGE
8-LEAD (3mm × 2mm) PLASTIC DFN
5
6
7
8
4
3
2
1FB
BIAS
BOOST
SW
SHDN
NC
VIN
GND
θJA = 180°C/W
EXPOSED PAD (PIN 9) IS GROUND (MUST BE SOLDERED TO PCB)
SHDN 1
NC 2
VIN 3
GND 4
8 FB
7 BIAS
6 BOOST
5 SW
TOP VIEW
TS8 PACKAGE
8-LEAD PLASTIC TSOT-23
θJA = 140°C/W
pin conFiguraTion
LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE
LT3470EDDB#PBF LT3470EDDB#TRPBF LBPN 8-Lead (3mm × 2mm) Plastic DFN –40°C to 85°C
LT3470IDDB#PBF LT3470IDDB#TRPBF LBPP 8-Lead (3mm × 2mm) Plastic DFN –40°C to 125°C
LT3470HDDB#PBF LT3470HDDB#TRPBF LCNR 8-Lead (3mm × 2mm) Plastic DFN –40°C to 150°C
LT3470ETS8#PBF LT3470ETS8#TRPBF LTBDM 8-Lead Plastic TSOT-23 –40°C to 85°C
LT3470ITS8#PBF LT3470ITS8#TRPBF LTBPW 8-Lead Plastic TSOT-23 –40°C to 125°C
LT3470HTS8#PBF LT3470HTS8#TRPBF LTCNQ 8-Lead Plastic TSOT-23 –40°C to 150°C
LEAD BASED FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE
LT3470EDDB LT3470EDDB#TR LBPN 8-Lead (3mm × 2mm) Plastic DFN –40°C to 85°C
LT3470IDDB LT3470IDDB#TR LBPP 8-Lead (3mm × 2mm) Plastic DFN –40°C to 125°C
LT3470HDDB LT3470HDDB#TR LCNR 8-Lead (3mm × 2mm) Plastic DFN –40°C to 150°C
LT3470ETS8 LT3470ETS8#TR LTBDM 8-Lead Plastic TSOT-23 –40°C to 85°C
LT3470ITS8 LT3470ITS8#TR LTBPW 8-Lead Plastic TSOT-23 –40°C to 125°C
LT3470HTS8 LT3470HTS8#TR LTCNQ 8-Lead Plastic TSOT-23 –40°C to 150°C
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/
LT347O L7 LJUW 3
LT3470
3
3470fd
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.
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.
elecTrical characTerisTics
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Input Voltage 4 V
Quiescent Current from VIN VSHDN = 0.2V
VBIAS = 3V, Not Switching
VBIAS = 0V, Not Switching
0.1
10
35
0.5
18
50
µA
µA
µA
Quiescent Current from Bias VSHDN = 0.2V
VBIAS = 3V, Not Switching
VBIAS = 0V, Not Switching
0.1
25
0.1
0.5
60
1.5
µA
µA
µA
FB Comparator Trip Voltage VFB Falling 1.228 1.250 1.265 V
FB Pin Bias Current (Note 3) VFB = 1V, E- and I-Grade
35
35
80
150
nA
nA
H-Grade 35 225 nA
FB Voltage Line Regulation 4V < VIN < 40V 0.0006 0.01 %/V
Minimum Switch Off-Time (Note 5) 500 ns
Switch Leakage Current 0.7 1.5 µA
Switch VCESAT ISW = 100mA (TS8 Package)
ISW = 100mA (DD8 Package)
215
215
300 mV
mV
Switch Top Current Limit VFB = 0V 250 325 435 mA
Switch Bottom Current Limit VFB = 0V 225 mA
Catch Schottky Drop ISH = 100mA (TS8 Package)
ISH = 100mA (DD8 Package)
630
630
775 mV
mV
Catch Schottky Reverse Leakage VSW = 10V 0.2 2 µA
Boost Schottky Drop ISH = 30mA 650 775 mV
Boost Schottky Reverse Leakage VSW = 10V, VBIAS = 0V 0.2 2 µA
Minimum Boost Voltage (Note 4) 1.7 2.2 V
BOOST Pin Current ISW = 100mA 7 12 mA
SHDN Pin Current VSHDN = 2.5V 1 5 µA
SHDN Input Voltage High 2.5 V
SHDN Input Voltage Low 0.2 V
LT347O HHH‘ \HHH V) ..\ \ \\ \ \ 4 L7LJ1W
LT3470
4
3470fd
Typical perForMance characTerisTics
Efficiency, VOUT = 3.3V
Efficiency, VOUT = 5V
VFB vs Temperature
Top and Bottom Switch Current
Limits (VFB = 0V) vs Temperature
VIN Quiescent Current
vs Temperature
BIAS Quiescent Current
(Bias > 3V) vs Temperature
SHDN Bias Current
vs Temperature
FB Bias Current (VFB = 1V)
vs Temperature
LOAD CURRENT (mA)
50
EFFICIENCY (%)
70
90
40
60
80
0.1 10 100
3470 G01
30
1
L = TOKO D52LC 47µH
TA = 25°C VIN = 7V
VIN = 12V
VIN = 36V
VIN = 24V
LOAD CURRENT (mA)
50
EFFICIENCY (%)
70
90
40
60
80
0.1 10 100
3470 G02
30
1
L = TOKO D52LC 47µH
TA = 25°C
VIN = 12V
VIN = 36V
VIN = 24V
TEMPERATURE (°C)
–50
1.240
VFB (V)
1.245
1.250
1.255
1.260
–25 0 25 50
3470 G03
75 100 125
TEMPERATURE (°C)
–50
CURRENT LIMIT (mA)
350
25
3470 G04
200
100
–25 0 50
50
0
400
300
250
150
75 100 150125
TEMPERATURE (°C)
–50 –25
0
VIN CURRENT (µA)
20
50
050 75
3470 G05
10
40
30
25 100 150125
BIAS < 3V
BIAS > 3V
TEMPERATURE (°C)
–50
BIAS CURRENT (µA)
20
25
30
25 75
3470 G06
15
10
–25 0 50 100 150125
5
0
TEMPERATURE (°C)
–50
0
SHDN CURRENT (µA)
1
3
4
5
50
9
3470 G07
2
0
–25 75 100
25 150125
6
7
8VSHDN = 36V
VSHDN = 2.5V
TEMPERATURE (°C)
–50 –25
0
FB CURRENT (nA)
20
60
50
050 75
3470 G08
10
40
30
25 100 150125
LT347O L7 LJUW 5
LT3470
5
3470fd
Typical perForMance characTerisTics
FB Bias Current (VFB = 0V)
vs Temperature
Switch VCESAT (ISW = 100mA)
vs Temperature
Boost Diode VF (IF = 50mA)
vs Temperature
Catch Diode VF (IF = 100mA)
vs Temperature
Diode Leakage (VR = 36V)
vs Temperature
Switch VCESAT
BOOST Pin Current
Catch Diode Forward Voltage
TEMPERATURE (°C)
–50
FB CURRENT (µA)
80
100
120
25 75
3470 G09
60
40
–25 0 50 100 150125
20
0
TEMPERATURE (°C)
–50
SWITCH VCESAT (mV)
200
250
300
25 75
3470 G10
150
100
–25 0 50 100 150125
50
0
TEMPERATURE (°C)
–50
SCHOTTKY VF (V)
0.7
25
3470 G11
0.4
0.2
–25 0 50
0.1
0
0.8
0.6
0.5
0.3
75 100 150125
TEMPERATURE (°C)
–50
0.4
0.5
0.7
25 75
3470 G12
0.3
0.2
–25 0 50 100 150125
0.1
0
0.6
SCHOTTKY VF (V)
TEMPERATURE (°C)
–50 –25
0
SCHOTTKY DIODE LEAKAGE (µA)
30
60
050 75
3470 G13
20
15
10
50
40
25
55
5
45
35
25 100 150125
CATCH
BOOST
SWITCH CURRENT (mA)
0
400
500
700
300
3470 G14
300
200
100 200 400
100
0
600
SWITCH VCESAT (mV)
SWITCH CURRENT (mA)
0
8
10
14
300
3470 G15
6
4
100 200 400
2
0
12
BOOST PIN CURRENT (mA)
CATCH DIODE CURRENT (mA)
0
SCHOTTKY VF (V)
0.4
0.6
400
3470 G16
0.2
0100 200 300
1.0
0.8
LT347O 0 RUN 6 L7LJ1‘JW
LT3470
6
3470fd
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
shutdown feature is not used, tie this pin to the VIN pin.
NC (Pin 2/Pin 7): This pin can be left floating or connected
to VIN.
VIN (Pin 3/Pin 6): The VIN pin supplies current to the
LT3470’s internal regulator and to the internal power
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.
SW (Pin 5/Pin 4): The SW pin is the output of the internal
power switch. Connect this pin to the inductor, catch diode
and boost capacitor.
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.
BIAS (Pin 7
/Pin 2):
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.
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).
Exposed Pad (
DD, Pin 9):
Ground. Must be soldered to
PCB.
Boost Diode Forward Voltage
Minimum Input Voltage, VOUT = 3.3V
Minimum Input Voltage, VOUT = 5V
Typical perForMance characTerisTics
BOOST DIODE CURRENT (mA)
0
SCHOTTKY VF (V)
500
600
700
200
3470 G17
400
300
050 100 150
100
200
900
800
LOAD CURRENT (mA)
0
3.0
INPUT VOLTAGE (V)
3.5
4.0
4.5
5.0
5.5
6.0
50 100 150 200
3470 G18
TA = 25°C VIN TO START
VIN TO RUN
LOAD CURRENT (mA)
0
INPUT VOLTAGE (V)
6
7
200
3470 G19
5
450 100 150
8TA = 25°C
VIN TO START
VIN TO RUN
pin FuncTions
(ThinSOT/DD)
LT347O ....| h} J J— U m J 1'" "HI -I|-[]-w L7 LJUW 7
LT3470
7
3470fd
block DiagraM
+
+
R Q
S Q
500ns
ONE SHOT
VREF
1.25V
Burst Mode
DETECT
SW
GND
3470 BD
FB
R2 R1
SHDN
ENABLE
VIN
VIN
NC
BIAS
BOOST
L1
C2
C3
VOUT
gm
C1
LT347O \ W # A ._‘ . ~ VA A _. /\/\/\/\/\A/ 4\_/\_/\_/L/\_ 8 L7HCU§QB
LT3470
8
3470fd
Figure 1. Operating Waveforms of the LT3470 Converting 12V to 5V Using a 33µH Inductor and 10µF Output Capacitor
The LT3470 uses a hysteretic control scheme in conjunction
with Burst Mode operation to provide low output ripple
and low quiescent current while using a tiny inductor and
capacitors.
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 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
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
completing a full cycle. The hysteretic action of this control
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.
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.
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.
(1a) Burst Mode Operation (1b) Continuous Operation
VOUT
20mV/DIV
IL
100mA/DIV
1ms/DIV
VOUT
20mV/DIV
IL
100mA/DIV
5µs/DIV 3470 F01a
NO LOAD
10mA LOAD
VOUT
20mV/DIV
IL
100mA/DIV
1µs/DIV
VOUT
20mV/DIV
IL
100mA/DIV
1µs/DIV 3470 F1b
200mA LOAD
150mA LOAD
operaTion
LT347O VOUT + VD ( )(VD VOUT) AIL Vom VD MAX L7 LJUW 9
LT3470
9
3470fd
applicaTions inForMaTion
Input Voltage Range
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:
DC =VOUT
+
VD
VIN – VSW +VD
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:
VIN(MIN) =VOUT +VD
DCMAX
+VSW – VD
This analysis assumes the part has started up such that the
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.
The maximum input voltage is limited by the absolute
maximum VIN rating of 40V, provided an inductor of suf-
ficient value is used.
Inductor Selection
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
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:
LMIN =VIN(MAX) tON-TIME(MIN)
I
MAX
where VIN(MAX) is the maximum input voltage for the ap-
plication, tON-TIME(MIN)
is ~150ns and IMAX is the maximum
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:
f=1– DC
( )
VD+VOUT
( )
LI
L
where f is the switching frequency, IL is the ripple current
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:
tOFF-TIME =1– DC
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.
Table 1. Recommended Inductors for Loads up to 200mA
VOUT VIN UP TO 16V VIN UP TO 40V
2.5V 10µH 33µH
3.3V 10µH 33µH
5V 15µH 33µH
12V 33µH 47µH
Choose an inductor that is intended for power applications.
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
LT3470 m Vom
LT3470
10
3470fd
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
CDRH3D16/HP
CDRH3D28
CDRH2D18/HP
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
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 www.cooperet.com SD10 10 to 47 1.0 × 5.0 × 5.0
Murata www.murata.com LQH43C
LQH32C
10 to 47
10 to 15
2.6 × 3.2 × 4.5
1.6 × 2.5 × 3.2
applicaTions inForMaTion
be used. It is important to note that inductor saturation
current is reduced at high temperatures—see inductor
vendors for more information.
Input Capacitor
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage ripple
at the 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.
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.
Output Capacitor and Output Ripple
The output capacitor filters the inductors 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
LT3470’s switching frequency. The capacitors equivalent
series resistance (ESR) determines this impedance. Choose
one with low ESR intended for use in switching regulators.
The contribution to ripple voltage due to the ESR is ap-
proximately ILIM ESR. ESR should 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:
COUT >50 LILIM
V
OUT
2
where ILIM is the top current limit with VFB = 0V (see Elec-
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.
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.
LT347O if] E L7 LJUW ‘I‘I
LT3470
11
3470fd
applicaTions inForMaTion
Ceramic Capacitors
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
capacitance resulting in much higher output voltage ripple
than expected.
Ceramic capacitors are piezoelectric. The LT3470’s switch-
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.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT3470. A ceramic
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-
age can ring to twice its nominal value, possibly exceeding
the LT3470’s rating. This situation is easily avoided; see
the Hot-Plugging Safely section.
BOOST and BIAS Pin Considerations
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
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 (864) 963-6300 www.kemet.com Ceramic,
Tantalum
T494, T495
Sanyo (408) 749-9714 www.sanyovideo.com Ceramic,
Polymer,
Tantalum
POSCAP
Murata (404) 436-1300 www.murata.com Ceramic
AVX www.avxcorp.com Ceramic,
Tantalum
TPS Series
Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com Ceramic
VIN BOOST
LT3470
(2a)
(2b)
SW
C3
0.22µF
VIN
VOUT
VBOOST – VSW VOUT
MAX VBOOST VIN + VOUT
BIAS
GND
VIN BOOST
LT3470
SWBIAS
C3
0.22µF
VIN
VOUT
3470 F02
VBOOST – VSW VIN
MAX VBOOST 2VIN
GND
LT347O 0 FUN JL J. Tm —I|I|—> 12 L7LJCUEN2
LT3470
12
3470fd
Figure 3. The Minimum Input Voltage Depends on Output
Voltage, Load Current and Boost Circuit
Minimum Input Voltage, VOUT = 3.3V
Minimum Input Voltage, VOUT = 5V
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
applicaTions inForMaTion
(Figure 2b). The circuit in Figure 2a is more efficient
because the BOOST pin current and BIAS pin quiescent
current comes from a lower voltage source. You must also
be sure that the maximum voltage ratings of the BOOST
and BIAS pins are not exceeded.
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 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.
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 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
charging 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 es-
sentially 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.
LOAD CURRENT (mA)
0
3.0
INPUT VOLTAGE (V)
3.5
4.0
4.5
5.0
5.5
6.0
50 100 150 200
3470 G18
TA = 25°C VIN TO START
VIN TO RUN
LOAD CURRENT (mA)
0
INPUT VOLTAGE (V)
6
7
200
3470 G19
5
450 100 150
8TA = 25°C
VIN TO START
VIN TO RUN
VIN BOOST
LT3470 SOT-23
SWSHDN
3470 F04
VIN
100k
D1
1M
VOUT
BACKUP
BIAS
FB
GND
LT347O LT EECC w W all 4WW1111 my“ uuuuuuuuuuuuuuuuuuuuuuuuuuuuu T mg" 13 IIHHQ
LT3470
13
3470fd
applicaTions inForMaTion
PCB Layout
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. 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, along with the inductor and output capacitor,
should be placed on the same side of the circuit board,
and their connections should be made on that layer. Place
a local, unbroken ground plane below these components,
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 instability 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 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.
Figure 5. A Good PCB Layout Ensures Proper, Low EMI Operation
SHDN
VIN
VOUT
(5a) (5b)
VOUT
3470 F05
GND
SHDN
VIN
GND
C1
C2
VIAS TO FEEDBACK DIVIDER
VIAS TO LOCAL GROUND PLANE
OUTLINE OF LOCAL GROUND PLANE
LT3470
LT3470
14
3470fd
applicaTions inForMaTion
Hot-Plugging Safely
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 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
capacitors 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 in the circuit. An alterna-
tive solution is shown in Figure 6c. A resistor is added
in series with the input to eliminate the 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.
High Temperature Considerations
The die junction temperature of the LT3470 must be
lower than the maximum rating of 125°C (150°C for the
H-grade). This is generally not a concern unless the ambi-
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
have significant leakage current (see Typical Performance
Characteristics) increasing the quiescent current of the
LT3470 converter.
LT347O L7HEJWEGR 1 5
LT3470
15
3470fd
applicaTions inForMaTion
Figure 6. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT3470 Is Connected to a Live Supply
+
LT3470
2.2µF
VIN
10V/DIV
IIN
10A/DIV
10µs/DIV
VIN
CLOSING SWITCH
SIMULATES HOT PLUG
IIN
(6a)
(6b)
(6c)
LOW
IMPEDANCE
ENERGIZED
24V SUPPLY
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
+
LT3470
2.2µF
10µF
35V
AI.EI.
LT3470
2.2µF0.1µF
1Ω
3470 F06
LT347O _|_ m— Tm m— 31' |—_|_ |—_|_ W— I!“ m— lvw‘. _L J.— : _T_§ TI : 417—; :: -I|-- UFF UN—
LT3470
16
3470fd
3.3V Step-Down Converter 5V Step-Down Converter
2.5V Step-Down Converter
VIN BOOST
LT3470
SWSHDN
C3
0.22µF, 6.3V
22pF C2
22µF
3470 TA03
C1
F
VIN
5.5V TO 40V
VOUT
3.3V
200mA
R1
324k
R2
200k
C1: TDK C3216JB1H105M
C2: CE JMK316 BJ226ML-T
L1: TOKO A993AS-270M=P3
L1
33µH
BIAS
FB
GND
OFF ON
VIN BOOST
LT3470
SWSHDN
C3
0.22µF, 6.3V
22pF C2
22µF
3470 TA04
C1
F
VIN
7V TO 40V
VOUT
5V
200mA
R1
604k
R2
200k
L1
33µH
BIAS
FB
GND
OFF ON
C1: TDK C3216JB1H105M
C2: CE JMK316 BJ226ML-T
L1: TOKO A914BYW-330M=P3
Typical applicaTions
1.8V Step-Down Converter
12V Step-Down Converter
VIN BOOST
LT3470
SWSHDN
C3
0.47µF, 6.3V
22pF C2
22µF
3470 TA07
C1
F
VIN
4.7V TO 40V
VOUT
2.5V
200mA
R1
200k
R2
200k
C1: TDK C3216JB1H105M
C2: TDK C2012JB0J226M
L1: SUMIDA CDRH3D28
L1
33µH
BIAS
FB
GND
OFF ON
VIN BOOST
LT3470
SWSHDN
BIAS
C3
0.22µF, 25V
22pF
C2
22µF
3470 TA05
C1
F
VIN
4V TO 23V
VOUT
1.8V
200mA
R1
147k
R2
332k
L1
22µH
FB
GND
OFF ON
C1: TDK C3216JB1H105M
C2: TDK C2012JB0J226M
L1: MURATA LQH32CN150K53
VIN BOOST
LT3470
SWSHDN
C3
0.22µF, 16V
22pF C2
10µF
3470 TA06
C1
F
VIN
15V TO 34V
VOUT
12V
200mA
R1
866k
R2
100k
C1: TDK C3216JB1H105M
C2: TDK C3216JB1C106M
L1: MURATA LQH32CN150K53
L1
33µH
BIAS
FB
GND
OFF ON
LT347O *II W WEE Ii [mag 17, Efli m; * A L 7 7+ ‘ ' w 1 + ‘I L7HCUWE/ég 1 7
LT3470
17
3470fd
package DescripTion
TS8 Package
8-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1637 Rev A)
1.50 – 1.75
(NOTE 4)
2.80 BSC
0.22 – 0.36
8 PLCS (NOTE 3)
DATUM ‘A’
0.09 – 0.20
(NOTE 3)
TS8 TSOT-23 0710 REV A
2.90 BSC
(NOTE 4)
0.65 BSC
1.95 BSC
0.80 – 0.90
1.00 MAX 0.01 – 0.10
0.20 BSC
0.30 – 0.50 REF
PIN ONE ID
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
3.85 MAX
0.40
MAX
0.65
REF
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
1.4 MIN
2.62 REF
1.22 REF
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
LT3470 i 7 ‘ j T i Lfififlfifi ‘ T U EUU 1‘ fl 777L777 ijqij L fifi “—1 ' film‘m‘ : »\ Lzm-D-D-D—‘L La$L4
LT3470
18
3470fd
package DescripTion
DDB Package
8-Lead Plastic DFN (3mm × 2mm)
(Reference LTC DWG # 05-08-1702 Rev B)
2.00 ±0.10
(2 SIDES)
NOTE:
1. DRAWING CONFORMS TO VERSION (WECD-1) IN JEDEC PACKAGE OUTLINE M0-229
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
0.40 ± 0.10
BOTTOM VIEW—EXPOSED PAD
0.56 ± 0.05
(2 SIDES)
0.75 ±0.05
R = 0.115
TYP
R = 0.05
TYP
2.15 ±0.05
(2 SIDES)
3.00 ±0.10
(2 SIDES)
14
85
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
0.200 REF
0 – 0.05
(DDB8) DFN 0905 REV B
0.25 ± 0.05
2.20 ±0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
0.61 ±0.05
(2 SIDES)
1.15 ±0.05
0.70 ±0.05
2.55 ±0.05
PACKAGE
OUTLINE
0.25 ± 0.05
0.50 BSC
PIN 1
R = 0.20 OR
0.25 × 45°
CHAMFER
0.50 BSC
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
LT347O L7HEJWEGR 1 9
LT3470
19
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.
revision hisTory
REV DATE DESCRIPTION PAGE NUMBER
D 09/11 Corrected lead-based tape and reel part numbers in the Order Information section. 2
(Revision history begins at Rev D)
LT347O 20 L7ELUEN2
LT3470
20
3470fd
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
LINEAR TECHNOLOGY CORPORATION 2004
LT 0911 REV D • PRINTED IN USA
relaTeD parTs
PART NUMBER DESCRIPTION COMMENTS
LT1616 25V, 500mA (IOUT), 1.4MHz, High Efficiency
Step-Down DC/DC Converter
VIN = 3.6V to 25V, VOUT = 1.25V, IQ = 1.9mA, ISD < 1µA,
ThinSOT Package
LT1676 60V, 440mA (IOUT), 100kHz, High Efficiency
Step-Down DC/DC Converter
VIN = 7.4V to 60V, VOUT = 1.24V, IQ = 3.2mA, ISD = 2.5µA,
S8 Package
LT1765 25V, 2.75A (IOUT), 1.25MHz, High Efficiency
Step-Down DC/DC Converter
VIN = 3V to 25V, VOUT = 1.2V, IQ = 1mA, ISD = 15µA,
S8, TSSOP16E Packages
LT1766 60V, 1.2A (IOUT), 200kHz, High Efficiency
Step-Down DC/DC Converter
VIN = 5.5V to 60V, VOUT = 1.2V, IQ = 2.5mA, ISD = 25µA,
TSSOP16/E Package
LT1767 25V, 1.2A (IOUT), 1.25MHz, High Efficiency
Step-Down DC/DC Converter
VIN = 3V to 25V; VOUT = 1.2V, IQ = 1mA, ISD = 6µA,
MS8/E Packages
LT1776 40V, 550mA (IOUT), 200kHz, High Efficiency
Step-Down DC/DC Converter
VIN = 7.4V to 40V; VOUT = 1.24V, IQ = 3.2mA, ISD = 30µA,
N8, S8 Packages
LT C
®
1877 600mA (IOUT), 550kHz, Synchronous
Step-Down DC/DC Converter
VIN = 2.7V to 10V; VOUT = 0.8V, IQ = 10µA, ISD ≤ 1µA,
MS8 Package
LTC1879 1.2A (IOUT), 550kHz, Synchronous
Step-Down DC/DC Converter
VIN = 2.7V to 10V; VOUT = 0.8V, IQ = 15µA, ISD ≤ 1µA,
TSSOP16 Package
LT1933 36V, 600mA, 500kHz, High Efficiency
Step-Down DC/DC Converter
VIN = 3.6V to 36V; VOUT = 1.25V, IQ = 2.5µA, ISD ≤ 1µA,
??? Package
LT1934 34V, 250mA (IOUT), Micropower, Step-Down
DC/DC Converter
VIN = 3.2V to 34V; VOUT = 1.25V, IQ = 12µA, ISD ≤ 1µA,
??? Package
LT1956 60V, 1.2A (IOUT), 500kHz, High Efficiency
Step-Down DC/DC Converter
VIN = 5.5V to 60V, VOUT = 1.2V, IQ = 2.5mA, ISD = 25µA,
TSSOP16/E Package
LTC3405/LTC3405A 300mA (IOUT), 1.5MHz, Synchronous
Step-Down DC/DC Converter
VIN = 2.7V to 6V, VOUT = 0.8V, IQ = 20µA, ISD ≤ 1µA,
ThinSOT Package
LTC3406/LTC3406B 600mA (IOUT), 1.5MHz, Synchronous
Step-Down DC/DC Converter
VIN = 2.5V to 5.5V, VOUT = 0.6V, IQ = 20µA, ISD ≤ 1µA,
ThinSOT Package
LTC3411 1.25A (IOUT), 4MHz, Synchronous
Step-Down DC/DC Converter
VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 60µA, ISD ≤ 1µA,
MS Package
LTC3412 2.5A (IOUT), 4MHz, Synchronous
Step-Down DC/DC Converter
VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 60µA, ISD ≤ 1µA,
TSSOP16E Package
LTC3430 60V, 2.75A (IOUT), 200kHz, High Efficiency
Step-Down DC/DC Converter
VIN = 5.5V to 60V, VOUT = 1.2V, IQ = 2.5mA, ISD = 30µA,
TSSOP16E Package

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