LT3470A Datasheet by Analog Devices Inc.

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LT3470A
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TYPICAL APPLICATION
FEATURES
APPLICATIONS
DESCRIPTION
Micropower Buck Regulator
with Integrated Boost and
Catch Diodes
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.
With its wide input range of 4V to 40V, the LT3470A 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 35µA 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 cur-
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.
Efficiency and Power Loss vs Load Current
n Low Quiescent Current: 35μA at 12VIN to 3.3VOUT
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
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) 2mm × 3mm Thermally
Enhanced 8-Lead DFN Package
n Automotive Battery Regulation
n
Power for Portable Products
n
Distributed Supply Regulation
n
Industrial Supplies
n Wall Transformer Regulation
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
VIN BOOST
LT3470A
SWSHDN
0.22µF
22pF
22µF
2.2µF
VIN
5.7V TO 40V
VOUT
5V
250mA
604k
1%
200k
1%
33µH
BIAS
FB
GND
OFF ON
3470a TA01a
LOAD CURRENT (mA)
30
EFFICIENCY (%)
POWER LOSS (mW)
40
60
80
90
0.1 10 100 300
3470a TA01b
20
1
70
50
10
1
1000
100
10
0.1
VIN = 12V
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PIN CONFIGURATION ABSOLUTE MAXIMUM RATINGS
VIN, SHDN Voltage ................................................... 40V
BOOST Pin Voltage .................................................. 47V
BOOST Pin Above SW Pin ........................................ 25V
FB Voltage .................................................................. 5V
BIAS Voltage .............................................................15V
SW Voltage ................................................................VIN
Maximum Junction Temperature
LT3470AE, LT3470AI ........................................ 125°C
LT3470AH ......................................................... 150°C
Operating Temperature Range (Note 2)
LT3470AE ............................................40°C to 85°C
LT3470AI ........................................... 40°C to 125°C
LT3470AH .......................................... 40°C to 150°C
Storage Temperature Range .................. 65°C to 150°C
Lead Temperature (Soldering, 10 sec) ..................300°C
(Note 1)
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 = 80°C/W
EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING*PACKAGE DESCRIPTION TEMPERATURE RANGE
LT3470AEDDB#PBF LT3470AEDDB#TRPBF LDPR 8-Lead (3mm × 2mm) Plastic DFN –40°C to 85°C
LT3470AIDDB#PBF LT3470AIDDB#TRPBF LDPR 8-Lead (3mm × 2mm) Plastic DFN –40°C to 125°C
LT3470AHDDB#PBF LT3470AHDDB#TRPBF LDPR 8-Lead (3mm × 2mm) Plastic DFN –40°C to 150°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
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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 4V
Quiescent Current from VIN VSHDN = 0.2V
VBIAS = 3V, Not Switching
VBIAS = 0V, Not Switching
0.1
10
40
0.5
18
55
µA
µA
µA
Quiescent Current from Bias VSHDN = 0.2V
VBIAS = 3V, Not Switching
VBIAS = 0V, Not Switching
0.1
30
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
H-Grade
35
35
35
80
150
225
nA
nA
nA
FB Voltage Line Regulation 4V < VIN < 40V 0.0006 0.02 %/V
Minimum Switch Off-Time (Note 5) 500 ns
Maximum Duty Cycle 90 95 %
Switch Leakage Current 0.7 1.5 µA
Switch VCESAT ISW = 100mA 150 mV
Switch VCESAT Without Boost VBOOST = VSW 0.9 1.2 V
Switch Top Current Limit VFB = 0V 320 440 560 mA
Switch Bottom Current Limit VFB = 0V 280 mA
Catch Schottky Drop ISW = 100mA 600 mV
Catch Schottky Reverse Leakage VSW = 10V 0.2 2 µA
Boost Schottky Drop IBIAS = 50mA 690 775 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 2.3 5 mA
Bias Pin Preload VBOOST = 10V 50 mA
SHDN Pin Current VSHDN = 2.5V 1 5 µA
SHDN Input Voltage High 2V
SHDN Input Voltage Low 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 specifi cations
from 0°C to 85°C. Specifi cations over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. The LT3470AI specifi cations are
guaranteed over the –40°C to 125°C temperature range. LT3470AH
specifi cations are guaranteed over the –40°C to 150°C temperature range.
Note 3: Bias current fl ows 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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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
LOAD CURRENT (mA)
50
EFFICIENCY (%)
70
90
40
60
80
0.1 10 100
3470a G01
30
1
L = TOKO D52LC 47µH
TA = 25°C VIN = 7V
VIN = 36V
VIN = 24V
VIN = 12V
TA = 25°C unless otherwise noted.
TEMPERATURE (°C)
–50
CURRENT LIMIT (mA)
550
25
3470a G04
400
300
–25 0 50
250
200
600
500
450
350
75 100 150125
TEMPERATURE (°C)
–50 –25
0
VIN CURRENT (µA)
20
50
050 75
3470a G05
10
40
30
25 100 150125
BIAS < 3V
BIAS > 3V
TEMPERATURE (°C)
–50
BIAS CURRENT (µA)
20
25
30
25 75
3470a G06
15
10
–25 0 50 100 150125
5
0
TEMPERATURE (°C)
–50
0
SHDN CURRENT (µA)
1
3
4
5
50
9
3470a G07
2
0
–25 75 100
25 150125
6
7
8VSHDN = 36V
VSHDN = 2.5V
TEMPERATURE (°C)
–50
1.240
VFB (V)
1.245
1.250
1.255
1.260
–25 0 25 50
3470a G03
75 100 150125
LOAD CURRENT (mA)
50
EFFICIENCY (%)
70
90
40
60
80
0.1 10 100
3470a G02
30
1
L = TOKO D52LC 47µH
TA = 25°C VIN = 12V
VIN = 36V
VIN = 24V
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LT3470A
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FB Bias Current (VFB = 1V)
vs Temperature
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
TA = 25°C unless otherwise noted.
TEMPERATURE (°C)
–50 –25
0
FB CURRENT (nA)
20
60
50
050 75
3470a G08
10
40
30
25 100 150125
TEMPERATURE (°C)
–50
FB CURRENT (µA)
80
100
120
25 75
3470a G09
60
40
–25 0 50 100 150125
20
0
TEMPERATURE (°C)
–50
SWITCH VCESAT (mV)
200
250
300
25 75
3470a G10
150
100
–25 0 50 100 150125
50
0
TEMPERATURE (°C)
–50
SCHOTTKY VF (V)
0.7
25
3470a 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
3470a 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 (mA)
30
60
050 75
3470a G13
20
15
10
50
40
25
55
5
45
35
25 100 150
CATCH
BOOST
125
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LT3470A
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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
3470a G17
400
300
050 100 150
100
200
900
800
Switch VCESAT BOOST Pin Current
Catch Diode Forward Voltage
SWITCH CURRENT (mA)
0
400
500
700
300 400
3470a G14
300
200
100 200 500
100
0
600
SWITCH VCESAT (mV)
SWITCH CURRENT (mA)
0
8
10
14
300 400
3470a G15
6
4
100 200 500
2
0
12
BOOST PIN CURRENT (mA)
CATCH DIODE CURRENT (mA)
0
SCHOTTKY VF (V)
0.4
0.6
400
3470a G16
0.2
0100 200 300
1.0
0.8
LOAD CURRENT (mA)
0
3.0
INPUT VOLTAGE (V)
3.5
4.0
4.5
5.0
5.5
6.0
50 100 150 200 250
3470a G18
TA = 25°C
VIN TO RUN/START
LOAD CURRENT (mA)
0
INPUT VOLTAGE (V)
6
7
250
3470a G19
5
450 100 150 200
8TA = 25°C
VIN TO RUN/START
TA = 25°C unless otherwise noted.
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LT3470A
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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
feature is not used, tie this pin to the VIN pin.
NC (Pin 7): This pin can be left floating, connected to VIN,
or tied to GND.
VIN (Pin 6): The VIN pin supplies current to the LT3470As
internal regulator and to the internal power switch. This
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.
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.
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.
BIAS (
Pin 2):
The BIAS pin connects to the internal boost
Schottky diode and to the internal regulator. Tie to VOUT
when VOUT > 2.5V or to VIN otherwise. When VBIAS > 3V
the BIAS pin will supply current to the internal regulator.
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).
Exposed Pad (
Pin 9):
Ground. Must be soldered to PCB.
PIN FUNCTIONS
BLOCK DIAGRAM
+
+
RQʹ
SQ
500ns
ONE SHOT
VREF
1.25V
BURST MODE
DETECT
SW
GND
3470a BD
FB
R2 R1
SHDN
ENABLE
VIN
VIN
NC
BIAS
BOOST
L1
C2
C3
VOUT
gm
C1
LT347OA /\/\/\/\ /\/\/V/\v/\/\/ L7Hߤߤ
LT3470A
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Figure 1. Operating Waveforms of the LT3470A Converting 12V to 5V Using a 33μH Inductor and 10μF Output Capacitor
OPERATION
The LT3470A 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 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 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 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 100nA.
(1a) Burst Mode Operation (1b) Continuous Operation
VOUT
20mV/DIV
IL
100mA/DIV
1ms/DIV
VOUT
20mV/DIV
IL
100mA/DIV
5µs/DIV 3470a F01a
NO LOAD
10mA LOAD
VOUT
20mV/DIV
IL
100mA/DIV
1µs/DIV
VOUT
20mV/DIV
IL
100mA/DIV
1µs/DIV 3470a F1b
200mA LOAD
150mA LOAD
VD UT VD sz VOUT VD DCMAX L7 LJUW LT347OA
LT3470A
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Input Voltage Range
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:
DC =VOUT +VD
V
IN V
SW +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:
V
IN(MIN) =VOUT +VD
DCMAX
+VSW –V
D
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 LT3470A 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 induc-
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:
LMIN =V
IN(MAX) •t
ON-TIME(MIN)
IMAX
APPLICATIONS INFORMATION
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=1DC
()
V
D+VOUT
()
L•ΔIL
where f is the switching frequency, ΔIL is the ripple current
in the inductor (~200mA), 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
f
The calculated tOFF-TIME should be more than LT3470As
minimum tOFF-TIME (See Electrical Characteristics), so the
application circuit is capable of delivering full rated output
current. If the full output current of 250mA 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 250mA
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
be used. It is important to note that inductor saturation
current is reduced at high temperatures—see inductor
vendors for more information.
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APPLICATIONS 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 LT3470A 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 LT3470A
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
LT3470A is quiescent. In order to keep output voltage
ripple low, the impedance of the capacitor must be low at
the LT3470As switching frequency. The capacitors equiva-
lent 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 approximately ILIM • ESR. ESR should be less than
~150m. 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 • L ILIM
VOUT
2
Where ILIM is the top current limit with VFB
= 0V (see Electri-
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 LT3470As
feedback comparator. This ripple can be reduced further
by adding a small (typically 22pF) phase lead capacitor
between the output and the feedback pin.
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
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APPLICATIONS INFORMATION
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3470A. 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 LT3470As
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
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 LT3470A. A ceramic
input capacitor combined with trace or cable inductance
forms a high quality (under damped) tank circuit. If the
LT3470A circuit is plugged into a live supply, the input volt-
age can ring to twice its nominal value, possibly exceeding
the LT3470As 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 2. 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
LT3470A
(2a)
(2b)
SW
C3
0.22µF
VIN
VOUT
VBOOST – VSW VOUT
MAX VBOOST VIN + VOUT
BIAS
GND
VIN BOOST
LT3470A
SWBIAS
C3
0.22µF
VIN
VOUT
3470a F02
VBOOST – VSW VIN
MAX VBOOST 2•VIN
GND
LT347OA ‘ _+f'—I |—_|_ m EMF START // § :: —§_|_I _ ? L7LJCUEN2
LT3470A
12
3470afb
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 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
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.
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 VIN = 40V. There is another situation to consider
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
LT3470As 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 LT3470As 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
LT3470A 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.
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 LT3470A Runs Only When the Input is
Present Hot-Plugging Safely
VIN BOOST
LT3470A
SWSHDN
3470a F04
VIN
100k
D1
1M
VOUT
BACKUP
BIAS
FB
GND
LOAD CURRENT (mA)
0
3.0
INPUT VOLTAGE (V)
3.5
4.0
4.5
5.0
5.5
6.0
50 100 150 200 250
3470a F03a
TA = 25°C
VIN TO RUN/START
LOAD CURRENT (mA)
0
INPUT VOLTAGE (V)
6
7
250
3470a F03b
5
450 100 150 200
8TA = 25°C
VIN TO RUN/START
LT347OA LT n ® " CCEC m .W .EEI_LL Amw 1111 E ............................. T Lywmg
LT3470A
13
3470afb
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
LT3470A to help remove heat from the LT3470A to the
ground plane.
Figure 5. A Good PCB Layout Ensures Proper, Low EMI Operation
VOUT
3470a F05
SHDN
VIN
GND
LT347OA L7LJCUEN2
LT3470A
14
3470afb
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 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-
age at the VIN pin of the LT3470A can ring to twice the
nominal input voltage, possibly exceeding the LT3470As
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 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 alternative solution is shown in Figure 6c. A 1
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 LT3470A must be
lower than the maximum rating of 125°C (150°C for
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 LT3470A. 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 LT3470A power
dissipation by the thermal resistance from junction to
ambient. Power dissipation within the LT3470A 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 DFN package with the exposed pad has a thermal
resistance of approximately 80°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 LT3470A converter.
L7 LJUW LT347OA
LT3470A
15
3470afb
APPLICATIONS INFORMATION
Figure 6. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT3470A is Connected to a Live Supply
+
LT3470A
2.2µF
VIN
10V/DIV
IIN
10A/DIV
10µs/DIV
VIN
10V/DIV
IIN
10A/DIV
10µs/DIV
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
+
LT3470A
2.2µF
10µF
35V
AI.EI.
LT3470A
2.2µF0.1µF
1Ω
3470a F06
LT347OA fl _—l 11 m Tm _: Lil I: _—I m him L7LJCUEN2
LT3470A
16
3470afb
APPLICATIONS INFORMATION
3.3V Step-Down Converter
5V Step-Down Converter
2.5V Step-Down Converter
VIN BOOST
LT3470A
SWSHDN
C3
0.22µF, 6.3V
22pF C2
22µF
3470a TA02
C1
F
VIN
4V TO 40V
VOUT
3.3V
250mA
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
LT3470A
SWSHDN
C3
0.22µF, 6.3V
22pF C2
22µF
3470a TA03
C1
F
VIN
5.7V TO 40V
VOUT
5V
250mA
R1
604k
R2
200k
L1
33µH
BIAS
FB
GND
OFF ON
C1: TDK C3216JB1H105M
C2: CE JMK316 BJ226ML-T
L1: TOKO A914BYW-330M=P3
VIN BOOST
LT3470A
SWSHDN
C3
0.47µF, 6.3V
22pF C2
22µF
3470a TA04
C1
F
VIN
4V TO 40V
VOUT
2.5V
250mA
R1
200k
R2
200k
C1: TDK C3216JB1H105M
C2: TDK C2012JB0J226M
L1: SUMIDA CDRH3D28
L1
33µH
BIAS
FB
GND
OFF ON
M— M— 11H L7 LJUW LT347OA
LT3470A
17
3470afb
TYPICAL APPLICATIONS
1.8V Step-Down Converter
12V Step-Down Converter
VIN BOOST
LT3470A
SWSHDN
BIAS
C3
0.22µF, 25V
22pF
C2
22µF
3470a TA05
C1
F
VIN
4V TO 23V
VOUT
1.8V
250mA
R1
147k
R2
332k
L1
22µH
FB
GND
OFF ON
C1: TDK C3216JB1H105M
C2: TDK C2012JB0J226M
L1: MURATA LQH32CN150K53
VIN BOOST
LT3470A
SWSHDN
C3
0.22µF, 16V
22pF C2
10µF
3470a TA06
C1
F
VIN
15V TO 34V
VOUT
12V
250mA
R1
866k
R2
100k
C1: TDK C3216JB1H105M
C2: TDK C3216JB1C106M
L1: MURATA LQH32CN150K53
L1
33µH
BIAS
FB
GND
OFF ON
LT3470A i ‘ 4 7‘ L 1 fgfifififi% 4 T LAU 1U U,:T J ‘ 13‘ {My}? ‘—\ i film W] _ ,wq : “ 9‘ L ii L” “A FWD—77 L7LJCUEN2
LT3470A
18
3470afb
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
LT347OA L7 LJUW
LT3470A
19
3470afb
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
B 01/11 Added H-grade part information. 2 to 5, 15
(Revision history begins at Rev B)
LT347OA L7LJCUEN2
LT3470A
20
3470afb
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2008
LT 0111 REV B • PRINTED IN USA
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