Linear LT3470 Micropower buck regulator with integrated boost and catch diode Datasheet

LT3470
Micropower Buck Regulator
with Integrated Boost and
Catch Diodes
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FEATURES
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DESCRIPTIO
Low Quiescent Current: 26µA at 12VIN to 3.3VOUT
Integrated Boost and Catch Diodes
Input Range: 4V to 40V
Low Output Ripple: <10mV
< 1µA in Shutdown Mode
Output Voltage: 1.25V to 16V
200mA Output Current
Hysteretic Mode Control
– Low Ripple Burst Mode® Operation at Light Loads
– Continuous Operation at Higher Loads
Solution Size as Small as 50mm2
Low Profile (1mm) ThinSOT Package
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APPLICATIO S
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Automotive Battery Regulation
Power for Portable Products
Distributed Supply Regulation
Industrial Supplies
Wall Transformer Regulation
The LT®3470 is a micropower step-down DC/DC converter
that integrates a 300mA power switch, catch diode and
boost diode into a low profile (1mm) ThinSOTTM package.
The LT3470 combines Burst Mode and continuous operation to allow the use of tiny inductor 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 leadacid 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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Burst Mode is a registered trademark of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
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TYPICAL APPLICATIO
Efficiency and Power Loss vs Load Current
90
VIN
7V TO 40V
80
0.22µF
BOOST
OFF ON
SHDN
70
33µH
VOUT
5V
200mA
SW
BIAS
22pF
2.2µF
604k
1%
FB
GND
22µF
200k
1%
100
60
50
10
40
30
POWER LOSS (mW)
LT3470
EFFICIENCY (%)
VIN
1000
VIN = 12V
1
20
10
0.1
0.1
1
10
LOAD CURRENT (mA)
100
3470 TA02
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LT3470
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ABSOLUTE
RATI GS
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PACKAGE/ORDER I FOR ATIO
(Note 1)
VIN, SHDN Voltage .................................................. 40V
BOOST Pin Voltage ................................................. 47V
BOOST Pin Above SW Pin ...................................... 25V
FB Voltage ................................................................ 5V
BIAS Voltage ............................................................ 25V
SW Voltage ................................................................VIN
Maximum Junction Temperature ......................... 125°C
Operating Temperature Range (Note 2) .. – 40°C to 85°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
TOP VIEW
SHDN 1
NC 2
VIN 3
GND 4
8 FB
7 BIAS
6 BOOST
5 SW
LT3470ETS8
LT3470ITS8
TS8 PART MARKING
TS8 PACKAGE
8-LEAD PLASTIC TSOT-23
TJMAX = 125°C, θJA = 140°C/ W
LTBDM
LTBPW
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.
VIN = 10V, VSHDN = 10V, VBOOST = 15V, VBIAS = 3V unless otherwise specified.
PARAMETER
CONDITIONS
TYP
MAX
4
V
0.5
18
50
µA
µA
µA
●
Minimum Input Voltage
Quiescent Current from VIN
MIN
UNITS
VSHDN = 0.2V
VBIAS = 3V, Not Switching
VBIAS = 0V, Not Switching
●
0.1
10
35
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
●
1.250
1.265
V
FB Pin Bias Current (Note 3)
VFB = 1V
35
35
80
150
nA
nA
0.0006
0.01
%/V
Quiescent Current from Bias
1.228
●
FB Voltage Line Regulation
4V < VIN < 40V
Minimum Switch Off-Time (Note 5)
500
Switch Leakage Current
Switch VCESAT
ISW = 100mA
Switch Top Current Limit
VFB = 0V
Switch Bottom Current Limit
VFB = 0V
250
ns
0.7
1.5
µA
215
300
mV
325
435
mA
225
mA
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LT3470
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.
VIN = 10V, VSHDN = 10V, VBOOST = 15V, VBIAS = 3V unless otherwise specified.
PARAMETER
CONDITIONS
Catch Schottky Drop
Catch Schottky Reverse Leakage
MIN
TYP
MAX
UNITS
ISH = 100mA
630
775
mV
VSW = 10V
0.2
2
µA
Boost Schottky Drop
ISH = 30mA
650
775
mV
Boost Schottky Reverse Leakage
VSW = 10V, VBIAS = 0V
●
Minimum Boost Voltage (Note 4)
0.2
2
µA
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
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: The LT3470E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. The LT3470I specifications are
guaranteed over the –40°C to 125°C temperature range.
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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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TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency, VOUT = 3.3V
Efficiency, VOUT = 5V
80
VFB vs Temperature
90
90
L = TOKO D52LC 47µH
TA = 25°C
VIN = 12V
VIN = 7V
80
1.260
L = TOKO D52LC 47µH
TA = 25°C
VIN = 12V
VIN = 36V
60
50
70
VIN = 24V
VIN = 36V
VFB (V)
VIN = 24V
EFFICIENCY (%)
EFFICIENCY (%)
1.255
70
60
1.250
50
1.245
40
40
30
0.1
10
1
LOAD CURRENT (mA)
100
3470 G01
30
0.1
10
1
LOAD CURRENT (mA)
100
3470 G02
1.240
–50
–25
75
0
25
50
TEMPERATURE (°C)
100
125
3470 G03
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LT3470
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TYPICAL PERFOR A CE CHARACTERISTICS
Top and Bottom Switch Current
Limits (VFB = 0V) vs Temperature
BIAS Quiescent Current
(Bias > 3V) vs Temperature
VIN Quiescent Current
vs Temperature
50
400
30
350
25
250
200
150
100
BIAS < 3V
BIAS CURRENT (µA)
VIN CURRENT (µA)
CURRENT LIMIT (mA)
40
300
30
20
10
0
50
75
25
TEMPERATURE (°C)
100
10
5
0
–50 –25
125
15
BIAS > 3V
50
0
– 50 – 25
20
50
25
0
75
TEMPERATURE (°C)
100
0
–50 –25
125
50
25
75
0
TEMPERATURE (°C)
100
3470 G05
3470 G04
SHDN Bias Current
vs Temperature
3470 G06
FB Bias Current (VFB = 1V)
vs Temperature
9
125
FB Bias Current (VFB = 0V)
vs Temperature
50
120
8
VSHDN = 36V
5
4
3
2
–25
0
25
50
75
TEMPERATURE (°C)
30
20
10
VSHDN = 2.5V
0
–50
FB CURRENT (µA)
FB CURRENT (nA)
SHDN CURRENT (µA)
6
1
100
40
7
100
50
25
0
75
TEMPERATURE (°C)
Switch VCESAT (ISW = 100mA)
vs Temperature
250
0
–50 –25
125
150
100
0.8
0.7
0.7
0.6
0.5
0.4
0.3
125
3470 G10
0
– 50 – 25
0.5
0.4
0.3
0.1
0.1
100
125
0.2
0.2
50
100
Catch Diode VF (IF = 100mA)
vs Temperature
SCHOTTKY VF (V)
200
50
25
75
0
TEMPERATURE (°C)
3470 G09
0.6
SCHOTTKY VF (V)
SWITCH VCESAT (mV)
100
Boost Diode VF (IF = 50mA)
vs Temperature
300
50
25
75
0
TEMPERATURE (°C)
40
3470 G08
3470 G07
0
–50 –25
60
20
0
–50 –25
125
80
0
50
75
25
TEMPERATURE (°C)
100
125
3470 G11
0
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
3470 G12
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LT3470
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TYPICAL PERFOR A CE CHARACTERISTICS
Diode Leakage (VR = 36V)
vs Temperature
BOOST
CATCH
700
14
600
12
15
10
BOOST PIN CURRENT (mA)
SWITCH VCESAT (mV)
20
500
400
300
200
5
50
25
0
75
TEMPERATURE (°C)
100
125
8
6
4
0
0
100
0
200
0
400
300
SWITCH CURRENT (mA)
3470 G13
100
200
300
SWITCH CURRENT (mA)
Catch Diode Forward Voltage
400
3470 G15
3470 G14
Boost Diode Forward Voltage
1.0
900
800
0.8
SCHOTTKY VF (V)
700
0.6
0.4
600
500
400
300
200
0.2
100
0
0
200
100
300
CATCH DIODE CURRENT (mA)
0
400
0
100
50
150
BOOST DIODE CURRENT (mA)
3470 G16
Minimum Input Voltage, VOUT = 3.3V
6.0
TA = 25°C
200
3470 G17
Minimum Input Voltage, VOUT = 5V
8
TA = 25°C
VIN TO START
VIN TO START
5.5
7
INPUT VOLTAGE (V)
–25
SCHOTTKY VF (V)
0
–50
10
2
100
INPUT VOLTAGE (V)
SCHOTTKY DIODE LEAKAGE (µA)
BOOST Pin Current
Switch VCESAT
25
5.0
4.5
4.0
VIN TO RUN
6
VIN TO RUN
5
3.5
3.0
0
50
100
150
LOAD CURRENT (mA)
200
3470 G18
4
0
100
150
50
LOAD CURRENT (mA)
200
3470 G19
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LT3470
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PI FU CTIO S
SHDN (Pin 1): The SHDN pin is used to put the LT3470 in
shutdown mode. Tie to ground to shut down the LT3470.
Apply 2V or more for normal operation. If the shutdown
feature is not used, tie this pin to the VIN pin.
NC (Pin 2): This pin can be left floating or connected to VIN.
VIN (Pin 3): The VIN pin supplies current to the LT3470’s
internal regulator and to the internal power switch. This
pin must be locally bypassed.
GND (Pin 4): Tie the GND pin to a local ground plane below
the LT3470 and the circuit components. Return the feedback divider to this pin.
BOOST (Pin 6): The BOOST pin is used to provide a drive
voltage, which is higher than the input voltage, to the
internal bipolar NPN power switch.
BIAS (Pin 7): The BIAS pin connects to the internal boost
Schottky diode and to the internal regulator. Tie to VOUT
when VOUT > 2V or to VIN otherwise. When VBIAS > 3V the
BIAS pin will supply current to the internal regulator.
FB (Pin 8): The LT3470 regulates its feedback pin to
1.25V. Connect the feedback resistor divider tap to this
pin. Set the output voltage according to VOUT = 1.25V
(1 + R1/R2) or R1 = R2 (VOUT/1.25 – 1).
SW (Pin 5): The SW pin is the output of the internal power
switch. Connect this pin to the inductor, catch diode and
boost capacitor.
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BLOCK DIAGRA
VIN
3
VIN
BIAS
7
C1
+
–
BOOST
500ns
ONE SHOT
R
Q′
S
Q
6
C3
SW
L1
VOUT
5
–
C2
1
SHDN
+
ENABLE
BURST MODE
DETECT
2 NC
VREF
1.25V
gm
FB
R2
GND
8
4
R1
3470 BD
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LT3470
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OPERATIO
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 bottom
current comparator trips and the minimum off-time oneshot 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 shortcircuit 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.
NO LOAD
200mA LOAD
VOUT
20mV/DIV
VOUT
20mV/DIV
IL
100mA/DIV
IL
100mA/DIV
1ms/DIV
1µs/DIV
10mA LOAD
150mA LOAD
VOUT
20mV/DIV
VOUT
20mV/DIV
IL
100mA/DIV
IL
100mA/DIV
5µs/DIV
(1a) Burst Mode Operation
3470 F01a
1µs/DIV
3470 F1b
(1b) Continuous Operation
Figure 1. Operating Waveforms of the LT3470 Converting 12V to 5V Using a 33µH Inductor and 10µF Output Capacitor
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LT3470
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APPLICATIO S I FOR ATIO
Input Voltage Range
The minimum input voltage required to generate a particular 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:
VOUT + VD
VIN(MIN) =
DCMAX – VD + VSW
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
sufficient 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)
IMAX
where VIN(MAX) is the maximum input voltage for the
application, 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 )
L • ∆IL
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
f
The calculated tOFF-TIME should be more than LT3470’s
minimum tOFF-TIME (See Electrical Characteristics), so the
application circuit is capable of delivering full rated output
current. If the full output current of 200mA is not required,
the calculated tOFF-TIME can be made less than minimum
tOFF-TIME possibly allowing the use of a smaller inductor.
See Table 1 for an inductor value selection guide.
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
be used. It is important to note that inductor saturation
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LT3470
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APPLICATIO S I FOR ATIO
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
current is reduced at high temperatures—see inductor
vendors for more information.
Input Capacitor
Step-down regulators draw current from the input supply
in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage ripple
at the VIN pin of the LT3470 and to force this switching
current into a tight local loop, minimizing EMI. The input
capacitor must have low impedance at the switching
frequency to do this effectively. A 1µF to 2.2µF ceramic
capacitor satisfies these requirements.
If the input source impedance is high, a larger value
capacitor may be required to keep input ripple low. In this
case, an electrolytic of 10µF or more in parallel with a 1µF
ceramic is a good combination. Be aware that the input
capacitor is subject to large surge currents if the LT3470
circuit is connected to a low impedance supply, and that
some electrolytic capacitors (in particular tantalum) must
be specified for such use.
Output Capacitor and Output Ripple
The output capacitor filters the inductor’s ripple current
and stores energy to satisfy the load current when the
LT3470 is quiescent. In order to keep output voltage ripple
low, the impedance of the capacitor must be low at the
LT3470’s switching frequency. The capacitor’s 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
approximately 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:
⎛I
⎞
COUT > 50 • L • ⎜ LIM ⎟
⎝ VOUT ⎠
2
Where ILIM is the top current limit with VFB = 0V (see
Electrical 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.
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LT3470
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Ceramic Capacitors
BOOST and BIAS Pin Considerations
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3470. Not all ceramic capacitors are
suitable. X5R and X7R types are stable over temperature
and applied voltage and give dependable service. Other
types, including Y5V and Z5U have very large temperature
and voltage coefficients of capacitance. In an application
circuit they may have only a small fraction of their nominal
capacitance resulting in much higher output voltage ripple
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
arrange 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
Ceramic capacitors are piezoelectric. The LT3470’s switching 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 BurstMode operation, 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
voltage can ring to twice its nominal value, possibly
exceeding the LT3470’s rating. This situation is easily
avoided; see the Hot Plugging Safely section.
VIN
3
C3
0.22µF
6
VIN
BOOST
LT3470
SW
BIAS
5
VOUT
7
GND
4
VBOOST – VSW ≅ VOUT
MAX VBOOST ≅ VIN + VOUT
(2a)
VIN
3
VIN
7
C3
0.22µF
6
BOOST
LT3470
BIAS
SW
5
VOUT
GND
4
3470 F02
VBOOST – VSW ≅ VIN
MAX VBOOST ≅ 2VIN
(2b)
Figure 2. Two Circuits for Generating the Boost Voltage
Table 3. Capacitor Vendors
Vendor
Phone
URL
Part Series
Comments
Panasonic
(714) 373-7366
www.panasonic.com
Ceramic,
Polymer,
Tantalum
EEF Series
Kemet
(864) 963-6300
www.kemet.com
Ceramic,
Tantalum
Sanyo
(408) 749-9714
www.sanyovideo.com
Ceramic,
Polymer,
Tantalum
Murata
(404) 436-1300
AVX
Taiyo Yuden
(864) 963-6300
www.murata.com
Ceramic
www.avxcorp.com
Ceramic,
Tantalum
www.taiyo-yuden.com
T494, T495
POSCAP
TPS Series
Ceramic
3470f
10
LT3470
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APPLICATIO S I FOR ATIO
(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 Input Voltage, VOUT = 3.3V
6.0
TA = 25°C
VIN TO START
INPUT VOLTAGE (V)
5.5
5.0
4.5
4.0
VIN TO RUN
3.5
3.0
0
50
100
150
LOAD CURRENT (mA)
200
3470 G18
Minimum Input Voltage, VOUT = 5V
8
TA = 25°C
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 excessively at the top switch current limit maximum of 450µA,
an LT3470 buck regulator will tolerate a shorted output
even if VIN = 40V. There is another situation to consider in
systems where the output will be held high when the input
to the LT3470 is absent. This may occur in battery 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
essentially zero. However, if the VIN pin is grounded while
the output is held high, then parasitic diodes inside the
LT3470 can pull large currents from the output through
the SW pin and the VIN pin. Figure 4 shows a circuit that
will run only when the input voltage is present and that
protects against a shorted or reversed input.
VIN TO START
D1
INPUT VOLTAGE (V)
7
VIN
3
VIN
100k
6
1
6
BOOST
LT3470 SOT-23
SHDN
SW
VIN TO RUN
BIAS
1M
5
FB
5
VOUT
7
8
GND
4
4
0
100
150
50
LOAD CURRENT (mA)
200
3470 G19
Figure 3. The Minimum Input Voltage Depends on Output
Voltage, Load Current and Boost Circuit
BACKUP
3470 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 LT3470 Runs Only When the Input is
Present Hot Plugging Safely
3470f
11
LT3470
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APPLICATIO S I FOR ATIO
PCB Layout
Hot Plugging Safely
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Note that large,
switched currents flow in the power switch, the internal
catch diode and the input capacitor. The loop formed by
these components should be as small as possible. Furthermore, the system ground should be tied to the regulator ground in only one place; this prevents the switched
current from injecting noise into the system ground.
These components, along with the inductor and output
capacitor, should be placed on the same side of the circuit
board, 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 of
the LT3470 to help remove heat from the LT3470 to the
ground plane.
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3470. However, these capacitors
can cause problems if the LT3470 is plugged into a live
supply (see Linear Technology Application Note 88 for a
complete discussion). The low loss ceramic capacitor
combined with stray inductance in series with the power
source forms an under damped tank circuit, and the
voltage at the VIN pin of the LT3470 can ring to twice the
nominal input voltage, possibly exceeding the LT3470’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LT3470 into an
energized supply, the input network should be designed to
prevent this overshoot. Figure 6 shows the waveforms
that result when an LT3470 circuit is connected to a 24V
supply through six feet of 24-gauge twisted pair. The first
plot is the response with a 2.2µF ceramic capacitor at the
input. The input voltage rings as high as 35V and the input
current peaks at 20A. One method of damping the tank
circuit is to add another capacitor with a series resistor to
the circuit. In Figure 6b an aluminum electrolytic capacitor
has been added. This capacitor’s high equivalent series
resistance damps the circuit and eliminates the voltage
overshoot. The extra capacitor improves low frequency
ripple filtering and can slightly improve the efficiency of
the circuit, though it is likely to be the largest component
SHDN
VIN
C1
GND
C2
VOUT
3470 F05
VIAS TO FEEDBACK DIVIDER
VIAS TO LOCAL GROUND PLANE
OUTLINE OF LOCAL GROUND PLANE
Figure 5. A Good PCB Layout Ensures Proper, Low EMI Operation
3470f
12
LT3470
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APPLICATIO S I FOR ATIO
CLOSING SWITCH
SIMULATES HOT PLUG
IIN
VIN
LT3470
+
VIN
10V/DIV
2.2µF
LOW
IMPEDANCE
ENERGIZED
24V SUPPLY
IIN
10A/DIV
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
10µs/DIV
(6a)
LT3470
10µF
35V
AI.EI.
+
2.2µF
(6b)
1Ω
LT3470
0.1µF
2.2µF
(6c)
3470 F06
Figure 6: A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT3470 is Connected to a Live Supply
in the circuit. An alternative solution is shown in Figure 6c.
A 1Ω resistor is added in series with the input to eliminate
the voltage overshoot (it also reduces the peak input
current). 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 temperature of the LT3470 must be lower than the
maximum rating of 125°C. This is generally 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 LT3470. The
maximum load current should be derated as the ambient
temperature approaches 125°C. The die temperature is
calculated by multiplying the LT3470 power dissipation
by the thermal resistance from junction to ambient.
Power dissipation within the LT3470 can be estimated by
calculating the total power loss from an efficiency measurement. Thermal resistance depends on the layout of
the circuit board, but a value of 150°C/W is typical. The
temperature rise for an LT3470 producing 5V at 200mA
is approximately 30°C, allowing it to deliver full load to
100°C ambient. Above this temperature the load current
should be reduced. For 3.3V at 200mA the temperature
rise is 20°C. Finally, be aware that at high ambient
temperatures the internal Schottky diode will have significant leakage current (See Typical Performance Characteristics) increasing the quiescent current of the LT3470
converter.
3470f
13
LT3470
U
TYPICAL APPLICATIO S
5V Step-Down Converter
3.3V Step-Down Converter
VIN
5.5V TO 40V
3
VIN
1
OFF ON
6
BOOST
LT3470
SHDN
5
SW
8
FB
22pF
GND
VOUT
3.3V
200mA
R1
324k
OFF ON
6
1
BOOST
LT3470
SHDN
SW
BIAS
C1
1µF
C2
22µF
R2
200k
4
3
VIN
7
BIAS
C1
1µF
VIN
7V TO 40V
C3
0.22µF
L1
33µH
FB
5
C3
0.22µF
L1
33µH
VOUT
5V
200mA
7
8
22pF
GND
R1
604k
R2
200k
4
C2
22µF
3470 TA04
3470 TA03
C1: TDK C3216JB1H105M
C2: CE JMK316 BJ226ML-T
L1: TOKO A914BYW-330M=P3
C1: TDK C3216JB1H105M
C2: CE JMK316 BJ226ML-T
L1: TOKO A993AS-270M=P3
2.5V Step-Down Converter
VIN
4.7V TO 40V
3
6
VIN
OFF ON
1
BOOST
LT3470
SHDN
SW
BIAS
C1
1µF
FB
5
C3
0.47µF
L1
33µH
VOUT
2.5V
200mA
7
8
22pF
GND
R1
200k
C2
22µF
R2
200k
4
3470 TA07
C1: TDK C3216JB1H105M
C2: TDK C2012JB0J226M
L1: SUMIDA CDRH3D28
12V Step-Down Converter
1.8V Step-Down Converter
VIN
4V TO 25V
3
VIN
OFF ON
1
7
C1
1µF
6
BOOST
LT3470
SHDN
SW
5
BIAS
22pF
FB
GND
4
8
VIN
15V TO 35V
C3
0.22µF
L1
22µH
3
VIN
VOUT
1.8V
200mA
R1
147k
R2
332k
C2
22µF
OFF ON
1
6
BOOST
LT3470
SHDN
SW
BIAS
C1
1µF
FB
GND
4
3470 TA04
C1: TDK C3216JB1H105M
C2: TDK C2012JB0J226M
L1: MURATA LQH32CN150K53
5
C3
0.22µF
L1
33µH
VOUT
12V
200mA
7
8
22pF
R1
866k
R2
100k
C2
10µF
3470 TA06
C1: TDK C3216JB1H105M
C2: TDK C3216JB1C106M
L1: MURATA LQH32CN150K53
3470f
14
LT3470
U
PACKAGE DESCRIPTIO
TS8 Package
8-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1637)
0.52
MAX
2.90 BSC
(NOTE 4)
0.65
REF
1.22 REF
1.4 MIN
3.85 MAX 2.62 REF
2.80 BSC
1.50 – 1.75
(NOTE 4)
PIN ONE ID
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
0.22 – 0.36
8 PLCS (NOTE 3)
0.65 BSC
0.80 – 0.90
0.20 BSC
0.01 – 0.10
1.00 MAX
DATUM ‘A’
0.30 – 0.50 REF
NOTE:
1. DIMENSIONS ARE IN MILLIMETERS
2. DRAWING NOT TO SCALE
3. DIMENSIONS ARE INCLUSIVE OF PLATING
0.09 – 0.20
(NOTE 3)
1.95 BSC
TS8 TSOT-23 0802
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
3470f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LT3470
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1613
550mA (ISW), 1.4MHz, High Efficiency Step-Up
DC/DC Converter
VIN: 0.9V to 10V, VOUT(MAX) = 34V, IQ = 3mA, ISD < 1µA,
ThinSOT Package
LT1615/LT1615-1
300mA/80mA (ISW), Constant Off-Time,
High Efficiency Step-Up DC/DC Converters
VIN: 1.2V to 15V, VOUT(MAX) = 34V, IQ = 20µA, ISD < 1µA,
ThinSOT Package
LT1944/LT1944-1 (Dual) Dual Output 350mA/100mA (ISW), Constant Off-Time,
High Efficiency Step-Up DC/DC Converters
VIN: 1.2V to 15V, VOUT(MAX) = 34V, IQ = 20µA, ISD < 1µA,
MS Package
LT1945 (Dual)
Dual Output, Pos/Neg, 350mA (ISW), Constant Off-Time, VIN: 1.2V to 15V, VOUT(MAX) = ±34V, IQ = 20µA, ISD < 1µA,
High Efficiency Step-Up DC/DC Converter
MS Package
LT1961
1.5A (ISW), 1.25MHz, High Efficiency Step-Up
DC/DC Converter
VIN: 3V to 25V, VOUT(MAX) = 35V, IQ = 0.9mA, ISD < 6µA,
MS8E Package
LTC®3400/LTC3400B
600mA (ISW), 1.2MHz, Synchronous Step-Up
DC/DC Converter
VIN: 0.85V to 5V, VOUT(MAX) = 5V, IQ = 19µA/300µA, ISD < 1µA,
ThinSOT Package
LTC3401
1A (ISW), 3MHz, Synchronous Step-Up
DC/DC Converter
VIN: 0.5V to 5V, VOUT(MAX) = 6V, IQ = 38µA, ISD < 1µA,
MS Package
LT3460
0.32A (ISW), 1.3MHz, High Efficiency Step-Up
DC/DC Converter
VIN: 2.5V to 16V, VOUT(MAX) = 36V, IQ = 2mA, ISD < 1µA,
MS8E Package
LT3461/LT3461A
0.3A (ISW), 1.3MHz/3MHz, High Efficiency Step-Up
DC/DC Converters
VIN: 2.5V to 16V, VOUT(MAX) = 38V, IQ = 2.8mA, ISD < 1µA,
SC70, ThinSOT Packages
LT3464
0.08A (ISW), High Efficiency Step-Up DC/DC Converter
with Integrated Schottky, Output Disconnect
VIN: 2.3V to 10V, VOUT(MAX) = 34V, IQ = 25µA, ISD < 1µA,
ThinSOT Package
3470f
16
Linear Technology Corporation
LT/TP 1104 1K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
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© LINEAR TECHNOLOGY CORPORATION 2004