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LT3470 Micropower Buck Regulator with Integrated Boost and Catch Diodes
FEATURES

DESCRIPTIO

Low Quiescent Current: 26A at 12VIN to 3.3VOUT Integrated Boost and Catch Diodes Input Range: 4V to 40V Low Output Ripple: <10mV < 1A in Shutdown Mode Output Voltage: 1.25V to 16V 200mA Output Current Hysteretic Mode Control - Low Ripple Burst Mode(R) Operation at Light Loads - Continuous Operation at Higher Loads Solution Size as Small as 50mm2 Low Profile (1mm) ThinSOT Package
The LT(R)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 26A 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.
APPLICATIO S

Automotive Battery Regulation Power for Portable Products Distributed Supply Regulation Industrial Supplies Wall Transformer Regulation
TYPICAL APPLICATIO
VIN 7V TO 40V VIN OFF ON SHDN BOOST
Efficiency and Power Loss vs Load Current
90 VIN = 12V 1000
0.22F LT3470 SW BIAS 22pF 2.2F FB GND 200k 1% 604k 1% 22F 33H VOUT 5V 200mA
80 70 100
EFFICIENCY (%)
60 50 40 30 20 10 0.1 0.1 1 10 LOAD CURRENT (mA) 100
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POWER LOSS (mW)
10 1
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ABSOLUTE
(Note 1)
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RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW SHDN 1 NC 2 VIN 3 GND 4 8 FB 7 BIAS 6 BOOST 5 SW
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 ......................... 125C Operating Temperature Range (Note 2) .. - 40C to 85C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
ORDER PART NUMBER LT3470ETS8 LT3470ITS8 TS8 PART MARKING LTBDM LTBPW
TS8 PACKAGE 8-LEAD PLASTIC TSOT-23 TJMAX = 125C, JA = 140C/ W
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 = 25C. VIN = 10V, VSHDN = 10V, VBOOST = 15V, VBIAS = 3V unless otherwise specified.
PARAMETER Minimum Input Voltage Quiescent Current from VIN VSHDN = 0.2V VBIAS = 3V, Not Switching VBIAS = 0V, Not Switching VSHDN = 0.2V VBIAS = 3V, Not Switching VBIAS = 0V, Not Switching VFB Falling VFB = 1V
CONDITIONS

MIN
TYP 0.1 10 35 0.1 25 0.1
MAX 4 0.5 18 50 0.5 60 1.5 1.265 80 150 0.01 1.5 300 435
UNITS V A A A A A A V nA nA %/V ns A mV mA mA
Quiescent Current from Bias

FB Comparator Trip Voltage FB Pin Bias Current (Note 3) FB Voltage Line Regulation Minimum Switch Off-Time (Note 5) Switch Leakage Current Switch VCESAT Switch Top Current Limit Switch Bottom Current Limit
1.228
1.250 35 35 0.0006 500 0.7
4V < VIN < 40V
ISW = 100mA VFB = 0V VFB = 0V 250
215 325 225
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LT3470
ELECTRICAL CHARACTERISTICS
The denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VIN = 10V, VSHDN = 10V, VBOOST = 15V, VBIAS = 3V unless otherwise specified.
PARAMETER Catch Schottky Drop Catch Schottky Reverse Leakage Boost Schottky Drop Boost Schottky Reverse Leakage Minimum Boost Voltage (Note 4) BOOST Pin Current SHDN Pin Current SHDN Input Voltage High SHDN Input Voltage Low 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 0C to 70C. Specifications over the -40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LT3470I specifications are guaranteed over the -40C to 125C temperature range. ISW = 100mA VSHDN = 2.5V 2.5 0.2 CONDITIONS ISH = 100mA VSW = 10V ISH = 30mA VSW = 10V, VBIAS = 0V
MIN
TYP 630 0.2 650 0.2 1.7 7 1
MAX 775 2 775 2 2.2 12 5
UNITS mV A mV A V mA A V V
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.
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency, VOUT = 3.3V
90 80 L = TOKO D52LC 47H TA = 25C VIN = 12V VIN = 7V 90 80 EFFICIENCY (%) 70 VIN = 24V 60 50 L = TOKO D52LC 47H TA = 25C VIN = 12V
EFFICIENCY (%)
70 VIN = 24V 60 50 40 30 0.1 VIN = 36V
VFB (V)
10 1 LOAD CURRENT (mA)
UW
100
3470 G01
Efficiency, VOUT = 5V
1.260
VFB vs Temperature
1.255
VIN = 36V
1.250
1.245
40 30 0.1
10 1 LOAD CURRENT (mA)
100
3470 G02
1.240 -50
-25
75 0 25 50 TEMPERATURE (C)
100
125
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LT3470 TYPICAL PERFOR A CE CHARACTERISTICS
Top and Bottom Switch Current Limits (VFB = 0V) vs Temperature
400 350
40 50
CURRENT LIMIT (mA)
300
VIN CURRENT (A)
BIAS CURRENT (A)
250 200 150 100 50 0 - 50 - 25 0 50 75 25 TEMPERATURE (C) 100 125
SHDN Bias Current vs Temperature
9 8 VSHDN = 36V 7 40 50
SHDN CURRENT (A)
FB CURRENT (nA)
6 5 4 3 2 1 0 -50 -25 VSHDN = 2.5V
FB CURRENT (A)
0 25 50 75 TEMPERATURE (C)
Switch VCESAT (ISW = 100mA) vs Temperature
300 250
SWITCH VCESAT (mV) SCHOTTKY VF (V)
200 150 100 50 0 -50 -25
0.5 0.4 0.3 0.2 0.1
SCHOTTKY VF (V)
50 25 75 0 TEMPERATURE (C)
4
UW
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VIN Quiescent Current vs Temperature
30 25
BIAS < 3V 30
BIAS Quiescent Current (Bias > 3V) vs Temperature
20 15 10 5 0 -50 -25
20 BIAS > 3V
10
0 -50 -25
50 25 0 75 TEMPERATURE (C)
100
125
50 25 75 0 TEMPERATURE (C)
100
125
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FB Bias Current (VFB = 1V) vs Temperature
120 100 80 60 40 20
FB Bias Current (VFB = 0V) vs Temperature
30
20
10
100
125
0 -50 -25
50 25 0 75 TEMPERATURE (C)
100
125
0 -50 -25
50 25 75 0 TEMPERATURE (C)
100
125
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Boost Diode VF (IF = 50mA) vs Temperature
0.8 0.7 0.6 0.7 0.6 0.5 0.4 0.3 0.2 0.1
Catch Diode VF (IF = 100mA) vs Temperature
100
125
0 - 50 - 25
0
50 75 25 TEMPERATURE (C)
100
125
0 -50 -25
50 25 75 0 TEMPERATURE (C)
100
125
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LT3470 TYPICAL PERFOR A CE CHARACTERISTICS
Diode Leakage (VR = 36V) vs Temperature
25 BOOST CATCH
SCHOTTKY DIODE LEAKAGE (A)
BOOST PIN CURRENT (mA)
20
15
SWITCH VCESAT (mV)
10
5
0 -50
-25
50 25 0 75 TEMPERATURE (C)
Catch Diode Forward Voltage
1.0 900 800 0.8 700
SCHOTTKY VF (V)
SCHOTTKY VF (V)
0.6
0.4
0.2
0
0
200 100 300 CATCH DIODE CURRENT (mA)
Minimum Input Voltage, VOUT = 3.3V
6.0 5.5
7
INPUT VOLTAGE (V)
TA = 25C
5.0 4.5 4.0 3.5 3.0 0 50 100 150 LOAD CURRENT (mA) 200
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INPUT VOLTAGE (V)
UW
100
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Switch VCESAT
700 600 500 400 300 200 100 0
125
BOOST Pin Current
14 12 10 8 6 4 2 0
0
100
300 SWITCH CURRENT (mA)
200
400
3470 G14
0
100
200 300 SWITCH CURRENT (mA)
400
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Boost Diode Forward Voltage
600 500 400 300 200 100 0
400
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0
100 50 150 BOOST DIODE CURRENT (mA)
200
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Minimum Input Voltage, VOUT = 5V
8 TA = 25C VIN TO START
VIN TO START
6 VIN TO RUN 5
VIN TO RUN
4
0
100 150 50 LOAD CURRENT (mA)
200
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LT3470
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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. 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. 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).
BLOCK DIAGRA
VIN C1 3 VIN
2 NC
BURST MODE DETECT
1
SHDN
VREF 1.25V gm
FB R2
8 R1
6
+
ENABLE
-
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BIAS
7
+ -
500ns ONE SHOT R S Q Q C3
BOOST
6
SW
L1 5 C2 VOUT
GND
4
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LT3470
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
NO LOAD VOUT 20mV/DIV VOUT 20mV/DIV IL 100mA/DIV IL 100mA/DIV 1ms/DIV 10mA LOAD VOUT 20mV/DIV VOUT 20mV/DIV
IL 100mA/DIV 5s/DIV
3470 F01a
(1a) Burst Mode Operation
Figure 1. Operating Waveforms of the LT3470 Converting 12V to 5V Using a 33H Inductor and 10F Output Capacitor
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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.
200mA LOAD 1s/DIV 150mA LOAD IL 100mA/DIV 1s/DIV
3470 F1b
(1b) Continuous Operation
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LT3470
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
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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:
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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 2.5V 3.3V 5V 12V VIN Up to 16V 10H 10H 15H 33H VIN Up to 40V 33H 33H 33H 47H
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 33H 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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APPLICATIO S I FOR ATIO
Table 2. Inductor Vendors
VENDOR Coilcraft URL www.coilcraft.com
Sumida
www.sumida.com
Toko Wurth Elektronik Coiltronics Murata
www.tokoam.com www.we-online.com www.cooperet.com www.murata.com
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 1F to 2.2F 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 10F or more in parallel with a 1F 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
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PART SERIES DO1605 ME3220 DO3314 CR32 CDRH3D16/HP CDRH3D28 CDRH2D18/HP DB320C D52LC WE-PD2 Typ S WE-TPC Typ S SD10 LQH43C LQH32C INDUCTANCE RANGE (H) 10 to 47 10 to 47 10 to 47 10 to 47 10 to 33 10 to 47 10 to 15 10 to 27 10 to 47 10 to 47 10 to 22 10 to 47 10 to 47 10 to 15 SIZE (mm) 1.8 x 5.4 x 4.2 2.0 x 3.2 x 2.5 1.4 x 3.3 x 3.3 3.0 x 3.8 x 4.1 1.8 x 4.0 x 4.0 3.0 x 4.0 x 4.0 2.0 x 3.2 x 3.2 2.0 x 3.8 x 3.8 2.0 x 5.0 x 5.0 3.2 x 4.0 x 4.5 1.6 x 3.8 x 3.8 1.0 x 5.0 x 5.0 2.6 x 3.2 x 4.5 1.6 x 2.5 x 3.2
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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 ~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:
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 = 33H requires 22F. 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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APPLICATIO S I FOR ATIO
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 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 2.2F 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.
Table 3. Capacitor Vendors
Vendor Panasonic Phone (714) 373-7366 URL
Kemet Sanyo
(864) 963-6300 (408) 749-9714
Murata AVX Taiyo Yuden
(404) 436-1300
(864) 963-6300
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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 0.22F 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 0.47F. For lower output voltages the boost diode can be tied to the input
VIN C3 0.22F 5 7 3 VIN LT3470 SW BIAS GND 4 VBOOST - VSW VOUT MAX VBOOST VIN + VOUT 6 BOOST VOUT
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(2a)
VIN 3 VIN 7 BIAS GND 4 VBOOST - VSW VIN MAX VBOOST 2VIN
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6 BOOST SW 5
C3 0.22F
LT3470
VOUT
(2b)
Figure 2. Two Circuits for Generating the Boost Voltage
Part Series Ceramic, Polymer, Tantalum Ceramic, Tantalum Ceramic, Polymer, Tantalum Ceramic Ceramic, Tantalum Ceramic
Comments EEF Series
www.panasonic.com
www.kemet.com www.sanyovideo.com
T494, T495 POSCAP
www.murata.com www.avxcorp.com www.taiyo-yuden.com
TPS Series
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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 5.5 TA = 25C VIN TO START
INPUT VOLTAGE (V)
5.0 4.5 4.0 3.5 3.0 0 50 100 150 LOAD CURRENT (mA) 200
3470 G18
VIN TO RUN
Minimum Input Voltage, VOUT = 5V
8 TA = 25C VIN TO START 7
INPUT VOLTAGE (V)
6 VIN TO RUN 5
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
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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 450A, 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.
D1 VIN 3 VIN 100k 1 SHDN 6 BOOST SW BIAS 1M GND 4
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LT3470 SOT-23
5 7 8
VOUT
FB
BACKUP
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
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LT3470
APPLICATIO S I FOR ATIO
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. 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.
SHDN VIN C1 GND
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
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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 2.2F 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
C2 VOUT
3470 F05
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LT3470
APPLICATIO S I FOR ATIO
CLOSING SWITCH SIMULATES HOT PLUG IIN VIN LT3470
+
2.2F
LOW IMPEDANCE ENERGIZED 24V SUPPLY
STRAY INDUCTANCE DUE TO 6 FEET (2 METERS) OF TWISTED PAIR
10F 35V AI.EI.
+
2.2F
1 LT3470 0.1F 2.2F
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 1 resistor is added in series with the input to eliminate the voltage overshoot (it also reduces the peak input current). A 0.1F 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 125C. This is generally not a concern unless the ambient temperature is above 85C. 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
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VIN 10V/DIV IIN 10A/DIV 10s/DIV
W
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(6a)
LT3470
(6b)
(6c)
3470 F06
temperature approaches 125C. 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 150C/W is typical. The temperature rise for an LT3470 producing 5V at 200mA is approximately 30C, allowing it to deliver full load to 100C ambient. Above this temperature the load current should be reduced. For 3.3V at 200mA the temperature rise is 20C. 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.
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LT3470
TYPICAL APPLICATIO S
3.3V Step-Down Converter
VIN 5.5V TO 40V 3 VIN OFF ON 1 SHDN LT3470 SW BIAS C1 1F FB GND 4 6 BOOST 5 7 8 22pF R1 324k R2 200k
3470 TA03
C1: TDK C3216JB1H105M C2: CE JMK316 BJ226ML-T L1: TOKO A993AS-270M=P3
VIN 4.7V TO 40V
1.8V Step-Down Converter
VIN 4V TO 25V 3 VIN OFF ON 1 7 C1 1F SHDN BIAS FB GND 4 8 LT3470 SW 6 BOOST 5 C3 0.22F L1 22H
C1: TDK C3216JB1H105M C2: TDK C2012JB0J226M L1: MURATA LQH32CN150K53
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5V Step-Down Converter
VIN 7V TO 40V 3 VIN LT3470 SHDN SW BIAS C1 1F FB GND 4 6 BOOST 5 7 8 22pF R1 604k R2 200k
3470 TA04
C3 0.22F L1 33H
VOUT 3.3V 200mA C2 22F
OFF ON
1
C3 0.22F L1 33H
VOUT 5V 200mA C2 22F
C1: TDK C3216JB1H105M C2: CE JMK316 BJ226ML-T L1: TOKO A914BYW-330M=P3
2.5V Step-Down Converter
3 VIN OFF ON 1 SHDN LT3470 SW BIAS C1 1F FB GND 4 6 BOOST 5 7 8 22pF R1 200k R2 200k
3470 TA07
C3 0.47F L1 33H
VOUT 2.5V 200mA C2 22F
C1: TDK C3216JB1H105M C2: TDK C2012JB0J226M L1: SUMIDA CDRH3D28
12V Step-Down Converter
VIN 15V TO 35V C3 0.22F L1 33H
3 VIN LT3470 SHDN
6 BOOST SW BIAS 5 7 8
VOUT 1.8V 200mA R1 147k R2 332k
3470 TA04
OFF ON
1
VOUT 12V 200mA R1 866k R2 100k
3470 TA06
22pF
22pF
C2 22F
C1 1F
FB GND 4
C2 10F
C1: TDK C3216JB1H105M C2: TDK C3216JB1C106M L1: MURATA LQH32CN150K53
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LT3470
PACKAGE DESCRIPTIO
0.52 MAX
3.85 MAX 2.62 REF
RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR
0.20 BSC 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
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.
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TS8 Package 8-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1637)
0.65 REF 2.90 BSC (NOTE 4) 1.22 REF 1.4 MIN 2.80 BSC 1.50 - 1.75 (NOTE 4) PIN ONE ID 0.65 BSC 0.22 - 0.36 8 PLCS (NOTE 3) 0.80 - 0.90 0.01 - 0.10 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
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LT3470 RELATED PARTS
PART NUMBER LT1613 LT1615/LT1615-1 DESCRIPTION 550mA (ISW), 1.4MHz, High Efficiency Step-Up DC/DC Converter 300mA/80mA (ISW), Constant Off-Time, High Efficiency Step-Up DC/DC Converters COMMENTS VIN: 0.9V to 10V, VOUT(MAX) = 34V, IQ = 3mA, ISD < 1A, ThinSOT Package VIN: 1.2V to 15V, VOUT(MAX) = 34V, IQ = 20A, ISD < 1A, ThinSOT Package VIN: 1.2V to 15V, VOUT(MAX) = 34V, IQ = 20A, ISD < 1A, MS Package
LT1944/LT1944-1 (Dual) Dual Output 350mA/100mA (ISW), Constant Off-Time, High Efficiency Step-Up DC/DC Converters LT1945 (Dual) LT1961 LTC(R)3400/LTC3400B LTC3401 LT3460 LT3461/LT3461A LT3464
Dual Output, Pos/Neg, 350mA (ISW), Constant Off-Time, VIN: 1.2V to 15V, VOUT(MAX) = 34V, IQ = 20A, ISD < 1A, High Efficiency Step-Up DC/DC Converter MS Package 1.5A (ISW), 1.25MHz, High Efficiency Step-Up DC/DC Converter 600mA (ISW), 1.2MHz, Synchronous Step-Up DC/DC Converter 1A (ISW), 3MHz, Synchronous Step-Up DC/DC Converter 0.32A (ISW), 1.3MHz, High Efficiency Step-Up DC/DC Converter 0.3A (ISW), 1.3MHz/3MHz, High Efficiency Step-Up DC/DC Converters 0.08A (ISW), High Efficiency Step-Up DC/DC Converter with Integrated Schottky, Output Disconnect VIN: 3V to 25V, VOUT(MAX) = 35V, IQ = 0.9mA, ISD < 6A, MS8E Package VIN: 0.85V to 5V, VOUT(MAX) = 5V, IQ = 19A/300A, ISD < 1A, ThinSOT Package VIN: 0.5V to 5V, VOUT(MAX) = 6V, IQ = 38A, ISD < 1A, MS Package VIN: 2.5V to 16V, VOUT(MAX) = 36V, IQ = 2mA, ISD < 1A, MS8E Package VIN: 2.5V to 16V, VOUT(MAX) = 38V, IQ = 2.8mA, ISD < 1A, SC70, ThinSOT Packages VIN: 2.3V to 10V, VOUT(MAX) = 34V, IQ = 25A, ISD < 1A, ThinSOT Package
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
LT/TP 1104 1K * PRINTED IN THE USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2004

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