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FEATURES
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LT3470A Micropower Buck Regulator with Integrated Boost and Catch Diodes DESCRIPTION
The LT(R)3470A is a micropower step-down DC/DC converter that integrates a 440mA power switch, catch diode and boost diode into low profile 2mm x 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 35A 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 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.
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners.
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Low Quiescent Current: 35A at 12VIN to 3.3VOUT Integrated Boost and Catch Diodes Input Range: 4V to 40V 3.3V at 250mA from 4V to 40V Input 5V at 250mA from 5.7V to 40V Input Low Output Ripple: <10mV < 1A in Shutdown Mode Output Voltage: 1.25V to 16V 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 (0.75mm) 2mm x 3mm Thermally Enhanced 8-Lead DFN Package
APPLICATIONS
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Automotive Battery Regulation Power for Portable Products Distributed Supply Regulation Industrial Supplies Wall Transformer Regulation
TYPICAL APPLICATION
Efficiency and Power Loss vs Load Current
90 VIN 5.7V TO 40V VIN OFF ON SHDN BOOST 33H SW BIAS 22pF 2.2F FB GND 200k 1%
3470a TA01
VIN = 12V
1000
80 0.22F 70 VOUT 5V 250mA 604k 1% 22F EFFICIENCY (%) 60 50 40 30 20 10 0.1 0.1 10 100 1 LOAD CURRENT (mA) 300 1 10 100 POWER LOSS (mW) LT3470A
3470a TA02
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LT3470A ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW FB 1 BIAS 2 BOOST 3 SW 4 9 8 7 6 5 SHDN NC VIN GND
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 ......................................... 125C Operating Temperature Range (Note 2) LT3470AE.............................................- 40C to 85C LT3470AI............................................ -40C to 125C Storage Temperature Range................... -65C to 150C Lead Temperature (Soldering, 10 sec) ................. 300C
DDB8 PACKAGE 8-LEAD (3mm x 2mm) PLASTIC DFN JA = 80C/W EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH LT3470AEDDB#PBF LT3470AIDDB#PBF TAPE AND REEL LT3470AEDDB#TRPBF LT3470AIDDB#TRPBF PART MARKING* LDPR LDPR PACKAGE DESCRIPTION 8-Lead (3mm x 2mm) Plastic DFN 8-Lead (3mm x 2mm) Plastic DFN TEMPERATURE RANGE -40C to 85C -40C to 125C
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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LT3470A ELECTRICAL CHARACTERISTICS
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 4V < VIN < 40V
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 10V, VSHDN = 10V, VBOOST = 15V, VBIAS = 3V unless otherwise specified.
CONDITIONS

MIN
TYP 0.1 10 40 0.1 30 0.1
MAX 4 0.5 18 55 0.5 60 1.5 1.265 80 150 0.02
UNITS V A A A A A A V nA nA %/V ns %
Quiescent Current from Bias

FB Comparator Trip Voltage FB Pin Bias Current (Note 3) FB Voltage Line Regulation Minimum Switch Off-Time (Note 5) Maximum Duty Cycle Switch Leakage Current Switch VCESAT Switch VCESAT Without Boost Switch Top Current Limit Switch Bottom Current Limit Catch Schottky Drop Catch Schottky Reverse Leakage Boost Schottky Drop Boost Schottky Reverse Leakage Minimum Boost Voltage (Note 4) BOOST Pin Current Bias Pin Preload SHDN Pin Current SHDN Input Voltage High SHDN Input Voltage Low
1.228
1.250 35 35 0.0006 500
90
95 0.7 1.5 1.2 560 150 0.9
A mV V mA mA mV
ISW = 100mA VBOOST = VSW VFB = 0V VFB = 0V ISW = 100mA VSW = 10V IBIAS = 50mA VSW = 10V, VBIAS = 0V
320
440 280 600 0.2 690 0.2 1.7 2.3 50 1
2 775 2 2.2 5 5 0.2
A mV A V mA mA A V V
ISW = 100mA VBOOST = 10V VSHDN = 2.5V 2
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LT3470AE is guaranteed to meet performance specifications from 0C to 85C. Specifications over the -40C to 85C operating temperature range are assured by design, characterization and correlation
with statistical process controls. The LT3470AI specifications are guaranteed over the -40C to 125C temperature range. Note 3: Bias current flows out of the FB pin. Note 4: This is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch. Note 5: This parameter is assured by design and correlation with statistical process controls.
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LT3470A TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency, VOUT = 3.3V
90 80 EFFICIENCY (%) 70 60 50 40 30 0.1 VIN = 24V VIN = 36V L = TOKO D52LC 47H TA = 25C VIN = 12V VIN = 7V 80 1.255 EFFICIENCY (%) 70 VIN = 24V 60 50 1.245 40 30 0.1 1.240 -50 VIN = 36V VFB (V) 1.250 90 L = TOKO D52LC 47H TA = 25C VIN = 12V
TA = 25C unless otherwise noted.
Efficiency, VOUT = 5V
1.260
VFB vs Temperature
1 10 LOAD CURRENT (mA)
100
3470a G01
1 10 LOAD CURRENT (mA)
100
3470a G02
-25
75 0 25 50 TEMPERATURE (C)
100
125
3470a G03
Top and Bottom Switch Current Limits (VFB = 0V) vs Temperature
600 550 CURRENT LIMIT (mA) 500 450 400 350 300 10 250 200 -50 -25 0 75 50 25 TEMPERATURE (C) 100 125 VIN CURRENT (A) 40 50
VIN Quiescent Current vs Temperature
BIAS < 3V 30
20 BIAS > 3V
0 -50
-25
50 25 0 75 TEMPERATURE (C)
100
125
3470a G04
3470a G05
BIAS Quiescent Current (Bias > 3V) vs Temperature
30 25 7 SHDN CURRENT (A) BIAS CURRENT (A) 20 15 10 5 1 0 -50 -25 50 25 75 0 TEMPERATURE (C) 100 125 6 5 4 3 2 9 8
SHDN Bias Current vs Temperature
VSHDN = 36V
VSHDN = 2.5V
0 -50
-25
0 25 50 75 TEMPERATURE (C)
100
125
3470a G06
3470a G07
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LT3470A TYPICAL PERFORMANCE CHARACTERISTICS
FB Bias Current (VFB = 1V) vs Temperature
60 50 FB CURRENT (nA) 40 30 20 10 0 -50 FB CURRENT (A) 120 100 80 60 40 20 0 -50
TA = 25C unless otherwise noted.
FB Bias Current (VFB = 0V) vs Temperature
-25
50 25 0 75 TEMPERATURE (C)
100
125
-25
50 25 75 0 TEMPERATURE (C)
100
125
3470a G08
3470a G09
Switch VCESAT (ISW = 100mA) vs Temperature
300 250 SWITCH VCESAT (mV) 200 150 100 50 0 -50 SCHOTTKY VF (V) 50 25 75 0 TEMPERATURE (C) 100 125 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 -25
Boost Diode VF (IF = 50mA) vs Temperature
0 -50
-25
0
75 50 25 TEMPERATURE (C)
100
125
3470a G10
3470a G11
Catch Diode VF (IF = 100mA) vs Temperature
0.7 SCHOTTKY DIODE LEAKAGE (mA) 0.6 0.5 0.4 0.3 0.2 0.1 0 -50 60 55 50 45 40 35 30 25 20 15 10 5 -25 50 25 75 0 TEMPERATURE (C) 100 125
Diode Leakage (VR = 36V) vs Temperature
CATCH BOOST
SCHOTTKY VF (V)
0 -50
-25
50 25 0 75 TEMPERATURE (C)
100
125
3470a G12
3470a G13
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LT3470A TYPICAL PERFORMANCE CHARACTERISTICS
Switch VCESAT
700 600 500 400 300 200 100 0 0 100 300 400 SWITCH CURRENT (mA) 200 500
3470a G14
TA = 25C unless otherwise noted.
BOOST Pin Current
14 12 BOOST PIN CURRENT (mA) 10 8 6 4 2 0 0 100 200 300 400 SWITCH CURRENT (mA) 500
3470a G15
SWITCH VCESAT (mV)
Catch Diode Forward Voltage
1.0 900 800 0.8 SCHOTTKY VF (V) SCHOTTKY VF (V) 200 100 300 CATCH DIODE CURRENT (mA) 700 600 500 400 300 200 100 0 0 400
3470a G16
Boost Diode Forward Voltage
0.6
0.4
0.2
0
0
100 50 150 BOOST DIODE CURRENT (mA)
200
3470a G17
Minimum Input Voltage, VOUT = 3.3V
6.0 5.5 INPUT VOLTAGE (V) 7 5.0 4.5 4.0 VIN TO RUN/START 3.5 3.0 0 50 100 150 200 LOAD CURRENT (mA) 250
3470a G18
Minimum Input Voltage, VOUT = 5V
8 TA = 25C
TA = 25C
INPUT VOLTAGE (V)
6 VIN TO RUN/START 5
4
0
50
100 150 200 LOAD CURRENT (mA)
250
3470a G19
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LT3470A PIN FUNCTIONS
SHDN (Pin 8): The SHDN pin is used to put the LT3470A in shutdown mode. Tie to ground to shut down the LT3470A. Apply 2V or more for normal operation. If the shutdown 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 LT3470A's 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.
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LT3470A BLOCK DIAGRAM
VIN C1 VIN BIAS
+ -
500ns ONE SHOT R S Q Q C3
BOOST
SW
L1 VOUT C2
NC
BURST MODE DETECT
SHDN
VREF 1.25V gm
FB R2 R1
8
+
GND
3470a BD
ENABLE
-
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LT3470A OPERATION
The LT3470A 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 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
NO LOAD VOUT 20mV/DIV VOUT 20mV/DIV IL 100mA/DIV IL 100mA/DIV 1ms/DIV 10mA LOAD VOUT 20mV/DIV VOUT 20mV/DIV 150mA LOAD 1s/DIV
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 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.
200mA LOAD
IL 100mA/DIV 5s/DIV
3470a F01a
IL 100mA/DIV 1s/DIV
3470a F1b
(1a) Burst Mode Operation
(1b) Continuous Operation
Figure 1. Operating Waveforms of the LT3470A Converting 12V to 5V Using a 33H Inductor and 10F Output Capacitor
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LT3470A APPLICATIONS INFORMATION
Input Voltage Range The minimum input voltage required to generate a particular 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: VOUT + VD DC = VIN - VSW + VD where VD is the forward voltage drop of the catch diode (~0.6V) and VSW is the voltage drop of the internal switch at maximum load (~0.4V). Given DCMAX = 0.90, this leads to a minimum input voltage of: VIN(MIN) = VOUT + VD + VSW - VD DCMAX 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 (~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: 1- DC tOFF-TIME = f The calculated tOFF-TIME should be more than LT3470A'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 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 2.5V 3.3V 5V 12V VIN Up to 16V 10H 10H 15H 33H VIN Up to 40V 33H 33H 33H 47H
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 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 inductor 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 = VIN(MAX) * tON-TIME(MIN) IMAX
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
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LT3470A APPLICATIONS INFORMATION
Table 2. Inductor Vendors
VENDOR Coilcraft URL www.coilcraft.com 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
Sumida
www.sumida.com
Toko Wurth Elektronik Coiltronics Murata
www.tokoam.com www.we-online.com www.cooperet.com www.murata.com
be used. It is important to note that inductor saturation current is reduced at high temperatures--see inductor vendors for more information. Input Capacitor Step-down regulators draw current from the input 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 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 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 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 inductor's ripple current and stores energy to satisfy the load current when the LT3470A is quiescent. In order to keep output voltage ripple low, the impedance of the capacitor must be low
at the LT3470A'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 LT3470A 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 LT3470A. Similar performance in traditional tantalum capacitors requires a larger package (C-case). With a high quality capacitor filtering the ripple current from the inductor, the output voltage ripple is determined by the delay in the LT3470A's feedback comparator. This ripple can be reduced further by adding a small (typically 22pF) phase lead capacitor between the output and the feedback pin.
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LT3470A 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 LT3470A's switching frequency depends on the load current, and at light loads the LT3470A can excite the ceramic capacitor at audio frequencies, generating audible noise. Since the LT3470A operates at a lower current limit during Burst Mode operation, the noise is typically very quiet to a 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 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 voltage can ring to twice its nominal value, possibly exceeding the LT3470A's rating. This situation is easily avoided; see the Hot-Plugging Safely section.
Table 2. Capacitor Vendors
Vendor Panasonic Phone (714) 373-7366 URL www.panasonic.com Part Series Ceramic, Polymer, Tantalum Ceramic, Tantalum Ceramic, Polymer, Tantalum Ceramic Ceramic, Tantalum Ceramic
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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 VIN BOOST SW BIAS GND C3 0.22F
LT3470A VOUT
VBOOST - VSW VOUT MAX VBOOST VIN + VOUT
(2a)
VIN VIN BIAS GND
3470a F02
BOOST SW
C3 0.22F
LT3470A VOUT
VBOOST - VSW VIN MAX VBOOST 2*VIN
(2b)
Figure 2. Two Circuits for Generating the Boost Voltage
Comments EEF Series
Kemet Sanyo
(864) 963-6300 (408) 749-9714
www.kemet.com www.sanyovideo.com
T494, T495 POSCAP
Murata AVX Taiyo Yuden
(404) 436-1300
www.murata.com www.avxcorp.com
TPS Series
(864) 963-6300
www.taiyo-yuden.com
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LT3470A 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 (typical) 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
Minimum Input Voltage, VOUT = 3.3V
6.0 5.5 INPUT VOLTAGE (V) 5.0 4.5 4.0 3.5 3.0 0 50 100 150 200 LOAD CURRENT (mA) 250
3470a F03a
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 capacitor. 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 excessively 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 LT3470A'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 LT3470A'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 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.
D1 VIN VIN BOOST SW BIAS VOUT
TA = 25C
VIN TO RUN/START
Minimum Input Voltage, VOUT = 5V
8 TA = 25C
7 INPUT VOLTAGE (V)
100k
LT3470A SHDN
6 VIN TO RUN/START 5
1M GND
FB BACKUP
3470a F04
4
0
50
100 150 200 LOAD CURRENT (mA)
250
3470a F03b
Figure 3. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
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
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LT3470A 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. 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, or pad, of the LT3470A to help remove heat from the LT3470A to the ground plane.
SHDN VIN
GND
VOUT
3470a F05
Figure 5. A Good PCB Layout Ensures Proper, Low EMI Operation
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LT3470A 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 voltage at the VIN pin of the LT3470A can ring to twice the nominal input voltage, possibly exceeding the LT3470A's rating and damaging the part. If the input supply is poorly controlled or the user will be plugging the LT3470A into an energized supply, the input network should be designed to prevent this overshoot. Figure 6 shows the waveforms that result when an LT3470A circuit is connected to a 24V supply through six feet of 24-gauge twisted pair. The first plot is the response with a 2.2F 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 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 junction temperature of the LT3470A 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 LT3470A. The maximum load current should be derated as the ambient temperature approaches the maximum junction rating. The die temperature 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 80C/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.
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LT3470A APPLICATIONS INFORMATION
CLOSING SWITCH SIMULATES HOT PLUG IIN VIN LT3470A VIN 10V/DIV
+
2.2F
LOW IMPEDANCE ENERGIZED 24V SUPPLY
STRAY INDUCTANCE DUE TO 6 FEET (2 METERS) OF TWISTED PAIR
IIN 10A/DIV 10s/DIV
(6a)
LT3470A 10F 35V AI.EI.
+
2.2F
VIN 10V/DIV
IIN 10A/DIV 10s/DIV
(6b)
1 LT3470A 0.1F 2.2F IIN 10A/DIV 10s/DIV
3470a F06
VIN 10V/DIV
(6c) Figure 6: A Well Chosen Input Network Prevents Input Voltage Overshoot and Ensures Reliable Operation When the LT3470A is Connected to a Live Supply
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16
LT3470A APPLICATIONS INFORMATION
3.3V Step-Down Converter
VIN 4V TO 40V VIN OFF ON SHDN BOOST SW BIAS 22pF C1 1F FB GND R2 200k
3470a TA03
LT3470A
C3 0.22F 6.3V , L1 33H
VOUT 3.3V 250mA R1 324k C2 22F
C1: TDK C3216JB1H105M C2: CE JMK316 BJ226ML-T L1: TOKO A993AS-270M=P3
5V Step-Down Converter
VIN 5.7V TO 40V VIN OFF ON SHDN BOOST SW BIAS 22pF C1 1F FB GND R2 200k
3470a TA04
LT3470A
C3 0.22F 6.3V , L1 33H
VOUT 5V 250mA R1 604k C2 22F
C1: TDK C3216JB1H105M C2: CE JMK316 BJ226ML-T L1: TOKO A914BYW-330M=P3
2.5V Step-Down Converter
VIN 4V TO 40V VIN OFF ON SHDN BOOST SW BIAS 22pF C1 1F FB GND R2 200k
3470a TA07
LT3470A
C3 0.47F 6.3V , L1 33H
VOUT 2.5V 250mA R1 200k C2 22F
C1: TDK C3216JB1H105M C2: TDK C2012JB0J226M L1: SUMIDA CDRH3D28
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LT3470A TYPICAL APPLICATIONS
1.8V Step-Down Converter
VIN 4V TO 23V VIN OFF ON SHDN BIAS C1 1F FB GND R2 332k
3470a TA05
BOOST SW
LT3470A
C3 0.22F 25V , L1 22H
VOUT 1.8V 250mA R1 147k C2 22F
22pF
C1: TDK C3216JB1H105M C2: TDK C2012JB0J226M L1: MURATA LQH32CN150K53
12V Step-Down Converter
VIN 15V TO 34V VIN OFF ON SHDN BOOST SW BIAS 22pF C1 1F FB GND R2 100k
3470a TA06
LT3470A
C3 0.22F 16V , L1 33H
VOUT 12V 250mA R1 866k C2 10F
C1: TDK C3216JB1H105M C2: TDK C3216JB1C106M L1: MURATA LQH32CN150K53
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LT3470A PACKAGE DESCRIPTION
DDB Package 8-Lead Plastic DFN (3mm x 2mm)
(Reference LTC DWG # 05-08-1702 Rev B)
0.61 0.05 (2 SIDES) 0.70 0.05 2.55 0.05 1.15 0.05 PACKAGE OUTLINE 0.25 0.05 0.50 BSC 2.20 0.05 (2 SIDES) PIN 1 BAR TOP MARK (SEE NOTE 6) 0.200 REF
3.00 0.10 (2 SIDES)
R = 0.05 TYP 2.00 0.10 (2 SIDES) 0.56 0.05 (2 SIDES) 0.75 0.05
R = 0.115 TYP 5
0.40 0.10 8
4 0.25 0.05 2.15 0.05 (2 SIDES)
1 0.50 BSC
PIN 1 R = 0.20 OR 0.25 x 45 CHAMFER
(DDB8) DFN 0905 REV B
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 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 - 0.05
BOTTOM VIEW--EXPOSED PAD
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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.
19
LT3470A RELATED PARTS
PART NUMBER LT1616 LT1676 LT1765 LT1766 LT1767 LT1776 LTC(R)1877 LTC1879 LT1933 LT1934 LT1956 LTC3405/LTC3405A LTC3406/LTC3406B LTC3411 LTC3412 LT3430 LT3470 DESCRIPTION 25V, 500mA (IOUT), 1.4MHz, High Efficiency Step-Down DC/DC Converter 60V, 440mA (IOUT), 100kHz, High Efficiency Step-Down DC/DC Converter 25V, 2.75A (IOUT), 1.25MHz, High Efficiency Step-Down DC/DC Converter 60V, 1.2A (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter 25V, 1.2A (IOUT), 1.25MHz, High Efficiency Step-Down DC/DC Converter 40V, 550mA (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter 600mA (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter 1.2A (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter 36V, 600mA, 500kHz, High Efficiency Step-Down DC/DC Converter 34V, 250mA (IOUT), Micropower, Step-Down DC/DC Converter 60V, 1.2A (IOUT), 500kHz, High Efficiency Step-Down DC/DC Converter 300mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter 600mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 2.5A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 60V, 2.75A (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter 40V, 200mA, Micropower Step-Down DC/DC Converter COMMENTS VIN = 3.6V to 25V, VOUT = 1.25V, IQ = 1.9mA, ISD = <1A, ThinSOT Package VIN = 7.4V to 60V, VOUT = 1.24V, IQ = 3.2mA, ISD = 2.5A, S8 Package VIN = 3V to 25V, VOUT = 1.2V, IQ = 1mA, ISD = 15A, S8, TSSOP16E Packages VIN = 5.5V to 60V, VOUT = 1.2V, IQ = 2.5mA, ISD = 25A, TSSOP16/E Package VIN = 3V to 25V; VOUT = 1.2V, IQ = 1mA, ISD = 6A, MS8/E Packages VIN = 7.4V to 40V; VOUT = 1.24V, IQ = 3.2mA, ISD = 30A, N8, S8 Packages VIN = 2.7V to 10V; VOUT = 0.8V, IQ = 10A, ISD = <1A, MS8 Package VIN = 2.7V to 10V; VOUT = 0.8V, IQ = 15A, ISD = <1A, TSSOP16 Package VIN = 3.6V to 36V; VOUT = 1.25V, IQ = 2.5A, ISD = <1A, ThinSOT and 2mm x 3mm DFN-6 Package VIN = 3.2V to 34V; VOUT = 1.25V, IQ = 12A, ISD = <1A, ThinSOT and 2mm x 3mm DFN-6 Package VIN = 5.5V to 60V, VOUT = 1.2V, IQ = 2.5mA, ISD = 25A, TSSOP16/E Package VIN = 2.7V to 6V, VOUT = 0.8V, IQ = 20A, ISD = <1A, ThinSOT Package VIN = 2.5V to 5.5V, VOUT = 0.6V, IQ = 20A, ISD = <1A, ThinSOT Package VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 60A, ISD = <1A, MS Package VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 60A, ISD = <1A, TSSOP16E Package VIN = 5.5V to 60V, VOUT = 1.2V, IQ = 2.5mA, ISD = 30A, TSSOP16E Package VIN = 4V to 40V, VOUT = 1.25V, IQ = 35A, ISD = <1A, DFN-8, ThinSOT Packages
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20 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 1108 REV A * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2008

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