LT®3480 adjustable frequency (200kHz 2.4MHz) monolithic buck switching
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LT3480 36V, 2.4MHz Step-Down Switching Regulator with 70µA Quiescent Current DESCRIPTION
LT®3480 adjustable frequency (200kHz 2.4MHz) monolithic buck switching regulator that accepts input voltages (60V maximum). high efficiency 0.25 switch included along with boost Schottky diode necessary oscillator, control, logic circuitry. Current mode topology used fast transient response good loop stability. ripple Burst Mode operation maintains high efficiency output currents while keeping output ripple below 15mV typical application. addition, LT3480 further enhance output current efficiency drawing bias current from output when VOUT above Shutdown reduces input supply current less than while resistor capacitor RUN/SS provide controlled output voltage ramp (soft-start). power good flag signals when VOUT reaches programmed output voltage. LT3480 available 10-Pin MSOP packages with exposed pads thermal resistance.
Lare registered trademarks Linear Technology Corporation. Burst Mode registered trademark Linear Technology Corporation. other trademarks property their respective owners.
Wide Input Range: Operation from 3.6V Over-Voltage Lockout Protects Circuits through Transients Maximum Output Current Ripple Burst Mode® Operation 12VIN 3.3VOUT Output Ripple 15mV Adjustable Switching Frequency: 200kHz 2.4MHz Shutdown Current: Integrated Boost Diode Synchronizable Between 250kHz 2MHz Power Good Flag Saturating Switch Design: 0.25 On-Resistance 0.790V Feedback Reference Voltage Output Voltage: 0.79V Soft-Start Capability Small 10-Pin Thermally Enhanced MSOP (3mm 3mm) Packages
APPLICATIONS
Automotive Battery Regulation Power Portable Products Distributed Supply Regulation Industrial Supplies
TYPICAL APPLICATION
3.3V Step-Down Converter
4.5V TRANSIENT 4.7F 470pF 40.2k SYNC 316k 100k
3480 TA01
Efficiency
VOUT 3.3V VOUT EFFICIENCY VOUT 3.3V
RUN/SS
BOOST 0.47F LT3480 4.7H
5.6H LOAD CURRENT
3480 TA01b
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LT3480 ABSOLUTE MAXIMUM RATINGS
(Note
VIN, RUN/SS Voltage (Note .60V BOOST Voltage .56V BOOST Above Pin.30V Voltage SYNC Voltage .30V Maximum Junction Temperature. 125°C
Operating Temperature Range (Note LT3480E -40°C 85°C LT3480I -40°C 125°C Storage Temperature Range. -65°C 150°C Lead Temperature (Soldering, sec) (MSE Only) 300°C
CONFIGURATION
VIEW VIEW BOOST RUN/SS SYNC BOOST RUN/SS SYNC
PACKAGE 10-LEAD (3mm 3mm) PLASTIC
45°C/W, 10°C/W EXPOSED (PIN GND, MUST SOLDERED
PACKAGE 10-LEAD PLASTIC MSOP
45°C/W, 10°C/W EXPOSED (PIN GND, MUST SOLDERED
ORDER INFORMATION
LEAD FREE FINISH LT3480EDD#PBF LT3480IDD#PBF LT3480EMSE#PBF LT3480IMSE#PBF LEAD BASED FINISH LT3480EDD LT3480IDD LT3480EMSE LT3480IMSE TAPE REEL LT3480EDD#TRPBF LT3480IDD#TRPBF LT3480EMSE#TRPBF LT3480IMSE#TRPBF TAPE REEL LT3480EDD#TR LT3480IDD#TR LT3480EMSE#TR LT3480IMSE#TR PART MARKING* LCTP LCTP LTCLTCPART MARKING* LCTP LCTP LTCLTCPACKAGE DESCRIPTION 10-Lead (3mm 3mm) Plastic 10-Lead (3mm 3mm) Plastic 10-Lead Plastic MSOP 10-Lead Plastic MSOP PACKAGE DESCRIPTION 10-Lead (3mm 3mm) Plastic 10-Lead (3mm 3mm) Plastic 10-Lead Plastic MSOP 10-Lead Plastic MSOP TEMPERATURE RANGE -40°C 85°C -40°C 125°C -40°C 85°C -40°C 125°C TEMPERATURE RANGE -40°C 85°C -40°C 125°C -40°C 85°C -40°C 125°C
Consult Marketing parts specified with wider operating temperature ranges. *The temperature grade identified label shipping container. more information lead free part marking, http://www.linear.com/leadfree/ more information tape reel specifications,
ELECTRICAL CHARACTERISTICS
PARAMETER Minimum Input Voltage Overvoltage Lockout
denotes specifications which apply over full operating temperature range, otherwise specifications 25°C. VRUN/SS VBOOST 3.3V unless otherwise noted. (Note
CONDITIONS
UNITS
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LT3480 ELECTRICAL CHARACTERISTICS
PARAMETER Quiescent Current from
denotes specifications which apply over full operating temperature range, otherwise specifications 25°C. VRUN/SS VBOOST 3.3V unless otherwise noted. (Note
CONDITIONS VRUN/SS 0.2V Switching Switching VRUN/SS 0.2V Switching Switching
0.01 0.01
0.01
UNITS
Quiescent Current from
Minimum Bias Voltage Pin) Feedback Voltage Bias Current (Note Voltage Line Regulation Error Error Gain Source Current Sink Current Switch Current Gain Clamp Voltage Switching Frequency 8.66k 29.4k 187k
0.002 1000
0.8V, 0.4V
0.02
1.15
Minimum Switch Off-Time Switch Current Limit Switch VCESAT Boost Schottky Reverse Leakage Minimum Boost Voltage (Note BOOST Current RUN/SS Current RUN/SS Input Voltage High RUN/SS Input Voltage Threshold Offset from Feedback Voltage Hysteresis Leakage Sink Current SYNC Threshold SYNC High Threshold SYNC Bias Current VSYNC 0.4V Rising VRUN/SS 2.5V Duty Cycle 10V,
Note Stresses beyond those listed under Absolute Maximum Ratings cause permanent damage device. Exposure Absolute Maximum Rating condition extended periods affect device reliability lifetime. Note LT3480E guaranteed meet performance specifications from 85°C. Specifications over -40°C 85°C operating temperature range assured design, characterization correlation with statistical process controls. LT3480I specifications guaranteed over -40°C 125°C temperature range.
Note Bias current flows pin. Note This minimum voltage across boost capacitor needed guarantee full saturation switch. Note Absolute Maximum Voltage RUN/SS pins nonrepetitive second transients, continious operation.
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LT3480 TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency
EFFICIENCY PLC-0745-5R6 800kHz VOUT 3.3V PLC-0745-5R6 800kHz EFFICIENCY POWER LOSS VOUT 3.3V 5.6H LOAD CURRENT
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Efficiency
Efficiency
EFFICIENCY
VOUT
0.01
LOAD CURRENT
LOAD CURRENT
Load Supply Current
SUPPLY CURRENT SUPPLY CURRENT INPUT VOLTAGE
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Load Supply Current
VOUT 3.3V CATCH DIODE: DIODES, INC. PDS360 TEMPERATURE (°C)
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Maximum Load Current
LOAD CURRENT MINIMUM INPUT VOLTAGE
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VOUT 3.3V INCREASED SUPPLY CURRENT CATCH DIODE LEAKAGE HIGH TEMPERATURE
TYPICAL
VOUT 3.3V 4.7H
Maximum Load Current
TYPICAL SWITCH CURRENT LIMIT(A) LOAD CURRENT INPUT VOLTAGE
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Switch Current Limit
SWITCH CURRENT LIMIT DUTY CYCLE
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Switch Current Limit
DUTY CYCLE
MINIMUM VOUT 4.7H 800kHz
DUTY CYCLE
TEMPERATURE (°C)
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LT3480 TYPICAL PERFORMANCE CHARACTERISTICS
Switch Voltage Drop
BOOST CURRENT (mA) VOLTAGE DROP (mV) 1000 2000 1500 SWITCH CURRENT (mA) 2500
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Boost Current
1000 1500 2000 SWITCH CURRENT (mA) 2500
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Feedback Voltage
FEEDBACK VOLTAGE (mV)
TEMPERATURE (°C)
4380
Switching Frequency
1.20 1.15 1.10 FREQUENCY (MHz) 1.05 1.00 0.95 0.90 0.85 0.80 TEMPERATURE (°C)
4380
Frequency Foldback
1200 1000 VOLTAGE (mV)
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Minimum Switch On-Time
MINIMUM SWITCH TIME (ns)
SWITCHING FREQUENCY (kHz)
TEMPERATURE (°C)
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Soft-Start
SWITCH CURRENT LIMIT RUN/SS VOLTAGE
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RUN/SS Current
RUN/SS VOLTAGE
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Boost Diode
BOOST DIODE
RUN/SS CURRENT
BOOST DIODE CURRENT
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LT3480 TYPICAL PERFORMANCE CHARACTERISTICS
Error Output Current
CURRENT INPUT VOLTAGE -200 -100 ERROR VOLTAGE
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Minimum Input Voltage
VOUT 3.3V 25°C 4.7H 800kHz 1000 LOAD CURRENT 10000
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Minimum Input Voltage
INPUT VOLTAGE
VOUT 4.7H 800kHz 1000 LOAD CURRENT 10000
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Voltages
2.50
Power Good Threshold
Switching Waveforms; Burst Mode
THRESHOLD VOLTAGE
2.00 CURRENT LIMIT CLAMP VOLTAGE 1.50
5V/DIV
0.2A/DIV
1.00
SWITCHING THRESHOLD
0.50
VOUT 10mV/DIV
TEMPERATURE (°C)
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TEMPERATURE (°C)
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5s/DIV 12V; FRONT PAGE APPLICATION ILOAD 10mA
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Switching Waveforms; Transition from Burst Mode Full Frequency
5V/DIV
Switching Waveforms; Full Frequency Continuous Operatio
5V/DIV
0.2A/DIV
0.5A/DIV
VOUT 10mV/DIV
VOUT 10mV/DIV
1s/DIV 12V; FRONT PAGE APPLICATION ILOAD 110mA
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1s/DIV 12V; FRONT PAGE APPLICATION ILOAD
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LT3480 FUNCTIONS
(Pin This connects anode boost Schottky diode. also supplies current internal regulator. BOOST (Pin This used provide drive voltage, higher than input voltage, internal bipolar power switch. (Pin output internal power switch. Connect this inductor, catch diode boost capacitor. (Pin supplies current LT3480's internal regulator internal power switch. This must locally bypassed. RUN/SS (Pin RUN/SS used LT3480 shutdown mode. ground shut down LT3480. 2.5V more normal operation. shutdown feature used, this pin. RUN/SS also provides soft-start function; Applications Information section. SYNC (Pin This external clock synchronization input. Ground this ripple Burst Mode operation output loads. clock source synchronization. Clock edges should have rise fall times faster than synchronizing section Applications Information. (Pin open collector output internal comparator. remains until within final regulation voltage. output valid when above 3.6V RUN/SS high. (Pin LT3480 regulates 0.790V. Connect feedback resistor divider this pin. (Pin output internal error amplifier. voltage this controls peak switch current. network from this ground compensate control loop. (Pin 10): Oscillator Resistor Input. Connecting resistor ground from this sets switching frequency. Exposed (Pin 11): Ground. Exposed must soldered PCB.
BLOCK DIAGRAM
INTERNAL 0.79V
RUN/SS SLOPE COMP SWITCH LATCH OSCILLATOR 200kHz-2.4MHz DISABLE BurstMode DETECT VOUT BOOST
SYNC
SOFT-START ERROR
0.7V
CLAMP
3480
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LT3480 OPERATION
LT3480 constant frequency, current mode stepdown regulator. oscillator, with frequency enables flip-flop, turning internal power switch. amplifier comparator monitor current flowing between pins, turning switch when this current reaches level determined voltage error amplifier measures output voltage through external resistor divider tied servos pin. error amplifier's output increases, more current delivered output; decreases, less current delivered. active clamp provides current limit. also clamped voltage RUN/SS pin; soft-start implemented generating voltage ramp RUN/SS using external resistor capacitor. internal regulator provides power control circuitry. bias regulator normally draws power from pin, connected external voltage higher than bias power will drawn from external source (typically regulated output voltage). This improves efficiency. RUN/SS used place LT3480 shutdown, disconnecting output reducing input current less than switch driver operates from either input from BOOST pin. external capacitor diode used generate voltage BOOST that higher than input supply. This allows driver fully saturate internal bipolar power switch efficient operation. further optimize efficiency, LT3480 automatically switches Burst Mode operation light load situations. Between bursts, circuitry associated with controlling output switch shut down, reducing input supply current typical application. oscillator reduces LT3480's operating frequency when voltage low. This frequency foldback helps control output current during startup overload. LT3480 contains power good comparator which trips when regulated value. output open-collector transistor that when output regulation, allowing external resistor pull high. Power good valid when LT3480 enabled above 3.6V. LT3480 overvoltage protection feature which disables switching action when goes above typical (36V minimum). When switching disabled, LT3480 safely sustain input voltages 60V.
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LT3480 APPLICATIONS INFORMATION
Resistor Network output voltage programmed with resistor divider between output pin. Choose resistors according 0.79 Reference designators refer Block Diagram. Setting Switching Frequency LT3480 uses constant frequency architecture that programmed switch from 200kHz 2.4MHz using resistor tied from ground. table showing necessary value desired switching frequency Figure
SWITCHING FREQUENCY (MHz) VALUE 88.7 68.1 56.2 46.4 40.2 29.4 23.7 19.1 16.2 13.3 11.5 9.76 8.66
where typical input voltage, VOUT output voltage, catch diode drop (~0.5V) internal switch drop (~0.5V load). This equation shows that slower switching frequency necessary safely accommodate high VIN/VOUT ratio. Also, shown next section, lower frequency allows lower dropout voltage. reason input voltage range depends switching frequency because LT3480 switch finite minimum times. switch turn minimum ~150ns turn minimum ~150ns. Typical minimum time 25°C 80ns. This means that minimum maximum duty cycles are: DCMIN tON(MIN) DCMAX tOFF(MIN) where switching frequency, tON(MIN) minimum switch time (~150ns), tOFF(MIN) minimum switch time (~150ns). These equations show that duty cycle range increases when switching frequency decreased. good choice switching frequency should allow adequate input voltage range (see next section) keep inductor capacitor values small. Input Voltage Range maximum input voltage LT3480 applications depends switching frequency, Absolute Maximum Ratings BOOST pins, operating mode. LT3480 operate from input voltages 38V, safely withstand input voltages 60V. Note that while VIN>38V (typical), LT3480 will stop switching, allowing output fall regulation. While output start-up, short-circuit, other overload conditions, switching frequency should chosen according following discussion. safe operation inputs switching frequency must enough satisfy VIN(MAX) according following equation. lower VIN(MAX) desired, this equation used directly.
Figure Switching Frequency Value
Operating Frequency Tradeoffs Selection operating frequency tradeoff between efficiency, component size, minimum dropout voltage, maximum input voltage. advantage high frequency operation that smaller inductor capacitor values used. disadvantages lower efficiency, lower maximum input voltage, higher dropout voltage. highest acceptable switching frequency (fSW(MAX)) given application calculated follows: fSW(MAX VOUT tON(MIN)
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LT3480 APPLICATIONS INFORMATION
VIN(MAX VOUT tON(MIN) frequency. reasonable starting point selecting ripple current 0.4(IOUT(MAX)) where IOUT(MAX) maximum output load current. guarantee sufficient output current, peak inductor current must lower than LT3480's switch current limit (ILIM). peak inductor current IL(PEAK) IOUT(MAX) IL/2 where IL(PEAK) peak inductor current, IOUT(MAX) maximum output load current, inductor ripple current. LT3480's switch current limit (ILIM) least 3.5A duty cycles decreases linearly 2.5A 0.8. maximum output current function inductor ripple current: IOUT(MAX) ILIM IL/2 sure pick inductor ripple current that provides sufficient maximum output current (IOUT(MAX)). largest inductor ripple current occurs highest VIN. guarantee that ripple current stays below specified maximum, inductor value should chosen according following equation: VIN(MAX where voltage drop catch diode (~0.4V), VIN(MAX) maximum input voltage, VOUT output voltage, switching frequency (set RT), inductor value. inductor's current rating must greater than maximum load current saturation current should about higher. robust operation fault conditions (start-up short circuit) high input voltage (>30V), saturation current should above 3.5A. keep efficiency high, series resistance (DCR) should less than core material should intended high frequency applications. Table lists several vendors suitable types.
where VIN(MAX) maximum operating input voltage, VOUT output voltage, catch diode drop (~0.5V), internal switch drop (~0.5V load), switching frequency (set RT), tON(MIN) minimum switch time (~150ns). Note that higher switching frequency will depress maximum operating input voltage. Conversely, lower switching frequency will necessary achieve safe operation high input voltages. output regulation short-circuit, startup, overload events expected, then input voltage transients acceptable regardless switching frequency. this mode, LT3480 enter pulse skipping operation where some switching pulses skipped maintain output regulation. this mode output voltage ripple inductor current ripple will higher than normal operation. Above switching will stop. minimum input voltage determined either LT3480's minimum operating voltage ~3.6V maximum duty cycle (see equation previous section). minimum input voltage duty cycle VIN(MIN) VOUT tOFF(MIN)
where VIN(MIN) minimum input voltage, tOFF(MIN) minimum switch time (150ns). Note that higher switching frequency will increase minimum input voltage. lower dropout voltage desired, lower switching frequency should used. Inductor Selection given input output voltage, inductor value switching frequency will determine ripple current. ripple current increases with higher VOUT decreases with higher inductance faster switching
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LT3480 APPLICATIONS INFORMATION
Table Inductor Vendors
VENDOR Murata Toko www.murata.com www.componenttdk.com www.toko.com PART SERIES LQH55D SLF7045 SLF10145 D62CB D63CB D75C D75F Sumida www.sumida.com CR54 CDRH74 CDRH6D38 CR75 TYPE Open Shielded Shielded Shielded Shielded Shielded Open Open Shielded Shielded
necessary. This provided with lower performance electrolytic capacitor. Step-down regulators draw current from input supply pulses with very fast rise fall times. input capacitor required reduce resulting voltage ripple LT3480 force this very high frequency switching current into tight local loop, minimizing EMI. 4.7F capacitor capable this task, only placed close LT3480 catch diode (see Layout section). second precaution regarding ceramic input capacitor concerns maximum input voltage rating LT3480. ceramic input capacitor combined with trace cable inductance forms high quality (under damped) tank circuit. LT3480 circuit plugged into live supply, input voltage ring twice nominal value, possibly exceeding LT3480's voltage rating. This situation easily avoided (see Plugging Safety section). space sensitive applications, 2.2F ceramic capacitor used local bypassing LT3480 input. However, lower input capacitance will result increased input current ripple input voltage ripple, couple noise into other circuitry. Also, increased voltage ripple will raise minimum operating voltage LT3480 ~3.7V. Output Capacitor Output Ripple output capacitor essential functions. Along with inductor, filters square wave generated LT3480 produce output. this role determines output ripple, impedance switching frequency important. second function store energy order satisfy transient loads stabilize LT3480's control loop. Ceramic capacitors have very equivalent series resistance (ESR) provide best ripple performance. good starting value COUT VOUT
course, such simple design guide will always result optimum inductor your application. larger value inductor provides slightly higher maximum load current will reduce output voltage ripple. your load lower than then decrease value inductor operate with higher ripple current. This allows physically smaller inductor, with lower resulting higher efficiency. There several graphs Typical Performance Characteristics section this data sheet that show maximum load current function input voltage inductor value several popular output voltages. inductance result discontinuous mode operation, which okay further reduces maximum load current. details maximum output current discontinuous mode operation, Linear Technology Application Note Finally, duty cycles greater than (VOUT/VIN 0.5), there minimum inductance required avoid subharmonic oscillations. AN19. Input Capacitor Bypass input LT3480 circuit with ceramic capacitor type. types have poor performance over temperature applied voltage, should used. 4.7F ceramic capacitor adequate bypass LT3480 will easily handle ripple current. Note that larger input capacitance required when lower switching frequency used. input power source high impedance, there significant inductance long wires cables, additional bulk capacitance
where MHz, COUT recommended output capacitance types. This choice will provide output ripple good transient response. Transient performance improved with higher value
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LT3480 APPLICATIONS INFORMATION
Table Capacitor Vendors
VENDOR Panasonic PHONE (714) 373-7366 www.panasonic.com PART SERIES Ceramic, Polymer, Tantalum Kemet Sanyo (864) 963-6300 (408) 749-9714 www.kemet.com www.sanyovideo.com Ceramic, Tantalum Ceramic, Polymer, Tantalum Murata Taiyo Yuden (864) 963-6300 (408) 436-1300 www.murata.com www.avxcorp.com www.taiyo-yuden.com Ceramic Ceramic, Tantalum Ceramic Series POSCAP T494, T495 Series COMMANDS
capacitor compensation network also adjusted maintain loop bandwidth. lower value output capacitor used save space cost transient performance will suffer. Frequency Compensation section choose appropriate compensation network. When choosing capacitor, look carefully through data sheet find what actual capacitance under operating conditions (applied voltage temperature). physically larger capacitor, with higher voltage rating, required. High performance tantalum electrolytic capacitors used output capacitor. important, choose that intended switching regulators. should specified supplier, should 0.05 less. Such capacitor will larger than ceramic capacitor will have larger capacitance, because capacitor must large achieve ESR. Table lists several capacitor vendors. Catch Diode catch diode conducts current only during switch time. Average forward current normal operation calculated from: ID(AVG) IOUT (VIN VOUT)/VIN where IOUT output load current. only reason consider diode with larger current rating than necessary nominal operation worst-case condition shorted output. diode current will then increase typical peak switch current. Peak reverse voltage equal
regulator input voltage. Schottky diode with reverse voltage rating greater than input voltage. overvoltage protection feature LT3480 will keep switch when which allows rated Schottky even when ranges 60V. Table lists several Schottky diodes their manufacturers.
Table Diode Vendors
PART NUMBER Semicnductor MBRM120E MBRM140 Diodes Inc. B120 B130 B220 B230 DFLS240L International Rectifier 10BQ030 20BQ030 IAVE (mV) (mV)
Ceramic Capacitors Ceramic capacitors small, robust have very ESR. However, ceramic capacitors cause problems when used with LT3480 their piezoelectric nature. When Burst Mode operation, LT3480's switching frequency depends load current, very light loads LT3480 excite ceramic capacitor audio frequencies, generating audible noise. Since LT3480 operates lower current limit during Burst Mode
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LT3480 APPLICATIONS INFORMATION
operation, noise typically very quiet casual ear. this unacceptable, high performance tantalum electrolytic capacitor output. final precaution regarding ceramic capacitors concerns maximum input voltage rating LT3480. ceramic input capacitor combined with trace cable inductance forms high quality (under damped) tank circuit. LT3480 circuit plugged into live supply, input voltage ring twice nominal value, possibly exceeding LT3480's rating. This situation easily avoided (see Plugging Safely section). Frequency Compensation LT3480 uses current mode control regulate output. This simplifies loop compensation. particular, LT3480 does require output capacitor stability, free ceramic capacitors achieve output ripple small circuit size. Frequency compensation provided components tied pin, shown Figure Generally capacitor (CC) resistor (RC) series ground used. addition, there lower value capacitor parallel. This capacitor (CF) part loop compensation used filter noise switching frequency, required only phase-lead capacitor used output capacitor high ESR. Loop compensation determines stability transient performance. Designing compensation network complicated best values depend application particular type output capacitor. practical approach start with circuits this data sheet that similar your application tune compensation network optimize performance. Stability should then checked across operating conditions, including load current, input voltage temperature. LT1375 data sheet contains more thorough discussion loop compensation describes test stability using transient load. Figure shows equivalent circuit LT3480 control loop. error amplifier transconductance amplifier with finite output impedance. power section, consisting modulator, power switch inductor, modeled transconductance amplifier generating output current proportional voltage pin. Note that output capacitor integrates this current, that capacitor (CC) integrates error amplifier output current, resulting poles loop. most cases zero required comes from either output capacitor from resistor series with This simple model works well long value inductor high loop crossover frequency much lower than switching frequency. phase lead capacitor (CPL) across feedback divider improve transient response. Figure shows transient response when load current stepped from 500mA 1500mA back 500mA.
LT3480 CURRENT MODE POWER STAGE 3.5mho ERROR AMPLIFIER 420mho POLYMER TANTALUM CERAMIC OUTPUT
Figure Model Loop Response
VOUT 100mV/DIV
0.5A/DIV 12V; FRONT PAGE APPLICATION 10s/DIV
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Figure Transient Load Response LT3480 Front Page Application Load Current Stepped from 500mA 1500mA. VOUT 3.3V
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0.8V
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LT3480 APPLICATIONS INFORMATION
5V/DIV
operation lower output load current than when Burst Mode. front page application circuit will switch full frequency output loads higher than about 60mA. BOOST BIAS Considerations
0.2A/DIV
VOUT 10mV/DIV
5s/DIV 12V; FRONT PAGE APPLICATION ILOAD 10mA
3480
Figure Burst Mode Operatio
Low-Ripple Burst Mode Pulse-Skip Mode LT3480 capable operating either Low-Ripple Burst Mode Pulse-Skip Mode which selected using SYNC pin. Synchronization section details. enhance efficiency light loads, LT3480 operated Low-Ripple Burst Mode operation which keeps output capacitor charged proper voltage while minimizing input quiescent current. During Burst Mode operation, LT3480 delivers single cycle bursts current output capacitor followed sleep periods where output power delivered load output capacitor. Because LT3480 delivers power output with single, current pulses, output ripple kept below 15mV typical application. addition, quiescent currents reduced typically respectively during sleep time. load current decreases towards load condition, percentage time that LT3480 operates sleep mode increases average input current greatly reduced resulting high efficiency even very loads. Figure higher output loads (above 140mA front page application) LT3480 will running frequency programmed resistor, will operating standard mode. transition between Low-Ripple Burst Mode seamless, will disturb output voltage. quiescent current required LT3480 operate Pulse-Skip mode. benefit this mode that LT3480 will enter full frequency standard
Capacitor internal boost Schottky diode (see Block Diagram) used generate boost voltage that higher than input voltage. most cases 0.22F capacitor will work well. Figure shows three ways arrange boost circuit. BOOST must more than 2.3V above best efficiency. outputs above, standard circuit (Figure best. outputs between 2.8V boost capacitor. 2.5V output presents special case because marginally adequate support boosted drive stage while using internal boost diode. reliable BOOST operation with 2.5V outputs good external Schottky diode (such Semi MBR0540), boost capacitor (see Figure 5b). lower output voltages boost diode tied input (Figure 5c), another supply greater than 2.8V. Tying reduces maximum input voltage 30V. circuit Figure more efficient because BOOST current quiescent current comes from lower voltage source. must also sure that maximum voltage ratings BOOST pins exceeded. minimum operating voltage LT3480 application limited minimum input voltage (3.6V) maximum duty cycle outlined previous section. proper startup, minimum input voltage also limited boost circuit. input voltage ramped slowly, LT3480 turned with RUN/SS when output already regulation, then boost capacitor fully charged. Because boost capacitor charged with energy stored inductor, circuit will rely some minimum load current boost circuit running properly. This minimum load will depend input output voltages, arrangement boost circuit. minimum load generally goes zero once circuit started. Figure shows plot minimum load start function input voltage. many cases discharged output capacitor will present load switcher, which will allow start. plots show
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LT3480 APPLICATIONS INFORMATION
VOUT BOOST INPUT VOLTAGE LT3480 VOUT 3.3V 25°C 8.2H 700kHz 1000 LOAD CURRENT 10000 START (WORST CASE)
4.7F
(5a) VOUT 2.8V
VOUT BOOST LT3480 INPUT VOLTAGE
START (WORST CASE)
4.7F
VOUT 25°C 8.2H 700kHz 1000 LOAD CURRENT 10000
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(5b) 2.5V VOUT 2.8V
VOUT BOOST LT3480
4.7F
Figure Minimum Input Voltage Depends Output Voltage, Load Current Boost Circuit
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(5c) VOUT 2.5V; VIN(MAX) Figure Three Circuits Generating Boost Voltage
Soft-Start RUN/SS used soft-start LT3480, reducing maximum input current during start-up. RUN/SS driven through external filter create voltage ramp this pin. Figure shows start-up shut-down waveforms with soft-start circuit. choosing large time constant, peak start-up current reduced current that required regulate output, with overshoot. Choose value resistor that supply when RUN/SS reaches 2.5V. Synchronization select Low-Ripple Burst Mode operation, SYNC below 0.3V (this ground logic output).
worst-case situation where ramping very slowly. lower start-up voltage, boost diode tied VIN; however, this restricts input range one-half absolute maximum rating BOOST pin. light loads, inductor current becomes discontinuous effective duty cycle very high. This reduces minimum input voltage approximately 300mV above VOUT. higher load currents, inductor current continuous duty cycle limited maximum duty cycle LT3480, requiring higher input voltage maintain regulation.
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LT3480 APPLICATIONS INFORMATION
MBRS140
RUN/SS 0.22F VRUN/ 2V/DI 1A/DI
RUN/SS
BOOST LT3480 VOUT
VOUT 2V/DI
BACKUP
2ms/DIV
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Figure Soft-Start LT3480, Resisitor Capacitor RUN/SS
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Synchronizing LT3480 oscillator external frequency done connecting square wave (with duty cycle) SYNC pin. square wave amplitude should have valleys that below 0.3V peaks that above 0.8V 6V). LT3480 will enter Burst Mode output loads while synchronized external clock, instead will skip pulses maintain regulation. LT3480 synchronized over 250kHz 2MHz range. resistor should chosen LT3480 switching frequency below lowest synchronization input. example, synchronization signal will 250kHz higher, should chosen 200kHz. assure reliable safe operation LT3480 will only synchronize when output voltage near regulation indicated flag. therefore necessary choose large enough inductor value supply required output current frequency resistor. Inductor Selection section. also important note that slope compensation value: When sync frequency much higher than slope compensation will significantly reduced which require larger inductor value prevent subharmonic oscillation. Shorted Reversed Input Protection inductor chosen that won't saturate excessively, LT3480 buck regulator will tolerate shorted output. There another situation consider systems where output will held high when input LT3480 absent. This occur battery charging applications
Figure Diode Prevents Shorted Input from Discharging Backup Battery Tied Output. Also Protects Circuit from Reversed Input. LT3480 Runs Only When Input Present
battery backup systems where battery some other supply diode OR-ed with LT3480's output. allowed float RUN/SS held high (either logic signal because tied VIN), then LT3480's internal circuitry will pull quiescent current through pin. This fine your system tolerate this state. ground RUN/SS pin, current will drop essentially zero. However, grounded while output held high, then parasitic diodes inside LT3480 pull large currents from output through pin. Figure shows circuit that will only when input voltage present that protects against shorted reversed input. Layout proper operation minimum EMI, care must taken during printed circuit board layout. Figure shows recommended component placement with trace, ground plane locations. Note that large, switched currents flow LT3480's pins, catch diode (D1) input capacitor (C1). loop formed these components should small possible. These components, along with inductor output capacitor, should placed same side circuit board, their connections should made that layer. Place local, unbroken ground plane below these components. BOOST nodes should small possible.
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LT3480 APPLICATIONS INFORMATION
VOUT
Finally, keep nodes small that ground traces will shield them from BOOST nodes. Exposed bottom package must soldered ground that acts heat sink. keep thermal resistance low, extend ground plane much possible, thermal vias under near LT3480 additional ground planes within circuit board bottom side. Plugging Safely small size, robustness impedance ceramic capacitors make them attractive option input bypass capacitor LT3480 circuits. However, these capacitors cause problems LT3480 plugged into live supply (see Linear Technology Application Note complete discussion). loss ceramic capacitor, combined with stray inductance series with power
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VIAS LOCAL GROUND PLANE VIAS VOUT VIAS SYNC
VIAS RUN/SS VIAS
VIAS OUTLINE LOCAL GROUND PLANE
Figure Good Layout Ensures Proper, Operatio
CLOSING SWITCH SIMULATES PLUG LT3480
DANGER 20V/DIV RINGING EXCEED ABSOLUTE MAXIMUM RATING
4.7F
IMPEDANCE ENERGIZED SUPPLY
STRAY INDUCTANCE FEET METERS) TWISTED PAIR
10A/DIV 20s/DIV
(10a)
LT3480 20V/DIV
0.1F 4.7F 10A/DIV
(10b)
20s/DIV
LT3480
20V/DIV
AI.EI.
4.7F 10A/DIV
(10c)
20s/DIV
3480
Figure Well Chosen Input Network Prevents Input Voltage Overshoot Ensures Reliable Operation when LT3480 Connected Live Supply
3480fb
LT3480 APPLICATIONS INFORMATION
source, forms under damped tank circuit, voltage LT3480 ring twice nominal input voltage, possibly exceeding LT3480's rating damaging part. input supply poorly controlled user will plugging LT3480 into energized supply, input network should designed prevent this overshoot. Figure shows waveforms that result when LT3480 circuit connected supply through feet 24-gauge twisted pair. first plot response with 4.7F ceramic capacitor input. input voltage rings high input current peaks 26A. good solution shown Figure 10b. resistor added series with input eliminate voltage overshoot also reduces peak input current). 0.1F capacitor improves high frequency filtering. high input voltages impact efficiency minor, reducing efficiency percent output full load operating from 24V. High Temperature Considerations must provide heat sinking keep LT3480 cool. Exposed bottom package must soldered ground plane. This ground should tied large copper layers below with thermal vias; these layers will spread heat dissipated LT3480. Place additional vias reduce thermal resistance further. With these steps, thermal resistance from junction) ambient reduced 35°C/W less. With LFPM airflow, this resistance fall another 25%. Further increases airflow will lead lower thermal resistance. Because large output current capability LT3480, possible dissipate enough heat raise junction temperature beyond absolute maximum 125°C. When operating high ambient temperatures, maximum load current should derated ambient temperature approaches 125°C. Power dissipation within LT3480 estimated calculating total power loss from efficiency measurement subtracting catch diode loss inductor loss. temperature calculated multiplying LT3480 power dissipation thermal resistance from junction ambient. Other Linear Technology Publications Application Notes contain more detailed descriptions design information buck regulators other switching regulators. LT1376 data sheet more extensive discussion output ripple, loop compensation stability testing. Design Note shows generate bipolar output supply using buck regulator.
TYPICAL APPLICATIONS
Step-Down Converter
6.8V TRANSIENT 60V* RUN/SS BOOST 0.47F 4.7F 16.2k 40.2k 470pF 800kHz DIODES INC. DFLS240L TAIYO YUDEN NP06DZB6R8M
3480fb
VOUT
6.8H
SYNC
LT3480
536k 100k
3480 TA02
LT3480 TYPICAL APPLICATIONS
3.3V Step-Down Converter
4.4V TRANSIENT 60V* VOUT 3.3V
RUN/SS
BOOST 0.47F 4.7H
4.7F 40.2k 470pF
SYNC 800kHz DIODES INC. DFLS240L TAIYO YUDEN NP06DZB4R7M
LT3480
316k 100k
3480 TA03
2.5V Step-Down Converter
TRANSIENT 60V* VOUT 2.5V 4.7H
RUN/SS
BOOST
4.7F 56.2k 330pF
LT3480
215k SYNC 600kHz DIODES INC. DFLS240L MBR0540 TAIYO YUDEN NP06DZB4R7M 100k
3480 TA04
3480fb
Information furnished Linear Technology Corporation believed accurate reliable. However, responsibility assumed use. Linear Technology Corporation makes representation that interconnection circuits described herein will infringe existing patent rights.
LT3480 TYPICAL APPLICATIONS
2MHz Step-Down Converter
8.6V TRANSIENT VOUT
RUN/SS
BOOST 0.47F 2.2H
2.2F 11.5k 470pF
SYNC 2MHz DIODES INC. DFLS240L SUMIDA CDRH4D22/HP-2R2
LT3480
536k 100k
3480 TA05
Step-Down Converter
TRANSIENT 60V* VOUT
RUN/SS
BOOST 0.47F
26.1k 40.2k 330pF
SYNC
LT3480
715k
3480 TA06
800kHz DIODES INC. DFLS240L NEC/TOKIN PLC-0755-100
3480fb
LT3480 TYPICAL APPLICATIONS
1.8V Step-Down Converter
3.5V RUN/SS BOOST 0.47F 4.7F 18.2k 68.1k 330pF 500kHz DIODES INC. DFLS240L TAIYO YUDEN NP06DZB3R3M SYNC 127k 100k
3480 TA08
VOUT 1.8V
3.3H
LT3480
3480fb
LT3480 PACKAGE DESCRIPTION
Package 10-Lead Plastic (3mm 3mm)
(Reference 05-08-1660)
0.115 0.675 0.05 0.38 0.10
3.50
0.05 1.65 0.05 2.15 0.05 SIDES) PACKAGE OUTLINE 0.25 0.05 0.50 2.38 0.05 SIDES) RECOMMENDED SOLDER PITCH DIMENSIONS NOTE: DRAWING MADE JEDEC PACKAGE OUTLINE M0-229 VARIATION (WEED-2). CHECK WEBSITE DATA SHEET CURRENT STATUS VARIATION ASSIGNMENT DRAWING SCALE DIMENSIONS MILLIMETERS DIMENSIONS EXPOSED BOTTOM PACKAGE INCLUDE MOLD FLASH. MOLD FLASH, PRESENT, SHALL EXCEED 0.15mm SIDE EXPOSED SHALL SOLDER PLATED SHADED AREA ONLY REFERENCE LOCATION BOTTOM PACKAGE MARK (SEE NOTE
3.00 0.10 SIDES)
1.65 0.10 SIDES)
(DD) 1103
0.200 0.75 0.05 2.38 0.10 SIDES)
0.25 0.05 0.50
0.00 0.05
BOTTOM VIEW-EXPOSED
3480fb
LT3480 PACKAGE DESCRIPTION
Package 10-Lead Plastic MSOP
(Reference 05-08-1660)
BOTTOM VIEW EXPOSED OPTION
2.794 (.110
0.102 .004)
0.889 (.035
0.127 .005)
2.06 0.102 (.081 .004) 1.83 0.102 (.072 .004)
5.23 (.206)
2.083 (.082
0.102 3.20 3.45 .004) (.126 .136)
0.50 0.305 0.038 (.0197) (.0120 .0015) RECOMMENDED SOLDER LAYOUT
3.00 0.102 (.118 .004) (NOTE
0.497 0.076 (.0196 .003)
4.90 0.152 (.193 .006) 0.254 (.010) GAUGE PLANE 0.53 0.152 (.021 .006) DETAIL 0.18 (.007) SEATING PLANE 1.10 (.043) DETAIL
3.00 0.102 (.118 .004) (NOTE
0.86 (.034)
0.17 0.27 (.007 .011)
NOTE: DIMENSIONS MILLIMETER/(INCH) DRAWING SCALE DIMENSION DOES INCLUDE MOLD FLASH, PROTRUSIONS GATE BURRS. MOLD FLASH, PROTRUSIONS GATE BURRS SHALL EXCEED 0.152mm (.006") SIDE DIMENSION DOES INCLUDE INTERLEAD FLASH PROTRUSIONS. INTERLEAD FLASH PROTRUSIONS SHALL EXCEED 0.152mm (.006") SIDE LEAD COPLANARITY (BOTTOM LEADS AFTER FORMING) SHALL 0.102mm (.004")
0.50 (.0197)
0.1016 (.004
0.0508 .002)
MSOP (MSE) 0307
3480fb
LT3480 TYPICAL APPLICATION
1.2V Step-Down Converter
3.6V RUN/SS BOOST 0.47F 4.7F 16.2k 68.1k 330pF 500kHz
3480 TA09
VOUT 1.2V
3.3H
SYNC
LT3480
52.3k 100k
DIODES INC. DFLS240L TAIYO YUDEN NP06DZB3R3M
RELATED PARTS
PART NUMBER LT1933 LT3437 LT1936 LT3493 LT1976/LT1977 LT1767 LT1940 LT1766 LT3434/LT3435 LT3481 LT3684 DESCRIPTION 500mA (IOUT), 500kHz Step-Down Switching Regulator SOT-23 60V, 400mA (IOUT), MicroPower Step-Down DC/DC Converter with Burst Mode 36V, 1.4A (IOUT), 500kHz High Efficiency Step-Down DC/DC Converter 36V, 1.2A (IOUT), 750kHz High Efficiency Step-Down DC/DC Converter 60V, 1.2A (IOUT), 200kHz/500kHz, High Efficiency Step-Down DC/DC Converter with Burst Mode 25V, 1.2A (IOUT), 1.1MHz, High Efficiency Step-Down DC/DC Converter Dual 25V, 1.4A (IOUT), 1.1MHz, High Efficiency Step-Down DC/DC Converter 60V, 1.2A (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter 60V, 2.4A (IOUT), 200/500kHz, High Efficiency Step-Down DC/DC Converter with Burst Mode 36V, (IOUT), 2.8MHz, High Efficiency Step-Down DC/DC Converter with Burst Mode 36V, (IOUT), 2.8MHz, High Efficiency Step-Down DC/DC Converter COMMENTS VIN: 3.6V 36V, VOUT(MIN) 1.2V, 1.6mA, <1A, ThinSOT Package VIN: 3.3V 80V, VOUT(MIN) 1.25V, 100A, <1A, 10-Pin 16-Pin TSSOP Packages VIN: 3.6V 36V, VOUT(MIN) 1.2V, 1.9mA, <1A, MS8E Package VIN: 3.6V 40V, VOUT(MIN) 0.8V, 1.9mA, <1A, 6-Pin Package VIN: 3.3V 60V, VOUT(MIN) 1.2V, 100A, <1A, 16-Pin TSSOP Package VIN: 25V, VOUT(MIN) 1.2V, 1mA, <6A, MS8E Package VIN: 3.6V 25V, VOUT(MIN) 1.2V, 3.8mA, <30A, 16-Pin TSSOP Package VIN: 5.5V 60V, VOUT(MIN) 1.2V, 2.5mA, 25A, 16-Pin TSSOP Package VIN: 3.3V 60V, VOUT(MIN) 1.2V, 100A, <1A, 16-Pin TSSOP Package VIN: 3.6V 34V, VOUT(MIN) 1.26V, 50A, <1A, 10-Pin 10-Pin MSOP Packages VIN: 3.6V 34V, VOUT(MIN) 1.26V, 1.5mA, <1A, 10-Pin 10-Pin MSOP Packages
3480fb
Linear Technology Corporatio(408) 432-1900 FAX: (408) 434-0507
0308 PRINTED
1630 McCarthy Blvd., Milpitas, 95035-7417
www.linear.com
LINEAR TECHNOLOGY CORPORATION 2008
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