LT1938 25V, 2.2A, 2.8MHz Step-Down Switching Regulator DESCRIPTION

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LT1938 25V, 2.2A, 2.8MHz Step-Down Switching Regulator DESCRIPTION

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LT1938 25V, 2.2A, 2.8MHz Step-Down Switching Regulator DESCRIPTION
LT®1938 adjustable frequency (300kHz 2.8MHz) monolithic buck switching regulator that accepts input voltages 25V. high efficiency 0.18 switch included along with boost Schottky diode necessary oscillator, control logic circuitry. Current mode topology used fast transient response good loop stability. LT1938's high operating frequency allows small, cost inductors ceramic capacitors resulting output ripple while keeping total solution size minimum. current shutdown mode 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. LT1938 available package with Exposed thermal resistance.
Lare registered trademarks Linear Technology Corporation. other trademarks property their respective owners.
Wide Input Voltage Range: 3.6V 2.2A Maximum Output Current Adjustable Switching Frequency: 300kHz 2.8MHz Shutdown Current: Integrated Boost Diode Power Good Flag Saturating Switch Design: 0.18 On-Resistance 1.265V Feedback Reference Voltage Output Voltage: 1.265V Soft-Start Capability Small 10-Pin Thermally Enhanced (3mm 3mm) Package
APPLICATIONS
Automotive Battery Regulation Power Portable Products Distributed Supply Regulation Industrial Supplies Wall Transformer Regulation
TYPICAL APPLICATION
3.3V Step-Down Converter
4.5V 16.2k 4.7F 680pF LT1938 60.4k BIAS 324k 200k RUN/SS BOOST EFFICIENCY 0.47F 4.7H VOUT 3.3V 2.2A PLC-0745-4R7 800kHz LOAD CURRENT
1938
Efficiency (VOUT 3.3V)
1938 TA01
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LT1938 ABSOLUTE MAXIMUM RATINGS
(Note
CONFIGURATION
VIEW BOOST RUN/SS BIAS
VIN, RUN/SS Voltage .25V BOOST Voltage .50V BOOST Above Pin.25V Voltage BIAS, Voltage .25V Operating Junction Temperature Range (Note LT1938E -40°C 125°C LT1938I -40°C 125°C Storage Temperature Range. -65°C 150°C
PACKAGE 10-LEAD (3mm 3mm) PLASTIC TJMAX 125°C, 45°C/W, 10°C/W EXPOSED (PIN GND, MUST SOLDERED
ORDER INFORMATION
LEAD FREE FINISH LT1938EDD#PBF LT1938IDD#PBF TAPE REEL LT1938EDD#TRPBF LT1938IDD#TRPBF PART MARKING* LDFT LDFT PACKAGE DESCRIPTION 10-Lead (3mm 3mm) Plastic 10-Lead (3mm 3mm) Plastic TEMPERATURE RANGE -40°C 125°C -40°C 125°C
Consult Marketing parts specified with wider operating temperature ranges. *The temperature grade identified label shipping container. Consult Marketing information non-standard lead based finish parts. more information lead free part marking, http://www.linear.com/leadfree/ more information tape reel specifications,
ELECTRICAL CHARACTERISTICS
PARAMETER Minimum Input Voltage Quiescent Current from
denotes specifications which apply over full operating temperature range, otherwise specifications 25°C. 10V, VRUN/SS 10V, VBOOST 15V, VBIAS 3.3V unless otherwise noted. (Note
CONDITIONS
0.01 0.01
1.28 1.29 0.02
UNITS
VRUN/SS 0.2V VBIAS Switching VBIAS Switching
Quiescent Current from BIAS
VRUN/SS 0.2V VBIAS Switching VBIAS Switching
0.85 1.25 1.24 1.265 1.265 0.002
Minimum Bias Voltage Feedback Voltage
Bias Current (Note Voltage Line Regulation
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LT1938 ELECTRICAL CHARACTERISTICS
PARAMETER Error Error Gain Source Current Sink Current Switch Current Gain Clamp Voltage Switching Frequency 8.66k 29.4k 187k
denotes specifications which apply over full operating temperature range, otherwise specifications 25°C. 10V, VRUN/SS VBOOST 15V, VBIAS 3.3V unless otherwise noted. (Note
CONDITIONS 1000 1.25 0.02
UNITS
1.55
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 0.4V Rising VRUN/SS 2.5V Duty Cycle 10V, VBIAS
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 LT1938E guaranteed meet performance specifications from 125°C. Specifications over -40°C 125°C operating temperature range assured design, characterization correlation with statistical process controls. LT1938I specifications guaranteed over -40°C 125°C temperature range.
Note Bias current measured regulation. Bias current flows into pin. Note This minimum voltage across boost capacitor needed guarantee full saturation switch.
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LT1938 TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency (VOUT 5.0V)
EFFICIENCY PLC-0745-4R7 800kHz LOAD CURRENT
1938
25°C unless otherwise noted) Efficiency
Efficiency (VOUT 3.3V)
EFFICIENCY
EFFICIENCY
PLC-0745-4R7 800kHz LOAD CURRENT
1938
VOUT 3.3V LOAD SWITCHING FREQUENCY (MHz)
1938
Maximum Load Current
LOAD CURRENT MINIMUM INPUT VOLTAGE
1938
Maximum Load Current
LOAD CURRENT
1938
Switch Current Limit
TYPICAL
TYPICAL MINIMUM VOUT 25°C 4.7H 800kHz INPUT VOLTAGE
VOUT 3.3V 25°C 4.7H 800kHz
SWITCH CURRENT LIMIT
DUTY CYCLE
1938
Switch Current Limit
SWITCH CURRENT LIMIT TEMPERATURE (°C) DUTY CYCLE DUTY CYCLE
Switch Voltage Drop
BOOST CURRENT (mA) 1000 1500 2000 2500 3000 3500 SWITCH CURRENT (mA)
1938
Boost Current
VOLTAGE DROP (mV)
1000 1500 2000 2500 3000 3500 SWITCH CURRENT (mA)
1938
1938
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LT1938 TYPICAL PERFORMANCE CHARACTERISTICS
Feedback Voltage
1.290 1.285 FEEDBACK VOLTAGE 1.280 FREQUENCY (MHz) 1.275 1.270 1.265 1.260 1.255 1.250 TEMPERATURE (°C) 1.20 1.15 1.10 1.05 1.00 0.95 0.90 0.85 0.80 TEMPERATURE (°C) SWITCHING FREQUENCY (kHz)
25°C unless otherwise noted) Frequency Foldback
1200 45.3k 1000 1000 1200 1400 VOLTAGE (mV)
1938
Switching Frequency
45.3k
1938
1938
Minimum Switch On-Time
MINIMUM SWITCH ON-TIME (ns) SWITCH CURRENT LIMIT TEMPERATURE (°C)
Soft-Start
RUN/SS VOLTAGE
1938
RUN/SS Current
RUN/SS CURRENT
RUN/SS VOLTAGE
1938
1938
Boost Diode
CURRENT BOOST DIODE BOOST DIODE CURRENT
1938
Error Output Current
1.065 1.165 1.365 1.265 VOLTAGE 1.465
1938
Minimum Input Voltage
INPUT VOLTAGE
VOUT 3.3V 25°C 4.7H 800kHz 0.01 LOAD CURRENT
1938
0.001
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LT1938 TYPICAL PERFORMANCE CHARACTERISTICS
Minimum Input Voltage
2.50
25°C unless otherwise noted) Power Good Threshold
1.200 RISING
Voltages
THRESHOLD VOLTAGE
INPUT VOLTAGE
CURRENT LIMIT CLAMP 1.50
THRESHOLD VOLTAGE
2.00
1.180 1.160
VOUT 25°C 4.7H 800kHz 0.01 LOAD CURRENT
1938
1.00 SWITCHING THRESHOLD 0.50
1.140
1.120
0.001
TEMPERATURE (°C)
1.100
TEMPERATURE (°C)
1938
1938
Switching Waveforms (Discontinuous Operation)
0.5A/DIV 0.5A/DIV
Switching Waveforms (Continuous Operation)
5V/DIV
5V/DIV
VOUT 10mV/DIV 12V, FRONT PAGE APPLICATION ILOAD 140mA 1s/DIV
1938
VOUT 10mV/DIV 12V, FRONT PAGE APPLICATION ILOAD 1s/DIV
1938
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LT1938 FUNCTIONS
(Pin This connects anode boost Schottky diode. 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 LT1938's internal regulator internal power switch. This must locally bypassed. RUN/SS (Pin RUN/SS used LT1938 shutdown mode. ground shut down LT1938. 2.3V more normal operation. shutdown feature used, this pin. RUN/SS also provides soft-start function; Applications Information section. (Pin open collector output internal comparator. remains until within final regulation voltage. output valid when above 3.5V RUN/SS high. BIAS (Pin BIAS supplies current LT1938's internal regulator. this lowest available voltage source above (typically VOUT). This architecture increases efficiency especially when input voltage much higher than output. (Pin LT1938 regulates 1.265V. 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.
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LT1938 BLOCK DIAGRAM
INTERNAL 1.265V BIAS
RUN/SS
SLOPE COMP
OSCILLATOR 300kHz-2.8MHz SOFT-START
ERROR
1.12V
1938
SWITCH LATCH CLAMP
BOOST
VOUT
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LT1938 OPERATION
LT1938 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, BIAS connected external voltage higher than bias power will drawn from external source (typically regulated output voltage). This improves efficiency. RUN/SS used place LT1938 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. oscillator reduces LT1938's operating frequency when voltage low. This frequency foldback helps control output current during startup overload. LT1938 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 LT1938 enabled above 3.6V.
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LT1938 APPLICATIONS INFORMATION
Resistor Network output voltage programmed with resistor divider between output pin. Choose resistors according VOUT 1.265 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 LT1938 switch finite minimum times. switch turn minimum ~150ns turn minimum ~150ns. 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 LT1938 applications depends switching frequency, Absolute Maximum Ratings BOOST pins, operating mode. output start-up short-circuit operating modes, then must below below result following equation: VIN(MAX VOUT tON(MIN)
Reference designators refer Block Diagram. Setting Switching Frequency LT1938 uses constant frequency architecture that programmed switch from 300kHz 2.8MHz using resistor tied from ground. table showing necessary value desired switching frequency Figure
SWITCHING FREQUENCY (MHz) VALUE 84.5 60.4 45.3 36.5 29.4 23.7 20.5 16.9 14.3 12.1 10.2 8.66
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)
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
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LT1938 APPLICATIONS INFORMATION
frequency will necessary achieve safe operation high input voltages. output regulation short-circuit start-up events expected, then input voltage transients acceptable regardless switching frequency. this mode, LT1938 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. minimum input voltage determined either LT1938's minimum operating voltage ~3.6V maximum duty cycle (see equation previous section). minimum input voltage duty cycle VIN(MIN) VOUT tOFF(MIN) 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: VOUT VOUT VIN(MAX
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 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 LT1938'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. LT1938's switch current limit (ILIM)
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 keep efficiency high, series resistance (DCR) should less than 0.1, core material should intended high frequency applications. Table lists several vendors suitable types.
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 Open
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LT1938 APPLICATIONS INFORMATION
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 LT1938 circuit with ceramic capacitor type. types have poor performance over temperature applied voltage, should used. 4.7F ceramic capacitor adequate bypass LT1938 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 necessary. This provided with 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 LT1938 force this very high frequency switching current into tight local loop, minimizing EMI. 4.7F capacitor capable this task, only placed close LT1938 catch diode (see Layout section). second precaution regarding ceramic input capacitor concerns maximum input voltage rating LT1938. ceramic input capacitor combined with trace cable inductance forms high quality (under damped) tank circuit. LT1938 circuit plugged into live supply, input voltage ring twice nominal value, possibly exceeding LT1938's voltage rating. This situation easily avoided (see Plugging Safety section). space sensitive applications, 2.2F ceramic capacitor used local bypassing LT1938 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 LT1938 ~3.7V. Output Capacitor Output Ripple output capacitor essential functions. Along with inductor, filters square wave generated LT1938 produce output. this role determines output ripple, impedance switching frequency important. second function store energy order satisfy transient loads stabilize LT1938's control loop. Ceramic capacitors have very equivalent series resistance (ESR) provide best ripple performance. good starting value COUT VOUT
where MHz, COUT recommended output capacitance types. This choice will provide output ripple good transient response. Transient performance improved with higher value 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.
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LT1938 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
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. diode with reverse voltage rating greater than input voltage. Table lists several Schottky diodes their manufacturers.
Table Diode Vendors
PART NUMBER Semiconductor MBRM120E Diodes Inc. B120 B130 B220 B230 DFLS230L International Rectifier 10BQ030 20BQ030 IAVE (mV) (mV)
Frequency Compensation LT1938 uses current mode control regulate output. This simplifies loop compensation. particular, LT1938 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.
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LT1938 APPLICATIONS INFORMATION
LT1938 CURRENT MODE POWER STAGE 3.5mho ERROR AMPLIFIER 330mho POLYMER TANTALUM CERAMIC 10s/DIV
1938
VOUT 12V, FRONT PAGE APPLICATION
OUTPUT 1A/DIV
Figure Model Loop Response
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 LT1938 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
1.265V
VOUT 100mV/DIV
Figure Transient Load Response LT1938 Front Page Application Load Current Stepped from 500mA 1500mA. VOUT 3.3V
1938
improve transient response. Figure shows transient response when load current stepped from 500mA 1500mA back 500mA. BOOST BIAS Considerations 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 4b). lower output voltages boost diode tied input (Figure 4c), another supply greater than 2.8V. circuit Figure more efficient because BOOST current BIAS quiescent current comes from lower voltage source. must also sure that maximum voltage ratings BOOST BIAS pins exceeded. minimum operating voltage LT1938 application limited minimum input voltage (3.6V) maximum duty cycle outlined previous section.
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LT1938 APPLICATIONS INFORMATION
VOUT BOOST INPUT VOLTAGE LT1938 START VOUT 3.3V 25°C 4.7H 800kHz 0.01 LOAD CURRENT
4.7F
(4a) VOUT 2.8V
VOUT BOOST LT1938 INPUT VOLTAGE
0.001
START
4.7F
0.001 VOUT 25°C 4.7H 800kHz 0.01 LOAD CURRENT
1938
(4b) 2.5V VOUT 2.8V
VOUT BOOST LT1938
4.7F
Figure Minimum Input Voltage Depends Output Voltage, Load Current Boost Circuit
1938
(4c) VOUT 2.5V Figure Three Circuits Generating Boost Voltage
proper start-up, minimum input voltage also limited boost circuit. input voltage ramped slowly, LT1938 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 minimum input start will same minimum input run. This occurs, example, RUN/SS asserted after applied. plots show 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 LT1938, requiring higher input voltage maintain regulation.
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LT1938 APPLICATIONS INFORMATION
Soft-Start RUN/SS used soft-start LT1938, reducing maximum input current during start-up. RUN/SS driven through external filter create voltage ramp this pin. Figure shows startup 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 2.3V.
MBRS140 RUN/SS BACKUP BOOST LT1938 VOUT
1938
Figure Diode Prevents Shorted Input from Discharging Backup Battery Tied Output. Also Protects Circuit from Reversed Input. LT1938 Runs Only When Input Present
RUN/SS 0.22F
1A/DIV
VRUN/SS 2V/DIV VOUT 2V/DIV
LT1938 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 LT1938'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. 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 LT1938 additional ground planes within circuit board bottom side.
2ms/DIV
1938
Figure Soft-Start LT1938, Resisitor Capacitor RUN/SS
Shorted Reversed Input Protection inductor chosen that won't saturate excessively, LT1938 buck regulator will tolerate shorted output. There another situation consider systems where output will held high when input LT1938 absent. This occur battery charging applications battery backup systems where battery some other supply diode OR-ed with LT1938's output. allowed float RUN/SS held high (either logic signal because tied VIN), then LT1938'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
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LT1938 APPLICATIONS INFORMATION
VOUT
1938
VIAS LOCAL GROUND PLANE VIAS VOUT
VIAS RUN/SS VIAS
VIAS OUTLINE LOCAL GROUND PLANE
Figure Good Layout Ensures Proper, Operation
Plugging Safely small size, robustness impedance ceramic capacitors make them attractive option input bypass capacitor LT1938 circuits. However, these capacitors cause problems LT1938 plugged into live supply (see Linear Technology Application Note complete discussion). loss ceramic capacitor, combined with stray inductance series with power source, forms under damped tank circuit, voltage LT1938 ring twice nominal input voltage, possibly exceeding LT1938's rating damaging part. input supply poorly controlled user will plugging LT1938 into energized supply, input network should designed prevent this overshoot. Figure shows waveforms that result when LT1938 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 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 LT1938 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 LT1938. 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 LT1938, possible dissipate enough heat raise junction temperature beyond absolute maximum 125°C. When operating high ambient temperatures,
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LT1938 APPLICATIONS INFORMATION
CLOSING SWITCH SIMULATES PLUG LT1938 DANGER 20V/DIV RINGING EXCEED ABSOLUTE MAXIMUM RATING
4.7F
IMPEDANCE ENERGIZED SUPPLY
STRAY INDUCTANCE FEET METERS) TWISTED PAIR
10A/DIV 20s/DIV
(9a)
LT1938 20V/DIV
0.1F 4.7F 10A/DIV
(9b)
20s/DIV
LT1938
20V/DIV
AI.EI.
4.7F 10A/DIV
(9c)
20s/DIV
1938
Figure Well Chosen Input Network Prevents Input Voltage Overshoot Ensures Reliable Operation when LT1938 Connected Live Supply
maximum load current should derated ambient temperature approaches 125°C. Power dissipation within LT1938 estimated calculating total power loss from efficiency measurement subtracting catch diode loss inductor loss. temperature calculated multiplying LT1938 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.
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LT1938 TYPICAL APPLICATIONS
Step-Down Converter
VOUT 2.2A RUN/SS BOOST 0.47F 4.7F 60.4k 680pF 800kHz DIODES INC. DFLS230L TAIYO YUDEN NP06DZB6R8M BIAS 590k 200k
1938 TA02
6.3V
6.8H
LT1938
3.3V Step-Down Converter
4.4V RUN/SS BOOST 0.47F 4.7F 16.2k 60.4k 680pF 800kHz DIODES INC. DFLS230L TAIYO YUDEN NP06DZB4R7M BIAS 324k 200k
1938 TA03
VOUT 3.3V 2.2A
4.7H
LT1938
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LT1938 TYPICAL APPLICATIONS
2.5V Step-Down Converter
RUN/SS BOOST 4.7F 22.1k 84.5k 680pF 600kHz DIODES INC. DFLS230L MBR0540 TAIYO YUDEN NP06DZB4R7M BIAS 196k 200k
3684 TA04
VOUT 2.5V 2.2A 4.7H
LT1938
2MHz Step-Down Converter
8.6V RUN/SS BOOST 0.47F 2.2F 16.9k 680pF 2MHz DIODES INC. DFLS230L SUMIDA CDRH4D22/HP-2R2 BIAS 590k 200k
1938 TA05
VOUT
2.2H
LT1938
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LT1938 TYPICAL APPLICATIONS
Step-Down Converter
RUN/SS BOOST 0.47F 60.4k 680pF 800kHz DIODES INC. DFLS230L NEC/TOKIN PLC-0755-100 BIAS 845k 100k
3684 TA06
VOUT 2.2A
LT1938
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LT1938 TYPICAL APPLICATIONS
1.8V Step-Down Converter
3.5V RUN/SS BOOST 0.47F 4.7F 15.4k 105k 680pF 500kHz DIODES INC. DFLS230L TAIYO YUDEN NP06DZB3R3M BIAS 84.5k 200k
1938 TA07
VOUT 1.8V 2.2A
3.3H
LT1938
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LT1938 PACKAGE DESCRIPTION
Package 10-Lead Plastic (3mm 3mm)
(Reference 05-08-1699)
0.675 ±0.05
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 0.115
0.38 0.10
3.00 ±0.10 SIDES) MARK (SEE NOTE
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 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
1938fa
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.
LT1938 TYPICAL APPLICATION
1.265V Step-Down Converter
VOUT 1.265V 2.2A RUN/SS BOOST 0.47F 4.7F 105k 680pF 500kHz
1938 TA08
3.6V
3.3H
LT1938
BIAS
DIODES INC. DFLS240L TAIYO YUDEN NP06DZB3R3M
RELATED PARTS
PART NUMBER LT1933 LT3437 LT1936 LT3493 LT1976/LT1977 LT1767 LT1940 LT1766 LT3434/LT3435 LT3481 DESCRIPTION 500mA (IOUT), 500kHz Step-Down Switching Regulator SOT-23 60V, 400mA (IOUT), MicroPower Step-Down DC/DC Converter with Burst Mode Operation 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 StepDown DC/DC Converter with Burst Mode Operation 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 Operation 36V, (IOUT), Micropower 2.8MHz, High Efficiency Step-Down DC/DC Converter COMMENTS VIN: 3.6V 36V, VOUT(MIN) 12V, 1.6mA, ThinSOTPackage VIN: 3.3V 80V, VOUT(MIN) 1.25V, 100A, Package VIN: 3.6V 36V, VOUT(MIN) 1.2V, 1.9mA, MS8E Package VIN: 3.6V 40V, VOUT(MIN) 0.8V, 1.9mA, Package VIN: 3.3V 60V, VOUT(MIN) 1.20V, 100A, TSSOP16E Package VIN: 25V, VOUT(MIN) 1.20V, 1mA, MS8E Package VIN: 3.6V 25V, VOUT(MIN) 1.20V, 3.8mA, 30A, TSSOP16E Package VIN: 5.5V 60V, VOUT(MIN) 1.20V, 2.5mA, 25A, TSSOP16E Package VIN: 3.3V 60V, VOUT(MIN) 1.20V, 100A, TSSOP16E Package VIN: 3.6V 36V, VOUT(MIN) 1.265V, MS10E Packages
1938fa
Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
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1630 McCarthy Blvd., Milpitas, 95035-7417
www.linear.com
LINEAR TECHNOLOGY CORPORATION 2007

LT1938 25V, 2.2A, 2.8MHz Step-Down Switching Regulator DESCRIPTION

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