DESCRIPTIO 1.5A, Internal Switch 2.7MHz Switching Frequency Integ
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LT1946A 2.7MHz Boost DC/DC Converter with 1.5A Switch Soft-Start
DESCRIPTIO
1.5A, Internal Switch 2.7MHz Switching Frequency Integrated Soft-Start Function Adjustable Output from VCESAT Switch: 300mV 1.5A (Typical) 430mA from Input Small Thermally Enhanced 8-Lead MSOP Package
APPLICATIO
TFT-LCD Bias Supplies Receivers Modems Local Power Supply
LT®1946A fixed frequency step-up DC/DC converter containing internal 1.5A, switch. Capable generating 430mA from input, LT1946A ideal powering large TFT-LCD panels. LT1946A switches 2.7MHz, allowing tiny, profile inductors value ceramic capacitors. Loop compensation either internal external, giving user flexibility setting loop compensation allowing optimized transient response with ceramic output capacitors. Soft-start controlled with external capacitor which determines input current ramp rate during start 8-lead MSOP package high switching frequency ensure profile overall solution less than 1.1mm high.
registered trademarks Linear Technology Corporation.
TYPICAL APPLICATIO
2.2µH SHDN 100nF 2.2µF 27.4k 270pF
VOUT 430mA 182k EFFICIENCY
LT1946A COMP GND*
2.2µF
2.2µF, X7R, 6.3V 2.2µF, X7R, MICROSEMI UPS120 EQUIVALENT SUMIDA CR43-2R2 EXPOSED MUST ALSO GROUNDED
1946A TA01
LOAD CURRENT (mA)
1946A TA01
Figure 12V, 430mA Step-Up DC/DC Converter
Efficiency
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LT1946A
ABSOLUTE
(Note
RATI
PACKAGE/ORDER ATIO
VIEW SHDN COMP
Voltage Voltage 0.4V Voltage 2.5V Current into ±1mA SHDN Voltage Maximum Junction Temperature 125°C Operating Temperature Range (Note 40°C 85°C Storage Temperature Range 65°C 150°C Lead Temperature (Soldering, sec). 300°C
ORDER PART NUMBER LT1946AEMS8E MS8E PART MARKING LTYZ
MS8E PACKAGE 8-LEAD PLASTIC MSOP EXPOSED GROUND (MUST SOLDERED PCB)
TJMAX 125°C, 40°C/W, 10°C/W
Consult Marketing parts specified with wider operating temperature ranges.
denotes specifications which apply over full operating temperature range, otherwise specifications 25°C. VSHDN unless otherwise noted. (Note
PARAMETER Minimum Operating Voltage Maximum Operating Voltage Feedback Voltage
ELECTRICAL CHARACTERISTICS
CONDITIONS
2.45
1.27 1.27
UNITS µmhos
1.23 1.22
1.25 0.01
Bias Current Error Transconductance Error Voltage Gain Quiescent Current Quiescent Current Shutdown Reference Line Regulation Switching Frequency
1.25V (Note VSHDN 2.5V, Switching VSHDN 2.6V
0.05
0.85
Switching Frequency Foldback Maximum Duty Cycle Switch Current Limit Switch VCESAT Switch Leakage Current Soft-Start Charging Current SHDN Input Voltage High SHDN Input Voltage SHDN Bias Current
0.01
(Note 0.5V
VSHDN VSHDN
Note Absolute Maximum Ratings those values beyond which life device impaired. Note LT1946AE guaranteed meet performance specifications from 70°C. Specifications over -40°C 85°C operating
temperature range assured design, characterization correlation with statistical process controls. Note Current flows pin. Note Current limit guaranteed design and/or correlation static test. Current limit independent duty cycle guaranteed design.
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LT1946A TYPICAL PERFOR CHARACTERISTICS
Feedback Voltage
1.28 1.27 3000 2700
OSCILLATOR FREQUENCY (kHz)
FEEDBACK VOLTAGE
1.26 1.25 1.24 1.23 1.22 1.21 1.20
1800 1500 1200 25°C
TEMPERATURE (°C)
FEEDBACK VOLTAGE
1946A
CURRENT LIMIT
Switch Saturation Voltage
0.35 0.30 QUIESCENT CURRENT (mA) 0.25 SWITCH CURRENT
VCESAT
0.20 0.15 0.10 0.05
Transient Response Figure Circuit
VOUT 100mV/DIV COUPLED VOUT 2V/DIV
0.5A/DIV ILOAD 250mA 150mA 50µs/DIV
1946A
Oscillator Frequency
Current Limit
2400 2100
-30°C
100°C
TEMPERATURE (°C)
1946A
1946A
Quiescent Current
VOUT 100mV/DIV COUPLED 10V/DIV
Switching Waveforms Figure Circuit
0.5A/DIV 100ns/DIV
1946A
TEMPERATURE (°C)
1946A
1946A
Start-Up Waveforms Figure Circuit
200mA/DIV VSHDN RLOAD 1ms/DIV
1946A
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LT1946A
CTIO
(Pin Error Amplifier Output Pin. external compensation network this internal compensation network shorting COMP pin. External compensation consists placing resistor capacitor series from GND. Typical capacitor range from 90pF 270pF. Typical resistor range from 120k. (Pin Feedback Pin. Reference voltage 1.25V. Connect resistive divider here. Minimize trace area VOUT according VOUT 1.25 (1+R1/R2). SHDN (Pin Shutdown Pin. 2.4V more enable device. Ground shut down. float this pin. (Pin Exposed Pad): Ground. both exposed directly local ground plane. ground metal exposed should wide better heat dissipation. Multiple vias (local ground plane ground backplane) placed close exposed further reducing thermal resistance. (Pin Switch Pin. This collector internal power switch. Minimize metal trace area connected this minimize EMI. (Pin Input Supply Pin. Must locally bypassed. COMP (Pin Internal Compensation Pin. Provides internal compensation network. directly internal compensation. used. (Pin Soft-Start Pin. Place soft-start capacitor here. Upon start-up, current charges capacitor 1.5V. larger capacitor slower start-up. Leave floating use.
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LT1946A
BLOCK DIAGRA
VOUT (EXTERNAL) (EXTERNAL) 0.5V
SHUTDOWN
SHDN
COMP 120k 90pF
1.25V REFERENCE
COMPARATOR DRIVER
0.01
RAMP GENERATOR
2.7MHz OSCILLATOR EXPOSED
1946A
Figure Block Diagram
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LT1946A
OPERATIO
LT1946A uses constant frequency, current mode control scheme provide excellent line load regulation. Please refer Figure following description part's operation. start oscillator cycle, latch set, turning power switch switch current flows through internal current sense resistor generating voltage. This voltage added stabilizing ramp resulting into positive terminal comparator When this voltage exceeds level negative input latch reset, turning power switch. level negative input pin) error amplifier (A1) simply amplified version difference between feedback voltage reference voltage 1.250V. this manner, error amplifier sets correct peak current level keep output regulation. functions provided enable very clean start-up LT1946A. Frequency foldback used reduce oscillator frequency one-third when below
APPLICATIO ATIO
Inductor Selection
Several inductors that work well with LT1946A listed Table This table complete, there many other manufacturers devices that used. Consult each manufacturer more detailed information their entire selection related parts, many different sizes shapes available. Ferrite core inductors should used obtain best efficiency, core losses 2.7MHz much lower ferrite cores than cheaper powdered-iron ones. Choose inductor that handle least 1.5A without saturating, ensure that inductor (copper-wire resistance) minimize power losses. 1.5µH 4.7µH inductor will best choice most LT1946A designs. Note that some applications, current handling requirements inductor lower, such SEPIC topology where each inductor only carries one-half total switch current. inductors shown Table were chosen small size. better efficiency, similar valued inductors with larger volume.
nominal value 0.5V. This accomplished comparator This feature reduces minimum duty cycle that part achieve thus allowing better control switch current during start-up. When voltage goes above 0.5V, oscillator returns normal frequency 2.7MHz. soft-start function also provided LT1946A. When part brought shutdown, current sourced pin. connecting external capacitor pin, rate voltage rise set. Typical values soft-start capacitor range from 10nF 200nF. directly limits rate rise pin, which turn limits peak switch current. Current limit shown Figure switch current constantly monitored allowed exceed nominal value 2.1A. switch current reaches 2.1A, latch reset regardless output comparator This current limit protects power switch well various external components connected LT1946A.
Table Recommended Inductors LT1946A
(µH) 10.0 Size LxWxH (mm) 5.2x5.6x1.8 PART RLF5018-1R5M2R1 RLF5018-2R7M1R8 RLF5018-4R7M1R4 RLF5018-100MR94 LPO1704-122MC LPO1704-222MC CR43-2R2 CR43-3R3 VENDOR (847) 803-6100 www.tdk.com Coilcraft (800) 322-2645 www.coilcraft.com Sumida (847) 956-0666 www.sumida.com 5.5x6.6x1.0 4.5x4.0x3.2
Capacitor Selection (equivalent series resistance) capacitors should used output minimize output ripple voltage. Multilayer ceramic capacitors excellent choice, they have extremely available very small packages. dielectrics preferred, followed X7R, these materials retain capacitance over wide voltage temperature ranges. 2.2µF 20µF output
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LT1946A
APPLICATIO ATIO
capacitor sufficient most applications, systems with very output currents need only smaller output capacitor. Solid tantalum OSCON capacitors used, they will occupy more board area than ceramic will have higher ESR. Always capacitor with sufficient voltage rating. Ceramic capacitors also make good choice input decoupling capacitor, which should placed close possible LT1946A. 2.2µF 4.7µF input capacitor sufficient most applications. Table shows list several ceramic capacitor manufacturers. Consult manufacturers detailed information their entire selection ceramic parts.
Table Ceramic Capacitor Manufacturers
Taiyo Yuden Murata (408) 573-4150 (803) 448-9411 (714) 852-2001 www.t-yuden.com www.avxcorp.com www.murata.com
Compensation compensate feedback loop LT1946A, series resistor-capacitor network should connected from COMP GND. most applications, capacitor range 90pF 470pF will suffice. good starting value compensation capacitor, 270pF. compensation resistor, usually range 100k. good technique compensate application 100k potentiometer place 270pF capacitor adjusting potentiometer while observing transient response, optimum value found. Figures 3a-3c illustrate this process circuit Figure Figure shows transient response with equal 2.5k. phase margin poor evidenced excessive ringing output voltage inductor current. Figure value increased 6.5k, which results more damped response. Figure shows results when increased further 27.4k. transient response nicely damped compensation procedure complete. COMP provides access internal resistor (120k) capacitor (90pF). some applications, these values will suffice external will needed.
VOUT 200mV/DIV COUPLED 0.5A/DIV 2.5k 50µs/DIV
1946A F03a
Figure Transient Response Shows Excessive Ringing
VOUT 200mV/DIV COUPLED
0.5A/DIV
6.5k
50µs/DIV
1946A F03b
Figure Transient Response Better
VOUT 200mV/DIV COUPLED
0.5A/DIV
27.4k
50µs/DIV
1946A F03c
Figure Transient Response Well Damped
Compensation-Theory Like other current mode switching regulators, LT1946A needs compensated stable efficient operation. feedback loops used LT1946A: fast current loop which does require compensation, slower voltage loop which does. Standard bode plot analysis used understand adjust voltage feedback loop. sn1946a 1946afs
LT1946A
APPLICATIO ATIO
Zero:
with feedback loop, identifying gain phase contribution various elements loop critical. Figure shows equivalent elements boost converter. Because fast current control loop, power stage inductor, diode have been replaced equivalent transconductance amplifier GMP. acts current source where output current proportional voltage. Note that maximum output current finite current limit From Figure gain, poles zeroes calculated follows: Output Pole:
Error Pole:
Error Zero:
Gain:
1.25 VOUT
VOUT 1.250V REFERENCE COUT
GMA: TRANSCONDUCTANCE AMPLIFIER INSIDE GMP: POWER STAGE TRANSCONDUCTANCE AMPLIFIER COUT: OUTPUT CAPACITOR OUTPUT RESISTANCE DEFINED VOUT DIVIDED ILOAD (MAX) FEEDBACK RESISTOR DIVIDER NETWORK OUTPUT RESISTANCE COMPENSATION RESISTOR COMPENSATION CAPACITOR
Figure Boost Converter Equivalent Model
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VOUT
Zero:
High Frequency Pole:
Using circuit Figure example, Table shows parameters used generate bode plot shown Figure
Table Bode Plot Parameters
Parameter COUT VOUT Value 27.4 Units µmho Comment Application Specific Application Specific Adjustable Adjustable Adjustable Application Specific Application Specific Adjustable Adjustable Application Specific Adjustable Adjustable
From Figure phase when gain reaches 122° giving phase margin 58°. This more than adequate. cross-over frequency 90kHz, which about times lower than frequency right half plane zero important that cross-over frequency least times lower than frequency zero achieve adequate phase margin.
LT1946A
APPLICATIO ATIO
GAIN
100k FREQUENCY (Hz)
1946A FO5a
PHASE
-100
-180 -200 100k FREQUENCY (Hz)
1946A FO5b
Figure Gain Phase Plots Figure Circuit
Diode Selection Schottky diode recommended with LT1946A. Microsemi UPS120 very good choice. Where input output voltage differential exceeds 20V, UPS140 diode). These diodes rated handle average forward current applications where average forward current diode less than 0.5A, Semiconductor MBR0520 diode used.
Setting Output Voltage output voltage, select values (see Figure according following equation:
1.25V
good range from 30k. Layout Hints high speed operation LT1946A demands careful attention board layout. will advertised performance with careless layouts. Figure shows recommended component placement boost converter.
GROUND PLANE SHUTDOWN MULTIPLE VIAs VOUT LT1946A
19949
NOTE: DIRECT HIGH CURRENT PATHS USING WIDE TRACES. MINIMIZE TRACE AREA 1(VC) 2(FB). MULTIPLE VIAS COPPER GROUND PLANE. VIAS LOCATION ONLY AVOID INTRODUCING SWITCHING CURRENTS INTO GROUND PLANE.
Figure Recommended Component Placement Boost Converter
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LT1946A
TYPICAL APPLICATIO
Profile 1.1mm Tall) Triple Output Supply (10V, -10V, 20V)
4.7µF 100nF
C1-C6: 4.7µF, 6.3V 10µF, 1µF, 2.2µF, C5-C6: 0.1µF, MICROSEMI UPS120 EQUIVALENT D2-D5: ZETEX BAT54S EQUIVALENT COILCRAFT LP01704-152MC EXPOSED MUST ALSO GROUNDED
Transient Response
AVDD 50mV/DIV COUPLED EFFICIENCY AVDD LOAD 350mA 200mA 100µs/DIV
1946A TA03
0.5A/DIV
0.1µF 1.5µH SHDN LT1946A GND* 10.5k 20µF AVDD 475mA
COMP 150pF
0.1µF
2.2µF VOFF -10V 10mA
1946A TA02
Efficiency
LOAD VOFF LOAD 10mA AVDD LOAD CURRENT (mA)
1946A TA04
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LT1946A
TYPICAL APPLICATIO
Triple Output Supply Uses SEPIC Topology Output Disconnect
0.22µF 0.22µF 10µH SHDN LT1946A GND* 9.76k 20µF COMP 10mA VOFF -12V 10mA
C1-C5: 2.2µF, 6.3V 10µF, 1µF, 0.22µF, 0.22µF,
2.794 0.102 (.110 .004)
0.889 0.127 (.035 .005)
5.23 (.206)
2.083 0.102 3.45 (.082 .004) (.126 .136)
0.42 0.04 (.0165 .0015)
0.65 (.0256)
GAUGE PLANE 0.53 0.015 (.021 .006) DETAIL 0.18 (.077) 1.10 (.043) 0.86 (.34)
RECOMMENDED SOLDER LAYOUT
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")
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.
AVDD 250mA 84.5k
2.2µF 100nF
10µH
MICROSEMI UPS120 EQUIVALENT D2-D3: CENTRAL SEMI CMDSH-3 L1-L2: RLF5018-100MR94 EXPOSED MUST ALSO GROUNDED
1946A TA09
MS8E Package 8-Lead Plastic MSOP
(Reference 05-08-1662)
3.00 0.102 (.118 .004) (NOTE 0.52 (.206) BOTTOM VIEW EXPOSED OPTION 2.06 0.102 (.080 .004) 1.83 0.102 (.072 .004)
0.254 (.010)
DETAIL
4.88 (.192 .004)
3.00 0.102 (.118 .004) NOTE
SEATING PLANE
0.22 0.38 (.009 .015)
0.65 (.0256)
0.13 0.05 (.005 .002)
MSOP (MS8E) 1001
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LT1946A
TYPICAL APPLICATIO
Profile 1.1mm Tall) Triple Output Supply (8V, 24V)
3.3V
1.2µH SHDN LT1946A GND*
EFFICIENCY
4.7µF 100nF
COMP
C1-C8: 4.7µF, 6.3V 10µF, 2.2µF, 1µF, 0.1µF, 0.1µF, MICROSEMI UPS120 EQUIVALENT D2-D7: ZETEX BAT54S EQUIVALENT COILCRAFT LP01704-122MC EXPOSED MUST ALSO GROUNDED
Transient Response
AVDD 50mV/DIV COUPLED AVDD 5V/DIV
0.5A/DIV VOFF 5V/DIV ILOAD 350mA 200mA 50µs/DIV
1946A TA07
RELATED PARTS
PART NUMBER LT1613 LT1615/LT1615-1 LT1930/LT1930A LT1946 LT1961 DESCRIPTION 550mA (ISW), 1.4MHz, Step-Up DC/DC Converter 300mA/0.75mA (ISW), Constant Off-Time Step-Up DC/DC Converter (ISW), 1.2MHz/2.2MHz, Step-Up DC/DC Converter 1.5A (ISW), 1.2MHz, Step-Up DC/DC Converter 1.5A (ISW), 1.25MHz, Step-Up DC/DC Converter COMMENTS 0.9V 10V, VOUT 34V, 3mA, 1µA, ThinSOTVIN 15V, VOUT 34V, 20µA, 1µA, ThinSOT 2.6V 16V, VOUT 34V, 4.2mA/5.5mA, 1µA, ThinSOT 2.45V 16V, VOUT 34V, 3.2mA, 1µA, 25V, VOUT 35V, 0.9mA, 6µA, MS8E
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ThinSOT trademark Linear Technology Corporation.
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, 95035-7417
(408) 432-1900
FAX: (408) 434-0507 www.linear.com
0.1µF
0.1µF
0.1µF
Efficiency
AVDD 375mA 28.7k
LOAD VOFF LOAD 10mA AVDD LOAD CURRENT (mA)
1946A TA06
20µF 5.23k 0.1µF
2.2µF VOFF 10mA
1946A TA05
Start-Up Waveforms
10V/DIV
0.5A/DIV 1ms/DIV
1946A TA08
LT/TP 1102 PRINTED
LINEAR TECHNOLOGY CORPORATION 2001
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