LM2637 Motherboard Power Supply Solution with 5-Bit Programmable Switc

The Datasheet Archive - 100 Million Datasheets from 7500 Manufacturers.

LM2637 Motherboard Power Supply Solution with 5-Bit Programmable Switc

Datasheet Thumbnail

Top Searches for this datasheet

LM2637 Motherboard Power Supply Solution with 5-Bit Programmable Switching Controller Linear Regulator Controllers
LM2637 Motherboard Power Supply Solution with 5-Bit Programmable Switching Controller Linear Regulator Controllers
LM2637 provides comprehensive embedded power supply solution motherboards hosting high performance MPUs such IITM, PentiumII, K6-2 other similar high performance MPUs. LM2637 incorporates 5-bit programmable, synchronous buck switching controller high-speed linear regulator controllers 24-pin package. Switching Section switching regulator controller features 5-bit programmable DAC, over-current overvoltage protection, under-voltage latch-off, power good signal, output enable. 5-bit typical tolerance There user-selectable over-current protection methods. provides accurate over-current protection with external sense resistor. other saves cost taking advantage rDS_ON highside FET. over voltage protection provides levels protection. first level keeps high-side low-side second provides gate signal that used fire external SCR. Linear Section linear regulator controllers feature wide control bandwidth, N-FET transistor driving capability, adjustable output voltage. wide control bandwidth makes meeting fast load transient response requirement such that GTL+ easy job. minimum configuration, controllers default 1.5V 2.5V respectively. Both linear controllers have under voltage latch-off.
Features
Provides regulated voltages Power Good flag output enable Under-voltage latch-off Switching Section Synchronous rectification 5-bit programmable from 3.5V 1.3V Typical tolerance Switching frequency: levels over-voltage protection methods over-current protection Adaptive non-overlapping gate drives Soft start without external capacitor Linear Section N-FET driving capability Ultra fast response speed Output voltages default 1.5V 2.5V adjustable
Applications
Embedded power supplies motherboards Triple DC/DC power supplies Programmable high current DC/DC power supply
Configuration
24-Lead SOIC
10084801
View Package Number M24B
IIis trademark Cyrix Corporation wholly owned subsidiary National Semiconductor Corporation. Pentiumis trademark Intel Corporation. trademark Advanced Micro Devices, Inc.
2004 National Semiconductor Corporation
DS100848
www.national.com
LM2637
Absolute Maximum Ratings (Note
Military/Aerospace specified devices required, please contact National Semiconductor Sales Office/ Distributors availability specifications. Junction Temperature Power Dissipation (Note 150°C 1.6W
Storage Temperature Susceptibility Soldering Time, Temperature sec.)
-65°C +150°C 300°C
Operating Ratings (Note
Junction Temperature Range 4.75V 5.25V +125°C
Electrical Characteristics
unless otherwise specified. Typicals limits appearing plain type apply +25°C. Limits appearing boldface type apply over +70°C range. Symbol IVID IQ_VCC VDACOUT IQ_VDD fOSC DMAX DMIN RSNS1 RDS_SRC RDS_SINK VCC_TH1 VCC_TH2 VDAC_IH VDAC_IL tPWGD tPWBAD VPWGD_HI Parameter internal Pull-Up Current Pins internal Pull-Up Current Operating VCCCurrent Shutdown Current 5-Bit Output Voltage Shutdown Current Oscillator Frequency Maximum Duty Cycle Minimum Duty Cycle SNS1 Resistance Ground Gate Driver Resistance When Sourcing Current Gate Driver Resistance When Sinking Current Rising Threshold Power-On Reset Falling Threshold Power-On Reset Input High Voltage Input Voltage PWGD Response Time PWGD Response Time PWGD High Trip Point SNS1 Rises from Rated Output Voltage SNS1 Falls from Rated Output Voltage Above Rated Output Voltage when output Voltage Above Rated Output Voltage when output Voltage (Note VPWGD_LO PWGD Trip Point Below Rated Output Voltage when output Voltage Below Rated Output Voltage when output Voltage (Note VOVP_TRP ICS+ VOCP IOVP Trip Point Sink Current Over-Current Trip Point (CS+ Differential Voltage) Source Current Error Amplifier Gain SNS1 Above Rated Output Drops from 11.5 10111 Pins Floating (Note Pins Floating N-1.5% Conditions 1000 N+1.5% Units
SWITCHING SECTION
www.national.com
LM2637
Electrical Characteristics
Symbol BWEA VRAMP_L VRAMP_H DSTEP_SS Parameter Error Amplifier Unity Gain Bandwidth Ramp Signal Valley Voltage Ramp Signal Peak Voltage Soft Start Time
(Continued)
unless otherwise specified. Typicals limits appearing plain type apply +25°C. Limits appearing boldface type apply over +70°C range. Conditions 1.25 3.25 4096 12.5 Units Clock Cycles
Duty Cycle Step Change Soft Start SNS2 Voltage 12V, 4.75V 5.25V, (Figure When Regulating (Note (Note 12V, 4.75V 5.25V, (Figure When Regulating (Note (Note
1.5V CONTROLLER SECTION VSNS2 1.463 0.63 0.44 1.538
ROUT2 ISNS2 VPWGD_HI VPWGD_LO VSNS3
Output Resistance SNS2 Bias Current PWGD High Trip Point PWGD Trip Point SNS3 Voltage
2.5V CONTROLLER SECTION 2.438 0.63 0.44 2.563
ROUT3 ISNS3 VPWGD_HI VPWGD_LO
Output Resistance SNS3 Bias Current PWGD High Trip Point PWGD Trip Point
Note Absolute Maximum Ratings limits beyond which damage device occur. Operating ratings conditions under which device operates correctly. Operating Ratings imply guaranteed performance limits. Note Maximum allowable power dissipation function maximum junction temperature, TJMAX, junction-to-ambient thermal resistance, ambient temperature, maximum allowable power dissipation ambient temperature calculated using: PMAX (TJMAX TA)/JA. junction-toambient thermal resistance, LM2637 78°C/W. TJMAX 150°C 25°C, maximum allowable power dissipation 1.6W. Note letter stands typical output voltages appearing italic boldface type Table Note output level PWGD logic power good function switching section, 1.5V section 2.5V section.
www.national.com
LM2637
TABLE 5-Bit Output Voltage Table (VCC 25°C, Test Mode) Symbol VDACOUT Parameter 5-Bit Output Voltages Different Codes Conditions VID4:0 01111 VID4:0 01110 VID4:0 01101 VID4:0 01100 VID4:0 01011 VID4:0 01010 VID4:0 01001 VID4:0 01000 VID4:0 00111 VID4:0 00110 VID4:0 00101 VID4:0 00100 VID4:0 00011 VID4:0 00010 VID4:0 00001 VID4:0 00000 VID4:0 11111 VID4:0 11110 VID4:0 11101 VID4:0 11100 VID4:0 11011 VID4:0 11010 VID4:0 11001 VID4:0 11000 VID4:0 10111 VID4:0 10110 VID4:0 10101 VID4:0 10100 VID4:0 10011 VID4:0 10010 VID4:0 10001 VID4:0 10000 Typical 1.30 1.35 1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00 2.05 (shutdown) Units
www.national.com
LM2637
Block Diagram
10084830
www.national.com
LM2637
Test Circuit
10084802
FIGURE Controller Test Circuit
Typical Applications
10084803
FIGURE Motherboard Power Supply Pentium Processor Core (1.3V 2.8V, 14.2A), GTL+ (1.5V, 4A), Legacy (2.5V, 0.3A). External sense resistor used provide both over-current limit dynamic voltage positioning.
www.national.com
LM2637
Typical Applications
(Continued)
10084804
FIGURE Motherboard Power Supply Pentium Processor Core (1.8V 2.8V, 14.2A), GTL+ (1.5V, 4A), Legacy (2.5V, 0.3A). High side used provide current limit.
www.national.com
LM2637
Description
PGND SNS2 SGND SNS1 FREQ PWGD VID4 VID3 VID2 VID1 VID0 SNS3 Name side N-FET gate driver output. Ground drivers switching section. Supply gate drivers. Usually tied +12V. Feedback 1.5V linear regulator. Gate drive output external N-MOS fast 1.5V linear regulator. Ground internal signal circuitry system ground reference. Supply voltage. Usually +5V. Output voltage monitor input switching regulator. Switching regulator current sense input, positive node. Switching regulator current sense input, negative node. Over-voltage protection output switching regulator. used fire external SCR. Switching frequency adjustment pin. external resistor needed desired frequency. Output error amplifier. Used compensating switching regulator. Inverting input error amplifier. Used compensating switching regulator. Open collector Power Good signal. 5-Bit input, MSB. 5-Bit input. 5-Bit input. 5-Bit input. 5-Bit input, LSB. Gate drive external N-MOS 2.5V linear regulator. Feedback 2.5V linear regulator. Output Enable. logic shuts whole chip down. High side N-FET gate driver output. Function
Preferred Startup Timing
10084831
Note: Start section Application Information.
Applications Information
OVERVIEW LM2637 provides control protection three voltage regulators. Namely, synchronous buck switching controller linear regulator controllers that drive external N-FET transistor.
Switching Section switching controller features VRM-compatible, 5-bit programmable output voltage, overcurrent over-voltage protection, under-voltage latch-off, power good signal, output enable. 5-bit typical tolerance There user-selectable over-current protection methods. provides accurate over-current protection with external sense resistor. other saves cost taking advantage rDS_ON high-side FET. over-voltage protection
www.national.com
LM2637
Applications Information
(Continued)
provides levels protection. first turns high-side turns low-side. second provides gate signal that used fire external SCR. frequency adjustable from beyond through external resistor. Soft start realized through internal digital counter. external soft start capacitor necessary. Dynamic positioning switcher output voltage reduces number output capacitors easily realized using same sense resistor over-current protection. Linear Section linear regulator controllers feature wide control bandwidth, N-FET transistor driving capability, adjustable output voltage typical tolerance. wide control bandwidth makes meeting GTL+ transient response requirement easy job. When external resistor divider used, controllers default 1.5V 2.5V respectively. Both linear sections have under-voltage latch-off. Should output voltage drop below 0.63V, corresponding gate drive will disabled PWGD will pulled low. THEORY OPERATION Start Switching Section soft start circuitry starts work when three conditions met, i.e., logic high, code valid voltage exceeds 4.2V. duration soft start determined internal digital counter switching frequency. During soft start, output error amplifier allowed increase gradually. When counter counted 4,096 clock cycles, soft start session ends output level error amplifier released allowed value that determined feedback loop. PWRGD always during soft start turned over output voltage monitoring circuitry after that. Before reaches internal logic power-on-reset state drivers disabled. power supply designer cautioned avoid situation when higher than while less than 1.8V. When less than 1.8V, internal circuitry LM2637 controlled state. this time greater than applied, pulled simultaneously, causing potential shoot-through thus possibly preventing come properly. Preferred Startup Timing recommended timing sequence. During normal operation, voltage drops below 3.6V, internal circuitry will into power-on-reset again. hysteresis helps decrease noise sensitivity pin. After soft start ends during normal operation, converter output voltage exceeds 118% output voltage, LM2637 will lock into over-voltage protection mode. high-side drive will low, low-side drive will high. There ways clear mode. cycle voltage once. other toggle level. After over-voltage protection mode cleared, LM2637 will enter soft start session start over. Linear Section linear section does through soft start. Whenever soft start switching section begins, linear section immediately applies required gate voltages base currents external power transistors. There under-voltage latch-off linear section.
after soft start ends, SNS2 SNS3 below 0.63V, corresponding gate drive will disabled PWGD will pulled low. Normal Operation Switching Section normal operation mode, LM2637 regulates converter output voltage adjusting duty ratio. output voltage determined 5-bit code user MPU. frequency external resistor between FREQ ground. resistance needed desired frequency determined following equation:
example, desired frequency kHz, resistance should around minimum allowable frequency kHz. Linear Section Under steady state operation, linear section supplies appropriate gate voltage base current correctly bias external pass transistor that voltage drop across transistor right value. Resetting LM2637 When LM2637 detects abnormal condition such switching regulator over voltage, will latch itself partially completely. reset LM2637, either voltage toggled. Another more subtle recover float pins reapply correct code. Gate Drives Switching Section switching controller gate drives that suitable driving external power N-FETs synchronous buck topology. voltage drivers supplied pin. This voltage should least VGS(th) higher than converter input voltage able fully enhance high-side FET. typical motherboard application, recommended that applied VDD, used input voltage switcher. charge pump recommended since linear sections need stable voltage minimize high frequency noise. 12V, peak gate charging current typically peak gate discharging current typically well suited high speed switching. LM2637 gate drives BiCMOS design. Unlike some bipolar control ICs, gate drive rail-to-rail swing that ensures spurious turn-on capacitive coupling. Another feature gate drives adaptive nonoverlapping mechanism. gate drive turned until other fully off. dead time between typically This avoids potential shoot-through problem helps improve efficiency. Linear Section gate drives linear section maximum continuous current about typical gate voltage 1.2V. Load Transient Response Switching Section typical modern application such Pentium K6-2 core power supply, load transient response critical issue. LM2637 utilizes conventional voltage feedback technology primary feedback control method. When load transient
www.national.com
LM2637
Applications Information
(Continued)
happens, error output voltage level error amplifier. output error amplifier then compared with internally generated ramp signal result comparison series pulses with certain duty ratios. These pulses then used control gate drives. this way, error output voltage gets corrected change duty ratio switches. During large load transient, depending compensation design, change duty ratio usually begins within switching cycle. Refer Design Considerations section more details. Besides voltage feedback control loop, LM2637 also pair fast comparators (the comparators) help maintain output voltage during large fast load transient. trip points comparators output voltage. When load transient large that output voltage goes outside window, comparator will bypass primary voltage control loop immediately duty ratio either 100% This provides fastest possible react such large load transient conventional buck converter. Linear Section linear section high control bandwidth. Depending external components selected, typical bandwidth high MHz. user choose lower this bandwidth have better noise immunity adding small capacitor between gate output ground. Power Good Signal power good signal indicate whether three output voltages within their corresponding range. range switching regulator typical window output voltage. range linear regulator 0.63V infinity. During soft start, power good signal kept low. completion soft start, three output voltages checked PWGD will asserted they within specified range. During normal operation, whenever voltage goes specified range more than about PWGD will pulled low. Over-Voltage Protection Switching Section When output voltage exceeds 118% output voltage time beyond soft start, switching section will enter over-voltage protection mode shuts itself down. upper gate drive will held while lower gate drive will held high. PWGD will low. There will also logic high signal that used fire external SCR. clear this mode, refer Resetting LM2637 section. Linear Section There over-voltage protection linear controllers. Under-Voltage Latch-Off completion soft start, controller starts monitor three output voltages. voltages goes below about 0.63V, controller will latch corresponding section, i.e., switching linear. mode cleared following procedures described Resetting LM2637 section.
Current Limit Switching Section Current limit realized methods. method through sensing high-side FET. other through separate sense resistor. first method cheaper more power efficient less accurate. second method more accurate dissipates additional power either more expensive requires special layout consideration. side benefit second method enables implementation technique called dynamic voltage positioning, which helps save number output capacitors. LM2637 tells which current limit mode supposed detecting voltage. When voltage 1.2V below voltage, sense resistor method assumed. Otherwise method chosen. method based typical rDS_ON high-side load current levels. Method High-Side Sensing This method detects high-side drain current sensing drain-source voltage when Figure Since rDS_ON known value, current through known measuring VDS. relationship between three parameters
implement current limit function, external resistor RIMAX needed. resistor should connected between drain high-side IMAX pin. constant current around forced flow into IMAX causes fixed voltage drop across RIMAX resistor. This voltage drop then compared with high-side latter higher, over current assumed. appropriate value RIMAX predetermined current limit level ILIM determined following equation:
example, suppose that rDS_ON desired current limit 20A, then RIMAX should
10084808
FIGURE Current Limit High-Side Sensing
www.national.com
LM2637
Applications Information
(Continued)
given current limit value, minimum RSENSE determined
Notice however, that rDS_ON positive temperature coefficient increase much when heated Also distribution rDS_ON fairly wide, 1.25 ratio uncommon. Consult MOSFET vendor further information distribution rDS_ON. designer should carefully choose value RIMAX that even under extreme case (largest rDS_ON highest temperature) current limit will trigger below preset value. provide greatest protection over high-side FET, cycle-by-cycle protection implemented. sampling starts early after turned Whenever over-current condition detected, highside immediately turned low-side turned This status remains rest cycle. same procedure applies next switching cycle. blanking time avoid switching noise that occurs whenever turned resistor between switching node (source high-side FET) important minimizing noise negative voltage present pin. resistance recommended. Method Current Sense Resistor This method uses sense resistor series with output inductor detect load current. SeeFigure voltage across sense resistor proportional load current. case that sense resistor discrete type (i.e., etch resistor) sense resistor value optimized dynamic voltage positioning (see Dynamic Positioning Load Voltage section), necessary signal level resistors, appropriately desired current limit.
where VOCP over-current trip voltage typically Electrical Characteristic table. example, current limit, minimum RSENSE 2.75 sense resistor used instead, appropriate values make voltage across VOCP when voltage across RSENSE discrete current sense resistor usually very good temperature coefficient tolerance. temperature coefficient ppm/°C typical. Tolerance usually Vishay Dale offer broad range discrete sense resistors. etch resistor also used RSENSE. advantage that approach flexible resistance, which will result minimum power loss. also eliminated. drawback high temperature coefficient, typically +4000 ppm/°C, which will result much less accurate current limit than discrete sense resistor. copper thickness usually tolerance. Linear Section There current limit function linear controllers. However, there ever severe overload, output voltage drop below 0.63V, which case under-voltage latch-off will provide protection. DESIGN CONSIDERATIONS Control Loop Compensation Switching Section switching regulator should properly compensated achieve stable operation, tight regulation good dynamic performance. synchronous buck regulator that needs meet stringent load transient requirement such that processor core voltage supply, 2-pole-1-zero compensation network should suffice, such shown Figure (C1, R2). This because zero typical output capacitors enough make control-to-output transfer function single-pole roll-off. example, figure values compensation network components Figure Assume following parameters: frequency kHz. Notice inductor resistance resistance FET's. control-to-output transfer function
zero frequency
10084809
double pole frequency
FIGURE Current Limit Current Sense Resistor
www.national.com
LM2637
Applications Information
(Continued)
corresponding Bode plots shown Figure Notice since zero frequency that phase doesn't even beyond -90°. This makes compensation easier Since gain cutoff frequency frequency) low, some compensation needed. Otherwise gain will cause poor line regulation, cutoff frequency hurt transient response performance. transfer function 2-pole-1-zero compensation network shown Figure
where
10084817
poles located origin help achieve highest gain. there three parameters determine, position zero, position second pole, constant determine cutoff frequency phase margin, loop bode plots need generated. loop transfer function -TF1 (10) choosing zero close double pole position second pole half switching frequency, closed loop transfer function turns very good. That 1.32 kHz, kHz, 10-6 then cutoff frequency will kHz, phase margin will 72°, gain will that error amplifier. Figure compensation network component values determined Equation (9), since values known. more conveniently calculate values, Equation rearranged follows:
FIGURE Buck Converter from Control Viewpoint different application different type output capacitors, different compensation scheme necessary. user either follow steps above figure appropriate component values contact National help.
(11) Notice there three equations four variables. variables chosen arbitrarily. Since current driving capability error amplifier limited around good idea have high impedance path from From Equation (11) told that larger will result smaller larger Calculations show that following combination good one: 0.022
10084818
FIGURE Control-to-Output Bode Plots
www.national.com
LM2637
Applications Information
(Continued)
switching (ZVS). That because every time just before low-side turned inductor current already flowing body diode, resulting drain-source voltage. When low-side turned off, current will shifted body diode temporarily, again clamping drain-source voltage value. difficult calculate switching loss complicated nature. Fortunately reasonable frequency such kHz, switching loss usually much less than Ohmic loss. designer initially ignore switching loss when trying meet efficiency specification. Ohmic loss high-side
(12) Ohmic loss low-side
10084819
FIGURE Loop Bode Plots Linear Section linear section designed high control bandwidth operation. phase margin cutoff frequency depends external N-FET, output capacitors their ESR. rule thumb, designer choose capacitance from 4000 with total larger capacitance, lower bandwidth. above capacitors usually result control bandwidth MHz. Selection Switching Section selection switches affects both efficiency whole converter current limit setting sensing mode selected). From efficiency standpoint suggested that high-side switch, only logic level FETs used. Standard FETs used low-side switch when used power pin. power loss associated with FETs twofold Ohmic loss switching loss. Ohmic loss relatively easy calculate whereas switching loss much more difficult estimate. switching loss synchronous buck converter usually happens only high-side FET. When high-side starts turn inductor current flowing low-side body diode. Since body diode undergoes reverse recovery before forced off, high-side will experience pulse drain current turn simultaneous presence high drain-source voltage high drain current high-side causes switching loss. Apparently switching loss proportional frequency. Having Schottky diode parallel with low-side body diode will large extent alleviate problem. This because Schottky diode does undergo reverse recovery lower forward voltage than body diode will take majority inductor current after low-side turned off. low-side benefits from what called zero voltage
(13) Notice when determining rDS_ON, gate-source voltages usually different FET's. high-side FET, minus drain voltage. low-side, VDD. This means low-side present lower rDS_ON when same type used both switches. Since rDS_ON positive temperature coefficient, actual Ohmic loss somewhat higher than calculated. power supply designer target 125°C operating temperature under maximum load highest ambient temperature then corresponding rDS_ON found datasheet. Linear Section things need considered, i.e., rDS_ON thermal capacity. Make sure that maximum possible rDS_ON N-FET lower than lowest input-output differential voltage divided maximum load current. typical motherboard 3.3V 1.5V 3.3V 2.5V application, this issue because maximum allowable rDS_ON higher than typical N-FET. thermal capacity cost that limits selection. example, consider 3.3V 1.5V, application. lowest input-output differential voltage 3.3V -1.5V 102% 1.605V, maximum allowable rDS_ON 1.605V Almost voltage discrete N-FET's meet this requirement. However, maximum power dissipation (3.3V 105% -1.5V 98%) least TO-220 package with beefy heat sink necessary handle thermal dissipation. When there load transient requirement such that GTL+ supply, make sure rDS_ON much lower than value calculated from steady state operation because headroom important transient performance. Capacitor Selection Switching Section Output Capacitors. selection capacitors extremely important step when designing converter load such core. Since typical slew rate load current during large load transient around A/µs A/µs, switching converter rely output capacitors take care first microseconds. Under such current slew rate, output capacitors more concern than terms voltage excursion. Depending kind capacitors being used, total output capacitance value important factor. When output capacitance low, converter
www.national.com
LM2637
Applications Information
(Continued)
have have small output inductor quickly supply current output capacitors when load suddenly kicks quickly stop supplying current when load suddenly removed. Multilayer ceramic (MLC) capacitors have very also capacitance value compared other kinds capacitors. aluminum electrolytic capacitors tend have large sizes capacitance. Tantalum electrolytic capacitors have fairly with much smaller size capacitance than aluminum capacitors. Certain OSCON capacitors present ultra long life span. time total output capacitor bank reaches around capacitance aluminum/tantalum/OSCON capacitors usually already millifarad range. those capacitors, only factor consider. MLCs have same amount total with much less capacitance, most probably under very small inductor, ultra fast control loop high switching frequency become necessary such case deal with fast charging/discharging rate output capacitor bank. From cost savings standpoint, aluminum electrolytic capacitors most popular choice output capacitors. They have reasonably long life span they tend have hugh capacitance withstand charging discharging process during load transient fairly long period. Sanyo MV-GX MV-DX series' give good performance when enough capacitors paralleled. 6MV1500GX capacitor typical capacitance 1500 voltage rating 6.3V. detailed procedure determining number output capacitors, refer application note Using Dynamic Voltage Positioning Technique Reduce Cost Output Capacitors Advanced Microprocessor Power Supplies associated spreadsheet automated design. Input Capacitors. challenge input capacitors ripple current. large ripple current drawn high-side switch tends generate quite some heat capacitor ESR. ripple current ratings capacitor catalogs usually specified under 105°C. case desktop applications, those ratings seem somewhat conservative. rule-of-thumb increase 105°C rating desktop applications. input ripple current value determined following equation:
inversely proportional square total number capacitors, which means power loss each capacitor quickly drops when number capacitors increases. Linear Section applications where there load transient requirement such that GTL+ supply, capacitors should considered. Make sure that total multiplied maximum load current smaller than half output voltage regulation window. output voltage regulation window should exclude tolerance LM2637. example, 3.3V 1.5V, design, initial regulation window Assume tolerance LM2637 plus margin then effective window left Therefore should less than Sanyo 6MV1200DX sufficient. applications where load static control bandwidth stability issue, refer guidelines control loop compensation section. Inductor Selection Output Inductor. size output inductor determined number parameters. Basically larger inductor, smaller output ripple voltage, slower converter's response speed during load transient. other hand, smaller inductor requires higher switching frequency maintain same level output ripple, probably results lossier converter, less inertia responding load transient. case core power supply, fast recovery load voltage from transient window back steady state window important. That limits highest inductance value that used. lowest inductance value limited highest switching frequency that practically employed. switching frequency increases, switching loss FETs tends increase, resulting lower overall efficiency larger heat sinks. good switching frequency probably frequency under which conduction loss much higher than switching loss because cost directly related rDS_ON. inductor size determined following equation:
(14) power loss each input capacitor
(16) where Vo_rip peak-peak output ripple voltage, switching frequency. commonly used rDS_ON FET's, reasonable switching frequency kHz. Assume peak-peak output ripple voltage total output capacitor input voltage output voltage 2.8V, then inductance value according above equation will highest slew rate inductor current when load changes from load full load determined follows:
(15) case Pentium power supply, maximum output current around 14A. Under worst case when duty cycle 50%, maximum input capacitor ripple current half output current, i.e., Therefore three Sanyo 16MV820GX capacitors necessary under room temperature (they rated 1.45A 105°C). maximum those capacitors maximum power loss each them less than (7A)2 m/32 0.24W. Note that power loss each capacitor
(17) where DMAX maximum allowed duty cycle, which around 0.95 LM2637. load transient from 14A, highest current slew rate inductor, according above equation, 0.97 A/µs, therefore shortest possible total recovery time 14A/(0.97 A/µs) 14.5 Notice that output voltage starts recover whenever inductor starts supply current. highest slew rate inductor current when load changes from full load load determined from same equation DMIN instead DMAX.
www.national.com
LM2637
Applications Information
(Continued)
Since DMIN LM2637 slew rate therefore -1.4 A/µs. approximate total recovery time will 14A/(1.4 A/µs) Often times power supply designer have custom-made inductor best performance/price ratio. Micrometals offers cost effective iron powder cores that widely adopted motherboard supplies OEMs. important rule when designing iron power inductor never saturate core else will exhibit extremely poor dynamic performance. Useful inductor design tools also found their page, www.micrometals.com. user LM2637 also contact National custommade inductor. Alternatively designer open core inductor, which lower cost ease mass production. However, open magnetic field cause some noise problems nearby circuitry cause issues. However, negative reports have been heard far. Coilcraft (www.coilcraft.com) offers wide range open core inductors. Custom-made parts also possible. Other than cost, advantages open core inductors less board space superior dynamic performance. Input Inductor. input inductor limiting input current slew rate during load transient normal operation. case that aluminum electrolytic capacitors used input capacitor bank, input capacitor voltage change capacitor charging/discharging usually negligible first dominant factor determining amount capacitor voltage undershoot/overshoot during fast load transient. worst case when load changes between load full load. Under that condition input inductor sees highest voltage change across input capacitors. Assume input capacitor bank consists three 16MV820GX, i.e., total Whenever there sudden load change, change input current initially supported input capacitor bank instead input inductor. fast load-swing between 14A, voltage change seen input inductor ramp from vice versa, whereas this situation just operating under heaviest load. following equation determine minimum inductance value:
tion steady state output voltage steady state regulation window with respect load current level that output voltage more headroom load transient response. This needs load current information. There least simple ways implement this idea with LM2637. utilize output inductor resistance, Figure average voltage across output inductor actually that across resistance, which proportional load current. Since switching node voltage toggles between input voltage ground switching frequency, impossible choose node feedback point, otherwise dynamic performance will suffer system have noise problems. Using pass filter network around inductor, such shown figure, seems good idea. feedback point node
10084827
FIGURE Dynamic Voltage Positioning Utilizing Output Inductor Resistance Since switching frequency impedance much less than toggling voltage node will mainly drop across resistor node will much quieter than However, average still majority average, because ratio resistor divider. steady state VCORE, where inductor resistance. load, output voltage equal full load, output voltage lower than further utilize steady state regulation window, resistor connected between ground increase no-load output voltage close upper limit window.
(18) where (di/dt)max maximum allowable input current slew rate, which A/µs case Pentium power supply equal maximum load current times input capacitor ESR. input inductor size, according above equation, should Dynamic Positioning Load Voltage following just quick overview technique called dynamic voltage positioning. detailed explanation examples please refer application note Using Dynamic Voltage Positioning Technique Reduce Cost Output Capacitors Advanced Microprocessor Power Supplies. associated spreadsheet also available automated design. Since typical core voltage's steady state regulation window fairly large, good idea dynamically posi15 www.national.com
LM2637
Applications Information
(Continued)
Layout Considerations There several points consider. copper ground plane tight load regulation desired. case dynamic voltage positioning, this concern because loose load regulation desired anyway. However, forget take into consideration voltage drop caused ground plane when calculating dynamic voltage positioning parameters. keep gate drive traces short. However, make them short else LM2637 placed close FETs heated them. same reason, wide traces, traces should enough. When employing dynamic voltage positioning, place feedback point connector pins have tight load regulation. embedded power supply, place feedback point Slot connector wherever closest MPU. Start component placement with power devices such FETs, inductors. place LM2637 directly underneath FETs other side PCB) when surface mount FETs used. Also avoid staying close output inductor, especially when using open core inductor. possible, keep capacitors some distance away from inductors heatsinks that capacitors will have better thermal environment. Keep mind that input capacitors usually much hotter than output capacitors. When implementing dynamic voltage positioning through trace, keep mind that trace heat source avoid placing trace directly underneath LM2637. place ceramic capacitor close possible pin. core supply, place output bulk capacitors fairly close lower inductance.
10084828
FIGURE Dynamic Voltage Positioning Using Stand-Alone Resistor possible drawback scheme Figure slow transient recovery speed. Since resistor capacitor have large time constant, settling node steady state value during load transient take milliseconds. Depends interaction between compensation network capacitor, VCORE take different routes reach steady state value. This undesired when load transient happens more than 1000 times second. Reducing time constant will result more fluctuating less effective pass filter. Fine tuning parameters generate acceptable design. Another implement dynamic voltage ppsitioning through separate resistor, such resistor Figure above. advantage this implementation over previous much faster recovery speed VCORE from transient level steady state level. finetuned compensation network will give good response shown Figure disadvantage additional power loss. total power loss 0.78W load current. cost resistor minimized using etch resistor.
10084829
FIGURE Load Transient Response with DVP: 14A, Droop Resistor
www.national.com
LM2637 Motherboard Power Supply Solution with 5-Bit Programmable Switching Controller Linear Regulator Controllers
Physical Dimensions
unless otherwise noted
inches (millimeters)
24-Lead Small Outline Package Order Number LM2637M Package Number M24B
National does assume responsibility circuitry described, circuit patent licenses implied National reserves right time without notice change said circuitry specifications. most current product information visit www.national.com. LIFE SUPPORT POLICY NATIONAL'S PRODUCTS AUTHORIZED CRITICAL COMPONENTS LIFE SUPPORT DEVICES SYSTEMS WITHOUT EXPRESS WRITTEN APPROVAL PRESIDENT GENERAL COUNSEL NATIONAL SEMICONDUCTOR CORPORATION. used herein: Life support devices systems devices systems which, intended surgical implant into body, support sustain life, whose failure perform when properly used accordance with instructions provided labeling, reasonably expected result significant injury user. BANNED SUBSTANCE COMPLIANCE National Semiconductor certifies that products packing materials meet provisions Customer Products Stewardship Specification (CSP-9-111C2) Banned Substances Materials Interest Specification (CSP-9-111S2) contain ``Banned Substances'' defined CSP-9-111S2.
National Semiconductor Americas Customer Support Center Email: new.feedback@nsc.com Tel: 1-800-272-9959 www.national.com National Semiconductor Europe Customer Support Center Fax: 180-530 Email: europe.support@nsc.com Deutsch Tel: 9508 6208 English Tel: 2171 Tel: 8790 National Semiconductor Asia Pacific Customer Support Center Email: ap.support@nsc.com National Semiconductor Japan Customer Support Center Fax: 81-3-5639-7507 Email: jpn.feedback@nsc.com Tel: 81-3-5639-7560
critical component component life support device system whose failure perform reasonably expected cause failure life support device system, affect safety effectiveness.

LM2637 Motherboard Power Supply Solution with 5-Bit Programmable Switc

Top Searches for this datasheet

Other recent searches