LT1931/LT1931A
ABSOLUTE AXI U RATI GS(Note 1)UUWPACKAGE/ORDER I FOR ATIOTOP VIEWSW 1GND 2NFB 34 SHDN5 VINVIN Voltage..............................................................16VSW Voltage................................................–0.4V to 36VNFB Voltage.............................................................–2VCurrent Into NFB Pin............................................ ±1mASHDN Voltage..........................................................16VMaximum Junction Temperature..........................125°COperating Temperature Range (Note 2)..–40°C to 85°CStorage Temperature Range.................–65°C to 150°CLead Temperature (Soldering, 10 sec)..................300°CELECTRICAL CHARACTERISTICSThe q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.VIN = 3V, VSHDN = VIN, unless otherwise noted. (Note 2)PARAMETERMinimum Operating VoltageMaximum Operating VoltageFeedback VoltageqCONDITIONSNFB Pin Bias CurrentQuiescent CurrentQuiescent Current in ShutdownReference Line RegulationSwitching FrequencyVNFB = –1.255VVSHDN = 2.4V, Not SwitchingVSHDN = 0V, VIN = 3V2.6V ≤ VIN ≤ 16VMaximum Duty CycleSwitch Current LimitSwitch VCESATSwitch Leakage CurrentSHDN Input Voltage, HighSHDN Input Voltage, LowSHDN Pin Bias CurrentVSHDN = 3VVSHDN = 0V(Note 3)ISW = 1AVSW = 5VNote 1: Absolute Maximum Ratings are those values beyond which the lifeof a device may be impaired.Note 2: The LT1931E/LT1931AE are guaranteed to meet performancespecifications from 0°C to 70°C. Specifications over the –40°C to 85°C2
UWWWORDER PARTNUMBERLT1931ES5LT1931AES5S5 PART MARKINGLTRALTSPS5 PACKAGE5-LEAD PLASTIC SOT-23TJMAX = 125°C, θJA = 256°C/WConsult factory for parts specified with wider operating temperature ranges.MINLT1931TYP2.45MAX2.616LT1931AMINTYPMAX2.452.616–1.275–1.255–1.235–1.280–1.23085.80.010.011.81.67512.2821.24000.012.42.560010.5350700.116810.052.62.9UNITSVVVVµAmAµA%/VMHzMHz%AmVµAVVµAµA–1.275–1.255–1.235–1.280–1.23044.20.010.0110.858411.2901.24000.012.40.5160320.1260018610.051.41.6qqqoperating temperature range are assured by design, characterization andcorrelation with statistical process controls.Note 3: Current limit guaranteed by design and/or correlation to static test.LT1931/LT1931ATYPICAL PERFOR A CE CHARACTERISTICS Quiescent Current7.06.5QUIESCENT CURRENT (mA)NOT SWITCHINGLT1931ASHDN PIN CURRENT (µA)FEEDBACK VOLTAGE (V)6.05.55.04.54.03.53.0–50–25LT19315025TEMPERATURE (°C)0Current Limit1.61.4CURRENT LIMIT (A)1.2VCESAT (V)1.00.80.60.40.2010203040506070DUTY CYCLE (%)80900.300.250.200.150.100.05000.20.40.60.8SWITCH CURRENT (A)1.01.2FREQUENCY (MHz)PI FU CTIO SSW (Pin 1): Switch Pin. Connect inductor/diode here.Minimize trace area at this pin to keep EMI down.GND (Pin 2): Ground. Tie directly to local ground plane.NFB (Pin 3): Feedback Pin. Reference voltage is –1.255V.Connect resistive divider tap here. Minimize trace area.The NFB bias current flows out of the pin. Set R1 and R2according to:ForLT1931:R1=|VOUT|–1.2551.255+4•10–6R2ForLT1931A:R1=|VOUT|–1.2551.255+8•10–6R2UW751931 G01Feedback Pin Voltage–1.28–1.27–1.26–1.25–1.24–1.23–1.22–509080706050403020100Shutdown Pin CurrentTA = 25°CLT1931ALT1931100–2502550TEMPERATURE (°C)751001931 G02–1001342SHDN PIN VOLTAGE (V)561931 G03Switch Saturation VoltageTA = 25°C0.450.400.35TA = 25°COscillator Frequency2.52.32.11.91.71.51.31.10.90.70.5–50–2525500TEMPERATURE (°C)751001931 G06LT1931ALT19311931 G041931 G05UUU()SHDN (Pin 4): Shutdown Pin. Tie to 2.4V or more to enabledevice. Ground to shut down.VIN (Pin 5): Input Supply Pin. Must be locally bypassed.()3
LT1931/LT1931A
BLOCK DIAGRA VIN5R580kR680kVIN1SWA1gmRCCC0.01Ω1.2MHzOSCILLATORSHDN4SHUTDOWN2GND1931 BDQ1VOUTR1(EXTERNAL)NFBR2(EXTERNAL)CPL(OPTIONAL)UOPERATIOThe LT1931 uses a constant frequency, current modecontrol scheme to provide excellent line and load regula-tion. Operation can be best understood by referring to theBlock Diagram in Figure 2. At the start of each oscillatorcycle, the SR latch is set, turning on the power switch Q3.A voltage proportional to the switch current is added to astabilizing ramp and the resulting sum is fed into thepositive terminal of the PWM comparator A2. When thisvoltage exceeds the level at the negative input of A2, the SRlatch is reset, turning off the power switch. The level at thenegative input of A2 is set by the error amplifier (gm) andis simply an amplified version of the difference betweenthe feedback voltage and the reference voltage of –1.255V.In this manner, the error amplifier sets the correct peak4
W+–Q2x10R330kR4150k3NFB–RAMPGENERATORCOMPARATORLATCHSDRIVERQQ3Σ+A2RFigure 2–+current level to keep the output in regulation. If the erroramplifier’s output increases, more current is taken fromthe output; if it decreases, less current is taken. Onefunction not shown in Figure 2 is the current limit. Theswitch current is constantly monitored and not allowed toexceed the nominal value of 1.2A. If the switch currentreaches 1.2A, the SR latch is reset regardless of the stateof comparator A2. This current limit protects the powerswitch as well as various external components connectedto the LT1931.The Block Diagram for the LT1931A is identical except thatthe oscillator is 2.2MHz and resistors R3 to R6 are one-halfthe LT1931 values.LT1931/LT1931A
APPLICATIO S I FOR ATIOLT1931A AND LT1931 DIFFERENCES:Switching FrequencyThe key difference between the LT1931A and LT1931 isthe faster switching frequency of the LT1931A. At 2.2MHz,the LT1931A switches at nearly twice the rate of theLT1931. Care must be taken in deciding which part to use.The high switching frequency of the LT1931A allowssmaller cheaper inductors and capacitors to be used in agiven application, but with a slight decrease in efficiencyand maximum output current when compared to theLT1931. Generally, if efficiency and maximum outputcurrent are critical, the LT1931 should be used. If applica-tion size and cost are more important, the LT1931A will bethe better choice. In many applications, tiny inexpensivechip inductors can be used with the LT1931A, reducingsolution cost.Duty CycleThe maximum duty cycle (DC) of the LT1931A is 75%compared to 84% for the LT1931. The duty cycle for agiven application using the dual inductor inverting topol-ogy is given by:DC=
|VOUT||VIN|+|VOUT|
For a 5V to –5V application, the DC is 50% indicating thatthe LT1931A can be used. A 5V to –16V application has aDC of 76.2% making the LT1931 the right choice. TheLT1931A can still be used in applications where the DC, ascalculated above, is above 75%. However, the part mustbe operated in the discontinuous conduction mode so thatthe actual duty cycle is reduced.INDUCTOR SELECTIONSeveral inductors that work well with the LT1931 are listedin Table 1 and those for the LT1931A are listed in Table 2.Besides these, there are many other inductors that can beused. Consult each manufacturer for detailed informationand for their entire selection of related parts. Ferrite coreinductors should be used to obtain the best efficiency, asUcore losses at frequencies above 1MHz are much lower forferrite cores than for powdered-iron units. When usingcoupled inductors, choose one that can handle at least 1Aof current without saturating, and ensure that the inductorhas a low DCR (copper-wire resistance) to minimize I2Rpower losses. If using uncoupled inductors, each inductorneed only handle one-half of the total switch current sothat 0.5A per inductor is sufficient. A 4.7µH to 15µHcoupled inductor or a 15µH to 22µH uncoupled inductorwill usually be the best choice for most LT1931 designs.For the LT1931A, a 2.2µH to 4.7µH coupled inductor or a3.3µH to 10µH uncoupled inductor will usually suffice. Incertain applications such as the “Charge Pump” invertingDC/DC converter, only a single inductor is used. In thiscase, the inductor must carry the entire 1A switch current.Table 1. Recommended Inductors—LT1931PARTCLS62-100CR43-150CR43-220CTX10-1CTX15-1LQH3C100K24LQH4C150K04L(µH)10152210151015Size(L × W × H) mm6.8 × 6.6 × 2.54.5 × 4.0 × 3.28.9 × 11.4 × 4.2VENDORSumida(847) 956-0666www.sumida.comCoiltronics(407) 241-7876www. coiltronics.comMurata(404) 436-1300www.murata.com3.2 × 2.5 × 2.0WUUTable 2. Recommended Inductors—LT1931APARTELJPC3R3MFELJPC4R7MFCLQ4D10-4R71CLQ4D10-6R82LB201R7MLB20163R3MLQH3C4R7K24LQH4C100K24L(µH)3.34.74.76.84.73.34.710Size(L × W × H) mm2.5 × 2.0 × 1.6VENDORPanasonic(408) 945-5660www.panasonic.comSumida(847) 956-0666www.sumida.comTaiyo Yuden(408) 573-4150www.t-yuden.comMurata(404) 436-1300www.murata.com7.6 × 4.8 × 1.82.0 × 1.6 × 1.63.2 × 2.5 × 2.01Use drawing #5382-T0392Use drawing #5382-T0415
LT1931/LT1931A
APPLICATIO S I FOR ATIOThe inductors shown in Table 2 for use with the LT1931Awere chosen for their small size. For better efficiency, usesimilar valued inductors with a larger volume. For in-stance, the Sumida CR43 series, in values ranging from3.3µH to 10µH, will give a LT1931A application a fewpercentage points increase in efficiency.CAPACITOR SELECTIONLow ESR (equivalent series resistance) capacitors shouldbe used at the output to minimize the output ripple voltage.Multilayer ceramic capacitors are an excellent choice, asthey have an extremely low ESR and are available in verysmall packages. X5R dielectrics are preferred, followed byX7R, as these materials retain their capacitance over widevoltage and temperature ranges. A 10µF to 22µF outputcapacitor is sufficient for most LT1931 applications whilea 4.7µF to 10µF capacitor will suffice for the LT1931A.Solid tantalum or OS-CON capacitors can be used, butthey will occupy more board area than a ceramic and willhave a higher ESR. Always use a capacitor with a sufficientvoltage rating.Ceramic capacitors also make a good choice for the inputdecoupling capacitor, which should be placed as close aspossible to the LT1931/LT1931A. A 1µF to 4.7µF inputcapacitor is sufficient for most applications. Table 3 showsa list of several ceramic capacitor manufacturers. Consultthe manufacturers for detailed information on their entireselection of ceramic parts.Table 3. Ceramic Capacitor ManufacturersTaiyo YudenAVXMurata(408) 573-4150www.t-yuden.com(803) 448-9411www.avxcorp.com(714) 852-2001www.murata.comThe decision to use either low ESR (ceramic) capacitors orthe higher ESR (tantalum or OS-CON) capacitors caneffect the stability of the overall system. The ESR of anycapacitor, along with the capacitance itself, contributes a6
Uzero to the system. For the tantalum and OS-CON capaci-tors, this zero is located at a lower frequency due to thehigher value of the ESR, while the zero of a ceramiccapacitor is at a much higher frequency and can generallybe ignored.A phase lead zero can be intentionally introduced byplacing a capacitor (C4) in parallel with the resistor (R1)between VOUT and VNFB as shown in Figure 1. Thefrequency of the zero is determined by the followingequation.ƒZ=12π•R1•C4By choosing the appropriate values for the resistor andcapacitor, the zero frequency can be designed to improvethe phase margin of the overall converter. The typicaltarget value for the zero frequency is between 20kHz to60kHz. Figure 3 shows the transient response of theinverting converter from Figure 1 without the phase leadcapacitor C4. The phase margin is reduced as evidencedby more ringing in both the output voltage and inductorcurrent. A 220pF capacitor for C4 results in better phasemargin, which is revealed in Figure 4 as a more dampedresponse and less overshoot. Figure 5 shows the transientresponse when a 22µF tantalum capacitor with no phaselead capacitor is used on the output. The higher outputvoltage ripple is revealed in the upper waveform as athicker line. The transient response is adequate whichimplies that the ESR zero is improving the phase margin.VOUT20mV/DIVAC COUPLEDIL1A + IL1B0.5A/DIVAC COUPLEDLOAD200mACURRENT100mA100µs/DIV1931 F03WUUFigure 3. Transient Response of Inverting ConverterWithout Phase Lead CapacitorLT1931/LT1931AAPPLICATIO S I FOR ATIOVOUT20mV/DIVAC COUPLEDIL1A + IL1B0.5A/DIVAC COUPLEDLOAD200mACURRENT100mA100µs/DIV1931 F04Figure 4. Transient Response of Inverting Converterwith 220pF Phase Lead CapacitorVOUT0.1V/DIVAC COUPLEDIL1A + IL1B0.5A/DIVAC COUPLEDLOAD200mACURRENT100mA50µs/DIV1931 F05Figure 5. Transient Response of Inverting Converter with 22µFTantalum Output Capacitor and No Phase Lead CapacitorSTART-UP/SOFT-STARTFor most LT1931/LT1931A applications, the start-up in-rush current can be high. This is an inherent feature ofswitching regulators in general since the feedback loop issaturated due to VOUT being far from its final value. TheCURRENTPROBEL1A10µHVIN5V+RSS15kVSSD21N4148CSS33nF/68nFVOUTC14.7µFC1: TAIYO YUDEN X5R JMK212BJ475MGC2: TAIYO YUDEN X5R LMK212BJ105MGC3: TAIYO YUDEN XR5 JMK325BJ226MMD1: ON SEMICONDUCTOR MBR0520L1: SUMIDA CLS62-100Figure 7. RSS and CSS at SHDN Pin Provide Soft-Start to LT1931 Inverting ConverterUVOUT2V/DIVIIN0.5A/DIVAC COUPLEDVSHDN5V0V500µs/DIV1931 F06WUUFigure 6. Start-Up Waveforms for 5V to –5V Application(Figure 1). No Soft-Start Circuit. VOUT Reaches –5V in500µs; Input Current Peaks at 800mAregulator tries to charge up the output capacitor as quicklyas possible, which results in a large inrush current. Fig-ure␣6 shows a typical oscillograph of the start-up wave-form for the application of Figure 1 starting into a load of33Ω. The lower waveform shows SHDN being pulsedfrom 0V to 5V. The middle waveform shows the inputcurrent, which reaches as high as 0.8A. The total timerequired for the output to reach its final value is approxi-mately 500µs. For some applications, this initial inrushcurrent may not be acceptable. If a longer start-up time isacceptable, a soft-start circuit consisting of RSS and CSS,as shown in Figure 7, can be used to limit inrush currentto a lower value. Figure 8 shows the relevant waveformswith RSS = 15k and CSS = 33nF. Input current, measuredat VIN, is limited to a peak value of 0.5A as the time requiredto reach final value increases to 1ms. In Figure 9, CSS isC21µFL1B10µHVINLT1931SHDNGNDSWR129.4kNFBR210kC4220pFD1VOUT–5VC322µF1931 F077
LT1931/LT1931A
APPLICATIO S I FOR ATIOVOUT2V/DIVIIN0.5A/DIVAC COUPLEDVSS5V0V200µs/DIV1931 F08Figure 8. RSS = 15k, CSS = 33nF; VOUT Reaches –5V in 1ms;Input Current Peaks at 500mAVOUT2V/DIVIIN0.5A/DIVAC COUPLEDVSS5V0V500µs/DIV1931 F09Figure 9. RSS = 15k, CSS = 68nF; VOUT Reaches –5V in 1.6ms;Input Current Peaks at 350mAGNDFigure 10. Suggested Component Placement.Note Cut in Ground Copper at D1’s Cathode8
+increased to 68nF, resulting in a lower peak input currentof 350mA with a VOUT ramp time of 1.6ms. CSS or RSS canbe increased further for an even slower ramp, if desired.Diode D2 serves to quickly discharge CSS when VSS isdriven low to shut down the device. D2 can be omitted,resulting in a “soft-stop” slow discharge of the outputcapacitor.UDIODE SELECTIONA Schottky diode is recommended for use with the LT1931/LT1931A. The Motorola MBR0520 is a very good choice.Where the input to output voltage differential exceeds 20V,use the MBR0530 (a 30V diode). These diodes are rated tohandle an average forward current of 0.5 A. In applicationswhere the average forward current of the diode exceeds0.5A, a Microsemi UPS5817 rated at 1A is recommended.LAYOUT HINTSThe high-speed operation of the LT1931/LT1931A de-mands careful attention to board layout. You will not getadvertised performance with careless layout. Figure 10shows the recommended component placement. Theground cut at the cathode of D1 is essential for low noiseoperation.L1B–VOUTD1C3123R2R11931 F10WUUL1AC1+VINC254SHUTDOWNLT1931/LT1931ATYPICAL APPLICATIO S5V to –12V Inverting ConverterL1A10µHC21µFL1B10µH1009590EFFICIENCY (%)VIN5VVINSHDNC14.7µFSWLT1931NFBGNDR210kR184.5kC1: TAIYO YUDEN X5R JMK212BJ475MGC2: TAIYO YUDEN X5R TMK316BJ105MLC3: TAIYO YUDEN X5R EMK325BJ106MMD1: ON SEMICONDUCTOR MBR0520L1: SUMIDA CLS62-100VIN5VVINSHDNC14.7µFC1: TAIYO YUDEN X5R JMK212BJ475MGC2: TAIYO YUDEN X5R LMK212BJ105MGC3: TAIYO YUDEN X5R JMK212BJ226MMD1: ON SEMICONDUCTOR MBR0520L1, L2: MURATA LQH3C100K042.2MHz, 5V to –5V Inverting ConverterL14.7µHC21µFL24.7µH8075EFFICIENCY (%)VIN5VVINSWLT1931ANFBGNDR210kR128.7kC4180pFSHDNC14.7µFC1: TAIYO YUDEN X5R JMK212BJ475MGC2: TAIYO YUDEN X5R LMK212BJ105MGC3: TAIYO YUDEN X5R JMK212BJ475MGD1: ON SEMICONDUCTOR MBR0520L1, L2: MURATA LQH3C4R7M24UEfficiencyD1VOUT–12V150mAC310µF858075706560551931 TA02500257510050LOAD CURRENT (mA)1251501931 TA035V to –5V Inverting Converter Using Uncoupled InductorsL110µHC21µFL210µHSWLT1931NFBGNDR210kR129.4k220pFD1VOUT–5V300mAC322µF1931 TA04EfficiencyD1VOUT–5V300mAC34.7µF706560551931 TA05a50050100150200250LOAD CURRENT (mA)3003501931 TA05b9
LT1931/LT1931A
TYPICAL APPLICATIO S2.2MHz, 5V to –5V Converter Uses Tiny Chip InductorsL13.3µHC21µFL23.3µH8075VIN5VVINSHDNC12.2µFLT1931ANFBGNDR128.7kR210kC468pFEFFICIENCY (%)SWC1: TAIYO YUDEN X5R JMK212BJ225MGC2: TAIYO YUDEN X5R LMK212BJ105MGC3: TAIYO YUDEN X5R JMK212BJ475MGD1: ON SEMICONDUCTOR MBR0520L1, L2: PANASONIC ELJPC3R3MFSLIC Power Supply with –33V and –68V Outputs, Uses Soft-StartL122µHC14.7µF16VRSS15kVSSVIN12V10
UEfficiencyD1VOUT–5V200mAC34.7µF706560551931 TA06a50050100150200LOAD CURRENT (mA)2501931 TA06bR11ΩC21µF35VD13NFB2C44.7µF35VVINLT1931SHDNGNDCSS68nFSWCOMR21kR325.5kC61000pFR42.7kC31µF35V31VOUT1–33V100mA*D221C54.7µF35V1931 TA08*TOTAL OUTPUT POWER NOT TO EXCEED 3.3WC1 TO C5: X5R OR X7RD1, D2: BAV99 OR EQUIVALENTL1: SUMIDA CR43-220VOUT2–66V48mA*LT1931/LT1931ATYPICAL APPLICATIO SSLIC Power Supply with –21.6V and –65V Outputs, Uses Soft-StartL110µHC14.7µF16VR11ΩC21µF35VD1LT1931SHDNGNDR21kCSS68nFR316.2kC81000pFR42.7kC31µF35V3*TOTAL OUTPUT POWER NOT TO EXCEED 1.3WC1 TO C7: X5R OR X7RD1, D2: BAV99 OR EQUIVALENTL1: SUMIDA CR43-100NFB132C54.7µF25VCOMVIN5VRSS15kVSSPACKAGE DESCRIPTIODATUM ‘A’L.09 – .20(.004 – .008)(NOTE 2)Information furnished by Linear Technology Corporation is believed to be accurate and reliable.However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.UUVINSWVOUT1–21.6V48mA*D221D3C.7µF25VC41µF35V321C74.7µF25V1931 TA09VOUT2–65V20mA*S5 Package5-Lead Plastic SOT-23(LTC DWG # 05-08-1633)(LTC DWG # 05-08-1635)11
LT1931/LT1931A
TYPICAL APPLICATIO2.2MHz, 12V to –5V Converter Uses Low Profile Coupled InductorL1A4.7µHC20.1µFL1B4.7µHVIN12VVINEFFICIENCY (%)RELATED PARTSPART NUMBERLT1307LT1316LT1317LT1610LT1611LT1613LT1615LT1617LT1930/LT1930ADESCRIPTIONSingle Cell Micropower 600kHz PWM DC/DC ConverterBurst ModeTM Operation DC/DC with Programmable Current Limit2-Cell Micropower DC/DC with Low-Battery DetectorSingle Cell Micropower DC/DC ConverterInverting 1.4MHz Switching Regulator in 5-Lead ThinSOT1.4MHz Switching Regulator in 5-Lead ThinSOTMicropower Constant Off-Time DC/DC Converter in 5-Lead ThinSOTMicropower Inverting DC/DC Converter in 5-Lead ThinSOT1.2MHz/2.2MHz, 1A Switching Regulators in 5-Lead ThinSOTCOMMENTS3.3V at 75mA from One Cell, MSOP Package1.5V Minimum, Precise Control of Peak Current Limit3.3V at 200mA from Two Cells, 600kHz Fixed Frequency3V at 30mA from 1V, 1.7MHz Fixed Frequency–5V at 150mA from 5V Input. Tiny SOT-23 Package5V at 200mA from 3.3V Input. Tiny SOT-23 Package20V at 12mA from 2.5V. Tiny SOT-23 Package–15V at 12mA from 2.5V. Tiny SOT-23 Package5V at 450mA from 3.3V Input. Tiny SOT-23 PackageBurst Mode operation is a trademark of Linear Technology Corporation.12
Linear Technology Corporation1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 q FAX: (408) 434-0507 q www.linear-tech.comUSWLT1931ANFBGNDR210k D1R128.7kSHDNC12.2µFVOUT–5V450mAC34.7µFC1: TAIYO YUDEN Y5V EMK212F225ZGC2: 0.1µF 25V X5RC3: TAIYO YUDEN X5R JMK212BJ475MGD1: ON SEMICONDUCTOR MBR0520L1: SUMIDA CLQ4D10-4R7 DRAWING #5382-T0391931 TA07aEfficiency807570656055500100200300400LOAD CURRENT (mA)5001931 TA07b 1931f LT/TP 0601 2K • PRINTED IN USA
© LINEAR TECHNOLOGY CORPORATION 2000
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