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## LT1959 Datasheet(PDF) 18 Page - Linear Technology

 Part No. LT1959 Description 4.5A, 500kHz Step-Down Switching Regulator Download 24 Pages Scroll/Zoom 100% Maker LINER [Linear Technology] Homepage http://www.linear.com Logo

## LT1959 Datasheet(HTML) 18 Page - Linear Technology

 18 / 24 page 18LT1959APPLICATIONS INFORMATIONformulas show how to calculate each of these losses.These formulas assume continuous mode operation, sothey should not be used for calculating efficiency at lightload currents.Switch loss:PRIVVns IVfSWSW OUTOUTINOUTIN=() ( )+()( )( )224Boost current loss:PVIVBOOSTOUTOUTIN=()250/Quiescent current loss:PVVVVQINOUTOUTIN=()+ ()+()0 0010 0050 0022...RSW = Switch resistance (≈ 0.07)24ns = Equivalent switch current/voltage overlap timef = Switch frequencyExample: with VIN = 10V, VOUT = 5V and IOUT = 3A:PWPWPWSWBOOSTQ= ()() ()+  ()( )=+== () ()==()+ ()+() ( ) =−007 351024 103 10 500 100 320 360 6853 501001510 0 0015 0 00550 0021000429322.••.../.....Total power dissipation is 0.68 + 0.15 + 0.04 = 0.87W.Thermal resistance for LT1959 package is influenced bythe presence of internal or backside planes. With a fullplane under the SO package, thermal resistance will beabout 80°C/W. No plane will increase resistance to about120°C/W. To calculate die temperature, use the properthermal resistance number for the desired package andadd in worst-case ambient temperature:TJ = TA + θJA (PTOT)With the SO-8 package (θJA = 80°C/W), at an ambienttemperature of 50°C,TJ = 50 + 80 (0.87) = 120°CDie temperature is highest at low input voltage, so uselowest continuous input operating voltage for thermalcalculations.FREQUENCY COMPENSATIONLoop frequency compensation of switching regulatorscan be a rather complicated problem because the reactivecomponents used to achieve high efficiency alsointroduce multiple poles into the feedback loop. Theinductor and output capacitor on a conventional step-down converter actually form a resonant tank circuit thatcan exhibit peaking and a rapid 180° phase shift at theresonant frequency. By contrast, the LT1959 uses a “cur-rent mode” architecture to help alleviate phase shift cre-ated by the inductor. The basic connections are shown inFigure 9. Figure 10 shows a Bode plot of the phase and gainof the power section of the LT1959, measured from the VCpin to the output. Gain is set by the 5.3A/V transconduc-tance of the LT1959 power section and the effectivecomplex impedance from output to ground. Gain rolls offsmoothly above the 600Hz pole frequency set by the100µF output capacitor. Phase drop is limited to about70°. Phase recovers and gain levels off at the zero fre-quency (≈16kHz) set by capacitor ESR (0.1Ω).Figure 9. Model for Loop Response1.21VVSWVCLT1959GND1959 F09R1OUTPUTESRCFCCRCERRORAMPLIFIERFBR2C1CURRENT MODEPOWER STAGEgm = 5.3A/V+