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LT1935 Datasheet(PDF) 11 Page - Linear Technology |
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LT1935 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 28 page  LT3580 11 3580fg APPLICATIONS INFORMATION Compensation—Theory Like all other current mode switching regulators, the LT3580 needs to be compensated for stable and efficient operation. Two feedback loops are used in the LT3580— a fast current loop which does not require compensation, and a slower voltage loop which does. Standard bode plot analysis can be used to understand and adjust the voltage feedback loop. As with any feedback loop, identifying the gain and phase contribution of the various elements in the loop is critical. Figure 4 shows the key equivalent elements of a boost converter. Because of the fast current control loop, the power stage of the IC, inductor and diode have been replaced by a combination of the equivalent transconductance amplifier gmp and the current controlled current source (which converts IVIN to ηVIN/VOUT • IVIN). gmp acts as a current source where the peak input current, IVIN, is proportional to the VC voltage. η is the efficiency of the switching regulator, and is typically about 88%. Note that the maximum output currents of gmp and gma are finite. The limits for gmp are in the Electrical Characteristics section (switch current limit), and gma is nominally limited to about ±12μA. From Figure 4, the DC gain, poles and zeros can be cal- culated as follows: Output Pole: P1 = 2 2• π •RL •COUT Error Amp Pole: P2 = 1 2• π •R O + RC ⎡⎣ ⎤⎦ •CC Error Amp Zero: Z1 = 1 2• π •R C •CC DC Gain: (Breaking Loop at FB Pin) ADC = AOL(0) = ∂V C ∂VFB • ∂IVIN ∂V C • ∂V OUT ∂IVIN • ∂VFB ∂V OUT = gma •R0 ( )•g mp • η • VIN VOUT • RL 2 ⎛ ⎝⎜ ⎞ ⎠⎟ • 0.5R2 R1 + 0.5R2 ESR Zero: Z2 = 1 2• π •R ESR •COUT RHP Zero: Z3 = VIN 2 •R L 2• π •V OUT 2 •L High Frequency Pole: P3 > fS 3 Phase Lead Zero: Z4 = 1 2• π •R1•CPL Phase Lead Pole: P4 = 1 2• π • R1• R2 2 R1 + R2 2 •CPL Error Amp Filter Pole: P5 = 1 2• π • RC •RO RC+RO •CF ,CF < CC 10 The current mode zero (Z3) is a right-half plane zero which can be an issue in feedback control design, but is manageable with proper external component selection. Figure 4. Boost Converter Equivalent Model – + gma RC RO R2 R2 CC: COMPENSATION CAPACITOR COUT: OUTPUT CAPACITOR CPL: PHASE LEAD CAPACITOR CF: HIGH FREQUENCY FILTER CAPACITOR gma: TRANSCONDUCTANCE AMPLIFIER INSIDE IC gmp: POWER STAGE TRANSCONDUCTANCE AMPLIFIER RC: COMPENSATION RESISTOR RL: OUTPUT RESISTANCE DEFINED AS VOUT DIVIDED BY ILOAD(MAX) RO: OUTPUT RESISTANCE OF gma R1, R2: FEEDBACK RESISTOR DIVIDER NETWORK RESR: OUTPUT CAPACITOR ESR 3580 F04 R1 FB COUT CPL RL RESR VOUT IVIN VC CC CF gmp 1.215V REFERENCE •VIN V OUT •IVIN |
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