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LT1737CS データシート(PDF) 11 Page - Linear Technology |
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LT1737CS データシート(HTML) 11 Page - Linear Technology |
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11 / 28 page  11 LT1737 1737f transformer secondary and output capacitor. This has been represented previously by the expression “ISEC • ESR.” However, it is generally more useful to convert this expression to an effective output impedance. Because the secondary current only flows during the off portion of the duty cycle, the effective output impedance equals the lumped secondary impedance times the inverse of the OFF duty cycle. That is: R ESR DC OUT OFF = 1 where ROUT = effective supply output impedance ESR = lumped secondary impedance DCOFF = OFF duty cycle Expressing this in terms of the ON duty cycle, remember- ing DCOFF = 1 – DC, R ESR DC OUT = 1 1– DC = ON duty cycle In less critical applications, or if output load current remains relatively constant, this output impedance error may be judged acceptable and the external FB resistor divider adjusted to compensate for nominal expected error. In more demanding applications, output impedance error may be minimized by the use of the load compensa- tion function. To implement the load compensation function, a voltage is developed that is proportional to average output switch current. This voltage is then impressed across the external ROCMP resistor, and the resulting current acts to decrease the voltage at the FB pin. As output loading increases, average switch current increases to maintain rough output voltage regulation. This causes an increase in ROCMP resistor current which effects a corresponding increase in flyback voltage amplitude. Assuming a relatively fixed power supply efficiency, Eff, Power Out = Eff • Power In VOUT • IOUT = Eff • VIN • IIN Average primary side current may be expressed in terms of output current as follows: OPERATIO I V V Eff I IN OUT IN OUT = • • combining the efficiency and voltage terms in a single variable: IIN = K1 • IOUT, where K V V Eff OUT IN 1 = • Switch current is converted to voltage by the external sense resistor and averaged/lowpass filtered by R3 and the external capacitor on RCMPC. This voltage is then impressed across the external ROCMP resistor by op amp A1 and transistor Q3. This produces a current at the collector of Q3 which is then mirrored around and then subtracted from the FB node. This action effectively in- creases the voltage required at the top of the R1/R2 feedback divider to achieve equilibrium. So the effective change in VOUT target is: ∆= ∆ () ∆ ∆ = VK I R R RR or V I K R R RR OUT OUT SENSE OCMP OUT OUT SENSE OCMP 11 2 11 2 •• ( || ) • ( || ) Nominal output impedance cancellation is obtained by equating this expression with ROUT: RK R R R R and RK R R R R where OUT SENSE OCMP OCMP SENSE OUT = = 11 2 11 2 • ( || ) • ( || ) K1 = dimensionless variable related to VIN, VOUT and efficiency as above RSENSE = external sense resistor ROUT = uncompensated output impedance (R1||R2) = impedance of R1 and R2 in parallel The practical aspects of applying this equation to deter- mine an appropriate value for the ROCMP resistor are found in the Applications Information section. |
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