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ISL6219ACAZ Datasheet(PDF) 14 Page - Intersil Corporation |
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ISL6219ACAZ Datasheet(HTML) 14 Page - Intersil Corporation |
14 / 17 page 14 FN9093.1 March 20, 2007 The feedback resistor, RFB, has already been chosen as out- lined in Load-Line Regulation Resistor. Select a target band- width for the compensated system, f0. The target bandwidth must be large enough to assure adequate transient perfor- mance, but smaller than 1/3 of the per-channel switching fre- quency. The values of the compensation components depend on the relationships of f0 to the L-C pole frequency and the ESR zero frequency. For each of the three cases defined in the following, there is a separate set of equations for the compensation components . In Equations 17, L is the per-channel filter inductance divided by the number of active channels; C is the sum total of all out- put capacitors; ESR is the equivalent-series resistance of the bulk output-filter capacitance; and VPP is the peak-to-peak sawtooth signal amplitude as described in Figure 5 and Electrical Specifications on page 5. Once selected, the compensation values in Equations 17 assure a stable converter with reasonable transient perfor- mance. In most cases, transient performance can be improved by making adjustments to RC. Slowly increase the value of RC while observing the transient performance on an oscilloscope until no further improvement is noted. Normally, CC will not need adjustment. Keep the value of CC from Equations 17 unless some performance issue is noted. The optional capacitor C2, is sometimes needed to bypass noise away from the PWM comparator (see Figure 5). Keep a position available for C2, and be prepared to install a high- frequency capacitor of between 22pF and 150pF in case any jitter problem is noted. COMPENSATION WITHOUT LOAD-LINE REGULATION The non load-line regulated converter is accurately modeled as a voltage-mode regulator with two poles at the L-C resonant frequency and a zero at the ESR frequency. A type III controller, as shown in Figure 12, provides the necessary compensation. The first step is to choose the desired bandwidth, f0, of the compensated system. Choose a frequency high enough to assure adequate transient performance but not higher than 1/3 of the switching frequency. The type-III compensator has an extra high-frequency pole, fHF. This pole can be used for added noise rejection or to assure adequate attenuation at the error-amplifier high-order pole and zero frequencies. A good general rule is to chose fHF =10 f0, but it can be higher if desired. Choosing fHF to be lower than 10 f0 can cause problems with too much phase shift below the system band- width. In the solutions to the compensation equations, there is a sin- gle degree of freedom. For the solutions presented in Equa- tions 18, RFB is selected arbitrarily. The remaining compensation components are then selected according to Equation 18. FIGURE 11. COMPENSATION CONFIGURATION FOR LOAD-LINE REGULATED ISL6219A CIRCUIT COMP CC RC RFB FB VSEN - + VDROOP C2 (OPTIONAL) 1 2 π LC ------------------- f 0 > R C R FB 2 πf 0Vpp LC 0.75V IN ------------------------------------ = C C 0.75V IN 2 πV PPRFBf0 ------------------------------------ = Case 1: 1 2 π LC ------------------- f 0 1 2 πCESR () ------------------------------ < ≤ R C R FB V PP 2 π ()2 f 0 2 LC 0.75 V IN -------------------------------------------- = C C 0.75V IN 2 π ()2 f 0 2 V PPRFB LC ------------------------------------------------------------- = Case 2: (EQ. 17) f 0 1 2 πCESR () ------------------------------ > R C R FB 2 π f 0VppL 0.75 V IN ESR () ------------------------------------------ = C C 0.75V IN ESR () C 2 πV PPRFBf0 L ------------------------------------------------- = Case 3: FIGURE 12. COMPENSATION CIRCUIT FOR ISL6219A BASED CONVERTER WITHOUT LOAD-LINE REGULATION. COMP CC RC RFB FB VSEN - + VDROOP C2 C1 R1 ISL6219A |
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