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FAN6520AM Datasheet(PDF) 9 Page - Fairchild Semiconductor |
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FAN6520AM Datasheet(HTML) 9 Page - Fairchild Semiconductor |
9 / 15 page ©2005 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN6520A Rev. 1.0.5 9 Figure 8. Asymptotic Bode Plot of Converter Gain An output capacitor is required to filter the output and supply the load transient current. The filtering require- ments are a function of the switching frequency and the ripple current. The load transient requirements are a function of the slew rate (di/dt) and the magnitude of the transient load current. These requirements are generally met with a mix of capacitors and careful layout. Component Selection Output Capacitors (COUT) Modern components and loads are capable of producing transient load rates above 1A/ns. High-frequency capac- itors initially supply the transient and slow the current load rate seen by the bulk capacitors. Effective Series Resistance (ESR) and voltage rating are typically the prime considerations for the bulk filter capacitors, rather than actual capacitance requirements. High-frequency decoupling capacitors should be placed as close to the power pins of the load as physically possible. Be careful not to add inductance in the circuit board wiring that could cancel the performance of these low-inductance components. Consult with the load manufacturer on spe- cific decoupling requirements. Use only specialized low- ESR capacitors intended for switching-regulator applica- tions for the bulk capacitors. The bulk capacitor’s ESR determines the output ripple voltage and the initial volt- age drop after a high slew-rate transient. An aluminum electrolytic capacitor’s ESR value is related to the case size with lower ESR available in larger case sizes; how- ever, the Equivalent Series Inductance (ESL) of these capacitors increases with case size and can reduce the usefulness of the capacitor to high slew-rate transient loading. Since ESL is not a specified parameter, work with the capacitor supplier and measure the capacitor’s impedance with frequency to select a suitable compo- nent. Generally, multiple small-case electrolytic capaci- tors perform better than a single large-case capacitor. Output Inductor (LOUT) The output inductor is selected to meet the output volt- age ripple requirements and minimize the converter’s response time to the load transient. The inductor value determines the converter’s ripple current and the ripple voltage is a function of the ripple current. The ripple volt- age ( ΔV) and current (ΔI) are approximated by the fol- lowing equations: Increasing the inductance value reduces the ripple cur- rent and voltage, but also reduces the converter’s ability to quickly respond to a load transient. One of the param- eters limiting the converter’s response to a load transient is the time required to change the inductor current. Given a sufficiently fast control-loop design, the FAN6520A provides either 0% or 100% duty cycle in response to a load transient. The response time is the time required to slew the inductor current from an initial current value to the transient current level. During this interval, the difference between the inductor current and the transient current level must be supplied by the output capacitor. Minimizing the response time can minimize the output capacitance required. Depending on whether there is a load application or a load removal, the response time to a load transient (ISTEP) is different. The following equations give the approximate response time interval for application and removal of a transient load: where TRISE is the response time to the application of a positive ISTEP and TFALL is the response time to a load removal (negative ISTEP). The worst-case response time can be either at application or removal of load. Check both of these equations at the minimum and maximum output levels for the worst-case response time. Input Capacitor Selection Use a mix of input bypass capacitors to control the volt- age overshoot across the MOSFETs. Use small ceramic capacitors for high-frequency decoupling and bulk capacitors to supply the current needed each time Q1 turns on. Place the small ceramic capacitors physically close to the MOSFETs and between the drain of Q1 and the source of Q2. The important parameters for the bulk input capacitor are the voltage rating and the RMS cur- rent rating. For reliable operation, select the bulk capaci- tor with voltage and current ratings above the maximum input voltage and the largest RMS current required by the circuit. The capacitor voltage rating should be at least 100 80 60 40 20 0 -20 -40 -60 10 100 1K 10K 100K FREQUENCY (Hz) OPEN LOOP ERROR AMP GAIN COMPENSATION GAIN CLOSED LOOP GAIN MODULATOR GAIN 20LOG (VIN/DVOSC) 20LOG (R2/R1) FZ1 FZ2 FP1 FLC FESR FP2 1M 10M ΔI V IN V OUT – F SW L × ------------------------------ ⎝⎠ ⎜⎟ ⎛⎞ V OUT V IN -------------- × = ΔVESR ΔI × ≈ (9) T RISE LI STEP × V IN V OUT – ------------------------------ = (10) T FALL LI STEP × V OUT ------------------------ = (11) |
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