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IPM6220EVAL1 Datasheet(PDF) 11 Page - Intersil Corporation |
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IPM6220EVAL1 Datasheet(HTML) 11 Page - Intersil Corporation |
11 / 14 page 11 determined by the ESR (Equivalent Series Resistance) and voltage rating requirements as well as actual capacitance requirements. The output voltage ripple is due to the inductor ripple current and the ESR of the output capacitors as defined by: where, ∆IL is calculated in the Inductor Selection section. 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 usefulness of these low inductance components. Consult with the manufacturer of the load circuitry for specific decoupling requirements. Use only specialized low-ESR capacitors intended for switching-regulator applications, at 300kHz, for the bulk capacitors. In most cases, multiple electrolytic capacitors of small case size perform better than a single large case capacitor. The stability requirement on the selection of the output capacitor is that the ‘ESR zero’, fZ, be between 1.2kHz and 30kHz. This range is set by an internal, single compensation zero at 6kHz. The ESR zero can be a factor of five on either side of the internal zero and still contribute to increased phase margin of the control loop. Therefore: In conclusion, the output capacitors must meet three criteria: By varying the values of the soft-start capacitors, it is possible to provide sequencing of the main outputs at start-up. 1. They must have sufficient bulk capacitance to sustain the output voltage during a load transient while the output inductor current is slewing to the value of the load transient 2. The ESR must be sufficiently low to meet the desired output voltage ripple due to the output inductor current, and 3. The ESR zero should be placed, in a rather large range, to provide additional phase margin. 3.3V ALWAYS and 5V ALWAYS Output Capacitors The output capacitors for the linear regulators insure stability and provide dynamic load current. The 3.3V ALWAYS and the 5V ALWAYS linear regulators should have, as a minimum, 10 µF capacitors on their outputs. 3.3V Main and 5V Main PWM Output Inductor Selection The PWM converters require output inductors. The output inductor is selected to meet the output voltage ripple requirements. The inductor value determines the converter’s ripple current and the ripple voltage is a function of the ripple current and output capacitor(s) ESR. The ripple voltage expression is given in the capacitor selection section and the ripple current is approximated by the following equation: Input Capacitor Selection The important parameters for the bulk input capacitor(s) are the voltage rating and the RMS current rating. For reliable operation, select bulk input capacitors with voltage and current ratings above the maximum input voltage and largest RMS current required by the circuit. The capacitor voltage rating should be at least 1.25 times greater than the maximum input voltage and 1.5 times is a conservative guideline. The AC RMS input current varies with load as shown in Figure 9. Depending on the specifics of the input power and it’s impedance, most (or all) of this current is supplied by the input capacitor(s). Figure 9 also shows the advantage of having the PWM converters operating out of phase. If the converters were operating in phase, the combined RMS current would be the algebraic sum, which is a much larger value as shown. The combined out-of-phase current is the square root of the sum of the square of the individual reflected currents and is significantly less than the combined in-phase current. Use a mix of input bypass capacitors to control the voltage ripple across the MOSFETs. Use ceramic capacitors for the high frequency decoupling and bulk capacitors to supply the RMS current. Small ceramic capacitors can be placed very close to the upper MOSFET to suppress the voltage induced in the parasitic circuit impedances. For board designs that allow through-hole components, the Sanyo OS-CON® series offer low ESR and good temperature performance. For surface mount designs, solid tantalum capacitors can be used, but caution must be exercised with regard to the capacitor surge current rating. These capacitors must be capable of handling the surge-current at power-up. The TPS series available from AVX is surge current tested. V RIPPLE ∆IL ESR × = C OUT 1 2 π ESR f Z × × × ------------------------------------------- = ∆IL V IN V OUT – F S L × -------------------------------- V OUT V IN ---------------- × = FIGURE 9. INPUT RMS CURRENT vs LOAD 12345 3.3V AND 5V LOAD CURRENT 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 0 IN PHASE OUT OF PHASE 3.3V 5V IPM6220 OS-CON® is a registered trademark of Sanyo Electric Company, Ltd. (Japan) |
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