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поискавой системы для электроныых деталей |
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поискавой системы для электроныых деталей |
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SC2446 датащи(PDF) 13 Page - Semtech Corporation |
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SC2446 датащи(HTML) 13 Page - Semtech Corporation |
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13 / 38 page  13 2004 Semtech Corp. www.semtech.com POWER MANAGEMENT SC2446 third term is voltage ripple due to ESL and the fourth term is the voltage ripple due to ESR. The total output voltage ripple is then a vector sum of the last three terms. Since the inductor current is a triangular waveform with peak-to-peak value δ*I o, the ripple-voltage caused by inductor current ripples is , f C 8 I v s o o C δ ≈ ∆ the ripple-voltage due to ESL is , D I f L v o s esl ESL δ = ∆ and the ESR ripple-voltage is . I R v o esr ESR δ = ∆ Aluminum capacitors (e.g. electrolytic, solid OS-CON, POSCAP, tantalum) have high capacitances and low ESL’s. The ESR has the dominant effect on the output ripple voltage. It is therefore very important to minimize the ESR. When determining the ESR value, both the steady state ripple-voltage and the dynamic load transient need to be considered. To keep the steady state output ripple-voltage < ∆V o, the ESR should satisfy . I V R o o 1 esr δ ∆ < To limit the dynamic output voltage overshoot/undershoot within α (say 3%) of the steady state output voltage) from no load to full load, the ESR value should satisfy . I V R o o 2 esr α < Then, the required ESR value of the output capacitors should be R esr = min{Resr1,Resr2 }. The voltage rating of aluminum capacitors should be at least 1.5V o. The RMS current ripple rating should also be greater than . 3 2 I o δ Usually it is necessary to have several capacitors of the same type in parallel to satisfy the ESR requirement. The voltage ripple cause by the capacitor charge/discharge Application Information (Cont.) should be an order of magnitude smaller than the voltage ripple caused by the ESR. To guarantee this, the capacitance should satisfy . R f 2 10 C esr s o π > In many applications, several low ESR ceramic capacitors are added in parallel with the aluminum capacitors in order to further reduce ESR and improve high frequency decoupling. Because the values of capacitance and ESR are usually different in ceramic and aluminum capacitors, the following remarks are made to clarify some practical issues. Remark 1: High frequency ceramic capacitors may not carry most of the ripple current. It also depends on the capacitor value. Only when the capacitor value is set properly, the effect of ceramic capacitor low ESR starts to be significant. For example, if a 10 µF, 4mΩ ceramic capacitor is connected in parallel with 2x1500 µF, 90mΩ electrolytic capacitors, the ripple current in the ceramic capacitor is only about 42% of the current in the electrolytic capacitors at the ripple frequency. If a 100 µF, 2mΩ ceramic capacitor is used, the ripple current in the ceramic capacitor will be about 4.2 times of that in the electrolytic capacitors. When two 100 µF, 2mΩ ceramic capacitors are used, the current ratio increases to 8.3. In this case most of the ripple current flows in the ceramic decoupling capacitor. The ESR of the ceramic capacitors will then determine the output ripple-voltage. Remark 2: The total equivalent capacitance of the filter bank is not simply the sum of all the paralleled capacitors. The total equivalent ESR is not simply the parallel combination of all the individual ESR’s either. Instead they should be calculated using the following formulae. ) C C ( C C ) C R C R ( ) C C ( C C ) R R ( : ) ( C b 1 a 1 b 1 a 1 2 b 1 2 b 1 a 1 2 a 1 2 b 1 a 1 2 b 1 2 a 1 2 2 b 1 a 1 eq + + ω + + + ω + = ω 2 b 1 a 1 2 b 1 2 a 1 2 2 b 1 a 1 2 a 1 a 1 2 b 1 b 1 2 b 1 2 a 1 2 b 1 a 1 b 1 a 1 eq ) C C ( C C ) R R ( ) C R C R ( C C ) R R ( R R : ) ( R + + ω + + + ω + = ω where R 1a and C1a are the ESR and capacitance of electrolytic capacitors, and R 1b and C1b are the ESR and capacitance of the ceramic capacitors respectively. (Figure 7) |
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