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ADM8660 Datasheet(PDF) 9 Page - Analog Devices |
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ADM8660 Datasheet(HTML) 9 Page - Analog Devices |
9 / 12 page REV. B ADM660/ADM8660 –9– current levels. The ADM660/ADM8660 is tested using low ESR, 10 µF, capacitors for both C1 and C2. Smaller values of C1 increase the output resistance, while increasing C1 will reduce the output resistance. The output resistance is also depen- dent on the internal switches on resistance as well as the capacitors ESR, so the effect of increasing C1 becomes negligible past a certain point. Figure 9 shows how the output resistance varies with oscillator frequency for three different capacitor values. At low oscillator frequencies, the output impedance is dominated by the 1/fC term. This explains why the output impedance is higher for smaller capacitance values. At high oscillator frequencies, the 1/fC term becomes insignificant and the output impedance is dominated by the internal switches on resistance. From an out- put impedance viewpoint, therefore, there is no benefit to be gained from using excessively large capacitors. OSCILLATOR FREQUENCY – kHz 500 400 0 0.1 100 110 300 200 100 C1 = C2 = 2.2 F C1 = C2 = 1 F C1 = C2 = 10 F Figure 9. Output Impedance vs. Oscillator Frequency Capacitor C2 The output capacitor size C2 affects the output ripple. Increas- ing the capacitor size reduces the peak-to-peak ripple. The ESR affects both the output impedance and the output ripple. Reducing the ESR reduces the output impedance and ripple. For convenience it is recommended that both C1 and C2 be the same value. Table III. Capacitor Selection Charge-Pump Capacitor Frequency C1, C2 25 kHz 10 µF 120 kHz 2.2 µF Power Efficiency and Oscillator Frequency Trade-Off While higher switching frequencies allow smaller capacitors to be used for equivalent performance, or improved performance with the same capacitors, there is a trade-off to consider. As the oscillator frequency is increased, the quiescent current increases. This happens as a result of a finite charge being lost at each switching cycle. The charge loss per unit cycle at very high frequencies can be significant, thereby reducing the power effi- ciency. Since the power efficiency is also degraded at low oscillator frequencies due to an increase in output impedance, this means that there is an optimum frequency band for maximum power transfer. Refer to the Typical Performance Characteristics section. Bypass Capacitor The ac impedance of the ADM660/ADM8660 may be reduced by using a bypass capacitor on the input supply. This capacitor should be connected between the input supply and GND. It will provide instantaneous current surges as required. Suitable capacitors of 0.1 µF or greater may be used. |
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