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ADM660AN Datasheet(PDF) 8 Page - Analog Devices |
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ADM660AN Datasheet(HTML) 8 Page - Analog Devices |
8 / 8 page ADM660/ADM8660 REV. A –8– output 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 re- duce the output resistance. The output resistance is also de- pendent 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 23 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 0 300 100 400 200 0.1 100 110 C1 = C2 = 2.2µF C1 = C2 = 10µF C1 = C2 = 1µF Figure 23. Output Impedance vs. Oscillator Frequency Capacitor C2 The output capacitor size C2 affects the output ripple. Increas- ing the capacitor size reduces the peak-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 Tradeoff While higher switching frequencies allow smaller capacitors to be used for equivalent performance, or improved performance with the same capacitors, there is a tradeoff to be considered. As the oscillator frequency is increased, the quiescent current in- creases. 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 oscil- lator frequencies, due to an increase in output impedance, this means that there is an optimum frequency band for maximum power transfer. Please refer to the Typical Performance Charac- teristics 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. OUTLINE DIMENSIONS Dimensions shown in inches and (mm). 8-Lead Plastic DIP (N-8) 8 14 5 0.430 (10.92) 0.348 (8.84) 0.280 (7.11) 0.240 (6.10) PIN 1 SEATING PLANE 0.022 (0.558) 0.014 (0.356) 0.060 (1.52) 0.015 (0.38) 0.210 (5.33) MAX 0.130 (3.30) MIN 0.070 (1.77) 0.045 (1.15) 0.100 (2.54) BSC 0.160 (4.06) 0.115 (2.93) 0.325 (8.25) 0.300 (7.62) 0.015 (0.381) 0.008 (0.204) 0.195 (4.95) 0.115 (2.93) 8-Lead Narrow-Body SOIC (SO-8) 0.1968 (5.00) 0.1890 (4.80) 8 5 4 1 0.2440 (6.20) 0.2284 (5.80) PIN 1 0.1574 (4.00) 0.1497 (3.80) 0.0688 (1.75) 0.0532 (1.35) SEATING PLANE 0.0098 (0.25) 0.0040 (0.10) 0.0192 (0.49) 0.0138 (0.35) 0.0500 (1.27) BSC 0.0098 (0.25) 0.0075 (0.19) 0.0500 (1.27) 0.0160 (0.41) 8 ° 0 ° 0.0196 (0.50) 0.0099 (0.25) x 45 ° 16-Lead TSSOP (RU-16) 16 9 8 1 0.201 (5.10) 0.193 (4.90) PIN 1 SEATING PLANE 0.006 (0.15) 0.002 (0.05) 0.0118 (0.30) 0.0075 (0.19) 0.0256 (0.65) BSC 0.0433 (1.10) MAX 0.0079 (0.20) 0.0035 (0.090) 0.028 (0.70) 0.020 (0.50) 8 ° 0 ° |
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