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ADM660

CMOS Switched-Capacitor Voltage Converters

Analog Devices 

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.

500

C1 = C2 = 2.2␮F

400

300

200

C1 = C2 = 10␮F

100

C1 = C2 = 1␮F

0

0.1

1

10

100

OSCILLATOR FREQUENCY – kHz

Figure 9. Output Impedance vs. Oscillator Frequency

ADM660/ADM8660

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

Frequency

25 kHz

120 kHz

Capacitor

C1, C2

10 µF

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.

REV. C

–9–

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