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

term. This explains why the output impedance is higher for

smaller capacitance values. At high oscillator frequencies, the

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

°