ADM8660 Analog Devices, ADM8660 Datasheet - Page 9

• Manufacturer Part Number: ADM8660
• Description: CMOS Switched-Capacitor Voltage Converter w/ Low Power Shutdown Pin
• Manufacturer: Analog Devices

Specifications of ADM8660

• Input Voltage (v): 1.5 - 7
• Vout Preset (v): -Vin
• Iout: 100mA
• Vout #2 (v): n/a
• Iout #2 (µa): n/a
• Vout #3 (v): n/a
• Iout #3 (µa): n/a
• Power Efficiency (%): 80%
• Supply Current: 4.5mA
• Shutdown Current, Max: 5µA
• Package: PDIP-8,SOIC-8

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/f
term. This explains why the output impedance is higher for
smaller capacitance values. At high oscillator frequencies, the
1/f
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.
REV.
C
Figure 9. Output Impedance vs. Oscillator Frequency
term becomes insignificant and the output impedance is
C
500
400
300
200
100
0
0.1
C1 = C2 = 10 F
C1 = C2 = 2.2 F
OSCILLATOR FREQUENCY – kHz
1
10
C1 = C2 = 1 F
100
C
–9–
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.
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.
Charge-Pump
Frequency
25 kHz
120 kHz
Table III. Capacitor Selection
ADM660/ADM8660
Capacitor
C1, C2
10 µF
2.2 µF