Data Sheet

ADP3605

Rev. B | Page 9 of 12

APPLICATIONS INFORMATION

CAPACITOR SELECTION

The high internal oscillator frequency of the ADP3605 permits

the use of small capacitors for both the pump and the output

capacitors. For a given load current, factors affecting the output

voltage performance are the following:

•

•

When selecting the capacitors, keep in mind that not all

manufacturers guarantee capacitor ESR in the range required

by the circuit. In general, the ESR of the capacitor is inversely

proportional to its physical size; therefore, larger capacitance

values and higher voltage ratings tend to reduce ESR. Because

the ESR is also a function of the operating frequency, when

selecting a capacitor, ensure that its value is rated at the

operating frequency of the circuit.

Temperature is another factor affecting capacitor performance.

Figure 14 illustrates the temperature effect on various capacitors. If

the circuit has to operate at temperatures significantly different

from 25°C, the capacitance and the ESR values must be carefully

selected to adequately compensate for the change. Various

capacitor technologies offer improved performance over

temperature; for example, certain tantalum capacitors provide

good low temperature ESR; however, at a higher cost. Table 3

provides the ratings for different types of capacitor technologies

to help the designer select the right capacitors for the application.

ESR capacitors, such as solid tantalum and multilayer ceramic

capacitors, are recommended to minimize voltage loss at high

currents. Table 4 shows a partial list of the recommended low

ESR capacitor manufacturers.

Figure 14. ESR vs. Temperature

INPUT CAPACITOR

A small 1 µF input bypass capacitor, preferably with low ESR,

such as tantalum or multilayer ceramic, is recommended to

reduce noise and supply transients and to supply part of the

peak input current drawn by the ADP3605. A large capacitor is

recommended if the input supply is connected to the ADP3605

through long leads, or if the pulse current drawn by the device

may affect other circuitry through supply coupling.

OUTPUT CAPACITOR

output. Figure 15 illustrates the output ripple voltage effect for

various capacitance and ESR values. Note that as the capacitor

value increases beyond the point where the dominant contribution

to the output ripple is due to the ESR, no significant reduction

one-half of the charge pump cycle, peak-to-peak output ripple

voltage is calculated by

O

RC

L

O

S

L

RIPPLE

ES

I

C

f

I

V

×

×

+

×

×

=

2

2

where:

mV

60

15

.

0

mA

120

2

F

μ

10

kHz

250

2

mA

0

12

=

×

×

+

×

×

=

RIPPLE

V

Multiple smaller capacitors can be connected in parallel to yield

lower ESR and lower cost. For lighter loads, proportionally smaller

capacitors are required. To reduce high frequency noise, bypass

the output with a 0.1 µF ceramic capacitor in parallel with the

output capacitor.

Figure 15. Output Ripple Voltage vs. Capacitance and ESR

10

1

0.1

0.01

–50

0

50

100

TEMPERATURE (°C)

ALUMINUM

CERAMIC

TANTALUM

ORGANIC SEMIC

100

80

60

40

20

0

0

20

40

60

80

100

150mΩ

100mΩ

50mΩ

120

140

160

CAPACITANCE (µF)

ADP3605

–3.0V OUTPUT