50mA, Frequency-Selectable,

Switched-Capacitor Voltage Converters

_______________________________________________________________________________________

7

__________Applications Information

Capacitor Selection

The MAX860/MAX861 are tested using 10µF capacitors

for both C1 and C2, although smaller or larger values

can be used (Table 3). Smaller C1 values increase the

output resistance; larger values reduce the output

resistance. Above a certain point, increasing the

capacitance of C1 has a negligible effect (because the

output resistance becomes dominated by the internal

switch resistance and the capacitor ESR). Low-ESR

capacitors provide the lowest output resistance and

ripple voltage. The output resistance of the entire circuit

(inverter or doubler) is approximately:

MAX861’s internal switches) is approximately 8

Ω when

using capacitors with 0.2

ues suggested in Table 3. When C1 and C2 are so

large (or the switching frequency is so high) that the

internal switch resistance dominates the output resis-

tance, estimate the output resistance as follows:

A typical design procedure is as follows:

1) Choose C1 and C2 to be the same, for convenience.

a) If you want to avoid a specific noise frequency,

b) If you want to minimize capacitor cost and size,

c) If you want to minimize current consumption,

3) Choose a capacitor based on Table 3, although

higher or lower values can be used to optimize per-

formance. Table 4 lists manufacturers who provide

low-ESR capacitors.

*In addition to Table 3, four graphs in the

Typical

Operating Characteristics section show typical output

current for C1 and C2 capacitances ranging from

0.33µF to 22µF. Output current is plotted for inputs of

4.5V (5V - 10%) and 3.0V (3.3V - 10%), and also for

ATTRIBUTE

LOWER

FREQUENCY

HIGHER

FREQUENCY

Output Ripple

Larger

Smaller

C1, C2 Values

Larger

Smaller

Supply Current

Smaller

Larger

C1, C2 (µF)

NOMINAL FREQUENCY (kHz)

6

68

13

47

50

10

100

4.7

130

4.7

250

2.2

Table 2. Switching-Frequency Trade-Offs

Table 3. Suggested Capacitor Values*

Table 4. Low-ESR Capacitor Manufacturers

MANUFACTURER–Series

PHONE

FAX

COMMENTS

AVX TPS Series

(803) 946-0629

(803) 626-3123

Low-ESR tantalum, SMT

AVX TAG Series

(803) 946-0629

(803) 626-3123

Low-cost tantalum, SMT

Matsuo 267 Series

(714) 969-2491

(714) 960-6492

Low-cost tantalum, SMT

Sprague 595 Series

(603) 224-1961

(613) 224-1430

Low-ESR tantalum, SMT

Sanyo MV-GX Series

(619) 661-6835

(619) 661-1055

Aluminum electrolytic, through hole

Sanyo CV-GX Series

(619) 661-6835

(619) 661-1055

Aluminum electrolytic, SMT

Nichicon PL Series

(847) 843-7500

(847) 843-2798

Aluminum electrolytic, through hole

United Chemicon (Marcon)

(847) 696-2000

(847) 696-9278

Ceramic SMT

TDK

(847) 390-4461

(847) 390-4405

Ceramic SMT