Rainbow Electronics MAX861 - Manual (Page 7)

MAX860/MAX861
50mA, Frequency-Selectable,
Switched-Capacitor Voltage Converters
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__________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:
R OUT = R O + 4 x ESR C1 + ESR C2 + 1 / (f S x C1)
where R O (the effective resistance of the MAX860/
MAX861’s internal switches) is approximately 8
Ω
and f S
is the switching frequency. R OUT is typically 12
Ω
when
using capacitors with 0.2
Ω
ESR and f S , C1, and C2 val-
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:
R OUT = R O + 4 x ESR C1 + ESR C2
A typical design procedure is as follows:
1) Choose C1 and C2 to be the same, for convenience.
2) Select f S :
a) If you want to avoid a specific noise frequency,
choose f S appropriately.
b) If you want to minimize capacitor cost and size,
choose a high f S .
c) If you want to minimize current consumption,
choose a low f S .
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
10% and 20% output droop from the ideal -V
IN value.
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