125mA, Frequency-Selectable,
Switched-Capacitor Voltage Converters
Operating Frequency Trade-Offs
It is important to recognize the trade-offs between
switching frequency, power consumption, noise, cost,
and performance. Higher frequency switching reduces
capacitor size while maintaining the same output
impedance, thus saving capacitor cost and board
space. Lower frequency designs use less supply cur-
rent. Table 2 summarizes the relative trade-offs.
resistance is a critical circuit component, as it deter-
mines the voltage drop that will occur at the output from
the ideal value of -V
INPUT
(or 2V
INPUT
when doubling).
To optimize performance, minimize overall resistance in
the system. In particular, equivalent series resistance
(ESR) in the capacitors produces significant losses as
large currents flow through them. Therefore, choose a
low-ESR capacitor for highest efficiency. Table 3 lists
recommended capacitors and their suppliers.
Calculate the output ripple voltage as follows:
V
RIPPLE
= [(I
OUT
) / (2 x f
OSC
x C2)] + 2 x (I
OUT
x ESR
C2
)
where I
OUT
is the load current, f
S
is the charge pump’s
operating frequency, C2 is the output capacitor, and
ESR
C2
is the output capacitor’s ESR.
Table 4 lists the minimum recommended capacitances
that allow for the maximum output current. The output
capacitor, C2, is normally equal to or greater than the
charge-pump capacitor, C1. Capacitor values can be
scaled directly proportional to the input voltage, fre-
quency, and load current. For example, for V
INPUT
=
5V, I
LOAD
= 125mA at f
OSC
= 125kHz, a 6.4µF mini-
mum capacitor is recommended. For an output of only
62.5mA, a 3.2µF capacitor is recommended. C1’s value
can be estimated as follows:
C1 = 6.4µF x (V
INPUT
/ 5.0V) x (125kHz / f
OSC
) x
(I
LOAD
/ 125mA)
where f
OSC
is the switching frequency (kHz) and I
LOAD
is the output current (mA) required.
MAX1680/MAX1681
Table 2. Switching-Frequency Trade-Offs
ATTRIBUTE
Output Ripple
C1, C2 Values
Supply Current
LOWER
FREQUENCY
Larger
Larger
Smaller
HIGHER
FREQUENCY
Smaller
Smaller
Larger
Capacitor Selection
The MAX1680/MAX1681 are tested with capacitor val-
ues of 10µF and 2.2µF, respectively. Capacitor size
and switching speed determine output resistance.
Larger C1 values decrease the output resistance until
the internal switch resistance (3.5Ω typ) becomes the
dominant term. Low-ESR capacitors minimize output
resistance and ripple voltage. The entire circuit’s output
resistance can be approximated by the following equa-
tion:
R
OUT
≅
R
O
+ 4 x ESR
C1
+ ESR
C2
+ [1 / (f
OSC
x C1)] +
[1 / (f
OSC
x C2)]
where R
O
is the device’s internal effective switch resis-
tance and f
OSC
is the switching frequency. Output
Table 3. Low-ESR Capacitor Suppliers
SUPPLIER
AVX
Marcon/United Chemi-Con
Matsuo
Nichicon
Sanyo
Sprague
TDK
United Chemi-Con
Vishay/Vitramon
PHONE
(803) 946-0690
(800) 282-4975
(847) 696-2000
(714) 969-2491
USA: (847) 843-7500
Japan: 81-7-5231-8461
USA: (619) 661-6835
Japan: 81-7-2070-6306
(603) 224-1961
(847) 390-4373
(714) 255-9500
(203) 268-6261
FAX
(803) 626-3123
(847) 696-9278
(714) 960-6492
USA: (847) 843-2798
Japan: 81-7-5256-4158
USA: (619) 661-1055
Japan: 81-7-2070-1174
(603) 224-1430
(847) 390-4428
(714) 255-9400
(203) 452-5670
DEVICE TYPE
Surface mount, TPS series
Ceramic capacitors
Surface mount, 267 series
Through-hole, PL series
Through-hole, OS-CON series
Surface mount, 595D series
Ceramic capacitors
Through-hole, LXF series
SMT ceramic chip capacitors
_______________________________________________________________________________________
7
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
