Rainbow Electronics MAX1681 - Manual (Page 7)

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.
Table 2. Switching-Frequency Trade-Offs
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 (3.5
Ω
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
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
125mA, Frequency-Selectable,
Switched-Capacitor Voltage Converters
_______________________________________________________________________________________ 7
ATTRIBUTE
C1, C2 Values Larger
LOWER
FREQUENCY
Output Ripple Larger
Smaller
Smaller
SUPPLIER
Marcon/United Chemi-Con (847) 696-2000
PHONE
AVX
(803) 946-0690
(800) 282-4975
(847) 696-9278
(803) 626-3123
HIGHER
FREQUENCY
Supply Current Smaller Larger
FAX DEVICE TYPE
Ceramic capacitors
Surface mount, TPS series
Matsuo (714) 969-2491 (714) 960-6492 Surface mount, 267 series
Nichicon
USA: (847) 843-7500
Japan: 81-7-5231-8461 USA: (847) 843-2798
Japan: 81-7-5256-4158
Sanyo
USA: (619) 661-6835
Japan: 81-7-2070-6306 USA: (619) 661-1055
Japan: 81-7-2070-1174
Through-hole, OS-CON series
Through-hole, PL series
Sprague (603) 224-1961 (603) 224-1430
TDK (847) 390-4373 (847) 390-4428 Ceramic capacitors
Surface mount, 595D series
United Chemi-Con (714) 255-9500 (714) 255-9400
Vishay/Vitramon (203) 268-6261 (203) 452-5670 SMT ceramic chip capacitors
Through-hole, LXF series
Table 3. Low-ESR Capacitor Suppliers