Datasheet MAX1682, MAX1683 (Maxim) - 6

Switched-Capacitor Voltage Doublers
Conversion losses occur during the charge transfer Using a larger flying capacitor reduces the output between C1 and C2 when there is a voltage difference impedance and improves efficiency (see the Efficiency between them. The power loss is: Considerations section). Above a certain point, increas- ing C1’s capacitance has a negligible effect because ⎡ ⎛ 2 2⎞ the output resistance becomes dominated by the inter- P = / ⎢ C1 4V ⎜ − V CONVERSION LOSS 1 2 IN OUT ⎟ + nal switch resistance and capacitor ESR (see the ⎢ ⎝ ⎠ ⎣ Output Resistance vs. Capacitance graph in the ⎛ 2 ⎞⎤ Typical Operating Characteristics). Table 2 lists the / 1 C2 2V V − V RIPPLE x f 2 OUT RIPPLE ⎥ OSC most desirable capacitor values—those that produce a ⎝⎜ ⎠⎟⎦ low output resistance. But when space is a constraint, it where VRIPPLE is the peak-to-peak output voltage ripple may be necessary to sacrifice low output resistance for determined by the output capacitor and load current the sake of small capacitor size. Table 3 demonstrates (see Output Capacitor section). Choose capacitor val- how the capacitor affects output resistance. ues that decrease the output resistance (see Flying Capacitor section).
Output Capacitor (C2)
Increasing the output capacitance reduces the output
Applications Information
ripple voltage. Decreasing its ESR reduces both output resistance and ripple. Smaller capacitance values can
Flying Capacitor (C1) MAX1682/MAX1683
be used with light loads. Use the following equation to To maintain the lowest output resistance, use capaci- calculate the peak-to-peak ripple: tors with low ESR. Suitable capacitor manufacturers are listed in Table 1. The charge-pump output resistance is VRIPPLE = IOUT / (fOSC x C2) + 2 x IOUT x ESRC2 a function of C1 and C2’s ESR and the internal switch
Input Bypass Capacitor
resistance, as shown in the equation for ROUT in the Bypass the incoming supply to reduce its AC imped- Efficiency Considerations section. ance and the impact of the MAX1682/MAX1683’s Minimizing the charge-pump capacitor’s ESR mini- switching noise. When loaded, the circuit draws a con- mizes the total resistance. Suggested values are listed tinuous current of 2 x IOUT. A 0.1µF bypass capacitor is in Tables 2 and 3. sufficient.
Table 1. Recommended Capacitor Manufacturers PRODUCTION METHOD MANUFACTURER SERIES PHONE FAX
AVX TPS 803-946-0690 803-448-2170 Surface-Mount Tantalum Matsuo 267 714-969-2491 714-960-6492 Sprague 593D, 595D 603-224-1961 603-224-1430 AVX X7R 803-946-0590 803-626-3123 Surface-Mount Ceramic Matsuo X7R 714-969-2491 714-960-6492
Table 2. Suggested Capacitor Values for Table 3. Suggested Capacitor Values for Low Output Resistance Minimum Size FREQUENCY CAPACITOR TYPICAL FREQUENCY CAPACITOR TYPICAL PART PART (kHz) VALUE (µF) ROUT (
Ω
) (kHz) VALUE (µF) ROUT (
Ω
)
MAX1682 12 10 20 MAX1682 12 3.3 35 MAX1683 35 3.3 20 MAX1683 35 1 35
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