Dual, High-Efficiency, PFM, Step-Up

DC-DC Controller

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13

Voltage ripple is the sum of contributions associated

with ESR and the capacitor value, as shown below:

To simplify selection, assume that 75% of the ripple

results from ESR and that 25% results from the capaci-

tor value. Voltage ripple as a consequence of ESR is

approximated by:

so:

Estimate input and output capacitor values for a given

voltage ripple as follows:

where V is the input or output voltage, depending on

which capacitor is being calculated.

Choose input capacitors with working voltage ratings

over the maximum input voltage, and output capacitors

with working voltage ratings higher than their respec-

tive outputs.

Bypass the MAX863 with 0.1µF or higher value ceramic

pins as possible.

Set the Output Voltage

DC-DC converter 1 operates with a 3.3V, 5V, or

adjustable output. For a preset output, connect

SENSE1 to OUT1 (Figures 2 and 4a), then set FB1 to

an adjustable output, connect a resistor voltage divider

to the FB1 pin (Figure 7). In adjustable output circuits,

connect SENSE1 to GND.

DC-DC converter 2 can be adjusted from very high

ed to the FB2 pin, as shown in Figure 7. Select feed-

back resistor R2 in the 10k

Ω to 500kΩ range. R1 is

given by:

where 1.25V is the voltage of the internal reference.

R1 = R2

V

1.25V









1

C

0.5L x I

V

x V

2

PEAK

RIPPLE,C

≥

R

V

I

ESR

RIPPLE,ESR

PEAK

≤

MAX863

FB1 OR FB2

C1

(OPTIONAL)

R1

R2

C2

(OPTIONAL FOR HIGH-

VOLTAGE CIRCUITS)

Figure 7. Adjustable Output Circuit

Table 1. Component Suppliers

PHONE

Inductors

SUPPLIER

Marcon/United

Chemi-Con

(847) 696-2000

TDK

(847) 390-4373

Vishay/Vitramon

(203) 268-6261

(847) 390-4428

(203) 452-5670

Large-Value Ceramic Capacitors

(847) 696-9278

Motorola

(602) 303-5454

AVX

(803) 946-0690

Sanyo USA

(619) 661-6835

Sprague

(603) 224-1961

(619) 661-1055

(603) 224-1430

Electrolytic Capacitors

(803) 626-3123

Dale/Vishay

(402) 564-3131

IRC

(512) 992-7900

(402) 563-6418

(512) 992-3377

(602) 994-6430

Current-Sense Resistors

Sumida USA

(847) 956-0666

Central Semiconductor

(516) 435-1110

International Rectifier

(310) 322-3331

(516) 435-1824

(310) 322-3232

(847) 956-0702

MOSFETs and Diodes

Coiltronics

(561) 241-7876

Dale Inductors

(605) 668-4131

(561) 241-9339

(605) 665-1627

FAX

(847) 639-1469

Coilcraft

(847) 639-6400