Rainbow-electronics MAX1724 Manual do Utilizador descarregar pdf (Página 10)

For most applications, the peak inductor current equals

the current limit. However, for applications using large

inductor values or low input voltages, the maximum on-

time limits the peak inductor current (see Inductor

Selection section).

Capacitor Selection

Choose input and output capacitors to supply the input

and output peak currents with acceptable voltage rip-

ple. The input filter capacitor (C

) reduces peak cur-

rents drawn from the battery and improves efficiency.

Low equivalent series resistance (ESR) capacitors are

recommended. Ceramic capacitors have the lowest

ESR, but low ESR tantalum or polymer capacitors offer

a good balance between cost and performance.

Output voltage ripple has two components: variations

in the charge stored in the output capacitor with each

LX pulse, and the voltage drop across the capacitor’s

ESR caused by the current into and out of the capaci-

tor:

where I

PEAK

is the peak inductor current (see Inductor

Selection section). For ceramic capacitors, the output

voltage ripple is typically dominated by V

RIPPLE(C)

. For

example, a 10µF ceramic capacitor and a 10µH induc-

tor typically provide 75mV of output ripple when step-

ping up from 3.3V to 5V at 50mA. Low input-to-output

voltage differences (i.e. two cells to 3.3V) require high-

er output capacitor values.

Capacitance and ESR variation of temperature should

be considered for best performance in applications

with wide operating temperature ranges. Table 2 lists

suggested capacitors and suppliers.

PC Board Layout Considerations

Careful PC board layout is important for minimizing

ground bounce and noise. Keep the IC’s GND pin and

the ground leads of the input and output capacitors

less than 0.2in (5mm) apart using a ground plane. In

addition, keep all connections to FB

(MAX1722/MAX1723 only) and LX as short as possible.

VV V

VVC

RIPPLE RIPPLE C RIPPLE ESR

RIPPLE ESR

RIPPLE C

OUT BATT OUT

≈

()













()

() ( )

()

PEAK ESR COUT

PEAK OUT

I R

I-

MAX1722/MAX1723/MAX1724

1.5µA I

, Step-Up DC-DC Converters

in Thin SOT23-5

10 ______________________________________________________________________________________

Table 2. Suggested Surface-Mount Capacitors and Manufacturers (C1 and C2)

MANUFACTURER

CAPACITOR

VALUE

DESCRIPTION

PHONE

WEBSITE

1µF to 10µF X7R Ceramic

AVX

10µF to 330µF

TAJ Tantalum Series

TPS Tantalum Series

843-448-9411

www.avxcorp.com

1µF to 22µF X5R/X7R Ceramic

10µF to 330µF T494 Tantalum SeriesKemet

68µF to 330µF T520 Tantalum Series

864-963-6300

www.kemet.com

Sanyo 33µF to 330µF TPC Polymer Series

408-749-9714

www.secc.co.jp

Taiyo Yuden 33µF to 330µF X5R/X7R Ceramic

800-368-2496

www.t-yuden.org

TDK 1µF to 10µF X7R Ceramic

847-803-6100

www.tdk.com

Vishay Sprague 10µF to 330µF

594D Tantalum Series

595D Tantalum Series

203-452-5664

www.vishay.com

Chip Information

TRANSISTOR COUNT: 863

1 2 ... 5 6 7 8 9 10 11 12

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