Rainbow-electronics MAX1734 Manual do Utilizador descarregar pdf (Página 7)

GND (Figure 1). Choose R2 to be less than 50kΩ:

where V

REF

= 1.25V.

Inductor Selection

The MAX1733/MAX1734 are optimized to use a 10µH

inductor over the entire operating range. A 300mA

rated inductor is enough to prevent saturation for out-

put currents up to 250mA. Saturation occurs when the

inductor’s magnetic flux density reaches the maximum

level the core can support and inductance falls.

Choose a low DC-resistance inductor to improve effi-

ciency. Tables 1 and 3 list some suggested inductors

and suppliers.

Capacitor Selection

The MAX1733/MAX1734 require output voltage ripple

(approximately 30mVp-p) for stable switching behavior.

Use a 10µF to 47µF tantalum output capacitor with

about 200mΩ to 300mΩ ESR to provide stable switch-

ing while minimizing output ripple. Choose input and

output capacitors to filter inductor currents for accept-

able voltage ripple. The input capacitor reduces peak

currents and noise at the voltage source. Input capaci-

tors must meet the input ripple requirements and volt-

age rating. Use the following equation to calculate the

maximum RMS input current:

Tables 2 and 3 list some suggested capacitors and

suppliers.

Using Ceramic C

OUT

with MAX1733

The circuit of Figure 3 is designed to allow the use of

ceramic output capacitors with the MAX1733.

Feedback is derived from the LX pin instead of the out-

put to remove the effects of phase lag in the feedback

loop. Compared to the standard applications circuit,

there are three benefits: 1) availability of ceramic vs.

tantalum; 2) size of 2.2µF 0805 vs. 22µF A-case; 3) out-

put ripple less than 10mVp-p vs. greater than 30mVp-p.

Increase the output capacitance to 4.7µF to further

reduce the output ripple. Note that this circuit exhibits

load regulation equal to the series resistance of the

inductor multiplied by the load current. This small

amount of load regulation is helpful in reducing over-

shoot of the output voltage during load transients.

I V V OUT

RMS

OUT OUT IN

−

()













12/

OUT

REF

12 1=× −













MAX1733/MAX1734

Low-Voltage, Step-Down DC-DC Converters

in SOT23

_______________________________________________________________________________________ 7

INDUCTOR VALUE

(µH)

MANUFACTURER

PART

NUMBER

CR43-100

10 Sumida

CDRH4D18-100

10 Coilcraft DT1608C-103

Table 1. Suggested Inductors

Table 2. Suggested Capacitors

CAPACITOR

TYPE

MANUFACTURER

PART

NUMBER

Tantalum

(22µF)

Taiyo Yuden LMK212BJ225MG

Ceramic

(2.2µF)

AVX TAJA226M006R

Table 3. Component Suppliers

COMPANY PHONE FAX

AVX 843-946-0238 843-626-3123

Coilcraft 847-639-6400 847-639-1469

Kemet 408-986-0424 408-986-1442

Murata 814-237-1431 814-238-0490

USA 847-956-0666 847-956-0702

Sumida

Japan 81-3-3607-5111 81-3-3607-5144

Taiyo Yuden 408-573-4150 408-573-4159

30k

1000pF

68k

2.2µF

X7R

OUTPUT

1.8V at 250mA

10µH

MAX1733

SHDN

OFFON

2.2µF

X7R

GND

Li+

2.7V TO 4.2V

Figure 3. Using a Ceramic Output Capacitor with the MAX1733

1 2 3 4 5 6 7 8

MAX1733/MAX1734 1

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