MAX1820 데이터 시트보기 (PDF) - Maxim Integrated

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MAX1820

WCDMA Cellular Phone 600mA Buck Regulators

Maxim Integrated 

WCDMA Cellular Phone 600mA

Buck Regulators

Table 1. Suggested Inductors

MANUFACTURER

Coilcraft

Coilcraft

Sumida

Sumida

Sumida

PART NUMBER

DO1606

LPT1606-472

CDRH4D18-4R7

CR43

CDRH5D18-4R1

INDUCTANCE

(µH)

4.7

4.7

4.7

4.7

4.1

ESR (mΩ)

120

240 (max)

125

108.7

57

SATURATION

CURRENT (A)

1.2

1.2

0.84

1.15

1.95

DIMENSIONS

(mm)

5.3 ✕ 5.3 ✕ 2.0

6.5 ✕ 5.3 ✕ 2.0

5✕5✕2

4.5 ✕ 4.0 ✕ 3.5

5.5 ✕ 5.5 ✕ 2.0

3) Calculate the equivalent load impedance, RL, by:

RL ≈

VOUT(MAX)

IOUT(MAX)

4) Calculate the compensation resistance (RC) value to

cancel out the dominant pole created by the output

load and the output capacitance:

1

=

1

2 × π × RL × COUT 2 × π × RC × C1

Solving for RC gives:

RC

=

RL × COUT

C1

=

⎛ 3.4V ⎞

⎝⎜ 0.6A ⎠⎟

⎛

⎝⎜

4.7µF ⎞

330pF ⎠⎟

=

80.8kΩ

5) Calculate the high-frequency compensation pole to

cancel the zero created by the output capacitor’s

equivalent series resistance (ESR):

1

=

1

`

2 × π × RESR × COUT 2 × π × R3 × C2

Solving for C2 gives:

C2 = RESR × COUT = 4.7µF × 0.01Ω = 0.55pF

R3

80.8kΩ

In this case, C2 can be omitted due to the use of

ceramic capacitors. Larger output capacitors and high-

er ESR may require the use of capacitor C2.

Inductor Selection

A 4µH to 6µH inductor with a saturation current of at

least 800mA is recommended for most applications.

For best efficiency, the inductor’s DC resistance should

be <200mΩ, and saturation current should be >1A. See

Table 1 for recommended inductors and manufacturers.

For most designs, a reasonable inductor value (LIDEAL)

can be derived from the following equation:

LIDEAL =

VOUT(VBATT - VOUT)

VBATT × LIR × IOUT(MAX) × ƒOSC

where LIR is the inductor current ripple as a percentage.

LIR should be kept between 20% and 40% of the maxi-

mum load current for best performance and stability.

The maximum inductor current is:

IL(MAX)

=

⎛⎝⎜1+

LIR

2

⎞⎠⎟IOUT(MAX)

The inductor current becomes discontinuous if IOUT

decreases to LIR/2 from the output current value used

to determine LIDEAL.

Input Capacitor Selection

The input capacitor reduces the current peaks drawn

from the battery or input power source and reduces

switching noise in the IC. The impedance of the input

capacitor at the switching frequency should be less

than that of the input source so high-frequency switch-

ing currents do not pass through the input source.

The input capacitor must meet the ripple-current

requirement (IRMS) imposed by the switching currents.

Nontantalum chemistries (ceramic, POSCAP, or OS-

CON) are preferred due to their resistance to power-up

surge currents:

⎛

IRMS = ILOAD ⎝⎜⎜

VOUT

(VBATT

VBATT

-

VOUT

)

⎞

⎠⎟⎟

For optimal circuit reliability, choose a capacitor that

has less than 10°C temperature rise at the peak ripple

current.

14 ______________________________________________________________________________________

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