MAX639 查看數據表（PDF） - Maxim Integrated

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MAX639

5V/3.3V/3V/Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Converters

Maxim Integrated 

5V/3.3V/3V/Adjustable, High-Efficiency,

Low IQ, Step-Down DC-DC Converters

Low-Battery Detector

The low-battery detector compares the voltage on the

LBI input with the internal 1.28V reference. LBO goes

low whenever the input voltage at LBI is less than

1.28V. Set the low-battery detection voltage with resis-

tors R1 and R2 (Figure 3) as determined by the follow-

ing formula:

R1 = R2 [(VLB / LBI Threshold) - 1]

where R2 is any resistance in the 10kΩ to 1MΩ range

(typically 100kΩ), the LBI threshold is typically 1.28V,

and VLB is the desired low-battery detection voltage.

The low-battery comparator remains active in shutdown

mode.

Shutdown Mode

Bringing SHDN below 0.8V places the MAX639/

MAX640/MAX643 in shutdown mode. LX becomes high

impedance, and the voltage at VOUT falls to zero. The

time required for the output to rise to its nominal regu-

lated voltage when brought out of shutdown (start-up

time) depends on the inductor value, input voltage, and

load current (see the Start-Up Time vs. Output Current

graph in the Typical Operating Characteristics). The

low-battery comparator remains active in shutdown

mode.

__________Applications Information

Inductor Selection

When selecting an inductor, consider these four factors:

peak-current rating, inductance value, series resistance,

and size. It is important not to exceed the inductor’s

peak-current rating. A saturated inductor will pull exces-

sive currents through the MAX639/MAX640/MAX653’s

switch, and may cause damage. Avoid using RF chokes

or air-core inductors since they have very low peak-cur-

rent ratings. Electromagnetic interference must not upset

nearby circuitry or the regulator IC. Ferrite-bobbin types

work well for most digital circuits; toroids or pot cores

work well for EMI-sensitive analog circuits.

Recall that the inductance value determines IPEAK for all

input voltages (Equation 3). If there are no resistive loss-

es and the diode is ideal, the maximum average current

that can be drawn from the MAX639/MAX640/MAX653

will be one-half IPEAK. With the real losses in the switch,

inductor, and diode taken into account, the real maxi-

mum output current typically varies from 90% to 50% of

the ideal. The following steps describe a conservative

way to pick an appropriate inductor.

Step 1: Decide on the maximum required output

current, in amperes: IOUTMAX.

Step 2: IPEAK = 4 x IOUTMAX.

Table 1. Component Suppliers

PART

NUMBER

MAXL001*

INDUCTORS — THROUGH HOLE

SIZE

(inches)

VALUE

(µH)

IMAX

(A)

0.65 x 0.33 dia. 100

1.75

SERIES R

(Ω)

0.2

7300-13** 0.63 x 0.26 dia. 100

0.89

0.27

7300-15** 0.63 x 0.26 dia. 150

0.72

0.36

7300-17** 0.63 x 0.26 dia. 220

0.58

0.45

7300-19** 0.63 x 0.26 dia. 330

0.47

0.58

7300-21** 0.63 x 0.26 dia. 470

0.39

0.86

7300-25** 0.63 x 0.26 dia. 1000

0.27

2.00

* Maxim Integrated Products

**Caddell-Burns

258 East Second Street

Mineola, NY 11501-3508

(516) 746-2310

PART

NUMBER

CD54

INDUCTORS — SURFACE MOUNT

SIZE

(mm)

VALUE

(µH)

IMAX

(A)

5.2 x 5.8 x 4.5

100

0.52

SERIES R

(Ω)

0.63

CD54

5.2 x 5.8 x 4.5

220

0.35

1.50

CDR74

7.1 x 7.7 x 4.5

100

0.52

0.51

CDR74

7.1 x 7.7 x 4.5

220

0.35

0.98

CDR105 9.2 x 10.0 x 5.0 100

0.80

0.35

CDR105 9.2 x 10.0 x 5.0 220

0.54

Sumida Electric (USA)

637 East Golf Road

Arlington Heights, IL 60005

(708) 956-0666

CAPACITORS — LOW ESR

PART

NUMBER

SIZE

(inches)

VALUE

(µF)

ESR

(Ω)

MAXC001* 0.49 x 0.394 dia. 150

0.2

0.69

VMAX

(V)

35

267 Series** D SM packages 47

0.2

10

267 Series** E SM packages 100

0.2

6.3

* Maxim Integrated Products

**Matsuo Electronics

2134 Main Street

Huntington Beach, CA 92648

(714) 969-2491

SCHOTTKY DIODES — SURFACE MOUNT

PART

NUMBER

SIZE

VF

IMAX

(V)

(A)

SE014

SOT89

0.55

1

SE024

SOT89

0.55

0.95

Collmer Semiconductor

14368 Proton Road

Dallas, TX 75244

(214) 233-1589

NOTE: This list does not constitute an endorsement by Maxim

Integrated Products and is not intended to be a comprehensive

list of all manufacturers of these components.

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