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 10k

Ω to 1MΩ range

(typically 100k

Ω), 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.

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

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

Step 2:

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

_______________________________________________________________________________________

9

Table 1. Component Suppliers

MAXL001*

0.65 x 0.33 dia.

100

1.75

7300-13**

0.63 x 0.26 dia.

100

0.89

7300-15**

0.63 x 0.26 dia.

150

0.72

7300-17**

0.63 x 0.26 dia.

220

0.58

7300-19**

0.63 x 0.26 dia.

330

0.47

7300-21**

0.63 x 0.26 dia.

470

0.39

7300-25**

0.63 x 0.26 dia.

1000

0.27

PART

NUMBER

SIZE

(mm)

VALUE

(µH)

(A)

CD54

5.2 x 5.8 x 4.5

100

0.52

CD54

5.2 x 5.8 x 4.5

220

0.35

CDR74

7.1 x 7.7 x 4.5

100

0.52

CDR74

7.1 x 7.7 x 4.5

220

0.35

CDR105

9.2 x 10.0 x 5.0

100

0.80

CDR105

9.2 x 10.0 x 5.0

220

0.54

PART

NUMBER

SIZE

(inches)

VALUE

(µF)

ESR

(

Ω)

MAXC001*

150

0.2

267 Series**

D SM packages

47

0.2

267 Series**

E SM packages

100

0.2

PART

NUMBER

SIZE

(V)

SE014

SOT89

0.55

SE024

SOT89

0.55

INDUCTORS — THROUGH HOLE

0.2

0.27

0.36

0.45

0.58

0.86

2.00

INDUCTORS — SURFACE MOUNT

SERIES R

(

Ω)

0.63

1.50

0.51

0.98

0.35

0.69

Sumida Electric (USA)

637 East Golf Road

Arlington Heights, IL 60005

(708) 956-0666

CAPACITORS — LOW ESR

(V)

35

10

6.3

* Maxim Integrated Products

**Matsuo Electronics

2134 Main Street

Huntington Beach, CA 92648

(714) 969-2491

SCHOTTKY DIODES — SURFACE MOUNT

(A)

1

0.95

PART

NUMBER

SIZE

(inches)

VALUE

(µH)

(A)

SERIES R

(

Ω)

* Maxim Integrated Products

**Caddell-Burns

258 East Second Street

Mineola, NY 11501-3508

(516) 746-2310

0.49 x 0.394 dia.

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.