Rainbow Electronics MAX653 - Manual (Page 9)

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 1M
Ω
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.
Recall that the inductance value determines I PEAK 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 I
PEAK . 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: I
OUTMAX .
Step 2: I PEAK = 4 x I OUTMAX .
MAX639/MAX640/MAX653
5V/3.3V/3V/Adjustable, High-Efficiency,
Low I
Q
, Step-Down DC-DC Converters
_______________________________________________________________________________________ 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)
I MAX
(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 F
(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 MAX
(V)
35
10
6.3
* Maxim Integrated Products
**Matsuo Electronics
2134 Main Street
Huntington Beach, CA 92648
(714) 969-2491
SCHOTTKY DIODES — SURFACE MOUNT
I MAX
(A)
1
0.95
PART
NUMBER
SIZE
(inches)
VALUE
(µH)
I MAX
(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.