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 resistors R1 and R2 (Figure 3) as determined by the following 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 regulated 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 excessive 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-current 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 losses 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 maximum 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 IQ, 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.