MAX640 MAXIM [Maxim Integrated Products], MAX640 Datasheet - Page 9

• Manufacturer Part Number: MAX640
• Description: 5V/3.3V/3V/Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Converters
• Manufacturer: MAXIM [Maxim Integrated Products]

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:
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.
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.
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
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
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: I
__________Applications Information
current, in amperes: I
PEAK
R1 = R2 [(VLB / LBI Threshold) - 1]
= 4 x I
PEAK
_______________________________________________________________________________________
. With the real losses in the switch,
OUTMAX
5V/3.3V/3V/Adjustable, High-Efficiency,
Low I
Low-Battery Detector
.
OUTMAX
Inductor Selection
Shutdown Mode
.
Q
, Step-Down DC-DC Converters
PEAK
for all
Table 1. Component Suppliers
MAXL001*
7300-13**
7300-15**
7300-17**
7300-19**
7300-21**
7300-25**
* Maxim Integrated Products
**Caddell-Burns
CD54
CD54
CDR74
CDR74
CDR105
CDR105
Sumida Electric (USA)
637 East Golf Road
Arlington Heights, IL 60005
[phone]
MAXC001*
267 Series**
267 Series**
* Maxim Integrated Products
**Matsuo Electronics
SE014
SE024
Collmer Semiconductor
14368 Proton Road
Dallas, TX 75244
[phone]
NOTE: This list does not constitute an endorsement by Maxim
NUMBER
NUMBER
NUMBER
NUMBER
258 East Second Street
Mineola, NY 11501-3508
[phone]
2134 Main Street
Huntington Beach, CA 92648
[phone]
PART
PART
PART
PART
Integrated Products and is not intended to be a comprehensive
list of all manufacturers of these components.
SCHOTTKY DIODES — SURFACE MOUNT
INDUCTORS — SURFACE MOUNT
INDUCTORS — THROUGH HOLE
0.65 x 0.33 dia.
0.63 x 0.26 dia.
0.63 x 0.26 dia.
0.63 x 0.26 dia.
0.63 x 0.26 dia.
0.63 x 0.26 dia.
0.63 x 0.26 dia.
5.2 x 5.8 x 4.5
5.2 x 5.8 x 4.5
7.1 x 7.7 x 4.5
7.1 x 7.7 x 4.5
9.2 x 10.0 x 5.0
9.2 x 10.0 x 5.0
0.49 x 0.394 dia.
D SM packages
E SM packages
SOT89
SOT89
CAPACITORS — LOW ESR
(inches)
(inches)
(mm)
SIZE
SIZE
SIZE
SIZE
VALUE
VALUE
VALUE
1000
(µH)
(µH)
(µF)
100
100
150
220
330
470
100
220
100
220
100
220
150
100
47
0.55
0.55
(V)
V
F
I
1.75
0.89
0.72
0.58
0.47
0.39
0.27
I
0.52
0.35
0.52
0.35
0.80
0.54
ESR
MAX
MAX
0.2
0.2
0.2
(A)
(A)
(Ω)
SERIES R
SERIES R
V
I
0.27
0.36
0.45
0.58
0.86
2.00
0.63
1.50
0.51
0.98
0.35
0.69
0.95
MAX
0.2
6.3
(A)
(Ω)
(Ω)
(V)
MAX
35
10
1
9