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LM2612 Datasheet(Fiches technique) 16 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Numéro de pièce LM2612
Description  400mA Sub-miniature, Programmable, Step-Down DC-DC Converter for Ultra Low-Voltage Circuits
Télécharger  19 Pages
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Fabricant  NSC [National Semiconductor (TI)]
Site Internet  http://www.national.com
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Soft-Start (Continued)
Soft start is implemented by ramping up the internal refer-
ence in the LM2612 to gradually increase the output voltage.
The reference ramps up in about 400µs. When powering up
in PWM mode, soft start may take an additional 200us to
allow time for the error amplifier compensation network to
charge.
Thermal Overload Protection
The LM2612 has a thermal overload protection function that
operates to protect itself from short-term misuse and over-
load conditions. When the junction temperature exceeds
about 155˚C, the device initiates a soft-start cycle which is
completed after the temperature drops below 130˚C. Pro-
longed operation in thermal overload conditions may dam-
age the device and is considered bad practice.
Application Information
Setting The Output Voltage
The LM2612 features pin-selectable output voltage to elimi-
nate the need for external feedback resistors. Select an
output voltage of 1.05V, 1.3V, 1.5V or 1.8V by configuring the
VID0 and VID1 pins, as directed in
Table 1.
TABLE 1. VID0 and VID1 Output Voltage Selection
Settings
V
OUT (V)
Logic Level
VID0
VID1
1.8
0
1
1.5
0
0
1.5
N.C.
N.C.
1.3
1
0
1.05
1
1
VID0 and VID1 are digital inputs. They may be set high by
connecting to VDD or low by connecting to SGND. Option-
ally, VID0 and VID1 may be driven by digital gates that
provide over 1.2V for a high state and less than 0.4V for a
low state to ensure valid logic levels. The VID0 and VID1
inputs each have an internal 1.8 µA pull-down that pulls them
low for a default 1.5V output, when left unconnected. Leav-
ing these pins open is acceptable, but setting the pins high or
low is recommended.
Inductor Selection
A 10µH inductor with a saturation current rating over 850mA
(980mA for B grade) is recommended for most applications.
The inductor’s resistance should be less than 0.3
Ω for good
efficiency.
Table 2 lists suggested inductors and suppliers.
TABLE 2. Suggested Inductors and Their Suppliers
Model
Vendor
Phone
FAX
DO1608C-103
Coilcraft
847-639-6400
847-639-1469
DO1606T-103
Coilcraft
UP1B-100
Coiltronics
561-241-7876
561-241-9339
UP0.4CB-100
Coiltronics
ELL6GM100M
Panasonic
714-373-7366
714-373-7323
ELL6PM100M
Panasonic
P1174.103T
Pulse Engineering
858-674-8100
858-674-8262
CDRH5D18-100
Sumida
847-956-0666
847-956-0702
CDRH4D28-100
Sumida
CDC5D23-100
Sumida
NP05D B100M
Taiyo Yuden
847-925-0888
847-925-0899
NP04S B100N
Taiyo Yuden
SLF6025T-100M1R0
TDK
847-803-6100
847-803-6296
SLF6020T-100MR90
TDK
A918CY-100M
Toko
847-297-0070
847-699-7864
A915AY-100M
Toko
For low-cost applications, an unshielded bobbin inductor is
suggested. For noise critical applications, a toroidal or
shielded-bobbin inductor should be used. A good practice is
to lay out the board with overlapping footprints of both types
for design flexibility. This allows substitution of a low-noise
toroidal inductor, in the event that noise from low-cost bobbin
models is unacceptable.
The saturation current rating is the current level beyond
which an inductor looses it’s inductance. Beyond this rating,
the inductor looses it’s ability to limit current through the
PFET switch to a ramp and allows the switch current to
increase rapidly. This can cause poor efficiency, regulation
errors or stress to DC-DC converters like the LM2612. Satu-
ration occurs when the magnetic flux density from current
through the windings of the inductor exceeds what the in-
ductor’s core material can support with energy storage in a
corresponding magnetic field.
TABLE 3. Suggested Capacitors and Their Suppliers
Model
Size
Vendor
Phone
FAX
22µF, X7R or X5R Ceramic Capacitor for C2 (Output Filter Capacitor)
C3225X5RIA226M
1210
TDK
847-803-6100
847-803-6296
www.national.com
16




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