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參數(shù)資料
| 型號(hào)： | LTC1148CN-3.3 |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類(lèi)： | 穩(wěn)壓器 |
| 英文描述： | CB 3C 3#16S SKT RECP WALL |
| 中文描述： | SWITCHING CONTROLLER, 250 kHz SWITCHING FREQ-MAX, PDIP14 |
| 封裝： | 0.300 INCH, PLASTIC, DIP-14 |
| 文件頁(yè)數(shù)： | 9/20頁(yè) |
| 文件大?。?/td> | 384K |
| 代理商： | LTC1148CN-3.3 |
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9
LTC1148
LTC1148-3.3/LTC1148-5
APPLICATIU
W
U
U
Inductor Core Selection
Once the minimum value for L is known, the type of
inductor must be selected. The highest efficiency will be
obtained using ferrite, Kool M
μ
on molypermalloy (MPP)
cores. Lower cost powdered iron cores provide suitable
performance but cut efficiency by 3% to 7%. Actual core
loss is independent of core size for a fixed inductor value,
but it is very dependent on inductance selected. As induc-
tance increases, core losses go down. Unfortunately,
increased inductance requires more turns of wire and
therefore copper losses increase.
Ferrite designs have very low core loss, so design goals
can concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductance collapses abruptly when the peak design cur-
rent is exceeded. This results in an abrupt increase in
inductor ripple current and consequent output voltage
ripple which can cause Burst Mode
operation to be falsely
triggered. Do not allow the core to saturate!
Kool M
μ
(from Magnetics, Inc.) is a very good, low loss
core material for toroids, with a “soft” saturation charac-
teristic. Molypermalloy is slightly more efficient at high
(>200kHz) switching frequencies, but quite a bit more
expensive. Toroids are very space efficient, especially
when you can use several layers of wire. Because they
generally lack a bobbin, mounting is more difficult. How-
ever, new designs for surface mount are available from
Coiltronics and Beckman Industrial Corp. which do not
increase the height significantly.
Power MOSFET and D1 Selection
Two external power MOSFETs must be selected for use
with the LTC1148 series: a P-channel MOSFET for the
main switch, and an N-channel MOSFET for the synchro-
nous switch. The main selection criteria for the power
MOSFETs are the threshold voltage V
GS(TH)
and on resis-
tance R
DS(ON)
.
The minimum input voltage determines whether standard
threshold or logic-level threshold MOSFETs must be used.
For V
IN
> 8V, standard threshold MOSFETs (V
GS(TH)
< 4V)
may be used. If V
IN
is expected to drop below 8V, logic-
FREQUENCY (kHz)
0
0
C
200
400
600
100
200
LTC1148 F03
800
1000
300
V
SENSE–
= V
OUT
= 5V
V
IN
= 12V
V
IN
= 10V
V
IN
= 7V
Figure 3. Timing Capacitor Value
)
f =
1
t
OFF
)
1 –V
OUT
V
IN
where:
t
OFF
= 1.3(10
4
)C
T
)
)
V
REG
V
OUT
V
REG
is the desired output voltage (i.e., 5V, 3.3V). V
OUT
is
the measured output voltage. Thus V
REG
/V
OUT
= 1 in
regulation.
Note that as V
IN
decreases, the frequency decreases.
When the input to output voltage differential drops
below 1.5V, the LTC1148 series reduces t
OFF
by in-
creasing the discharge current in C
T
. This prevents
audible operation prior to dropout.
Once the frequency has been set by C
T
, the inductor L
must be chosen to provide no more than 25mV/R
SENSE
of peak-to-peak inductor ripple current. This results in
a minimum required inductor value of:
L
MIN
= 5.1(10
5
)R
SENSE
(C
T
)V
REG
As the inductor value is increased from the minimum
value, the ESR requirements for the output capacitor
are eased at the expense of efficiency. If too small an
inductor is used, the inductor current will decrease past
zero and change polarity.
A consequence of this is that
the LTC1148 series may not enter Burst Mode
operation
and efficiency will be severely degraded at low currents.
Kool M
μ
is a registered trademark of Magnetics, Inc.
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| LTC1148CN-5 | ER 3C 3#16S SKT RECP WALL |
| LTC1148CS-3.3 | RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS |
| LTC1148CS-5 | RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS |
| LTC1148LCS-3.3 | RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS |
| LTC1148CN | RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
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