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參數(shù)資料
| 型號(hào)： | LT1432CS8-3.3 |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS |
| 中文描述： | SWITCHING CONTROLLER, PDSO8 |
| 封裝： | 0.150 INCH, PLASTIC, SO-8 |
| 文件頁數(shù)： | 17/28頁 |
| 文件大?。?/td> | 611K |
| 代理商： | LT1432CS8-3.3 |
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LT1432
function of duty cycle. Care must be taken to ensure that
the sum of output current plus 1/2 inductor p-p ripple
current does not exceed the switch current limit at the
highest duty cycle (lowest input voltage).
Duty Cycle (maximum)
Vf = Diode forward voltage
1/2 p-p Ripple Current
(Use minimum V
IN
+2V)
A 2.5A design using an LT1271 at 60kHz, with a minimum
input voltage of 7V and a 50
μ
H inductor, would have a
maximum duty cycle of (5 + 0.5)/7 = 79%. 1/2 p-p ripple
current would be:
( )
(
2 – 5
2 7+2 60E
50E
0.37A
3
–6
+
)
=
Output current plus 1/2 ripple current = 2.5 + 0.37 = 2.9A.
The switch current rating for the LT1271 is shown on the
data sheet as 4A for duty cycle below 50% and 2.67 (2–
DC) for duty cycles greater than 50%. With DC = 79%,
switch current rating would be 2.67 (2 – 0.79) = 3.23A, so
this meets the guidelines. It should be noted that if normal
running load current conditions result in switch currents
that are close to the maximum switch ratings, efficiency
will drop. Switch voltage loss at maximum switch current
rating is typically 0.7V, and this represents a significant
loss, especially at low input voltages. In most laptop
computer designs, surge currents from hard or floppy
disks require an oversized switcher, so normal running
currents are typically less than one half rated switch
current and efficiency is high except during the short
surge periods.
2. Ferrite designs have very low core loss, so design
goals can concentrate on copper loss and preventing
saturation. The downside is that the finished unit will
almost surely be larger than a molypermalloy toroid de-
sign because of the basic topological limitations of the
ferrite/bobbin arrangement. Newer low-profile ferrite cores
are even less space efficient than older configurations.
Cost may also be higher. Ferrite core material saturates
“hard,” which means that inductance collapses abruptly
when peak design current is exceeded. This may be a
problem in current limit or if peak load requirements are
not well characterized.
3. Molypermalloy (from Magnetics, Inc.) is a very
good, low loss core material for toroids, but it is (naturally)
rather expensive. A reasonable substitute is KoolM
μ
(same
manufacturer). Toroids are very space efficient, especially
when you can convince the manufacturer to use several
layers of wire. Because they generally lack a bobbin,
mounting is more difficult. Newer designs for surface
mount are available (Coiltronics), which are nested in a
ring that does not increase the height significantly.
Catch Diode
The catch diode carries load current only during switch
“off” time. Its average current is therefore dependent on
switch duty cycle. At high input voltages, the diode con-
ducts most of the time, and as V
IN
approaches V
OUT
, it
conducts only a small fraction of the time. The current
rating of the diode should be higher than maximum load
current for two reasons. First, conservative diode current
improves efficiency because the diode forward voltage is
lower, and second, short circuit conditions result in near
100% diode duty cycle at currents higher than full load
unless some form of foldback current limiting is used.
Schottky diodes are a must for their low forward drop and
fast switching times.
Maximum diode reverse voltage is equal to maximum
input voltage. However, do not over-specify the diode for
breakdown voltage. Schottky diodes are made with lighter
silicon doping as breakdown ratings increase. This gives
higher forward voltage and degrades regulator efficiency.
An MBR350 (3A, 50V) has almost 100mV higher forward
voltage than the MBR330 (3A, 30V).
Diode current ratings are predicated on proper thermal
mounting techniques. Check the manufacturers assump-
tions carefully before assuming that a 3A diode is actually
capable of carrying 3A continuously. Pad size may have to
be larger than normal to meet the mounting requirements
for full current capability.
U
S
A
O
PPLICATI
U
U
=
+
)
V
Vf
V
OUT
IN MIN
=
(
)(
)
)( )( )
V
V – V
f L
IN
2 V
OUT
IN
OUT
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