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參數(shù)資料
| 型號(hào)： | LTC1779 |
| 廠(chǎng)商： | Linear Technology Corporation |
| 英文描述： | 250mA Low Dropout Current Mode Step-Down DC/DC Converter in SOT-23(2.5V輸出,250mA電流模式，低壓差,步降DC/DC變換器（SOT-23封裝）) |
| 中文描述： | 250mA的低壓差電流模式降壓DC / DC轉(zhuǎn)換器采用SOT - 23（2.5V的輸出，250mA的電流模式，低壓差，步降直流/直流變換器（采用SOT - 23封裝）） |
| 文件頁(yè)數(shù)： | 7/12頁(yè) |
| 文件大?。?/td> | 174K |
| 代理商： | LTC1779 |
7
LTC1779
where f is the operating frequency. Accepting larger values
of I
RIPPLE
allows the use of low inductances, but results in
higher output voltage ripple and greater core losses. A
reasonable starting point for setting ripple current is
I
RIPPLE
= 0.4(I
OUT(MAX)
). Remember, the maximum I
RIPPLE
occurs at the maximum input voltage.
In Burst Mode operation on the LTC1779, the ripple
current is normally set such that the inductor current is
continuous during the burst periods. Therefore, the peak-
to-peak ripple current must not exceed:
I
M
R
(
RIPPLE
SENSE
≤
+
2
(0 030
)
)
This implies a minimum inductance of:
L
V
V
f
M
R
V
V
V
V
MIN
IN
OUT
SENSE
OUT
D
IN
D
=
+
2
+
+
(0 030
)
(
)
(Use V
IN(MAX)
= V
IN
)
A smaller value than L
MIN
could be used in the circuit;
however, the inductor current will not be continuous
during burst periods.
Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool Mu
cores. Actual core loss is independent of core
size for a fixed inductor value, but it is very dependent on
inductance selected. As inductance increases, core losses
go down. Unfortunately, increased inductance requires
more turns of wire and therefore copper losses will
increase. Ferrite designs have very low core losses and are
preferred at high switching frequencies, 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. Do not allow the core to saturate!
Molypermalloy (from Magnetics, Inc.) is a very good, low
loss core material for toroids, but it is more expensive than
ferrite. A reasonable compromise from the same manu-
facturer is Kool Mu. Toroids are very space efficient,
especially when you can use several layers of wire.
Because they generally lack a bobbin, mounting is more
difficult. However, new designs for surface mount that do
not increase the height significantly are available.
Output Diode Selection
The catch diode carries load current during the off-time.
The average diode current is therefore dependent on the
internal P-channel switch duty cycle. At high input volt-
ages the diode conducts most of the time. As V
IN
ap-
proaches V
OUT
the diode conducts only a small fraction of
the time. The most stressful condition for the diode is
when the output is short-circuited. Under this condition
the diode must safely handle I
PEAK
at close to 100% duty
cycle. Therefore, it is important to adequately specify the
diode peak current and average power dissipation so as
not to exceed the diode ratings.
Under normal load conditions, the average current con-
ducted by the diode is:
I
V
V
V
V
I
D
IN
OUT
IN
D
OUT
=
+
The allowable forward voltage drop in the diode is calcu-
lated from the maximum short-circuit current as:
V
P
I
F
D
SC MAX
(
≈
)
where P
D
is the allowable power dissipation and will be
determined by efficiency and/or thermal requirements.
A fast switching diode must also be used to optimize
efficiency. Schottky diodes are a good choice for low
forward drop and fast switching times. Remember to keep
lead length short and observe proper grounding (see
Board Layout Checklist) to avoid ringing and increased
dissipation.
Kool Mu is a registered trademark of Magnetics, Inc.
APPLICATIOU
W
U
U
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