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參數(shù)資料
| 型號(hào)： | LTC1772 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Constant Frequency Current Mode Step-Down DC/DC Controller in SOT-23 |
| 中文描述： | 恒定頻率電流模式降壓DC / DC控制器，采用SOT - 23 |
| 文件頁(yè)數(shù)： | 7/16頁(yè) |
| 文件大小： | 188K |
| 代理商： | LTC1772 |
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7
LTC1772
APPLICATIO
S I
FOR
ATIO
U
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.
W
U
U
Power MOSFET Selection
An external P-channel power MOSFET must be selected
for use with the LTC1772. The main selection criteria for
the power MOSFET are the threshold voltage V
GS(TH)
and
the “on” resistance RDS(ON), reverse transfer capaci-
tance C
RSS
and total gate charge.
Since the LTC1772 is designed for operation down to low
input voltages, a sublogic level threshold MOSFET (R
DS(ON)
guaranteed at V
GS
= 2.5V) is required for applications that
work close to this voltage. When these MOSFETs are used,
make sure that the input supply to the LTC1772 is less than
the absolute maximum V
GS
rating, typically 8V.
The required minimum R
DS(ON)
of the MOSFET is gov-
erned by its allowable power dissipation. For applications
that may operate the LTC1772 in dropout, i.e., 100% duty
cycle, at its worst case the required R
DS(ON)
is given by:
R
P
I
p
DS ON
(
P
)
OUT MAX
(
DC
)
)
%
=
=
(
+
(
)
100
2
1
δ
where P
P
is the allowable power dissipation and
δ
p is the
temperature dependency of R
DS(ON)
. (1 +
δ
p) is generally
given for a MOSFET in the form of a normalized R
DS(ON)
vs
temperature curve, but
δ
p = 0.005/
°
C can be used as an
approximation for low voltage MOSFETs.
In applications where the maximum duty cycle is less than
100% and the LTC1772 is in continuous mode, the R
DS(ON)
is governed by:
P
DC I
OUT
where DC is the maximum operating duty cycle of the
LTC1772.
R
p
DS ON
(
P
2
1
δ
)
(
+
(
)
Output Diode Selection
The catch diode carries load current during the off-time.
The average diode current is therefore dependent on the
P-channel switch duty cycle. At high input voltages the
diode conducts most of the time. As V
IN
approaches 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
D
IN
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.
Kool Mu is a registered trademark of Magnetics, Inc.
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相關(guān)代理商/技術(shù)參數(shù) |
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|---|---|
| LTC1772BES6 | 制造商:Linear Technology 功能描述:IC CTRLR DC/DC S-DWN SOT23-6 |
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