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參數(shù)資料
| 型號(hào)： | LTC3700 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Constant Frequency Step-Down DC/DC Controller with LDO Regulator |
| 中文描述： | 恒頻降壓DC / DC控制器LDO穩(wěn)壓器 |
| 文件頁(yè)數(shù)： | 12/16頁(yè) |
| 文件大?。?/td> | 209K |
| 代理商： | LTC3700 |
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12
LTC3700
3700f
APPLICATIO
S I
FOR
ATIO
U
In applications where the maximum duty cycle is less than
100% and the buck is in continuous mode, the R
DS(ON)
is
governed by:
W
U
U
R
P
DC I
(
p
DS ON
(
P
2
1
δ
OUT
)
)
+
(
)
where DC is the maximum operating duty cycle of the
buck.
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.
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.
C
IN
and C
OUT
Selection
In continuous mode, the source current of the P-channel
MOSFET is a square wave of duty cycle (V
OUT
+ V
D
)/
(V
IN
+ V
D
). To prevent large voltage transients, a low ESR
input capacitor sized for the maximum RMS current must
be used. The maximum RMS capacitor current is given by:
C
I
I
V
V
V
V
IN
RMS
MAX
OUT
IN
OUT
IN
Required
≈
(
)
[
]
1 2
This formula has a maximum value at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst-case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that capacitor manufacturer’s
ripple current ratings are often based on 2000 hours of life.
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may be paralleled to meet the
size or height requirements in the design. Due to the high
operating frequency of the LTC3700, ceramic capacitors
can also be used for C
IN
. Always consult the manufacturer
if there is any question.
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