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參數(shù)資料
| 型號： | LTC1174-3.3 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | High Efficiency Step-Down and Inverting DC/DC Converter(高效率,步降DC/DC變換器) |
| 中文描述： | 高效率降壓和倒轉(zhuǎn)DC / DC轉(zhuǎn)換器（高效率，步降直流/直流變換器） |
| 文件頁數(shù)： | 8/20頁 |
| 文件大?。?/td> | 246K |
| 代理商： | LTC1174-3.3 |
8
LTC1174
LTC1174-3.3/LTC1174-5
I
PGM
= V
IN
I
PGM
= 0
GND
L = 100
μ
H
V
IN
= 13.5V
20
μ
s/DIV
1174 F02
Figure 2. Inductor's Current with Output Shorted
the maximum RMS current must be used. The C
IN
RMS
current is given by:
I
I
V
V
V
V
A
RMS
OUT
OUT
IN
OUT
IN
RMS
≈
(
)
[
]
(
)
1 2
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
=
I
OUT
/2. This simple worst case is commonly used for design
because even significant deviations do not offer much relief.
Note that ripple current directly affects capacitor’s lifetime.
DO NOT UNDERSPECIFY THIS COMPONENT. An additional
0.1
μ
F ceramic capacitor is also required on V
IN
for high
frequency decoupling.
C
OUT
To avoid overheating, the output capacitor must be sized to
handle the ripple current generated by the inductor. The
worst case RMS ripple current in the output capacitor is
given by:
I
I
A
mA
RMS
PEAK
2
170
RMS
≈
(
)
=
or 300mA
Although the output voltage ripple is determined by the
hysteresis of the voltage comparator, ESR of the output
capacitor is also a concern. Too high of an ESR will create
a higher ripple output voltage and at the same time cause the
LTC1174 to sleep less often. This will affect the efficiency of
the LTC1174. For a given technology, ESR is a direct
function of the volume of the capacitor. Several small-sized
capacitors can also be paralleled to obtain the same ESR as
one large can. Manufacturers such as Nichicon, Chemicon
and Sprague should be considered for high performance
capacitors. The OS-CON semiconductor dielectric capaci-
tor available from Sanyo has the lowest ESR for its size, at
a higher price.
Catch 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
APPLICATIOU
W
U
U
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. 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.
Most LTC1174 circuits will be well served by either a 1N5818,
a MBRS140T3 or a MBR0520L Schottky diode.
Short-Circuit Protection
The LTC1174 is protected from output short by its internal
current limit. Depending on the condition of I
PGM
pin, the
limit is either set to 340mA or 600mA. In addition, the off-
time of the switch is increased to allow the inductor’s
current to decay far enough to prevent any current build-up
(see Figure 2).
Low-Battery Detector
The low-battery indicator senses the input voltage through
an external resistive divider. This divided voltage connects
to the “–” input of a voltage comparator (Pin 3) which is
compared with a 1.25V reference voltage. With the current
going into Pin 3 being negligible, the following expression
is used for setting the trip limit:
V
R
R
LBTRIP
=
+
1 25 1
.
4
3
When the LTC1174 is shut down, the low-battery detector
is inactive.
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