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參數(shù)資料
| 型號(hào)： | LTC1174HVCS8#TR-5 |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING REGULATOR, 200 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | 0.150 INCH, PLASTIC, SO-8 |
| 文件頁數(shù)： | 19/20頁 |
| 文件大?。?/td> | 241K |
| 代理商： | LTC1174HVCS8#TR-5 |
8
LTC1174
LTC1174-3.3/LTC1174-5
IPGM = VIN
IPGM = 0
GND
L = 100
H
VIN = 13.5V
20
s/DIV
1174 F02
Figure 2. Inductor's Current with Output Shorted
the maximum RMS current must be used. The CIN RMS
current is given by:
I
IV
V
A
RMS
OUT
IN
OUT
IN
RMS
≈
()
[]
()
12
/
This formula has a maximum at VIN = 2VOUT, where IRMS =
IOUT/2.Thissimpleworstcaseiscommonlyusedfordesign
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
F ceramic capacitor is also required on VIN for high
frequency decoupling.
COUT
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
A
mA
RMS
PEAK
RMS
≈
()
=
2
170
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 VIN approaches VOUT
APPLICATIO S I FOR ATIO
WU
UU
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
handleIPEAKatcloseto100%dutycycle.Afastswitchingdiode
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 IPGM 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
LBTRIP =+
125 1
4
3
.
When the LTC1174 is shut down, the low-battery detector
is inactive.
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