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參數(shù)資料
| 型號： | LTC1265-3.3 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | 1.2A, High Efficiency Step-Down Current Mode DC/DC Converter(3.3V,12A,高效率,電流模式步降DC/DC變換器) |
| 中文描述： | 1.2A的，高效率降壓型電流模式DC / DC轉(zhuǎn)換器（3.3V，12A條，高效率，電流模式步降直流/直流變換器） |
| 文件頁數(shù)： | 8/16頁 |
| 文件大?。?/td> | 233K |
| 代理商： | LTC1265-3.3 |
8
LTC1265/LTC1265-3.3/LTC1265-5
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
SC(PK)
at close to 100% duty cycle. Most
LTC1265 circuits will be well served by either a 1N5818 or
a MBRS130LT3 Schottky diode. An MBRS0520 is a good
choice for I
OUT(MAX)
≤
500mA.
C
IN
In continuous mode, the input current of the converter is
a square wave of duty cycle V
OUT
/V
IN
. To prevent large
voltage transients, a low ESR input capacitor must be
used. In addition, the capacitor must handle a high RMS
current. The C
IN
RMS current is given by:
I
RMS
≈
I
OUT
[V
OUT
(V
IN
–
V
OUT
)]
1
/
2
IN
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 capacitor manufacturer’s ripple
current ratings are often based on only 2000 hours life-
time. This makes it advisable to further derate the capaci-
tor, or to choose a capacitor rated at a higher temperature
than required.
Do not underspecify this component
. An
additional 0.1
μ
F ceramic capacitor is also required on
PWR V
IN
for high frequency decoupling.
(A
RMS
)
C
OUT
The selection of C
OUT
is based upon the effective series
resistance (ESR) for proper operation of the LTC1265. The
required ESR of C
OUT
is:
ESR
COUT
< 50mV/I
RIPPLE
where I
RIPPLE
is the ripple current of the inductor. For the
case where the I
RIPPLE
is 25mV/R
SENSE
, the required ESR
of C
OUT
is:
ESR
COUT
< 2(R
SENSE
)
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:
150mV
2(R
SENSE
)
I
RMS
≈
(A
RMS
)
Generally, once the ESR requirement for C
OUT
has been
met, the RMS current rating far exceeds the I
RIPPLE(P-P)
requirement.
ESR is a direct function of the volume of the capacitor.
Manufacturers such as Nichicon, AVX and Sprague should
be considered for high performance capacitors. The
OS-CON semiconductor dielectric capacitor available
from Sanyo has the lowest ESR for its size at a somewhat
higher price.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the capacitance, ESR or RMS
current handling requirement of the application. Alumi-
num electrolyte and dry tantalum capacitors are both
available in surface mount configurations. In the case of
tantalum, it is critical that the capacitors are both available
in surface mount configuration and are surge tested for
use in switching power supplies. An excellent choice is the
AVX TPS series of surface mount tantalums, available in
case heights ranging from 2mm to 4mm. Consult the
manufacturer for other specific recommendations.
When the capacitance of C
OUT
is made too small, the
output ripple at low frequencies will be large enough to trip
the voltage comparator. This causes Burst Mode opera-
tion to be activated when the LTC1265 would normally be
in continuous operation. The effect will be most pro-
nounced with low value of R
SENSE
and can be improved at
higher frequencies with lower values of L.
Low-Battery Detection
The low-battery comparator senses the input voltage
through an external resistive divider. This divided voltage
connects to the (–) input of a voltage comparator (Pin 4)
which is compared with a 1.25V reference voltage. Ne-
glecting Pin 4 bias current, the following expression is
used for setting the trip limit:
R3
)
V
LB_TRIP
= 1.25 1 + R4
)
APPLICATIOU
W
U
U
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