參數(shù)資料
型號(hào)： LTC3713
廠商： Linear Technology Corporation
英文描述： Low Input Voltage, High Power, No RSENSE Synchronous Buck DC/DC Controller
中文描述： 低輸入電壓，高功率，無(wú)檢測(cè)電阻同步降壓DC / DC控制器
文件頁(yè)數(shù)： 13/24頁(yè)
文件大?。?/td> 325K
代理商： LTC3713
13
LTC3713
3713fa
APPLICATIOU
specific application. A good starting point is to feed about
25% of the voltage change at the I
TH
pin to the V
ON
pin as
shown in Figure 4a. Place capacitance on the V
ON
pin to
filter out the I
TH
variations at the switching frequency. The
resistor load on I
TH
reduces the DC gain of the error amp
and degrades load regulation, which can be avoided by
using the PNP emitter follower of Figure 4b.
W
U
U
Inductor L1 Selection
Given the desired input and output voltages, the inductor
value and operating frequency determine the ripple
current:
=
fL
V
IN
Lower ripple current reduces cores losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Highest efficiency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, efficiency and operating frequency.
A reasonable starting point is to choose a ripple current
that is about 40% of I
OUT(MAX)
. The largest ripple current
occurs at the highest V
IN
. To guarantee that ripple current
does not exceed a specified maximum, the inductance
should be chosen according to:
(
)
(
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron
cores, forcing the use of more expensive ferrite,
molypermalloy or Kool M
μ
cores. A variety of inductors
designed for high current, low voltage applications are
available from manufacturers such as Sumida, Panasonic,
Coiltronics, Coilcraft and Toko.
I
V
V
L
OUT
OUT
1
L
V
f I
V
V
OUT
L MAX
OUT
IN MAX
=
)
1
Schottky Diode D1 Selection
The Schottky diode D1 shown in Figure 1 conducts during
the dead time between the conduction of the power
MOSFET switches. It is intended to prevent the body diode
of the bottom MOSFET from turning on and storing charge
during the dead time, which can cause a modest (about
1%) efficiency loss. The diode can be rated for about one
half to one fifth of the full load current since it is on for only
a fraction of the duty cycle. In order for the diode to be
effective, the inductance between it and the bottom MOSFET
must be as small as possible, mandating that these
components be placed adjacently. The diode can be omit-
ted if the efficiency loss is tolerable.
C
IN
and C
OUT
Selection
The input capacitance C
IN
is required to filter the square
wave current at the drain of the top MOSFET. Use a low
ESR capacitor sized to handle the maximum RMS current.
I
I
V
V
V
V
RMS
OUT MAX
(
OUT
IN
IN
OUT
)
–1
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT(MAX)
/2. This simple worst-case condition is
commonly used for design because even significant
deviations do not offer much relief. Note that ripple
current ratings from capacitor manufacturers are often
based on only 2000 hours of life which makes it advisable
to derate the capacitor.
The selection of C
OUT
is primarily determined by the ESR
required to minimize voltage ripple and load step
transients. The output ripple
V
OUT
is approximately
bounded by:
fC
OUT
8
Since
I
L
increases with input voltage, the output ripple is
highest at maximum input voltage. Typically, once the ESR
requirement is satisfied, the capacitance is adequate for
filtering and has the necessary RMS current rating.
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic and
ceramic capacitors are all available in surface mount
packages. Special polymer capacitors offer very low ESR
but have lower capacitance density than other types.
Kool M
μ
is a registered trademark of Magnetics, Inc.
≤
+
V
I
ESR
OUT
L
1
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