參數(shù)資料
型號: LTC3714EG
廠商: LINEAR TECHNOLOGY CORP
元件分類: 穩(wěn)壓器
英文描述: Intel Compatible, Wide Operating Range, Step-Down Controller with Internal Op Amp
中文描述: 2 A SWITCHING CONTROLLER, PDSO28
封裝: 5.30 MM, PLASTIC, SSOP-28
文件頁數(shù): 14/28頁
文件大?。?/td> 345K
代理商: LTC3714EG
LTC3714
14
3714f
APPLICATIOU
W
U
U
Inductor Selection
Given the desired input and output voltages, the inductor
value and operating frequency determine the ripple
current:
=
I
V
fL
V
V
L
OUT
OUT
IN
1
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:
L
V
f I
V
V
OUT
L MAX
(
OUT
IN MAX
(
=
)
)
1
Once the value for L is known, the type of inductor must be
selected. A variety of inductors designed for high current,
low voltage applications are available from manufacturers
such as Sumida and Panasonic.
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, causing 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.
C
0.01
μ
F
R
100k
R
VON1
30k
C
C
3714 F04a
V
OUT
R
C
V
ON
I
TH
LTC3714
C
0.01
μ
F
R
VON2
10k
Q1
2N5087
R
VON1
3k
10k
C
C
3714 F04b
V
OUT
INTV
CC
R
C
V
ON
I
TH
LTC3714
(4a)
Figure 4. Correcting Frequency Shift with Load Current Changes
(4b)
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