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參數(shù)資料
| 型號： | LTC1435AI |
| 廠商： | Linear Technology Corporation |
| 英文描述： | High Efficiency Low Noise Synchronous Step-Down Switching Regulator |
| 中文描述： | 高效率低噪聲同步降壓型開關(guān)穩(wěn)壓器 |
| 文件頁數(shù)： | 8/20頁 |
| 文件大小： | 427K |
| 代理商： | LTC1435AI |
8
LTC1435A
APPLICATIO
S I
N
FOR
ATIO
U
The value of C
OSC
is calculated from the desired operating
frequency:
W
U
C
pF
OSC
(
)
–
=
1.37(10 )
Frequency (kHz)
11
A graph for selecting C
OSC
vs frequency is given in Figure
2. As the operating frequency is increased the gate charge
losses will be higher, reducing efficiency (see Efficiency
Considerations). The maximum recommended switching
frequency is 400kHz.
greater core losses. A reasonable starting point for setting
ripple current is
I
L
= 0.4(I
MAX
). Remember, the maximum
I
L
occurs at the maximum input voltage.
The inductor value also has an effect on low current opera-
tion. The transition to low current operation begins when
the inductor current reaches zero while the bottom MOSFET
is on. Lower inductor values (higher
I
L
) will cause this to
occur at higher load currents, which can cause a dip in
efficiency in the upper range of low current operation. In
Burst Mode operation, lower inductance values will cause
the burst frequency to decrease.
The Figure 3 graph gives a range of recommended induc-
tor values vs operating frequency and V
OUT
.
OPERATING FREQUENCY (kHz)
C
O
300
250
200
150
100
50
0
100
200
300
400
1435A F02
500
0
Figure 2. Timing Capacitor Value
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with
larger components The answer is efficiency. A higher
frequency generally results in lower efficiency because of
MOSFET gate charge losses. In addition to this basic trade-
off, the effect of inductor value on ripple current and low
current operation must also be considered.
The inductor value has a direct effect on ripple current. The
inductor ripple current
I
L
decreases with higher induc-
tance or frequency and increases with higher V
IN
or V
OUT
:
I
V
V
L
OUT
OUT
IN
=
( )( )
1
1–
Accepting larger values of
I
L
allows the use of low induc-
tances, but results in higher output voltage ripple and
Kool M
μ
is a registered trademark of Magnetics, Inc.
For low duty cycle, high frequency applications where the
required minimum on-time,
t
V
V
f
ON MIN
(
OUT
IN MAX
(
)
)
,
=
(
)( )
is less than 350ns, there may be further restrictions on the
inductance to ensure proper operation. See Minimum On-
Time Considerations section for more details.
Inductor Core Selection
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, forc-
ing the use of more expensive ferrite, molypermalloy or Kool
M
μ
cores. Actual core loss is independent of core size for
OPERATING FREQUENCY (kHz)
0
0
I
μ
H
10
20
30
40
60
50
100
150
200
1435A F03
250
300
50
V
OUT
= 5.0V
V
OUT
= 3.3V
V
OUT
≤
2.5V
Figure 3. Recommended Inductor Values
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| LTC1435AIG | High Efficiency Low Noise Synchronous Step-Down Switching Regulator |
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| LTC1435C | High Efficiency Low Noise Synchronous Step-Down Switching Regulator |
| LTC1435CG | High Efficiency Low Noise Synchronous Step-Down Switching Regulator |
| LTC1435CS | High Efficiency Low Noise Synchronous Step-Down Switching Regulator |
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