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| 型号： | LTC3717EUH-1#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 11/20页 |
| 文件大小： | 0K |
| 描述： | IC CTRLR STEP-DOWN SYNC 32QFN |
| 标准包装： | 73 |
| 应用： | 控制器，DDR，QDR |
| 输入电压： | 4 V ~ 36 V |
| 输出数： | 1 |
| 输出电压： | 2 V ~ 18 V |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 32-WFQFN 裸露焊盘 |
| 供应商设备封装： | 32-QFN 裸露焊盘（5x5） |
| 包装： | 管件 |
| 产品目录页面： | 1335 (CN2011-ZH PDF) |
LTC3717-1
APPLICATIO S I FOR ATIO
R VON1
30k
R VON1
3k
V OUT
R VON2
100k
R C
C VON
0.01 μ F
V ON
LTC3717-1
I TH
V OUT
INTV CC
10k
Q1
2N5087
R VON2
10k
C VON
0.01 μ F
R C
V ON
LTC3717-1
I TH
C C
(3a)
(3b)
C C
37171 F03
Figure 3. Adjusting Frequency Shift with Load Current Changes
? I L = ? OUT ? ? 1 ? OUT ?
? ? ? V OUT ?
L = ?
1 ?
? ?
?
? I
I RMS OUT ( MAX )
V OUT V IN
25%ofthevoltagechangeattheI TH pintotheV ON pinas
shown in Figure 3a. 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 3b.
Inductor L1 Selection
Given the desired input and output voltages, the inductor
value and operating frequency determine the ripple
current:
? V ? ? V ?
? 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:
V OUT
? f ? I L ( MAX ) ? ? V IN ( MAX ) ?
Once the value for L is known, the type of inductor must be
selected. High efficiency converters generally cannot af-
ford 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 manu-
facturers such as Sumida, Panasonic, Coiltronics, Coil-
craft and Toko.
Schottky Diode D1, D2 Selection
The Schottky diodes, D1 and D2, shown in Figure 1
conduct during the dead time between the conduction of
the power MOSFET switches. It is intended to prevent the
body diodes of the top and bottom MOSFETs from turning
on and storing charge during the dead time, which can
cause a modest (about 1%) efficiency loss. The diodes can
be rated for about one half to one fifth of the full load current
since they are 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 diodes can be omitted 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.
–1
V IN V OUT
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
Kool M μ is a registered trademark of Magnetics, Inc.
sn37171 37171fs
11
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