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| 型号： | LTC3642IDD-3.3#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 11/22页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK SYNC 3.3V 50MA 8DFN |
| 标准包装： | 121 |
| 类型： | 降压（降压） |
| 输出类型： | 固定 |
| 输出数： | 1 |
| 输出电压： | 3.3V |
| 输入电压： | 4.5 V ~ 45 V |
| PWM 型： | Burst Mode? |
| 电流 - 输出： | 50mA |
| 同步整流器： | 是 |
| 工作温度： | -40°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 8-WFDFN 裸露焊盘 |
| 包装： | 管件 |
| 供应商设备封装： | 8-DFN-EP（3x3） |
LTC3642
APPLICATIONS INFORMATION
700
600
500
400
V OUT = 5V
I SET OPEN
L = 47μH
L = 68μH
L = 100μH
where board area is not a limiting factor, inductors with
larger cores can be used, which extends the recommended
range of Figure 4 to larger values.
Inductor Core Selection
L = 150μH
300
200 L = 220μH
100 L = 470μH
0
5 10 15 20 25 30 35 40 45
INPUT VOLTAGE (V)
3642 F02
Figure 2. Switching Frequency for V OUT = 5V
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 the more expensive ferrite cores. Actual
core loss is independent of core size for a fixed inductor
value but is very dependent of the inductance selected.
As the inductance increases, core losses decrease. Un-
fortunately, increased inductance requires more turns of
wire and therefore copper losses will increase.
500
450
400
350
300
250
200
150
V OUT = 3.3V
I SET OPEN
L = 47μH
L = 68μH
L = 100μH
L = 150μH
L = 220μH
Ferrite designs have very low core losses and are pre-
ferred at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductance collapses abruptly when the peak design current
is exceeded. This results in an abrupt increase in inductor
ripple current and consequently output voltage ripple. Do
not allow the core to saturate!
100
L = 470μH
50
0
5 10 15 20 25 30 35 40 45
INPUT VOLTAGE (V)
3642 F03
Figure 3. Switching Frequency for V OUT = 3.3V
10000
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials
are small and do not radiate energy but generally cost
more than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
mainly depends on the price vs size requirements and any
radiated field/EMI requirements. New designs for surface
mount inductors are available from Coiltronics, Coilcraft,
Toko, Sumida and Vishay.
1000
100
10
100
C IN and C OUT Selection
The input capacitor, C IN , is needed to filter the trapezoidal
current at the source of the top high side MOSFET. To
prevent large ripple voltage, a low ESR input capacitor
sized for the maximum RMS current should be used.
Approximate RMS current is given by:
PEAK INDUCTOR CURRENT (mA)
3642 F04
Figure 4. Recommended Inductor Values for Maximum Efficiency
I RMS = I OUT(MAX) ?
V OUT
V IN
?
V IN
V OUT
? 1
3642fc
11
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