参数资料
| 型号： | SC171MLTRT |
| 厂商： | Semtech |
| 文件页数： | 15/27页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK SYNC ADJ 1A 10MLPD |
| 标准包装： | 1 |
| 系列： | EcoSpeed® |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 0.75 V ~ 5.23 V |
| 输入电压： | 3 V ~ 5.5 V |
| 频率 - 开关： | 200kHz ~ 1MHz |
| 电流 - 输出： | 1A |
| 同步整流器： | 是 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 10-VFDFN 裸露焊盘 |
| 包装： | 标准包装 |
| 供应商设备封装： | 10-MLPD（3x3） |
| 其它名称： | SC171MLDKR |
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SC171
Applications Information (continued)
der zero load condition.
Design Procedure
R TON = (
V OUT
V FB
) ?
1
100pF ? f SW
When designing a switch mode supply the input voltage
range, load current, switching frequency, and inductor
ripple current must be specified.
The maximum input voltage (V INMAX ) is the highest speci-
fied input voltage. The minimum input voltage ( V INMIN ) is
determined by the lowest input voltage after evaluating
the voltage drops due to connectors, fuses, switches, and
PCB traces.
The following parameters define the design.
Since the f SW may increase by about 100kHz per Ampere,
R TON should be programmed at 700kHz under zero load
to achieve 800kHz at full load.
Calculating R TON results in the following solution.
R TON =19.1k W, we use R TON =19.1k W in real application.
Inductor Selection
In order to determine the inductance, the ripple cur-
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Nominal output voltage (V OUT )
Static or DC output tolerance
Transient response
Maximum load current (I OUT )
rent must first be defined. Low inductor values result in
smaller size but create higher ripple current which can
reduce efficiency. Higher inductor values will reduce the
ripple current/voltage and for a given DC resistance are
more efficient. However, larger inductance translates di-
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T ON =
There are two values of load current to evaluate — con-
tinuous load current and peak load current. Continuous
load current relates to thermal stresses which drive the
selection of the inductor and input capacitors. Peak load
current determines instantaneous component stresses and
filtering requirements such as inductor saturation, output
capacitors, and design of the current limit circuit.
The following values are used in this design.
V IN = 5V + 10%
V OUT = 1.0V + 4%
f SW = 800kHz
Load = 1A maximum
Frequency Selection
Selection of the switching frequency requires making a
trade-off between the size and cost of the external filter
components (inductor and output capacitor) and the
power conversion efficiency.
The desired switching frequency is 800kHz which results
from using components selected for optimum size and
cost .
rectly into larger packages and higher cost. Cost, size,
output ripple, and efficiency are all used in the selection
process.
The ripple current will also set the boundary for power-
save operation. The switching will typically enter pow-
er-save mode when the load current decreases to 1/2 of
the ripple current. For example, if ripple current is 0.5A
then power-save operation will typically start for loads
less than 0.25A. If ripple current is set at 40% of maxi-
mum load current, then power-save will start for loads
less than 20% of maximum current.
The inductor value is typically selected to provide a rip-
ple current that is between 25% to 50% of the maximum
load current. This provides an optimal trade-off between
cost, efficiency, and transient performance.
During the DH on-time, voltage across the inductor is
(V IN - V OUT ). The equation for determining inductance is
shown next.
V OUT
V INMAX ? f SW
A resistor (R TON ) is used to program the on-time (indirectly
setting the frequency) using the following equation un-
? 2010 Semtech Corporation
L =
(V IN - V OUT ) × T ON
I RIPPLE
15
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