参数资料
| 型号： | SC171MLTRT |
| 厂商： | Semtech |
| 文件页数： | 16/27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)
voltage across the ESR. The maximum ESR value allowed
is shown by the following equations.
Example
ESR MAX =
V RIPPLE
I RIPPLEMAX
=
40mV
0.51A
T ON_VINMAX =
= 227ns
L =
= 2 m H
In this example, the inductor ripple current is set equal
to 50% of the maximum load current. Therefore ripple
current will be 50% x 1A or 0.5A. To find the minimum
inductance needed, use the V IN and T ON values that cor-
respond to V INMAX .
1V
5.5V ? 800 kHz
(5.5V - 1V) ? 227ns
0.5A
So a 2μH inductor is selected.
ESR MAX = 78.4 m?
The output capacitance is chosen to meet transient re-
quirements. A worst-case load release, from maximum
load to no load at the exact moment when inductor cur-
rent is at the peak, determines the required capacitance.
If the load release is instantaneous (load changes from
maximum to zero in < 1μs), the output capacitor must
absorb all the inductor’s stored energy. This will cause a
peak voltage on the capacitor according to the following
equation.
× I RIPPLEMAX ) 2
L × (I OUT +
Note that the inductor must be rated for the maximum
DC load current plus 1/2 of the ripple current.
COUT MIN =
1
2
(V PEAK ) 2 - (V OUT ) 2
The ripple current under minimum V IN conditions is also
checked using the following equations.
Assuming a peak voltage V PEAK of 1.050V (50mV rise upon
load release), and a 1A load release, the required capaci-
= 277ns
2 m H × (1A +
× 0.511A)
COUT MIN =
T ON_VINMIN =
I RIPPLE =
1V
4.5V × 800kHz
(V IN - V OUT ) × T ON
L
tance is shown by the next equation.
1
2
(1.05V) 2 - (1.0V) 2
2
= 31 m F
I RIPPLE_VIN MIN =
(4.5V - 1V) × 277ns
2 m H
= 0.485A
If the load release is relatively slow, the output capacitance
can be reduced. At heavy loads during normal switching,
when the FB pin is above the 750mV reference, the DL
Capacitor Selection
The output capacitors are chosen based on required ESR
and capacitance. The maximum ESR requirement is con-
trolled by the output ripple requirement and the DC tol-
erance. The output voltage has a DC value that is equal
to the valley of the output ripple plus 1/2 of the peak-
to-peak ripple. Change in the output ripple voltage will
lead to a change in DC voltage at the output.
The design goal is for the output voltage regulation to be
±4% under static conditions. The internal 750mV refer-
ence tolerance is 1%. Assuming a 1% tolerance from the
FB resistor divider, this allows 2% tolerance due to V OUT
ripple. Since this 2% error comes from 1/2 of the ripple
voltage, the allowable ripple is 4%, or 40mV for a 1V out-
put.
The maximum ripple current of 0.511A creates a ripple
? 2010 Semtech Corporation
output is high and the low-side MOSFET is on. During this
time, the voltage across the inductor is approximately
-V OUT . This causes a down-slope or falling di/dt in the
inductor. If the load di/dt is not much faster than the
-di/dt in the inductor, then the inductor current will tend
to track the falling load current. This will reduce the
excess inductive energy that must be absorbed by the
output capacitor, therefore a smaller capacitance can be
used.
The following can be used to calculate the needed ca-
pacitance for a given dI LOAD /dt. Peak inductor current is
shown by the next equation.
16
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