参数资料
| 型号： | SC493ULTRT |
| 厂商： | Semtech |
| 文件页数： | 17/28页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM MLPQ-20 |
| 标准包装： | 1 |
| 系列： | EcoSpeed®, SmartDrive™ |
| PWM 型： | 控制器 |
| 输出数： | 1 |
| 频率 - 最大： | 1MHz |
| 电源电压： | 3 V ~ 5.5 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | 20-MLPQ |
| 包装： | 标准包装 |
| 其它名称： | SC493ULTRTDKR |
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SC493
Applications Information (continued)
Power Stage Design Procedure
When designing a switch mode supply the input voltage
implemented is calculated according to the next
equation.
range, load current, switching frequency, and inductor
ripple current must be specified.
D MAX
t ON
t ON
t OFF ( MIN )
I
L
V UV
L
I
I
t ON
V OV
Themaximuminputvoltage(V INMAX )isthehighestspeci-
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:
? Nominal output voltage (V OUT )
? Static or DC output tolerance
? Transient response
? Maximum load current (I OUT )
There are two values of load current to evaluate — con-
tinuous load current and peak load current. Continuous
load current relates to thermal limitations 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.
Inductor Selection
Low inductor values result in smaller size but create higher
ripple current, and are less efficient because of that ripple
current flowing in the inductor. Higher inductor values
will reduce the ripple current/voltage and are more effi-
cient, but are larger and more costly. The inductor selec-
If a maximum load step occurs instantly, the voltage
undershoot can be derived by the next equation.
2
2 C D MAX V IN V OUT
For load release, the worst case happens when the
maximum load release occurs at the same time as the high
side turns on. The over-shoot in this situation can be
derived by the next equation.
2
2 C V OUT 2 C
Using the previous two equations, the output capacitor
can be calculated based upon the required performance
metrics (under-shoot or over-shoot voltage during the
transient). Note that the above equations show the worst
case analysis. In practice, the load normally changes with
certain slew rate limits, so the required capacitance value
may be much smaller than the value calculated using
these equations.
Input Capacitor Selection
The input capacitor should be chosen to handle the RMS
ripple current of a synchronous buck converter. This value
is shown by the next equation.
tion is based upon the ripple current which is typically set
between 20% and 50% of the maximum load current.
I RMS
( I D ) I IN
2
D ( I OUT I IN ) 2
( V IN V OUT ) t ON
I RIPPLE
V OUT V OUT I OUT
V IN
V IN
Cost,size,outputripple,andefficiencyareallusedinthe
selection process. The equation for determining the induc-
tance is shown by the next equation.
L
Output Capacitor Selection
where
D , I IN
When the input voltage is also used as VDD, it is desirable to
limit the input voltage ripple to less than 20mV. The input
voltage ripple can be calculated by the next equation.
Two parameters need to be determined in order to select
the output capacitor — the output capacitance and the
capacitor ESR. These two parameters are determined
based upon the dynamic and the static regulation require-
V IN _ RIPPLE
I OUT I IN
t ON
C IN
ments. On a load step, the maximum duty ratio that is
17
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