参数资料
| 型号： | SC419ULTRT |
| 厂商： | Semtech |
| 文件页数： | 16/24页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM 20-MLPQ |
| 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 ~ 28 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： | SC419ULTRTDKR |
SC419
Applications Information (continued)
T ON
DesignProcedure
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.
To select R TON , use the maximum value for V IN , and for T ON
use the value associated with maximum V IN .
V OUT
V INMAX f SW
T ON = 318 ns at 13.2V IN , 1.05V OUT , 250kHz
Substituting for R TON results in the following solution.
R TON = 154.9k?, use R TON = 154k?
Inductor Selection
?
?
?
?
Nominal output voltage (V OUT )
Static or DC output tolerance
Transient response
Maximum load current (I OUT )
In order to determine the inductance, the ripple current
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
( V IN V OUT ) T ON
I RIPPLE
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 = 12V + 10%
? V OUT = 1.05V + 4%
? f SW = 250kHz
? Load = 10A 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 250kHz.
A resistor, R TON is used to program the on-time (indirectly
setting the frequency) using the following equation.
efficient. However, larger inductance translates directly
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 power-
save mode when the load current decreases to 1/2 of the
ripple current. For example, if ripple current is 4A then
Power-save operation will typically start for loads less than
2A. If ripple current is set at 40% of maximum load current,
then power-save will start for loads less than 20% of
maximum current.
The inductor value is typically selected to provide a ripple
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 following equation is used to determine the
inductance.
L
In this example the inductor ripple current is set equal to
R TON
( T ON 10ns ) V IN
25 pF V OUT
50% of the maximum load current. Thus ripple current
will be 50% x 10A or 5A.
16
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