参数资料
| 型号： | A8447SLJTR-T |
| 厂商： | Allegro Microsystems Inc |
| 文件页数： | 6/9页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 3A 8SOIC |
| 标准包装： | 1 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 0.8 V ~ 24 V |
| 输入电压： | 8 V ~ 50 V |
| PWM 型： | 电流模式 |
| 电流 - 输出： | 3A |
| 同步整流器： | 无 |
| 工作温度： | -20°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 8-SOIC（0.154"，3.90mm Width）裸露焊盘 |
| 包装： | 标准包装 |
| 供应商设备封装： | 8-SOIC-EP |
| 产品目录页面： | 1142 (CN2011-ZH PDF) |
| 其它名称： | 620-1174-6 |
A8447
High Voltage Step Down Regulator
Component Selection
L1. The inductor must be rated to handle the total load current.
The value should be chosen to keep the ripple current to a reason-
able value. The ripple current, I RIPPLE , can be calculated by:
Substituting into equation 7:
f SW = 1 / (7 μ s +1.12 μ s) = 123 kHz
I RIPPLE = V L(OFF) × t OFF / L
V L(OFF) = V OUT + V f + I L(AV) × R L
(3)
(4)
Higher inductor values can be chosen to lower the ripple cur-
rent. This may be an option if it is required to increase the total
maximum current available above that drawn from the switching
Example:
Given V OUT = 5 V, V f = 0.55 V, V IN = 42 V, I LOAD = 0.5 A, power
inductor with L = 180 μ H and R L = 0.5 Ω Rdc at 55°C, t OFF =
7 μ s, and R DS(on) = 1 Ω .
Substituting into equation 4:
V L(OFF) = 5 V + 0.55 V+ 0.5 A × 0.5 Ω = 5.8 V
Substituting into equation 3:
I RIPPLE = 5.8 V × 7 μ s / 180 μ H = 225 mA
The switching frequency, f SW , can then be estimated by:
regulator. The maximum total current available, I LOAD(MAX) , is:
I LOAD(MAX) = I CL (min) – I RIPPLE / 2 (8)
where I CL (min) is 3.5 A, from the Electrical Characteristics table.
D1. The Schottky catch diode should be rated to handle 1.2 times
the maximum load current. The voltage rating should be higher
than the maximum input voltage expected during all operating
conditions. The duty cycle for high input voltages can be very
close to 100%.
COUT. The main consideration in selecting an output capacitor
f SW = 1 / ( t ON + t OFF )
t ON = I RIPPLE × L / V L(ON)
V L(ON) = V IN – I L(AV) × R DS(on) – I L(AV) × R L – V OUT
Substituting into equation 7:
(5)
(6)
(7)
is voltage ripple on the output. For electrolytic output capacitors,
a low-ESR type is recommended.
The peak-to-peak output voltage ripple is simply I RIPPLE ×
ESR. Note that increasing the inductor value can decrease the
ripple current. The minimum voltage rating of the capacitor is
V L(ON) = 42 V – 0.5 A × 1 Ω – 0.5 A × 0.5 Ω – 5 V = 36 V
Substituting into equation 6:
t ON = 225 mA × 180 μ H / 36 V = 1.12 μ s
10 V. However, because ESR decreases with voltage, the most
cost-effective choice may be rated higher in voltage. It is recom-
mended that the ESR be less than 100 m Ω .
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
6
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