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| 型号： | LM3445MX/NOPB |
| 厂商： | National Semiconductor |
| 文件页数： | 22/40页 |
| 文件大小： | 0K |
| 描述： | IC LED DRIVER TRIAC DIMM 14SOIC |
| 标准包装： | 2,500 |
| 系列： | PowerWise® |
| 恒定电流： | 是 |
| 拓扑： | PWM，降压（降压） |
| 输出数： | 1 |
| 内部驱动器： | 无 |
| 类型 - 次要： | 高亮度 LED（HBLED） |
| 频率： | 30kHz ~ 1MHz |
| 电源电压： | 8 V ~ 12 V |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 14-SOIC（0.154"，3.90mm 宽） |
| 供应商设备封装： | 14-SOICN |
| 包装： | 带卷 (TR) |
| 工作温度： | -40°C ~ 125°C |
| 其它名称： | LM3445MX |
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SNVS570L – JANUARY 2009 – REVISED MAY 2013
C12
D10
V BUCK
V LED
-
L2
V L2
-
Q2
R3
Figure 27. LM3445 External Components of the Buck Converter
The equation for an ideal inductor is:
Q = L di
dt
(14)
Given a fixed inductor value, L, this equation states that the change in the inductor current over time is
proportional to the voltage applied across the inductor.
During the on-time, the voltage applied across the inductor is,
V L(ON-TIME) = V BUCK - (V LED + V DS(Q2) + I L2 x R3)
(15)
Since the voltage across the MOSFET switch (Q2) is relatively small, as is the voltage across sense resistor R3,
we can simplify this to approximately,
V L(ON-TIME) = V BUCK - V LED
During the off-time, the voltage seen by the inductor is approximately:
V L(OFF-TIME) = V LED
(16)
(17)
The value of V L(OFF-TIME) will be relatively constant, because the LED stack voltage will remain constant. If we
rewrite the equation for an inductor inserting what we know about the circuit during the off-time, we get:
V L(OFF-TIME) = V LED = L x ' ' t i
' i # t OFF x V L2
V L(OFF-TIME) = V LED = L x
Re-arranging this gives:
LED
(I (MAX) - I (MIN) )
' t
(18)
(19)
From this we can see that the ripple current ( Δ i) is proportional to off-time (t OFF ) multiplied by a voltage which is
dominated by V LED divided by a constant (L2).
These equations can be rearranged to calculate the desired value for inductor L2.
L2 # t OFF x ' LED
Where:
V
i
(20)
22
Product Folder Links: LM3445
Copyright ? 2009–2013, Texas Instruments Incorporated
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