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| 型号： | LM3445-EDSNEV/NOPB |
| 厂商： | National Semiconductor |
| 文件页数： | 20/40页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL LED DRIVER LM3445 |
| 标准包装： | 1 |
| 系列： | PowerWise® |
| 电流 - 输出 / 通道： | 205 ~ 340 mA |
| 输出及类型： | 1，非隔离 |
| 特点： | 可调光 |
| 输入电压： | 85 ~ 135 VAC |
| 已供物品： | 板 |
| 已用 IC / 零件： | LM3445 |
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SNVS570L – JANUARY 2009 – REVISED MAY 2013
DETERMINING DUTY-CYCLE (D)
Duty cycle (D) approximately equals:
DESIGN GUIDE
V LED
V BUCK
=D= t ON + t OFF
t ON
= t ON x f SW
(4)
With efficiency considered:
1 u V LED = D
K V BUCK
(5)
For simplicity, choose efficiency between 75% and 85%.
CALCULATING OFF-TIME
The “Off-Time” of the LM3445 is set by the user and remains fairly constant as long as the voltage of the LED
stack remains constant. Calculating the off-time is the first step in determining the switching frequency of the
converter, which is integral in determining some external component values.
PNP transistor Q3, resistor R4, and the LED string voltage define a charging current into capacitor C11. A
constant current into a capacitor creates a linear charging characteristic.
i = C dv
dt
(6)
Resistor R4, capacitor C11 and the current through resistor R4 (i COLL ), which is approximately equal to V LED /R4,
are all fixed. Therefore, dv is fixed and linear, and dt (t OFF ) can now be calculated.
t OFF = C11 x 1.276V x
R4
V LED
(7)
Common equations for determining duty cycle and switching frequency in any buck converter:
f SW =
1
t OFF + t ON
D= t
= V LED
'? = t
Therefore:
t ON
ON + t OFF
t OFF
ON + t OFF
V
BUCK
(8)
D t -
f SW = t ON , and f SW = 1 OFF D
With efficiency of the buck converter in mind:
V LED
V BUCK = K u D
Substitute equations and rearrange:
(9)
(10)
V LED ·
K u V BUCK ?1
f SW =
§
¨ 1
?
1
t OFF
(11)
Off-time, and switching frequency can now be calculated using the equations above.
SETTING THE SWITCHING FREQUENCY
Selecting the switching frequency for nominal operating conditions is based on tradeoffs between efficiency
(better at low frequency) and solution size/cost (smaller at high frequency).
20
Product Folder Links: LM3445
Copyright ? 2009–2013, Texas Instruments Incorporated
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LM3445EVM-695 | 功能描述:LM3445 1, Non-Isolated Output LED Driver Evaluation Board 制造商:texas instruments 系列:- 零件状态:有效 电流 - 输出/通道:500mA 输出和类型:1，非隔离 电压 - 输出:- 特性:可调光 电压 - 输入:90 ~ 135 VAC 所含物品:板 使用的 IC/零件:LM3445 标准包装:1 |
| LM3445M | 制造商:NSC 制造商全称:National Semiconductor 功能描述:Triac Dimmable Offline LED Driver |
| LM3445M/NOPB | 功能描述:LED照明驱动器 RoHS:否 制造商:STMicroelectronics 输入电压:11.5 V to 23 V 工作频率: 最大电源电流:1.7 mA 输出电流: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:SO-16N |
| LM3445M/NOPB NOPB | 制造商:Texas Instruments 功能描述:TRIAC DIMMABLE LED DRIVER |
| LM3445MM | 制造商:Texas Instruments 功能描述:LED DRIVER, TRIAC DIMMABLE, 10MSOP |
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