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| 型号： | LM3445-208277EV/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： | 500mA |
| 输出及类型： | 1，非隔离 |
| 特点： | 可调光，带标准型三端双向可控硅调光器，功率因数修正 |
| 输入电压： | 176 ~ 305 VAC |
| 已供物品： | 板 |
| 已用 IC / 零件： | LM3445 |
| 产品目录页面： | 1285 (CN2011-ZH PDF) |
| 其它名称： | LM3445-208277EV |
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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 |
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|---|---|
| LM3445-220VEVAL/NOPB | 功能描述:BOARD EVAL LM3445 220V RoHS:是 类别:编程器，开发系统 >> 过时/停产零件编号 系列:PowerWise® 标准包装:1 系列:- 传感器类型:CMOS 成像，彩色（RGB） 传感范围:WVGA 接口:I²C 灵敏度:60 fps 电源电压:5.7 V ~ 6.3 V 嵌入式:否 已供物品:成像器板 已用 IC / 零件:KAC-00401 相关产品:4H2099-ND - SENSOR IMAGE WVGA COLOR 48-PQFP4H2094-ND - SENSOR IMAGE WVGA MONO 48-PQFP |
| LM3445-220VEVAL/NOPB | 制造商:Texas Instruments 功能描述:LED Driver Eval. Board |
| LM3445-230VFLBK/NOPB | 功能描述:LED 照明开发工具 LM3445-230VFLBK EVAL BOARD RoHS:否 制造商:Fairchild Semiconductor 产品:Evaluation Kits 用于:FL7732 核心: 电源电压:120V 系列: 封装: |
| LM3445A | 制造商:NSC 制造商全称:National Semiconductor 功能描述:A19 Edison Retrofit Evaluation Board Can drive up to 18 series LEDs |
| LM3445-EDSNEV/NOPB | 功能描述:LED 照明开发工具 LM3445-EDSN EVAL BOARD RoHS:否 制造商:Fairchild Semiconductor 产品:Evaluation Kits 用于:FL7732 核心: 电源电压:120V 系列: 封装: |
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