PDF《operating point of an LED》

Texas Instruments

22 AAJ 1Q

2015 Personal Electronics Analog Applications Journal Optimal operating point of an LED Achieving optimal performance of an LED luminaire or LED backlight design requires numerous trade-offs.

Understanding an LED'

s power transfer characteristics empowers intelligent choices regarding cost, power consump- tion, and weight. While most LED data- sheets publish pertinent data that can be used to make these decisions, data may not be formatted in a way that is readily applicable to the chosen applica- tion. Optimal performance requires find- ing pertinent information from manufacturer'

s LED datasheets and utilizing methods to capture, reformat and analyze the data. A relevant case study involves a typical tablet LCD backlight application that drives a 10-inch display with a 16:9 aspect ratio. Driving the backlight, the LED chosen for our example is the Nichia NNSW208CT[1] . Typical dis- plays in modern mobile devices emit approximately

650 nits of light when driven at maximum brightness. Most of the LED light produced is lost as it passes through the physi- cal elements integrated into the display (light diffuser, polarizers, RGB color filter, touch-panel ITO, and so on). Modern display stack-ups loose approximately 95% of the light produced by the LED. This device in this case study emits 10.398 lumens when driven at the recommended continuous drive current of

25 mA. Calculate the mini- mum number of LEDs using Equation 1. Using a conversion constant of K = 1550.0031 and the design requirements listed above, the calculated minimum number of LEDs is 35. While seven strings of five LEDs satisfies the design requirements, most LED driver ICs in this market are tailored to drive only six strings of LEDs. Adjusting the LED count to

36 enables an off-the-shelf LED driver. Assuming 100% driver efficiency, driving

36 LEDs at maximum brightness consumes 2.56 W of power. LED efficacy, color shift, and thermal properties are key data metrics. Efficacy versus forward current is rarely pro- vided in an LED datasheet. Tabulated efficacy data is also difficult to find in specifications. Calculating this key met- ric is relatively easy using available IF versus VF and lumi- nosity versus IF curves. Also required is a typical lumen output at a given IF (8.4 lumens at IF =

20 mA). All required data is readily available in the manufacturers'

datasheets. Start by importing/digitizing the datasheet graphs (Figure 1) into a spreadsheet using pre- defined increments of LED current. Free software tools speed the process and digitize Y data on pre-determined X increments[2] , enabling the calculations required to derive efficacy. By Donald Schelle Analog Field Applications Aspect Ratio 16:9 Screen Area (m2) Screen Size(inches) Conversion Constant (inches2 to m2) Max Brightness (nits) Lumped Stackup Losses Lumen Output of Single LED

2 S X Y min V(disp)

2 2 disp V X Y L A A

1 1 #LED M K

1 V A A ? ? * >

* * * ? ? ? ? + ? ? (1) Figure 1. Cornerstone plots used to derive the optimal LED operating point Relative Luminous Flux (a.u.) Nichia (NNSW208CT) 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 2.5

0 2.75

20 3.25

60 3.5

100 80

3 40 Forward Voltage (V) Forward Current (mA)

100 10

1 Forward Current ( mA ) 8.4 Lumens at

20 mA 图1:用于推导最佳 LED 操作点的基础曲线图 AAJ

2015 年第一季度 德州仪器 模拟应用期刊 个人电子产品 LED 的最佳操作点 作者:Donald Schelle 模拟现场应用 实现 LED 灯具或 LED 背光源设计的最 优性能需要进行诸多的权衡折衷.了解 LED 的功率传输特性能够就成本、功 耗和效率做出明智的选择.虽然大多数 LED 产品手册均公布了可用于做出此类 决定的相关数据,但是可能没有采取一 种可使此类数据容易地适用于选定应用 的方式对其进行格式化.实现最优性能 需要从制造商的 LED 产品手册中找到 首先,采用预先定义的 LED 电流增量把产品手册中的曲 线图(图1)导入 / 数字化为 一张电子表格.免费提供的 软件工具加快了这一过程, 并针对预定的 X 增量进行了 Y 数据的数字化处理[2] ,从而实 现了推导效率所需的计算. 有关的信息,并运用合适的方法对这些数据进行捕获、 重新格式化和分析. 一个相关的案例研究涉及一种典型的平板电脑 LCD 背 光源应用,其驱动一个宽高比为 16:9 的10 英寸显示 屏.在我们举的例子中,用于驱动背光源所选的 LED 是Nichia NNSW208CT[1] .当以最大亮度驱动时,新式移 动设备中的典型显示屏的发光亮度大约为

650 尼特.产 生的大部分 LED 光都在通过集成于显示屏中的物理元件 (散光器、偏光器、RGB 滤色器、ITO 触摸屏,等等) 时损失掉了.新的堆叠式显示屏会使 LED 产生的光损失 95% 左右.在本案例研究中,该器件在以推荐的

25 mA 连续驱动电流进行驱动的情况下发出 10.398 流明的光能 量.采用 (1) 式来计算最小的 LED 数量. 采用转换常数 K = 1550.0031 和上面列出的设计要求, 计算得出的最小 LED 数量为

35 个.虽然采用

7 个LED 灯串(每串

5 个LED)就能满足设计要求,但是该市场 中的大多数 LED 驱动器 IC 都是只为驱动

6 个LED 灯串 而定制的.把LED 的数量调整为

36 个即可使用现成有 售的 LED 驱动器.假设 LED 驱动器的效率为 100%,那 么以最大亮度驱动

36 个LED 时的功耗为 2.56 W. LED 效率、色移和热性能是关键的数据指标.LED 产品 手册中很少提供 效率与 LED 正向电流的关系 信息, 在规格指标中也很难找到列成表格的效率数据.采用提 供的 IF 与VF 的关系 曲线和 发光度与 IF 的关系 曲 线可以相对容易地计算此类关键的指标.另外,还需要 了解某种给定 IF 条件下的典型流明输出(在IF =

20 mA 时为 8.4 流明).所有需要的数据都可在制造商的产品 手册中容易地获得. 屏幕尺寸(英寸) 宽高比 屏幕面积 (m2 ) 最大亮度(尼特) 集总堆叠损失 单个LED的流明 输出 转换常数 (平方英寸至平方米) Texas Instruments

23 AAJ 1Q

2015 Personal Electronics Analog Applications Journal Once digitized and tabulated, LED flux output (ΦV), LED power consumption (PLED) and efficacy (η) are cal- culated versus LED forward current (IF) (Figure 2). Peak efficacy is reached at a relatively low forward current and drops off steadily as forward current approaches the maxi- mum rated amount. Battery-powered applications greatly benefit by reduc- ing these power requirements. Operating more LEDs at a lower forward current results in a net reduction of power for a given fixed-light output. Table

1 summarizes the orig- inal application requirements while comparing three alter- native LED configurations. Cost and mechanical volume requirements may limit the final configuration;

however, doubling the number of LEDs yields a power savings of

160 mW. This equates to a 6.3% net power reduction. Additionally, the backlight can be operated at a much higher brightness (with increased power consumption) when ambient light conditions (out- doors/daylight) dictate a brighter image. Figure 2. LED flux output, power consumption and efficacy can be calculated and plotted Flux (lm) Power (mW) Efficacy (lm/W) LED Forward Current (mA)

0 0

20 20

40 40

60 60

80 80

100 100

0 20

40 60

80 100 40.0 30.0 20.0 10.0 0.0

500 400

300 200

100 0

225 200

175 150

125 100 Table 1. A comparative backlight design study Number of LEDs LED Operating Point (mA) Total Light Output (lm) Total Power Consump- tion of LED Array (W) Net Reduction in Operat- ing Power (%) Decreasing Number of LEDs

24 32.6 373.2 2.65 C3.5 Control

36 25 374.2 2.56

0 Increasing Number of LEDs

42 (16%) 21.2 373.2 2.50 2.2

54 (50%) 16.4 ........