環境光傳感器(ALS)集成電路正越來越多地用於各種顯示器和照明設備
，以節省電能，改善用戶體驗。借助ALSjiejuefangan
，xitongshejishikegenjuhuanjingguangqiangdu，zidongtiaojiexianshipingdeliangdu。yinweibeiguangzhaomingdehaodianliangzaixitongdezonghaodianliangzhongzhanjuhendadebili，shixingdongtaidebeiguangliangdukongzhi
，kejieshengdaliangdedianneng。ciwai，tahainenggougaishanyonghutiyan
，rangxianshipingliangdugenjuhuanjingguangtiaojianzixingtiaozhengdaozuijiazhuangtai。

 

係統實現需要三大部分：監測環境光強的光傳感器、數據處理裝置(通常是微控製器)、控製背光輸入電流的執行器。

 

 

圖1是shi實shi施shi背bei光guang控kong製zhi的de係xi統tong示shi範fan框kuang圖tu。在zai這zhe套tao組zu合he中zhong
，光guang傳chuan感gan器qi是shi關guan鍵jian的de組zu成cheng部bu分fen，因yin為wei它ta要yao向xiang係xi統tong的de其qi他ta模mo塊kuai提ti供gong環huan境jing光guang強qiang信xin息xi。光guang傳chuan感gan器qi必bi須xu具ju備bei將jiang光guang信xin號hao轉zhuan換huan成cheng電dian信xin號hao的de信xin號hao轉zhuan換huan器qi(譬如光電二極管或CdS光敏電阻)和信號放大和/或調節裝置以及模/數轉換器(ADC)。

 

圖1. 實施背光控製的係統框圖

 

圖2所示為分立光電二極管電路，從圖中可以看出，該電路需要一個或多個運算放大器：一yi個ge用yong於yu電dian流liu到dao電dian壓ya的de轉zhuan換huan，可ke能neng還hai需xu要yao一yi級ji放fang大da，提ti供gong附fu加jia增zeng益yi。它ta還hai包bao括kuo一yi些xie分fen支zhi電dian路lu
，用yong於yu供gong電dian，確que保bao高gao度du可ke靠kao的de信xin號hao鏈lian。而er在zai空kong間jian極ji其qi寶bao貴gui的de應ying用yong中zhong，所suo需xu元yuan件jian的de數shu量liang過guo多duo可ke能neng導dao致zhi空kong間jian受shou限xian問wen題ti。

 

圖2. 光電二極管電路分立設計

 

這裏還存在一個更細微的問題。具體而言，理想情況下
，應確保環境光的測量模擬了人眼對光線的響應機製。這通常借助CIE提供的視覺亮度曲線(圖3)。然而，光電二極管很少能夠完全模擬這種響應機製
，因為它們通常具有很高的紅外(IR)靈敏度。在IR強度較大的光照條件(譬如白熾燈或日光)下，這種紅外靈敏度會造成錯誤地判斷光線強度。

 

解決上述問題的方法之一是使用兩個光電二極管：一yi個ge采cai用yong對dui可ke見jian光guang和he紅hong外wai光guang都dou很hen敏min感gan的de元yuan件jian，另ling一yi個ge采cai用yong隻zhi對dui紅hong外wai光guang敏min感gan的de元yuan件jian。最zui終zhong用yong前qian者zhe的de響xiang應ying值zhi減jian去qu後hou者zhe的de響xiang應ying值zhi，將jiang紅hong外wai幹gan擾rao降jiang至zhi最zui小xiao，獲huo得de準zhun確que的de可ke見jian光guang響xiang應ying。

 

這種解決方案雖然有效，卻增加了分立電路的占用空間。通常還很難、甚至不可能讓兩個分立的光電二極管配合得足夠緊密
，以實現消除紅外幹擾的目的。如果不配備精密放大器(譬如對數放大器)，動態範圍可能很小。換句話說，很難利用這種組合獲得可重複的結果。

 

圖3. CIE曲線和典型的光電二極管

 

高集成度解決方案不僅能夠獲得比人眼光學係統更真實的光強數據，還能夠節省大量空間MAX44009等環境光傳感器，可將所有信號調節和模/數轉換器集成在一個小封裝(2mm x 2mm UTDFN封裝)內

，從而在空間受限應用中有效節省電路板麵積。

 

圖4提供了MAX44009的功能框圖
，采用I²C通tong信xin協xie議yi，使shi其qi與yu微wei控kong製zhi器qi的de連lian接jie方fang式shi更geng簡jian單dan
，數shu據ju傳chuan輸shu速su度du更geng快kuai。除chu此ci之zhi外wai，該gai解jie決jue方fang案an的de高gao集ji成cheng特te性xing使shi其qi能neng夠gou置zhi於yu柔rou性xing電dian纜lan，安an裝zhuang在zai離li主zhu電dian路lu板ban距ju離li合he適shi的de位wei置zhi。

 

圖4. MAX44009功能框圖

 

背光控製：調節顯示屏亮度

該(gai)控(kong)製(zhi)方(fang)案(an)的(de)第(di)二(er)部(bu)分(fen)是(shi)調(tiao)節(jie)顯(xian)示(shi)屏(ping)的(de)背(bei)光(guang)亮(liang)度(du)。這(zhe)可(ke)通(tong)過(guo)多(duo)種(zhong)方(fang)式(shi)實(shi)現(xian)
，具(ju)體(ti)取(qu)決(jue)於(yu)設(she)備(bei)中(zhong)的(de)顯(xian)示(shi)屏(ping)模(mo)塊(kuai)。有(you)兩(liang)種(zhong)最(zui)簡(jian)單(dan)的(de)方(fang)式(shi)
，一(yi)種(zhong)是(shi)借(jie)助(zhu)脈(mai)衝(chong)寬(kuan)度(du)調(tiao)製(zhi)(PWM)方案的直接調節方式，另一種是采用顯示屏控製器的間接調節方式。

 

許(xu)多(duo)顯(xian)示(shi)屏(ping)模(mo)塊(kuai)如(ru)今(jin)都(dou)配(pei)有(you)一(yi)個(ge)集(ji)成(cheng)控(kong)製(zhi)器(qi)
，用(yong)戶(hu)可(ke)以(yi)通(tong)過(guo)向(xiang)控(kong)製(zhi)器(qi)發(fa)送(song)串(chuan)行(xing)命(ming)令(ling)，直(zhi)接(jie)設(she)置(zhi)背(bei)光(guang)亮(liang)度(du)。如(ru)果(guo)顯(xian)示(shi)屏(ping)模(mo)塊(kuai)未(wei)配(pei)備(bei)集(ji)成(cheng)控(kong)製(zhi)器(qi)
，還(hai)可(ke)安(an)裝(zhuang)一(yi)個(ge)簡(jian)單(dan)的(de)背(bei)光(guang)控(kong)製(zhi)執(zhi)行(xing)器(qi)
，控(kong)製(zhi)顯(xian)示(shi)屏(ping)後(hou)麵(mian)用(yong)於(yu)背(bei)光(guang)照(zhao)明(ming)的(de)白(bai)光(guang)LED燈的輸入電流。實現這種控製的一種簡單辦法是：直接給LED串聯一個場效應晶體管(FET)，使用PWM信號快速打開
、關閉FET (圖5)。然而，也可以利用單一芯片(用於LED控製的MAX1698升壓轉換器)準確

、可靠地調節(圖6)，請參考應用筆記3866“Low-power PWM output controls LED brightness”，獲取詳細信息。

 

圖5. 簡單的PMW控製電路

 

圖6. 基於MAX1698的LED亮度調節器

 

 

最後一步就是在傳感器和執行器之間建立連接，通過微控製器實現。有人可能首先要問：“環境光強如何映射到背光亮度？”事實上，有些文獻專門介紹了相關方案。其中一種映射方式是，Microsoft®針對運行Windows® 7¹操作係統的計算機提出的。圖7所示曲線是由Microsoft提供的
，它可以將環境光強度映射到顯示屏亮度(以全部亮度的百分比表示)。

 

圖7. 將環境光強映射為最佳顯示屏亮度的曲線示例

 

這種特殊曲線可以用以下函數表示：

 

 

如果設備采用的是已集成亮度控製功能的LCD控製芯片，就可通過向芯片發送指令，輕鬆設置背光亮度。如果設備采用的是PWM直接控製亮度，則要考慮如何將比例信號映射至顯示屏亮度。

 

在MAX1698示例中，根據其產品說明書的介紹，可以將驅動電流映射為電壓。通過這個示例，我們可以假設LED電流強度幾乎與其電流呈線性關係。這樣，我們就可以在上述等式中乘上一個係數

，計算出PWM所映射的有效電壓，該電壓再被映射至LED電流

，最後轉化成顯示屏亮度。

 

 

最zui好hao不bu要yao從cong一yi個ge亮liang度du級ji直zhi接jie跳tiao轉zhuan到dao另ling一yi個ge亮liang度du級ji，而er是shi平ping滑hua上shang調tiao和he下xia調tiao背bei光guang亮liang度du，確que保bao不bu同tong亮liang度du等deng級ji之zhi間jian無wu縫feng過guo渡du。為wei了le達da到dao這zhe一yi目mu的de
，最zui好hao采cai用yong帶dai有you固gu定ding或huo不bu同tong亮liang度du步bu長chang、可逐步調節亮度的定時中斷，也可采用帶有可控製LED輸入電流的PWM值的定時中斷，或者是能夠發送到顯示屏控製器的串行指令的定時中斷。圖8提供了這種算法的一個示例。

 

圖8. 步進式亮度調節的算法示例

 

另一個問題是，係統響應環境光強變化的速度。我們應盡量避免過快地改變亮度等級。這是因為光強的瞬間變化(譬如一扇窗戶打開或瞬間有一束光掃過)可ke能neng導dao致zhi背bei光guang亮liang度du發fa生sheng不bu必bi要yao的de變bian化hua，這zhe往wang往wang會hui造zao成cheng用yong戶hu感gan覺jiao不bu適shi。並bing且qie
，較jiao長chang的de響xiang應ying時shi間jian還hai有you助zhu於yu減jian少shao微wei控kong製zhi器qi對dui光guang傳chuan感gan器qi的de檢jian測ce次ci數shu，從cong而er可ke以yi釋shi放fang一yi定ding的de微wei控kong製zhi器qi資zi源yuan。

 

最(zui)初(chu)級(ji)的(de)方(fang)法(fa)就(jiu)是(shi)每(mei)隔(ge)一(yi)兩(liang)秒(miao)鍾(zhong)檢(jian)查(zha)一(yi)次(ci)光(guang)傳(chuan)感(gan)器(qi)，然(ran)後(hou)相(xiang)應(ying)地(di)調(tiao)整(zheng)背(bei)光(guang)亮(liang)度(du)。更(geng)好(hao)的(de)方(fang)法(fa)是(shi)
，隻(zhi)有(you)光(guang)線(xian)強(qiang)度(du)偏(pian)離(li)特(te)定(ding)範(fan)圍(wei)一(yi)定(ding)時(shi)間(jian)後(hou)，才(cai)對(dui)背(bei)光(guang)亮(liang)度(du)進(jin)行(xing)調(tiao)節(jie)。譬(pi)如(ru)，如(ru)果(guo)正(zheng)常(chang)光(guang)強(qiang)是(shi)200lux，我們可能隻會在光強降到180lux以下或升至220lux以上，而且持續時間超過數秒的情況下才調節亮度。幸運的是，MAX44009集成了中斷引腳和閾值寄存器
，可輕鬆實現這個目的。

 

 

#define MAX44009_ADDR 0x96

// begin definition of slave addresses for MAX44009

#define INT_STATUS 0x00

#define INT_ENABLE 0x01

#define CONFIG_REG 0x02

#define HIGH_BYTE 0x03

#define LOW_BYTE 0x04

#define THRESH_HIGH 0x05

#define THRESH_LOW 0x06

#define THRESH_TIMER 0x07

// end definition of slave addresses for MAX44009

 

extern float SCALE_FACTOR; // captures scaling factors to map from % brightness to PWM

float currentBright_pct; // the current screen brightness, in % of maximum

float desiredBright_pct; // the desired screen brightness, in % of maximum

float stepSize; // the step size to use to go from the current 

// brightness to the desired brightness

uint8 lightReadingCounter;

 

/**

 * Function: SetPWMDutyCycle

 *

 * Arguments: uint16 dc - desired duty cycle

 *

 * Returns: none

 *

 * Description: Sets the duty cycle of a 16-bit PWM, assuming that in this 

 * architecture, 0x0000 = 0% duty cycle

 * 0x7FFF = 50% and 0xFFFF = 100%

**/

extern void SetPWMDutyCycle(uint16 dc);

 

/**

 * Function: I2C_WriteByte

 *

 * Arguments: uint8 slaveAddr - address of the slave device

 * uint8 command - destination register in slave device

 * uint8 data - data to write to the register

 *

 * Returns: ACK bit

 *

 * Description: Performs necessary functions to send one byte of data to a 

 * specified register in a specific device on the I2C bus

**/

uint8 2C_WriteByte(uint8 slaveAddr, uint8 command, uint8 data);

 

/**

 * Function: I2C_ReadByte

 *

 * Arguments: uint8 slaveAddr - address of the slave device

 * uint8 command - destination register in slave device

 * uint8 *data - pointer data to read from the register

 * 

 * Returns: ACK bit

 *

 * Description: Performs necessary functions to get one byte of data from a 

 * specified register in a specific device on the I2C bus

**/

uint8 I2C_ReadByte(uint8 slaveAddr, uint8 command, uint8* data);

 

/**

 * Function: getPctBrightFromLuxReading

 *

 * Arguments: float lux - the pre-computed ambient light level

 *

 * Returns: The % of maximum brightness to which the backlight should be set 

 * given the ambient light (0 to 1.0)

 *

 * Description: Uses a function to map the ambient light level to a backlight 

 * brightness by using a predetermined function

**/

float getPctBrightFromLuxReading(float lux);

 

/**

 * Function: mapPctBrighttoPWM

 *

 * Arguments: float pct

 *

 * Returns: PWM counts needed to achieve the specified % brightness (as 

 * determined by some scaling factors)

**/

uint16 mapPctBrighttoPWM(float pct);

 

/**

 * Function: getLightLevel

 *

 * Arguments: n/a

 *

 * Returns: the ambient light level, in lux

 *

 * Description: Reads both the light registers on the device and returns the 

 * computed light level

**/

float getLightLevel(void);

 

/**

 * Function: stepBrightness

 *

 * Arguments: n/a

 *

 * Returns: n/a

 *

 * Description: This function would be called by an interrupt. It looks at the 

 * current brightness setting, then the desired brightness setting. 

 * If there is a difference between the two, the current brightness 

 * setting is stepped closer to its goal.

**/

void stepBrightness(void);

 

/**

 * Function: timerISR

 *

 * Arguments: n/a

 *

 * Returns: n/a

 *

 * Description: An interrupt service routine which fires every 100ms or so. This 

 * handles all the ambient light sensor and backlight

 * control code.

**/

void timerISR(void);

 

void main() {

SetupMicro(); // some subroutine which initializes this CPU

I2C_WriteByte(MAX44009_ADDR, CONFIG_REG, 0x80); // set to run continuously

lightReadingCounter = 0;

stepSize = .01;

currentBright_pct = 0.5;

desiredBright_pct = 0.5;

SetPWMDutyCycle(mapPctBrighttoPWM(currentBright_pct));

InitializeTimerInterrupt(); // set this to fire every 100ms

while(1) {

// do whatever else you need here, the LCD control is done in interrupts

Idle();

}

} // main routine

 

// the point at which the function clips to 100%

#define MAXIMUM_LUX_BREAKPOINT 1254.0

float getPctBrightFromLuxReading(float lux) {

if (lux > MAXIMUM_LUX_BREAKPOINT)

return 1.0;

else

return (9.9323*log(x) + 27.059)/100.0;

} // getPctBrightFromLuxReading

 

uint16 mapPctBrighttoPWM(float pct) {

return (uint16)(0xFFFF * pct * SCALE_FACTOR);

} // mapPctBrighttoPWM

 

float getLightLevel(void) {

uint8* lowByte;

uint8* highByte;

uint8 exponent;

uint8 mantissa;

float result;

I2C_ReadByte(MAX44009_ADDR, HIGH_BYTE, highByte);

I2C_ReadByte(MAX44009_ADDR, LOW_BYTE, lowByte);

exponent = (highByte & 0xF0) >> 4;// upper four bits of high byte register

mantissa = (highByte & 0x0F) << 4;// lower four bits of high byte register = 

   // upper four bits of mantissa

mantissa += lowByte & 0x0F;  // lower four bits of low byte register = 

 // lower four bits of mantissa

result = mantissa * (1 << exponent) * 0.045;

return result;

} //getLightLevel

 

void stepBrightness(void) { 

// if current is at desired, don''t do anything

if (currentBright_pct == desiredBright_pct)

return;

// is the current brightness above the desired brightness?

else if (currentBright_pct > desiredBright_pct) {

// is the difference between the two less than one step?

if ( (currentBright_pct-stepSize) < desiredBright_pct)

currentBright_pct = desiredBright_pct;

else

currentBright_pct -= stepSize;

} // else if

else if (currentBright_pct < desiredBright_pct) {

// is the difference between the two less than one step?

if ( (currentBright_pct+stepSize) > desiredBright_pct)

currentBright_pct = desiredBright_pct;

else

currentBright_pct += stepSize;

} // else if

SetPWMDutyCycle(mapPctBrighttoPWM(currentBright_pct));

return;

} // stepBrightness

 

void timerISR(void) {

float lux;

float pctDiff;

stepBrightness();

if (lightReadingCounter)

lightReadingCounter--;

else {

lightReadingCounter = 20; // 2 second delay

lux = getLightLevel();

desiredBright_pct = getPctBrightFromLuxReading(lux);

pctDiff = abs(desiredBright_pct - currentBright_pct);

stepSize = (pctDiff <= 0.01) ? 0.01:pctDiff/10;

} // else

ClearInterruptFlag();

} // timerISR

 

本文來源於Maxim。