吴忠躺衫网络科技有限公司

您好,歡迎來電子發(fā)燒友網(wǎng)! ,新用戶?[免費(fèi)注冊]

您的位置:電子發(fā)燒友網(wǎng)>電子元器件>發(fā)光二極管>

Protect LED driver in backlit displays

2011年02月02日 12:22 電子發(fā)燒友網(wǎng) 作者:大毛 用戶評論(0

Several circuit ideas are presented for protecting a boost-converter LED driver when the LEDs are disconnected. (Without protection, the driver output can destroy the external MOSFET and Schottky diode.) An LED-driver IC (MAX1698) and comparator (MAX9060 or MAX9028) are included.

LEDs often serve as the light source in a backlighted display, and they usually operate with a low battery voltage such as that produced by two NiCd cells or one lithium-ion cell. An IC (MAX1698/MAX1698A) can simplify these applications by boosting the battery voltage to a level suitable for LEDs. The chip also regulates LED current, and includes brightness-control circuitry for dimming the LEDs. The LED array and IC should always remain connected (Figure 1).

Figure 1. This schematic illustrates the application of a typical LED-backlight driver.
Figure 1. This schematic illustrates the application of a typical LED-backlight driver.

If you disconnect the LED array from the IC, the loss of LED current in RFB allows the voltage at FB (pin 6) to drop below the internal current-controller threshold, causing the device to begin increasing its output voltage. Unfortunately, the MAX1698 (like many similar devices) cannot sense the disconnected-LED condition, so its output voltage increases to a level that can destroy the external MOSFET and Schottky diode. This problem is present for any boost converter; not just LED drivers.

The simplest solution is a zener diode connected across the LEDs (Figure 2). A 16V zener works fine in this case (the four white LEDs drop about 12V), but it must be capable of dissipating power. When the LEDs are drawing 100mA or more and someone disconnects them, the zener must dissipate ~1/6W. A possible alternative to this circuit is shown in Figure 3.

Figure 2. The simplest protection for the Figure 1 circuit adds just a zener diode.
Figure 2. The simplest protection for the Figure 1 circuit adds just a zener diode.

Figure 3. Adding a zener diode and transistor to the Figure 1 circuit provides low-power protection for the MOSFET and Schottky diode.
Figure 3. Adding a zener diode and transistor to the Figure 1 circuit provides low-power protection for the MOSFET and Schottky diode.

It requires the addition of two resistors and one transistor, but the Figure 3 circuit doesn't dissipate extra power when the LEDs are disconnected. It also saves space—the zener can be a 0.5W device, and the resistor and BJT can be standard low-power devices available in small packages like the SOT23-3, or smaller. The circuit senses output voltage at the MOSFET drain, and deactivates the driver (MAX1698) by controlling its Shutdown input. You can choose a zener voltage that ensures this voltage is within the MOSFET's operating characteristics.

In other words, the circuit doesn't "work" except when a user removes the LED array. In that event the output voltage starts to rise, and when it reaches the zener voltage the circuit trips and shuts down the IC. As in shutdown mode, the inductor begins to discharge when the driver turns off the external MOSFET, which allows the output voltage to drop below the zener voltage and bring the driver out of shutdown. The driver re-starts, and if the LED array remains unconnected, the output voltage increases until it exceeds the zener voltage, and triggers the protection again.

Because the output voltage regulates around the zener voltage, this circuit does not generate a damaging current spike when the LED array is reconnected. To save battery energy, it also permits external control of the shutdown mode (using a microcontroller, for instance, as shown in Figure 3), to switch off the backlight array.

Another alternative is the circuit of Figure 4, which requires an additional comparator and three resistors. This approach also uses small, low-cost components and dissipates negligible power. It senses output voltage at the Schottky-diode cathode, and limits circuit operation to a voltage set by the resistor divider and the driver's VREF output (1.25V typical).

Figure 4. Better yet, this tiny comparator protects the Figure 1 circuit, dissipates little power, and requires little space on the pc board.
Figure 4. Better yet, this tiny comparator protects the Figure 1 circuit, dissipates little power, and requires little space on the pc board.

This protection circuit remains inactive until the LED array is removed, and (again) its operating voltages remain well within limits for the chosen MOSFET. The comparator should have an open-drain output (MAX9060/MAX9061 or MAX9028) to permit external control of the shutdown mode by a microcontroller—as before, to switch off the backlight array when needed.

The circuit also consumes less power, according to values selected for the resistor divider. (Its quiescent current is a few tens of microamps.) Last but not least, this circuit is smaller than the other two because the comparator comes in a tiny SOT23-5 package (MAX9060/MAX9061) or 1x1.52mm UCSP? package (MAX9028). All three circuits protect the external MOSFET and diode in an LED-backlight application, when the LED array is disconnected.

UCSP is a trademark of Maxim Integrated Products, Inc.

非常好我支持^.^

(2) 66.7%

不好我反對

(1) 33.3%

( 發(fā)表人:admin )

      發(fā)表評論

      用戶評論
      評價:好評中評差評

      發(fā)表評論,獲取積分! 請遵守相關(guān)規(guī)定!

      ?
      大发888娱乐场漏洞| 大发888娱乐场下载远程| 百家乐注码调整| 南宁百家乐官网赌机| 888真人娱乐城| 蓝盾百家乐娱乐场开户注册| 百家乐官网软件辅助器| 永城市| 大发888明星婚讯| 安桌百家乐游戏百家乐| 百家乐不能视频| 百家乐官网网站程序| 浦江县| 娱乐城注册送钱| 大发888备用地址| 新澳门百家乐的玩法技巧和规则| 可以玩百家乐的博彩网站| 临汾玩百家乐的人在那里找| 百家乐里和的作用| 百家乐实战玩法| 金道百家乐游戏| 利记百家乐现金网| 百家乐在线直播| 在线百家乐官网作弊| 安桌百家乐官网游戏百家乐官网 | 百家乐官网策略详解| 海南博彩业| 全讯网a3322.com| 休闲百家乐的玩法技巧和规则| 百家乐投注技巧公式| 永利高百家乐开户| 属猪的人做生意摆放什么聚财| 百家乐官网博彩公| 加多宝百家乐官网的玩法技巧和规则 | 金榜百家乐官网现金网| 百家乐官网转盘技巧| 百家乐官网投注注技巧| 永利高百家乐官网会员| 永福县| 奉贤区| 澳门百家乐官网真人娱乐城 |