NS的LM3421/LM3423是恒流LED驱动器的N沟控制器，可配置成降压，升压，降-升压(反激)或SEPIC，输入电压可高达75V，非常适合各种情况下的LED照明应用. 采用LM3423的环境光自适应LED驱动器采用4.5V-12V的电池输入，在15-20V时LED保持100mA的恒流，效率高达85%，PWM标LED调光高达25kHz，可使LED的亮度正比与环境光，从而延长了电池使用寿命.本文介绍了LM3421/LM3423的主要特性，方框图以及相应的应用电路.同时还介绍了采用LM3423的环境光自适应LED驱动器的主要特性，详细的电路图和材料清单(BOM).

LM3421， LM3423: N-Channel Controllers for Constant Current LED Drivers
The LM3421/LM3423 devices are versatile high voltage LED driver controllers. With the capability to be configured in a Buck， Boost， Buck-Boost (Flyback)， or SEPIC topology， and an input operating voltage rating of 75V， these controllers are ideal for illuminating LEDs in a very diverse， large family of applications.

Adjustable high-side current sense with a typical sense voltage of 100mV allows for tight regulation of the LED current with the highest efficiency possible. Output LED current regulation is based on peak current-mode control with predictive Off-Time Control. This method of control eases the design of loop compensation while providing inherent input voltage feed-forward compensation.

The LM3421/LM3423 include a high-voltage startup regulator that operates over a wide input range of 4.5V to 75V. The internal PWM controller is designed for adjustable switching frequencies of up to 2.0MHz， thus enabling compact solutions.

Additional features include: “zero” current shutdown， precision reference， logic compatible DIM input suitable for fast PWM dimming， cycle-by-cycle current limit， and thermal
shutdown.

The LM3423 also includes an LED output status flag， a fault flag， a programmable fault timer， and a logic input to select the polarity of the dimming output driver.

LM3421/LM3423主要特性:

■VIN range from 4.5V to 75V

■2% Internal reference voltage (1.235V)

■urrent sense voltage adjustable from 20mV

■High-side current sensing

■2Ω MOSFET gate driver

■Dimming MOSFET gate driver

■Input under-voltage protection

■Over-voltage protection

■Low shutdown current， IQ < 1μA

■Fast (50kHz) PWM dimming

■Cycle-by-cycle current limit

■Programmable switching frequency

■LED output status flag (LM3423 only)

■Fault timer pin (LM3423 only)

■TSSOP EP 16-lead package (LM3421)

■TSSOP EP 20-lead package (LM3423)

LM3421/LM3423应用:

■LED Drivers

■Constant-Current Buck-Boost Regulator

■Constant-Current Boost Regulator

■Constant-Current Flyback Regulator

■Constant-Current SEPIC Regulator

■Thermo-Electric Cooler (Peltier) Driver

图1.LM3421典型应用电路

图2. LM3421/LM3423方框图

图3. 带高速调光的LM3421降-升压电路图

图4.带高速调光的LM3421 SEPIC电路图

图5.带高速PWM调光和对VLED恒定的波纹电流的LM3421降压电路图

图6.带高速PWM调光和对VIN恒定的波纹电流的LM3421降压电路图

图7.LM3423全特性应用电路图:带高速PWM调光，故障保护和输入断开开关

图8.带高速PWM调光的LM3423降压电路图

采用LM3423的环境光自适应LED驱动器

Ambient Light Adaptive LED Driver
The ambient light adaptive LED driver automatically adjusts display brightness for changing ambient light conditions， providing a more viewable display with better contrast. The design drives five series connected LEDs from an input voltage of 4.5V to 12V. The LED current is nominally 100 mA and the output voltage is 15 to 20V.

The heart of the design is National’s PowerWise LM3423 LED driver which has all the features needed to implement a boost converter with output current regulation and PWM dimming of the current. The LM3423 achieves output current regulation by sensing and appropriately adjusting the voltage across a resistor connected in series with the LED string.

Dimming or brightening the backlight of a display in response to ambient light condition can greatly increase the life of batteries in portable devices， and improves the visibility of the display under different viewing conditions. In this design a photodiode circuit is used to measure ambient light and provide a signal for dimming the LEDs.

PWM dimming is the preferred method of dimming because it allows the light’s color temperature to remain constant irrespective of its intensity. PWM dimming is done by keeping the amplitude of the LED current constant， but periodically turning the LED driver on and off， thus varying the LEDs’ average current and light intensity. The switching is at done at a low frequency， but one which is high enough for the human eye to sense only changes in the light’s average intensity. Linear dimming， where the amplitude of the current in the LEDs is modulated， is not suitable because the LEDs’ light color temperature is sensitive to the current’s amplitude.

The dimming range of the average LED current is from 20 to nearly 100% percent of its nominal value as ambient light conditions increase from dark to bright. To reduce power consumption and prolong battery life the output current is clamped to no more than 50% of the nominal value when the input (battery) voltage falls below a predetermined level.

环境光自适应LED驱动器主要特性:

■4.5-12V battery input voltage range

■100 mA Constant current LED at 15-20V

■High switching frequency

■High efficiency (85%)

■PWM LED dimming up to 25kHz

■LED brightness limited to less than 50% of maximum at low input voltage to prolong battery life

■LED brightness proportional to ambient light

图9. 环境光自适应LED驱动器外形图

图10. 环境光自适应LED驱动器电路图

材料清单(BOM):