AMBIENTLIGHTADAPTIVELED-REF

Ambient Light Adaptive LED Driver

AMBIENTLIGHTADAPTIVELED-REF

Design files

Overview

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 100mA 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.

Features
InputsOutputs #1
VinMin=6V Vout1=20V
VinMax=12V Iout1=0.1A
  • 4.5-12V battery input voltage range 
  • 100mA Constant current LED at 15-20V 
  • High switching frequency 
  • High efficienty (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

A fully assembled board has been developed for testing and performance validation only, and is not available for sale.

Design files & products

Design files

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SNVC025.ZIP (123 K)

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Products

Includes TI products in the design and potential alternatives.

Automotive LED drivers

LM3423-Q1Automotive N-channel constant current LED controller with series DIM FET driver and fault flag

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Automotive LED drivers

LM3424-Q1Automotive constant current N-channel controller with Thermal Foldback for driving LEDs

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Illumination LED drivers

LM3423N-channel controllers for constant current LED drivers with series DIM FET driver and fault flag

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Illumination LED drivers

LM3424Constant current N-channel controller with Thermal Foldback for driving LEDs

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Shunt voltage references

LMV4311.5%, low-voltage (1.24-V) adjustable precision shunt regulator

Data sheet document-pdfAcrobat PDF

Technical documentation

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* User guide Ambient Light Adaptive LED Driver User Guide Jan. 27, 2012

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