Design Specifications
| Inputs |
Outputs #1 |
| VinMin=6V |
Vout1=20V |
| VinMax=12V |
Iout1=0.1A |
Design Description
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.
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Design Resources
Additional Product Resources
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LM3423
- N-Channel Controllers for Constant Current LED Drivers from the
PowerWise® Family |
| • |
LMV431 -
Low-Voltage (1.24V) Adjustable Precision Shunt Regulators |
- datasheet
- application notes
- design tools
- samples
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Board Photo
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Features
- 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
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