LP39542 Datasheet, PDF (29/58 Pages) National Semiconductor (TI) – Advanced Lighting Management Unit

Ambient Light and Temperature Measurement with LP39542
The Analog-to-Digital converter (ADC) in the Audio Syn-
cronization block can be also used for ambient light measure-
ment or temperature measurement.
The selection between these modes is controlled with input
selector bits INPUT_SEL[1:0] in register 2AH as seen on the
following table. Internal averaging function can be used to fil-
ter unwanted noise from the measured signal. Averaging
function can be enabled with EN_AVG bit in register 2BH.
INPUT_SEL[1:0]
Mode
00
Audio synchronization
01
Temperature measurement
(voltage input)
10
Ambient light measurement
(current input)
11
No input
EN_AVG = 0
EN_AVG = 1
Averaging disabled. f_sample = 122 Hz,
data in register changes every 8.2 ms.
Averaging enabled. f_sample = 244 Hz,
averaging of 64 samples, data in
register changes every 262 ms
(3.2Hz).
AMBIENT LIGHT MEASUREMENT
The ambient light measurement requires only one external
component: Ambient light sensor (photo transistor or diode).
The ADC reads the current level at ASE pin and converts the
result into a digital word. User can read the ADC output from
the ADC output register. The known ambient light condition
allows user to set the backlight current to optimal level thus
saving power especially in low light and bright sunlight con-
dition.
30008564
ADC Code vs Input Current
in Light Measurement Mode
TEMPERATURE MEASUREMENT
The temperature measurement requires two external compo-
nents: resistor and thermistor (resistor that has known tem-
perature vs resistance curve). The ADC reads the voltage
level at ASE pin and converts the result into a digital word.
User can read the ADC output from register. The known tem-
perature allows for example to monitor the temperature inside
the display module and decrease the current level of the LEDs
if temperature raises too high. This function may increase
lifetime of LEDs in some applications.
30008541
ASE Input Configuration for Light Measurement
30008542
Temperature sensor connection example
29
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