LM3532TMX-40ANOPB National Semiconductor Corporation, LM3532TMX-40ANOPB Datasheet - Page 19

• Manufacturer Part Number: LM3532TMX-40ANOPB
• Description: High Efficiency White Led Driver With Programmable Ambient Light Sensing Capability And I2c-compatible Interface
• Manufacturer: National Semiconductor Corporation

ALS AVERAGING
ALS averaging is used to filter out any fast changes in the
ambient light sensor inputs. This prevents the backlight cur-
rent from constantly changing due to rapid fluctuations in the
ambient light. There are 8 separate averaging periods avail-
able for the ALS inputs (see
period the ADC continually samples at 7.143 ksps. Therefore,
during an average period, the ALS Averager output will be the
average of 7143/t
ALS ADC AVERAGE READBACK
The output of the LM3532's averager is read back via the Av-
erage ADC Register. This data is the ADC register data,
averaged over the programmed ALS average time.
ASL OPERATION
The LM3532's Ambient Light Sensor Interface has 3 different
algorithms that can be used to control the ambient light to
backlight current response.
ALS Algorithms
1.
2.
For each algorithm, the ALS follows these basic rules:
ALS Rules
1.
2.
Direct ALS Control
Down Delay
For the ALS Interface to force a change in the backlight
current (to a higher zone target), the averager output
must have shown an increase for 3 consecutive average
periods, or an increase and a remain at the new zone for
3 consecutive average periods.
For the ALS Interface to force a change in the backlight
current (to a lower zone target), the averager output must
have shown a decrease for 3 consecutive average
AVE
.
Table
17). During an average
FIGURE 9. ALS Startup Sequence
19
INITIALIZING THE ALS
On initial startup of the ALS Block, the Ambient Light Zone will
default to Zone 0. This allows the ALS to start off in a pre-
dictable state. The drawback is that Zone 0 is often not
representative of the true ALS Brightness Zone since the ALS
inputs can get to their ambient light representative voltage
much faster then the backlight is allowed to change. In order
to avoid a multiple average time wait for the backlight current
to get to its correct state, the LM3532 switches over to a fast
average period (1.1 ms) on ALS startup. This will quickly bring
the ALS Brightness Zone (and the backlight current) to its
correct setting (see
3.
4.
5.
6.
7.
periods, or a decrease and remain at the new zone for 3
consecutive average periods.
If condition #1 or #2 is satisfied, and during the next
average period, the averager output changes again in the
same direction as the last change, the LED current will
immediately change at the beginning of the next average
period.
If condition #1 or #2 is satisfied and the next average
period shows no change in the average zone, or shows
a change in the opposite direction, then the criteria in step
#1 or #2 must be satisfied again before the ALS interface
can force a change in the backlight current.
The Averager Output (see
that is determined from the most recent full average
period.
The ALS Interface only forces a change in the backlight
current at the beginning of an average period.
When the ALS forces a change in the backlight current
the change will be to the brightness target pointed to by
the zone in the Averager Output.
Figure
9).
Figure
7) contains the zone
www.national.com
30115413