LM3537_13 Datasheet, PDF (15/33 Pages) Texas Instruments – 8-Channel WLED Driver with Four Integrated LDOs

LM3537
www.ti.com
SNVS634B – JUNE 2011 – REVISED MAY 2013
The ambient light sensing circuit has 4 configurable Ambient Light Boundaries (ZB0 – ZB3) programmed through
the four 8-bit Zone Boundary Registers. These zone boundaries define 5 ambient brightness zones.
The ambient light sensor input has a 0 to 1V operational input voltage range. The Typical Application Circuit
shows the LM3537 with an ambient light sensor (ROHM, BH1621FVC). If the internal ALS Resistor Select
Register is set to 0x14 (1.44 kΩ), this circuit will convert 0 to 1000 LUX light into approximately a 0 to 850 mV
linear output voltage (high-gain mode). The voltage at the active ambient light sensor input is compared against
the 8-bit values programmed into the Zone Boundary Registers (ALS ZONE BOUNDARY#0 - ALS ZONE
BOUNDARY#3 ). When the ambient light sensor output crosses one of the programmed thresholds the internal
ALS circuitry will smoothly transition the LED current to the new 7-bit brightness level as programmed into the
appropriate Zone Target Register (ALS BRIGHTNESS ZONE#0 to ALS BRIGHTNESS ZONE#4).
Ambient light sensor samples are averaged and then further processed by the discriminator block to provide
rejection of noise and transient signals. The averager is configurable with 8 different averaging times to provide
varying amounts of noise and transient rejection. The discriminator block algorithm has a maximum latency of
two averaging cycles; therefore, the averaging time selection determines the amount of delay that will exist
between a steady state change in the ambient light conditions and the associated change of the backlight
illumination. For example, the A/D converter samples the ALS inputs at 16 kHz. If the averaging time is set to
800 ms, the averager will send the updated zone information to the discriminator every 800 ms. This zone
information contains the average of approximately 12800 samples (800 ms × 16 kHz). Due to the latency of 2
averaging cycles, when there is a steady state change in the ambient light, the LED current will begin to
transition to the appropriate target value after approximately 1600 ms have elapsed.
ALS Zone to LED Brightness Mapping principle without AutoGain is shown in Figure 3 below. Here, the
exponential dimming scheme is used.
Vals_ref
= 1V
ZB3
Zone 4
Full
Scale
Zone 3
ZB2
ZB1
Zone 2
ZB0
Zone 1
Zone 0
Ambient Light (lux)
Z0T
Z1T
Z2T
Z3T
Z4T
LED Driver Input Code (0-127)
Figure 3. ALS Zone to LED Brightness Mapping
ALS Zone transitions with AutoGain is shown in Figure 4. When the light intensity increases, the LM3537
configures the sensor for low-gain mode. Transition from Zone2 to Zone3 triggers the shift to lower gain mode.
When the light intensity decreases, the LM3537 configures the sensor to high-gain mode. The trip point to this
transition is set by the ALS LOW_to_HIGH_TP register, and it should be set lower than the Zone2 to Zone3
transition, in order to have hysteresis. Zone3 to Zone2 transition trip point must be set separately for lower gain
mode, by the ALS ZONE BOUNDARY Z3_to_Z2 register. This register value should be set higher than the ALS
LOW_to_HIGH_TP. In low-gain mode the sensor will have a lower output current which helps save battery
power. High-gain mode will allow better resolution, but will result higher output current. Thus, there is a trade-off
between increased resolution and increased power consumption. High-gain mode is the default mode of
operation after enabling the autogain.
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