Ambient Light (lux)

Vals_ref

= 1V

ZB3

ZB0

ZB1

ZB2

Zone 0

Zone 4

Zone 3

Zone 2

Zone 1

LED Driver Input Code (0-127)

Full

Scale

Z0T

Z4T

Z3T

Z2T

Z1T

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 k

Ω), 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.

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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