LM3530 datasheet(24/50 Pages) TI1

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1.3 mA

22.5 mA x 5.781%

ZoneTarget2 =

LED

LED =

LM3530

SNVS606L – JUNE 2009 – REVISED DECEMBER 2014

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Example: ALS Averaging:

As an example, suppose the LM3530 ALS Configuration Register is loaded with 0x3B. This configures the device

for:

ALS1 as the active ALS input (bits [6:5] = 01)

Enables Ambient Light Current Control (bit 4 = 1)

Enables the ALS circuitry (bit 3 = 1)

Sets the ALS Averaging Time to 256 ms (bits [2:0] = 011)

Next, the ALS Resistor Select Register is loaded with 0x04. This configures the ALS2 input as high impedance

and configures the ALS1 input with a 2.27-k

Ω internal pull-down resistor. The Zone Boundary Registers and

Zone Target Registers are left with their default values. The Brightness Ramp Rate Register is loaded with 0x2D.

This sets up the LED current ramp rate at 16.384 ms/step. Finally, the General Configuration Register is loaded

with 0x15. This sets up the device with:

Simple Enable OFF (bit 7 = 0)

PWM Polarity High (bit 6 = 0)

PWM Input Disabled (bit 5 = 0)

Full-Scale Current = 22.5mA (bits [4:2] = 101)

Brightness Mapping Mode as Exponential (bit 1 = 0)

Device Enabled via I2C (bit 0 = 1)

As the device starts up the APDS-9005 ambient light sensor (connected to the ALS1 input) detects 500 LUX.

This puts approximately 437.5 mV at ALS1 (see Figure 37). This places the measured ambient light between

Zone Boundary Registers 1 and 2, thus corresponding to Zone Target Register 2. The default value for this

(10)

Referring to Figure 37, initially the Averager is loaded with Zone 0 so it takes 2 averaging periods for the LM3530

to change to the new zone. After the ALS1 voltage remains at 437.5 mV for two averaging periods (end of period

2) the LM3530 repeats Zone 2 and signals the LED current to begin ramping to the Zone 2 target beginning at

average period 3. Since the ramp rate is set at 16.384 ms/step the LED current goes from 0 to 1.3 mA in 76 ×

16.384 ms = 1.245s (approximately 5 average periods).

After the LED current has been at its steady state of 1.3 mA for a while, the ambient light suddenly steps to 900

LUX for 500 ms and then steps back to 500 LUX. In this case the 900 LUX will place the ALS1 voltage at

approximately 979 mV corresponding to Zone 4 somewhere during average period 10 and fall back to 437.5 mV

somewhere during average period 12. The averager output during period #10 goes to 3, and then during period

11, goes to 4. Since there have been 2 increases in the average during period 10 and period 11, the beginning of

average period #12 shows a change in the brightness zone to Zone 4. This results in the LED current ramping to

the new value of 22.5 mA (the Zone 4 target). During period #12 the ambient light steps back to 500 LUX and

forces ALS1 to 437.5 mV (corresponding to Zone 2). After average period 12 and period 13 have shown that the

averager transitioned lower two times, the brightness zone changes to the new target at the beginning of period

14. This signals the LED current to ramp down to the zone 2 target of 1.3 mA. Looking back at average period 12

and period 13, the LED current was only able to ramp up to 7.38 mA due to the ramp rate of 16.384 ms/step (2

average periods of 256 ms each) before it was instructed to ramp back to the Zone 2 target at the start of period

14. This example demonstrates not only the averaging feature, but how additional filtering of transient events on

the ALS inputs can be accomplished by using the LED current ramp rates.

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