1000

0

LOW INTENSITY

HIGH INTENSITY

750

500

250

Z0

Z1

Z2

Z3

Z4

Z0

Z1

Z2

Z3

Z4

LIGHT INTENSITY INCREASES

LIGHT INTENSITY DECREASES

t0-1

t1-2

t2-3

t3-2

t3-4

LOW-to-HIGH gain

transition

HIGH-to-LOW gain

transition

LM3537

SNVS634B – JUNE 2011 – REVISED MAY 2013

www.ti.com

The higher X-axis is for increasing light intensity, while the lower axis is for decreasing light intensity

There are some limits in Zone transitions when the autogain is enabled, for example a direct transition from the lowest

Zone0 to the highest Zone4 (and vice versa) is not possible, because the device must go through the gain change

process first.

Figure 4. ALS Zone Transitions with AutoGain

Countdown Timer

The ALS engine includes a pre-defined countdown timer function. This function is targeted to applications where

it's favorable to only increase through the zones; i.e., the LM3537 will stick to the highest zone reached, but won't

allow transitions to lower Zones until the countdown has completed. At the end of every countdown, the timer

sets the countdown timer flag (reg 40H), and after that, any Zone transition to a lower Zone re-loads the timer

and starts the next timer period. See Table 3 and Figure 5 for details.

Table 3. Countdown Timer

Pre-defined Countdown Timer Function

TIMER[1]

TIMER[0]

Timer Function

0

0

Countdown timer is disabled

0

1

10s countdown timer is enabled (stick to the highest zone for 10s).

1

0

Always stick to the highest zone the ALS reached.

1

1

Always stick to the highest zone the ALS reached.

16

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