ZONE0

ZONE1

ZONE2

ZONE3

ZONE4

5

10

15

20

25

30

35

40

45

50

55

60

THE END OF THE

COUNTDOWN PERIOD

TIMER PERIOD

STARTED

TIMER PERIOD

STARTED

TIMER PERIOD

STARTED

TIMER PERIOD

STARTED

ELAPSED TIME (s)

THE END OF THE

COUNTDOWN

PERIOD

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

LM3538

SNVS674A – JUNE 2011 – REVISED MAY 2013

www.ti.com

(1)

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

A.

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 19. 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 LM3538 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 timerflag (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 20 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.

Solid line shows the ALS operation when the timer is disabled. Dashed line shows the operation when the 10s timer

is enabled. Dotted line shows the operation when the device sticks to the highest zone.

Figure 20. Countdown Timer Principle.

16

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