SC668 datasheet(41/46 Pages) SEMTECH

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SC668

Table 22 — ALS Function Enable Bits

AD_AUTO AD_EN

Comments

ADC is disabled. Bank #1 will not change

brightness with ambient conditions.

ADC is enabled. ADC output is stored in the

OUT register. Bank #1 does not respond

to the AD

OUT value.

ADC is disabled. Bank #1 current is set to

the start or target current. AD

OUT is com-

pared to AD

RISE and ADFALL registers. A new

value may be written to AD

OUT via the I

interface.

ADC is enabled. Bank #1 current is set to

the start or target current. AD

OUT is com-

pared to AD

RISE and ADFALL registers.

The ALS comparator functions are shown in Table 23.

Table 23 — ALS Comparator Function(1)

Conditions

OUT Effect on Bank #1

OUT > ADRISE

Brightness changes to target value

OUT < ADFALL

Brightness changes to start value

FALL ≤ ADOUT ≤ ADRISE

Brightness does not change

FALL ≥ ADRISE

Hysteresis between the rising and falling

thresholds is disabled, and . . .

FALL → has no effect on brightness

O > ADRISE → brightness changes to

target value

O < ADRISE → brightness changes to

start value

O = ADRISE → brightness does not

change

Notes:

1) When AD_AUTO bit is high.

The state diagram of Figure 29 shows how the ADC is used

to change the brightness of backlight bank #1. No auto-

matic change in brightness occurs when AD_AUTO = 0.

ADOUT > ADRISE

ADOUT < ADFALL

ADOUT < ADRISE

AD_AUTO = 1 or 0

AD_AUTO = 1

AD_AUTO = 0

ADOUT > ADRISE

Bank #1 Brightness

= Value of Register 02h

ADOUT > ADFALL

AD_AUTO = 1 or 0

Bank #1 Brightness

= Value of Register 06h

ADOUT < ADFALL

Figure 29 — ADC function at Bank #1 Brightness

ADC Output and Threshold Register (13h)

This register contains the value AD

OUT which is compared

with AD

RISE and ADFALL. The contents of ADOUT may originate

automatically from the ADC, when the AD_EN bit is a logic

one. Alternatively, AD

OUT may be written to the register via

the I2C interface when the AD_EN bit is a logic zero.

When AD_EN = 1, the ADC receives its analog voltage

input signal from the external photo-detector circuit con-

nected to the ADI pin. V

ADI is then converted into digital

and stored as the 8 bit word AD

OUT. The reference voltage

for the ADC is provided internally by the output of LDO4

LDO4). When the voltage VADI is equal to VLDO4 , the ADC is

at full-scale value FFh, and AD

OUT will equal FFh.

When using an external photo-detection circuit, the ADC

requires the LDO output voltage V

LDO4 to remain constant.

LDO4 can drop-out if the supply voltage becomes too low.

LDO4 should be set to a value sufficiently low to guard

against drop-out.

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