ADP8863ACPZ-R7 datasheet(18/48 Pages) AD

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ADP8863

Rev. 0 | Page 18 of 48

L2_HYS

L2_TRIP

L3_TRIP

L3_HYS

FILTER

SETTINGS

ADC

PHOTO

SENSOR

OUTPUT

L2_EN

L3_EN

L2_OUT

L3_OUT

Figure 40. Ambient Light Sensing and Trip Comparators

The L2 comparator, L2_CMPR, detects when the photosensor

output has dropped below the programmable L2_TRP point

(Register 0x1D). If this event occurs, then the L2_OUT status

signal is set. L2_CMPR contains programmable hysteresis,

meaning that the photosensor output must rise above L2_TRP +

L2_HYS before L2_OUT clears. L2_CMPR is enabled via the

L2_EN bit. The L2_TRP and L2_HYS values of L2_CMPR can

be set between 0 μA and 1080 μA (typical) in steps of 4.3 μA

(typical).

The L3 comparator, L3_CMPR, detects when the photosensor

output drops below the programmable L3_TRP point (Register

0x1F). If this event occurs, the L3_OUT status signal is set.

L3_CMPR contains programmable hysteresis, meaning that the

photosensor output must rise above L3_TRP + L3_HYS before

L3_OUT clears. L3_CMPR is enabled via the L3_EN bit. The

L3_TRP and L3_HYS values of L3_CMPR can be set between

0 μA and 137.7 μA (typical) in steps of 0.54 μA (typical).

L2_TRP

L2_HYS

L3_TRP

L3_HYS

100

1000

ADC RANGE (µA)

Figure 41. Comparator Ranges

Note that the full-scale value of the L2_TRP and L2_HYS

registers is 250 (decimal). Therefore, if the value of L2_TRP +

L2_HYS exceeds 250, the comparator output is unable to

deassert. For example, if L2_TRP is set to 204 (80% of the full-

scale value, or approximately 0.80 × 1080 μA = 864 μA), then

L2_HYS must be set to less than 46 (250 − 204 = 46). If it is not,

then L2_HYS + L2_TRP exceeds 250 and the L2_CMPR

comparator is never allowed to go low.

When both phototransistors are enabled and programmed in

automatic mode (through Bit CMP_AUTOEN in Register

0x01), the user application must determine which comparator

outputs to use, by selecting Bit SEL_AB in Register 0x04 for

automatic light sensing transitions. For example, the user soft-

ware may select the comparator of the phototransistor that is

exposed to higher light intensity to control the transition

between the programmed backlight intensity levels.

The L2_CMPR and L3_CMPR comparators can be enabled

independently of each other, or they can operate simultaneously. A

single conversion from each ADC takes 80 ms (typical). When

CMP_AUTOEN is set for automatic backlight adjustment (see

the Automatic Backlight Adjustment section), the ADC and

comparators run continuously. If the backlight is disabled and

at least one independent sink is enabled, it is possible to use the

light sensor comparators in a single-shot mode. A single-shot read

of the photocomparators is performed by setting the FORCE_RD

bit in Register 0x1B. After the single-shot measurement is com-

pleted, the internal state machine clears the FORCE_RD bit.

The interrupt flags (CMP_INT and CMP_INT2) can be used to

notify the system when either L2 or L3 changes state. See the

Interrupts section for more information.

AUTOMATIC BACKLIGHT ADJUSTMENT

The ambient light sensor comparators can automatically transi-

tion the backlight between one of its three operating levels. To

enable this mode, set the CMP_AUTOEN bit in Register 0x01.

When I2C selection is enabled, the internal state machine takes

control of the BLV bits and changes them based on the L2_OUT

and L3_OUT status bits. When L2_OUT is set high, it indicates

that the ambient light conditions have dropped below the

L2_TRP point and that the backlight should move to its office

(L2) level. When L3_OUT is set high, it indicates that ambient light

conditions have dropped below the L3_TRP point and that the

backlight should move to its dark (L3) level. Table 6 shows the

relationship between backlight operation and the ambient light

sensor comparator outputs.

The L3_OUT status bit has greater priority; therefore, if

L3_OUT is set, the backlight operates at L3 (dark) even if

L2_OUT is also set.

Filter times from 80 ms to 10 sec can be programmed for the

comparators (Register 0x1B and Register 0x1C) before they

change state.

Table 6. Comparator Output Truth Table

CMP_AUTOEN

L3_OUT

L2_OUT

Backlight Operation

BLV can be manually set

by the user

BLV = 00, backlight

operates at L1 (daylight)

BLV = 01, backlight

operates at L2 (office)

BLV = 10, backlight

operates at L3 (dark)

1 X is the don’t care bit.

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