Datasheet

ADP8863
Rev. A | Page 18 of 52
L2_HYS
L2_TRIP
L3_TRIP
L3_HYS
FILTER
SETTINGS
ADC
PHOTO
SENSOR
OUTPUT
L2_EN
L3_EN
L2_OUT
L3_OUT
08392-045
Figure 41. 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
1 10 100 1000
ADC RANGE (µA)
08392-046
Figure 42. 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 A = 864 A), 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 I
2
C 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
0 X
1
X
1
BLV can be manually set
by the user
1 0 0
BLV = 00, backlight
operates at L1 (daylight)
1 0 1
BLV = 01, backlight
operates at L2 (office)
1 1 X
1
BLV = 10, backlight
operates at L3 (dark)
1
X is the don’t care bit.