SC668 datasheet(42/46 Pages) SEMTECH

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SC668

To prevent drop-out, the maximum LDO4 output setting

should be limited to:

LDO4(MAX) = VIN(MIN) - [1.33 × ILDO4(MAX)],

where V

IN(MIN) is the minimum battery voltage and ILDO4(MAX)

is the maximum load current of LDO4. LDO4 load current

includes current to the external photo-detection circuit.

AD_O7 through AD_O0 [D7:D0]

These are the 8 bits of AD

OUT. AD_O7 [D7] is the Most

Significant Bit (MSB), and AD_O0 [D0] is the Least

Significant Bit (LSB). Binary weights of these bits are

shown in Table 24.

Table 24 — AD

OUT Bits [D7:D0]

Name

Bit

Binary Weight

AD_O7

1/2

AD_O6

1/4

AD_O5

1/8

AD_O4

1/16

AD_O3

1/32

AD_O2

1/64

AD_O1

1/128

AD_O0

1/256

To calculate the voltage at the ADC input, the weights of

only the bits set to one are summed together and multi-

plied by the voltage provided by LDO4. The result is equal

to the analog voltage applied to the ADI pin. For example,

if the value of register 13h reads AD

OUT = 01111111,

and V

LDO4 = 2.8V, the ADC input is at:

2.8 x (4-1 + 8-1 + 16-1 + 32-1 + 64-1 + 128-1 + 256-1) = 1.38V.

ADC Rising Threshold (14h)

This register contains the value AD

RISE. When ADOUT rises

above AD

RISE and AD_AUTO = 1, backlight bank #1 changes

to the target value in the bank #1 target register (06h).

AD_R7 through AD_R0 [D7:D0]

These are the 8 bits of AD

RISE. AD_R7 is the MSB, and AD_R0

is the LSB.

ADC Falling Threshold (15h)

This register contains the value AD

FALL. When ADOUT falls

below AD

FALL and AD_AUTO = 1, backlight bank #1 changes

to the starting value in the bank #1 register (02h).

AD_F7 through AD_F0 [D7:D0]

These are the 8 bits of AD

FALL. AD_F7 is the MSB, and AD_F0

is the LSB.

ADP and OLE Functions (16h)

ADP (Automatic Drop-out Protection) and OLE (Other

Lighting Effects) are controlled with this register. ADP

applies to bank #1 only.

ADP ensures current matching in the LEDs by responding

to a low battery voltage. ADP limits the maximum back-

light current to a level which ensures that backlight LEDs

maintain matched currents. The ADP feature also prevents

the ripple on a low battery from inducing flicker in the

LEDs. As the battery voltage gradually proceeds lower,

ADP gradually dims the backlights. The normal backlight

brightness is restored after the battery is recharged by

writing a logic zero to the ADP_EN bit.

Bank #1 will try to resume the original backlight current

setting whenever ADP_EN = 0. Also, if any bank #1 current

sink is floating, the ADP_EN bit is cleared automatically.

Any write operations to change the following bit combi-

nations in register 01h will also cause bank #1 to resume

the original setting:

DIS = 1, EN = 0, BANK1EN = 1

DIS = 0, EN = 1, BANK1EN = 1

DIS = 1, EN = 1, BANK1EN = 1

ADP_ACT Bit D6

This bit is a flag. A logic one indicates that ADP has been

activated. Once activated, the ADP is limiting the backlight

current. This flag bit is reset when ADP_EN = 0, or when

any bank#1 current sink is floating, or by writing any of

the following bit combinations in register 01h:

DIS = 1, EN = 0, BANK1EN = 1

DIS = 0, EN = 1, BANK1EN = 1

DIS = 1, EN = 1, BANK1EN = 1

•

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