NCP5608

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10

OUTPUT LED PROGRAMMING SEQUENCE

Once the maximum output current has been set up by the

external resistor (see Load Current Calculation paragraph

above), the I2C protocol can be used to dynamically adjust

the brightness of the selected block.

At this point, the dimming of each block depends upon

the content of the appropriate register (PWRLD_BK[4..0]

or

PWRLED_FL[6..0]).

The

LED current can be

calculated according to the digital value stored into the

registers.

The LED can be programmed in four steps:

1. Define the maximum ILEDBK−MAX and

ILEDFL−MAX currents requested by the Back

Light and Flash applications (set by external

resistors). This is the maximum current that will

be reached when the registers will be at their

respective full range (PWRLD_BK[4..0] = $1F =

31 Decimal, PWRLED_FL[6..0] = $7F= 127

decimal).

2. Calculate the reference current (Irefbk and Ireffl ):

Irefbk = ILED−BK/1200 and

Ireffl = ILED−FL/4000

3. Calculate the external resistor value

RBK = 0.6/Irefbk

RFL = 0.6/Ireffl

4. The dimming of flash and backlight LED will be

now achieved by changing the PWRLD_BK[4..0]

and PWRLED_FL[6..0] registers content to get the

operating LED current along the curves 0 mA to

ILED−BK−MAX mA and 0 mA to ILED−FL mA:

BK−NSteps = number of steps stored into the

PWRLD_BK register (value, in decimal, of the

PWRLD_BK[4..0] register)

FL−NSteps = number of steps stored into the

PWRLED_FL register (value, in decimal, of the

PWRLD_FL[6..0] register)

ILEDBK = (ILEDBK−MAX/31) * BK−NSteps

ILEDFL = (ILEDFL−MAX/127) * FL−NSteps

PHYSICAL ADDRESS

The physical I2C address dedicated to the NCP5608 to support the I2C protocol is: 0111 001X

→ $72. The external

controller must fulfill the I2C protocol to drive the chip: see I2C−BUS SPECIFICATION, Version 2.1. The NCP5608

operates as a Slave only and never takes over the I2C control.

Table 3. NCP5608 Operation Truth Table

PWRLED_BK (0−7)

PWRLED_FL (0−7)

Output Voltage

Comments

$00

$00

Forced to zero

DC−DC = OFF

>$80

X

Vfbk + Vsense

DC−DC = ON, LED1 to LED4 active

>$00

X

Vfbk + Vsense

DC−DC = ON, LED1 to LED3 active

LED4 deactivated

X

>$00

Vffl + Vsense

DC−DC = ON

The I2C protocol is based on the standard format defined

in the industry. Basically, the DATA is transferred from the

MCU to the NCP5608 registers by means of the SDA

message associated to the SCL clock. The MCU presents

the 8 bits during the low state of the SCK signal and the

peripheral device ( in our case, the NCP5608) shall reads

the bits during the high state of the same clock. The transfer

is MSB first as depicted in Figure 5.

DATA

CLOCK

MPU send bit

The NCP5608 reads one bit

MPU enables clock

The NCP5608 send ACK

B0

B1

B2

B3

B4

B5

B6

B7

Start

ACK

B0

B1

B2

B3

B4

B5

B6

B7

ACK

Stop

PHYSICAL ADDRESS FRAME

DATA FRAME

NOTE: See I2C−BUS SPECIFICATION, Version 2.1, January 2000, for further timing details.

Figure 5. Basic I2C Timings