NCP5608

http://onsemi.com

8

LOAD CURRENT CALCULATION

The load current is derived from the 600 mV reference

voltage provided by the internal band gap associated to the

pins and GND (see Figure 4). In any case, no voltage shall

upward. The backlight block, LED1 − LED4, is powered by

current can be dimmed by means of a dynamic modulation

constant ka to yield the output backlight LED load current.

Since the reference voltage is based on a temperature

compensated bandgap, a tight tolerance resistor will

provide a very accurate load current.

The ka parameter is derived from the constant 40

multiplied by the binary defined in the PWRLED_BK

register. Consequently, ka varies from 40 (1.0 mA

output/LED) to 1200 to support the full output current

range. The resistor is calculated from the Ohm’s law

arranged to define the resistor value for a given output

current:

RBK + (Vref * ka * 30) Iout

(eq. 1)

RBK + (0.6 * 1200) Iout

RBK + 720 Iout

Consequently, the resistor value will range between

W and

W for the low power block.

Similarly, the PowerFlash block, LED5−LED8, is

pin. The same calculation as before applies, assuming

kb = 40, the maximum output current being 100 mA/LED:

RFL + (Vref * kb * 100) Iout

(eq. 2)

RFL + (0.6 * 4000) Iout

RFL + 2400 Iout

Finally, the resistor value will range between

W and

W for the High Power Flash

block.

On the other hand, the output currents can be dimmed by

means of a dynamic modulation of their respective

tolerance of such resistors must be 1% or better, with a

100 ppm thermal coefficient, to get the expected overall

tolerance.

Figure 4. Typical Backlight

Reference Current Operation

(Similar Circuit Applies for Power Flash Section)

Vbat

+

−

600 mV

GND

GND

R1

PWRGND

PIN 13

PIN 3

LEDx

ANALOG

CONTROL

PIN 18

Note: Due to relative high impedance connected at the

reference current pins, cares must be observed to minimize

the noise pick−up and stray current present at PCB level.

Multi layer layout, with dedicated ground screen, is

mandatory.

SERIAL LINK I2C PROTOCOL

The chip is remotely controlled by means of a byte

transferred along a serial link between the MCU and the

NCP5608. The industrial standard I2C protocol is used,

although one can drive the SCL and SDA signal from

standard MCU I/O pins . Two dedicated internal registers

are used to decode the SDA content and to store the output

currents.

The I2C message carries three bytes within the same

frame:

Byte #1:

The content of this byte is the physical address

of the NCP5608 in the I2C bus.

Byte #2:

The content of this byte contains the address

of the selected block.

Byte #3:

This byte contains the output current value to

set up the selected block.