NCP5608 查看數據表（PDF） - ON Semiconductor

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NCP5608

Multiple LED Charge Pump Driver

ON Semiconductor 

NCP5608

LOAD CURRENT CALCULATION

The load current is derived from the 600 mV reference

voltage provided by the internal band gap associated to the

external resistor connected across the IREFBK and IREFFL

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

be forced at IREFBK or IREFFL pins, either downward or

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

the current reference defined at the IREFBK pin. The output

current can be dimmed by means of a dynamic modulation

of the IREFBK pin.

The IREFBK reference current is multiplied by the

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

(R = Vref/IREF) and a more practical equation can be

arranged to define the resistor value for a given output

current:

Let Iout = 4*ILED, then:

RBK + (Vref * ka * 30)ńIout

RBK + (0.6 * 1200)ńIout

RBK + 720ńIout

(eq. 1)

Consequently, the resistor value will range between

RBK = 720/(30 mA*4) = 6000 W and

RBK = 720/(0.5 mA*4) = 360 kW for the low power block.

Similarly, the PowerFlash block, LED5−LED8, is

powered by the current reference defined at the IREFFL

pin. The same calculation as before applies, assuming

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

Let Iout = 4*ILED, then:

RFL + (Vref * kb * 100)ńIout

RFL + (0.6 * 4000)ńIout

RFL + 2400ńIout

(eq. 2)

Finally, the resistor value will range between

RFL = 2400/(100 mA*4) = 6000 W and

RFL = 2400/(1 mA*4) = 600 kW 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

IREFBK/IREFFL pins current references. Obviously, the

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

100 ppm thermal coefficient, to get the expected overall

tolerance.

Vbat

VOUT

PIN 18

+ 600 mV

−

PIN 3

IREFBK

R1

ANALOG

CONTROL

GND

LEDx

PWRGND

PIN 13

GND

Figure 4. Typical Backlight

Reference Current Operation

(Similar Circuit Applies for Power Flash Section)

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.

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