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
external resistor connected across the I
REFBK
and I
REFFL
pins and GND (see Figure 4). In any case, no voltage shall
be forced at I
REFBK
or I
REFFL
pins, either downward or
upward. The backlight block, LED1 − LED4, is powered by
the current reference defined at the I
REFBK
pin. The output
current can be dimmed by means of a dynamic modulation
of the I
REFBK
pin.
The I
REFBK
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/I
REF
) and a more practical equation can be
arranged to define the resistor value for a given output
current:
Let Iout = 4*I
LED
, then:
R
BK
+ (Vref * ka * 30)ńI
out
(eq. 1)
R
BK
+ (0.6 * 1200)ńI
out
R
BK
+ 720ńI
out
Consequently, the resistor value will range between
R
BK
= 720/(30 mA*4) = 6000 W and
R
BK
= 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 I
REFFL
pin. The same calculation as before applies, assuming
kb = 40, the maximum output current being 100 mA/LED:
Let Iout = 4*I
LED
, then:
R
FL
+ (Vref * kb * 100)ńI
out
(eq. 2)
R
FL
+ (0.6 * 4000)ńI
out
R
FL
+ 2400ńI
out
Finally, the resistor value will range between
R
FL
= 2400/(100 mA*4) = 6000 W and
R
FL
= 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
I
REFBK
/I
REFFL
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.
Figure 4. Typical Backlight
Reference Current Operation
(Similar Circuit Applies for Power Flash Section)
Vbat
+
−
600 mV
GND
REFBK
GND
R1
V
OUT
PWRGND
PIN 13
PIN 3
BOOST
CONVERTER
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.
