NCP5608 ON Semiconductor, NCP5608 Datasheet - Page 8

• Manufacturer Part Number: NCP5608
• Description: Multiple LED Charge Pump Driver
• Manufacturer: ON Semiconductor

LOAD CURRENT CALCULATION
voltage provided by the internal band gap associated to the
external resistor connected across the I
pins and GND (see Figure 4). In any case, no voltage shall
be forced at I
upward. The backlight block, LED1 − LED4, is powered by
the current reference defined at the I
current can be dimmed by means of a dynamic modulation
of the I
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.
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
arranged to define the resistor value for a given output
current:
R
R
powered by the current reference defined at the I
pin. The same calculation as before applies, assuming
kb = 40, the maximum output current being 100 mA/LED:
R
R
block.
means of a dynamic modulation of their respective
I
tolerance of such resistors must be 1% or better, with a
100 ppm thermal coefficient, to get the expected overall
tolerance.
REFBK
BK
BK
FL
FL
The load current is derived from the 600 mV reference
The I
The ka parameter is derived from the constant 40
Let Iout = 4*I
Consequently, the resistor value will range between
Similarly, the PowerFlash block, LED5−LED8, is
Let Iout = 4*I
Finally, the resistor value will range between
On the other hand, the output currents can be dimmed by
= 2400/(100 mA*4) = 6000 W and
= 2400/(1 mA*4) = 600 kW for the High Power Flash
= 720/(30 mA*4) = 6000 W and
= 720/(0.5 mA*4) = 360 kW for the low power block.
/I
REFBK
REFBK
REFFL
REF
REFBK
pin.
R BK + (Vref * ka * 30) I out
R BK + (0.6 * 1200) I out
R BK + 720 I out
R FL + (Vref * kb * 100) I out
R FL + (0.6 * 4000) I out
R FL + 2400 I out
) and a more practical equation can be
pins current references. Obviously, the
reference current is multiplied by the
LED
LED
, then:
, then:
or I
REFFL
pins, either downward or
REFBK
REFBK
pin. The output
and I
http://onsemi.com
REFFL
REFFL
(eq. 1)
(eq. 2)
NCP5608
8
I
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
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.
frame:
REFBK
Note: Due to relative high impedance connected at the
The chip is remotely controlled by means of a byte
The I2C message carries three bytes within the same
R1
GND
Byte #1:
Byte #2:
Byte #3:
PIN 3
(Similar Circuit Applies for Power Flash Section)
The content of this byte is the physical address
of the NCP5608 in the I2C bus.
The content of this byte contains the address
of the selected block.
This byte contains the output current value to
set up the selected block.
CONTROL
ANALOG
+
−
Reference Current Operation
Vbat
Figure 4. Typical Backlight
600 mV
GND
PIN 18
PIN 13
LEDx
PWRGND
V
OUT