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NCP5623C
ILED + IREF @ k (eq. 1)
@ 2400
ILED +
(eq. 2)
DC/DC OPERATION
The converter is based on a charge pump technique to
generate a DC voltage capable to supply the RGB LED load.
The system regulates the current flowing into each LED, not
the DC Vout value, by means of internal current mirrors
associated with the diodes.
Consequently, Vout = Vbat * Mode, with Mode = 1 or
Mode = 2, the extra voltage Vout ? Vf being sustained by the
current mirror structure.
The average forward current of each LED can be
independently programmed (by means of the associated
PWM ) to achieve the RGB function. The maximum LED
current, setup by the external bias resistor connected across
IREF pin and Ground, is associated to the digital content of
the I 2 C register (see Table 1). This peak current applies to the
three LED simultaneously, but, thanks to the RGB function,
the average output current of each LED is controlled by the
independent PWM controllers. Consequently, the
luminosity of each RGB diode can be independently
adjusted to cope with a given illumination need. Since the
peak current is constant, the color of the RGB diodes is the
one defined by the specifications of each individual LED.
The built ? in OVP circuit continuously monitors the V out
voltage and stops the converter when the voltage is above
5.7 V. The converter resumes to normal operation when the
voltage drops below 4.4 V (no latch ? up mechanism).
Consequently, the chip can operate under no load conditions
during any test procedures.
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 I REF pin and Ground.
Note : due to the internal structure of this pin, no voltage,
either downward or upward, shall be forced at the I REF pin.
The reference current is multiplied by the constant
k = 2400 to yield the output load current. Since the reference
voltage is based on a temperature compensated Band Gap,
a tight tolerance resistor will provide a very accurate load
current. The resistor is calculated from the Ohm’s law (R bias
= Vref/I REF ) and a more practical equation can be arranged
to define the resistor value for a given maximum output
current:
R bias = (Vref*k)/Iout [ 1 ]
R bias = (0.6*2400)/Iout
R bias = 1440/Iout [ 2 ]
Since the Iref to ILED ratio is very high, it is strongly
recommended to set up the reference current at 12.5 m A to
optimize the tolerance of the output current. Although it is
possible to use higher or lower value, as defined in the
analog section, a 48 k W / 1% resistor will provide the best
compromise, the dimming being performed by the
appropriate PWM registers.
On the other hand, care must be observed to avoid leakage
current flowing into either the IREF pin of the bias resistor
network.
Finally, for any desired ILED current, the curve provided
Figure 4 can be recalculated according to the equation:
31 * n
Vref
R
31 * n
with: n = step value @ 1 ≤ n ≤ 31
with: R = reference resistance
with: k = internal multiplier constant = 2400
Note: n = 0 forces ILED to zero
with: n = 30 and n = 31 yields the same LED current
LOAD CONNECTION
The primary function of the NCP5623C is to control three
LED arranged in the RGB color structure (reference
OSRAM LATB G66x). The brightness of each LED is
independently controlled by a set of dedicated PWM
structure embedded into the silicon chip. The peak current,
identical for each LED, is programmable by means of the
I 2 C data byte. With 32 steps per PWM, the chip provides
32768 colors hue in a standard display.
Moreover, a built ? in gradual dimming provides a smooth
brightness transition for any current level, in both Upward
and Downward direction. The dimming function is
controlled by the I 2 C interface: see Table 2.
The NCP5623C chip is capable to drive the three LED
simultaneously, as depicted in Figure 1, but the load can be
arranged to accommodate several LED if necessary in the
application. Finally, the three current mirrors can be
connected in parallel to drive a single power full LED, thus
yielding 90 mA current capability in a single LED.
I 2 C PROTOCOL
The NCP5623C is programmed by means of the standard
I 2 C protocol controlled by an external MCU. The
communication takes place with two serial bytes sharing the
same I 2 C frame:
? Byte#1 3 physical I 2 C address
? Byte#2 3 Selected internal registers & function
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