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NCP5623C Datasheet(PDF) 7 Page - ON Semiconductor |
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NCP5623C Datasheet(HTML) 7 Page - ON Semiconductor |
7 / 11 page NCP5623C http://onsemi.com 7 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 I2C 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 Vout 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 IREF pin and Ground. Note : due to the internal structure of this pin, no voltage, either downward or upward, shall be forced at the IREF 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 (Rbias = Vref/IREF) and a more practical equation can be arranged to define the resistor value for a given maximum output current: Rbias = (Vref*k)/Iout [ 1 ] Rbias = (0.6*2400)/Iout Rbias = 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 mA 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: ILED + IREF @ k 31 * n (eq. 1) ILED + Vref R @ 2400 31 * n (eq. 2) 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 I2C 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 I2C 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. I2C PROTOCOL The NCP5623C is programmed by means of the standard I2C protocol controlled by an external MCU. The communication takes place with two serial bytes sharing the same I2C frame: − Byte#1 ³ physical I2C address − Byte#2 ³ Selected internal registers & function |
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