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NCP5612
APPLICATION INFORMATION
t wkp
t on
t off
V IH
t f
90%
t r
90%
V IL
10%
Bit = 1
Bit = 0
Bit = 0
Figure 3. Timings Reference
V OH @ V ccio = 3.0 V 2600 mV
V OH @ V ccio = 2.6 V 2400 mV
1400 mV
V IHsw
V OL @ MOTOROLA: 500 mV
V OL @ QUALCOMM: 450 mV
V OL @ INTEL: 400 mV
V IL
600 mV
100 mV/step
GROUND
Figure 4. Basic Cellular Phone Chip Set Digital Output Levels
DC/DC Operation
The converter is based on a charge pump technique to
generate a DC voltage capable to supply the White LED
load. The system regulates the current flowing into each
LED by means of internal current mirrors associated with
the white diodes. Consequently, the output voltage will be
equal to the V f of the LED, plus the drop voltage (ranging
from 150 mV to 400 mV, depending upon the output
current and V bat / V f ratio) developed across the internal
NMOS mirror. Typically, assuming a standard white LED
forward biased at 10 mA, the output voltage will be 3.6 V.
The built?in OVP circuit continuously monitors the
output voltage and stops the converter when the voltage is
above 5.0 V typical. The converter resumes to normal
operation when the voltage drops below the typical 5.0 V
(no latch?up mechanism). Consequently, the chip can
operate with no load 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 (see
Figure 5). In any case, no voltage shall be forced at I REF pin,
either downward or upward.
The reference current is multiplied by the internal
current mirror, associated to the number of pulses as
depicted Figure 9, 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 = V ref /I REF ) and define the maximum
current flowing into the LED when 20 pulses have been
counted at the CNTL pin.
Since the reference current must be between the
minimum and maximum specified, the resistor value will
range between R bias = 300/30 mA = 10 k W and R bias =
300/0.5 mA = 600 k W . Obviously, the tolerance of such a
resistor must be 1% or better, with a 100 ppm thermal
coefficient, to get the expected overall tolerance.
Typical applications will run with R bias = 10 k W to make
profit of the full dynamic range provided by the S?Wire
DATA byte.
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