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NCP5602
APPLICATION INFORMATION
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 Vf of the LED, plus the drop voltage (ranging
from 200 mV to 400 mV, depending upon the output
current) developed across the internal NMOS mirror.
Typically, assuming a standard white LED forward biased
at 10 mA, the output voltage will be 3.8 V.
The built ? in OVP circuit continuously monitor each
output and stops the converter when the voltage is above
Rbias + 300 Iout (eq. 2)
Consequently, 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.
VBandgap
5.0 V. The converter resumes to normal operation when the
voltage drops below 5.0 V (no latchup mechanism).
Consequently, the chip can operate with no load during any
test procedures.
Load Current Calculation
I REF
+
-
600mV
LED Return
Pin 2 & 3
The load current is derived from the 600 mV reference
voltage provided by the internal Bandgap associated to the
external resistor connected across I REF pin and Ground (see
Figure 3). In any case, no voltage shall be forced at I REF
pin, either downward or upward.
The reference current is multiplied by the constant
R1
GND
Pin 4
GND
Note: The I REF pin must never be
biased by an external voltage.
Figure 3. Basic Reference Current Source
Rbias + (Vref * k) Iout
k = 500 to yield the output 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 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 output
current:
(eq. 1)
Rbias + (0.6 * 500) Iout
7
Load Connection
The NCP5602 chip is capable to drive the two LED
simultaneously, as depicted in Figure 1, but the load can be
arranged to accommodate one or two LED if necessary in
the application (see Figure 4). In this case, the two current
mirrors can be connected in parallel to drive a single power
full LED, thus yielding 60 mA current capability in a single
LED.
7
2
3
GND
2
3
GND
Figure 4. Typical Single and Double LED Connections
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6
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