NCP5602(2008) Просмотр технического описания (PDF) - ON Semiconductor

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NCP5602
(Rev.:2008)

High Efficiency Ultra Small Thinnest White LED Driver

ON Semiconductor 

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

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.

Rbias + 300ńIout

(eq. 2)

Consequently, the resistor value will range between

Rbias = 300/30 mA = 10 kW and Rbias = 300/0.5 mA =

600 kW. 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

+ 600mV

-

LED Return

Pin 2 & 3

Load Current Calculation

The load current is derived from the 600 mV reference

voltage provided by the internal Bandgap associated to the

external resistor connected across IREF pin and Ground (see

Figure 3). In any case, no voltage shall be forced at IREF

pin, either downward or upward.

The reference current is multiplied by the constant

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

(Rbias = Vref/IREF) and a more practical equation can be

arranged to define the resistor value for a given output

current:

Rbias + (Vref * k)ńIout

Rbias + (0.6 * 500)ńIout

(eq. 1)

IREF

Pin 4

R1

GND

GND

Note: The IREF pin must never be

biased by an external voltage.

Figure 3. Basic Reference Current Source

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

7

2

GND

2

3

3

Figure 4. Typical Single and Double LED Connections

http://onsemi.com

6

GND

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