FAN5645 Datasheet, PDF (7/10 Pages) Fairchild Semiconductor – Indicator LED Blinker with Single-Wire Interface

Application Information
Setting the LED Current
The LED forward current is established by the external
resistor, RSET, according to:
ILED = 400 • 1.235V / RSET
where
1.235V = Typical ISET voltage, and
400 = Typical current mirror ratio.
EQ.1
The ILED accuracy is determined by the tolerance of the
above parameters over the input voltage and the
ambient temperature range as well the tolerance of the
RSET resistance. For best accuracy, RSET should be a
precision resistor, connected close to the IC pins, so
that the voltage across RSET is identical to the voltage
between the ISET and GND pins.
Programming the LED ON times below 20 - 30ms
appears as a flash, rather than as a blink with an
observable ON time, to the observer. The brightness of
such a flash is determined by the energy delivered
during the flash. It is possible to change the apparent
brightness by varying the ON / OFF time ratio, keeping
the frequency higher than 40Hz.
Since the resolution of the ON time is in 1ms
increments, a dimming function for 20 discrete levels of
brightness can be implemented via software.
Selecting External Components
A ceramic capacitor of 0.1µF or more can be added
between VIN and GND to reduce the electrical noise at
the power supply line.
The minimum value of VIN should exceed Vf by as little
as 40mV to ensure the LED current is regulated at the
right value. Conversely, the LED should be selected to
have the Vf at the specified current, at least 40mV lower
than the minimum VIN.
The LEDs utilized as blinking indicators, driven by the
FAN5645, may be white or any color. The forward
current voltage characteristics and the absolute
maximum ratings are provided by the manufacturer in
their technical specifications. The typical forward
voltage at 15mA current is 1.9V for red and orange,
2V for yellow, 2.1V for green, and 3.3V for bright white,
bright non-yellowish green, and most blue types.
Meet the maximum rated current of the LED only under
favorable conditions with little or no heat buildup. Some
LED current ratings assume really favorable test
conditions - such as surrounded by air no warmer than
25°C and decent thermal conduction from where the
leads are mounted. Running LEDs at specified
laboratory conditions used for maximum current rating
can cause LEDs to lose half the light output after rated
life expectancy (20,000 to 100,000 hours), best-case.
© 2006 Fairchild Semiconductor Corporation
FAN5645 • Rev. 1.0.0
7
www.fairchildsemi.com