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 LED
Lighting Overview
Regardless of type, color, size, or power, all LEDs work best
when driven with a constant current. LED manufacturers specify
the characteristics (such as lumens, beam pattern, color)
of their devices at a specified forward current (IF)
not at a specific forward voltage (VF). Most power
supply ICs are designed to provide constant voltage outputs
over a range of currents, hence it can be difficult to ascertain
which parts will work for a given application from the device
datasheet alone. With an array of LEDs, the main challenge
is to ensure that every LED in the array is driven with the
same current. Placing all the LEDs in a series string ensures
that exactly the same current flows through each device.
Low-Power LEDs
Low-power LEDs are ideal for lighting portable electronics
because they are efficient, easy to drive, small, thin, robust,
and low noise. When running off a Lithium-Ion battery (typically
3.7V output voltage), each low-power LED requires up to 4V
at 30 mA. To operate more than one LED for a lighting solution,
an LED driver is needed to boost the voltage and regulate
the current to optimize LED output.
For low-powered LEDs, National offers LED drivers for both
parallel and series solutions. Parallel drivers with built-in,
actively-matched, high- and low-side current sources, high
efficiency, and a low total component count are available
with inductive-boost converters, switched-capacitor boost
converters, or no boost at all. National’s series of
LED drivers with inductive-boost converters provide solutions
that combine very high efficiency with low noise and a small
footprint. All of these solutions are optimized to drive 2
to 10 LEDs, and are available in the industry’s smallest
packaging: tiny micro-SMD BGA and versatile LLP (Leadless
Leadframe Package) and CSP-leadless packages.
High-Brightness LEDs: Input Voltage and Forward Voltage
Sources of input voltage for LED arrays come from batteries
or power supplies that have a certain tolerance. An automotive
battery, for example, may supply 8V to 16V depending on the
load and the age of the battery. The ‘silver box’
power supply inside a desktop CPU may supply 12V ±10%.
High-brightness (HB) LEDs also give a range of forward voltage.
A typical HB LED might be characterized at a forward current
of 350 mA. The forward voltage of the LED when IF
= 350 mA is specified with a range that includes a typical
value as well as over-temperature maximum and minimum values.
To ensure that a true constant current is delivered to each
LED in an array, the power topology must be able to deliver
an output voltage equal to the sum of the maximum forward
voltages of every device placed in the string.
Manufacturers bin their devices for color, brightness, and
forward voltage. Binning for all three characteristics is
expensive, and forward voltage is often the specification
that is allowed to vary the most. Adding this to the shift
in forward voltage as the LED die temperature changes gives
rise to the need for constant current regulators that have
a wide range of output voltage.
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