Datasheet

www.microchip.com/lighting LED Lighting Solutions Design Guide 3
LED Lighting Solutions
LED Lighting
LEDs are no longer used just for providing the pretty red and
green indicator lights on electronic equipment. Advances
in technology have allowed LEDs to be used as practical
sources of illumination. The primary benefits of LEDs are
long life, durability and efficiency. When driven properly, a
power LED can last tens of thousands of hours without a
degradation of light output. The typical efficacy of a power
LED, measured in lumens per watt, is 40-80. This is several
times greater than incandescent light sources and is only
exceeded by fluorescent light sources. Since the LED is a
solid-state device, it can withstand shock and vibration that
would damage a filament bulb.
LED Applications
The benefits of LED lighting are helpful in many types of
lighting applications:
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Automotive and aircraft cabin lighting
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Automotive and aircraft instrument panel lighting
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 Architectural emergency exit lighting
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 Architectural color effect lighting
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 Industrial and outdoor lighting
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 Traffic and railway signals
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 Automotive brake lights
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 Dot matrix signs and video displays
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 LCD display backlighting
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 Personal flashlights
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 Medical instrument and tool lighting
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 Digital camera flash and video light
Efficient LED Control
LEDs must be driven with a source of constant current. Most
LEDs have a specified current level that will achieve the
maximum brightness for that LED without premature failures.
An LED could be driven with a linear voltage regulator
configured as a constant current source. However, this
approach is not practical for higher power LEDs due to power
dissipation in the regulator circuit. A switch-mode power
supply (SMPS) provides a much more efficient solution to
drive the LED.
An LED will have a forward voltage drop across its terminals
for a given current drive level. The power supply voltage and
the LED forward voltage characteristics determine the SMPS
topology that is required. Multiple LEDs can be connected
in series to increase the forward voltage drop at the chosen
drive current level.
The SMPS circuit topologies adopted to regulate current
in LED lighting applications are the same used to control
voltage in a power supply application. Each type of SMPS
topology has its advantages and disadvantages as presented
in the table below.
This design guide presents two types of LED driver solutions.
First, an analog driver IC can be used independently or
together with a MCU for added intelligence. Second, the LED
drive function can be integrated into the MCU application.
Literature on the Web
■
AN1114 – Switch Mode Power Supply (SMPS)
Topologies (Part I), DS01114
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AN1207 – Switch Mode Power Supply (SMPS)
Topologies (Part II), DS01207
Common SMPS Topologies Useful for LED Lighting
Regulator
Topology
V
IN to VOUT
Relationship
Complexity
Component
Count
Comments
Charge Pump -V
OUT < VIN < VOUT Low Medium
– Limited IOUT range
– No inductors
Buck VIN > VOUT Medium Medium
– Chopped IIN
– High side drive
Boost VIN < VOUT Medium Medium
– Extra parts needed to isolate
output from input
SEPIC V
OUT < VIN < VOUT Medium High
– Smooth IIN
– Multiple outputs
– Two inductors
Buck-Boost V
OUT < VIN < VOUT Medium Medium
– Single inductor
– Up to four switches
Flyback
Depends on
transformer
Medium Medium
– Transformer can provide electrical
isolation
– Multiple outputs possible