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LED Lighting Solutions Design Guide

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:

■Automotive and aircraft cabin lighting

■Automotive and aircraft instrument panel lighting

■ Architectural emergency exit lighting

■ Architectural color effect lighting

■ Industrial and outdoor lighting

■ Traffic and railway signals

■ Automotive brake lights

■ Dot matrix signs and video displays

■ LCD display backlighting

■ Personal flashlights

■ Medical instrument and tool lighting

■ 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

■ AN1207 – Switch Mode Power Supply (SMPS)

Topologies (Part II), DS01207

Common SMPS Topologies Useful for LED Lighting

Regulator

Topology

VIN to VOUT

Relationship

Complexity

Component

Count

Comments

Charge Pump

-VOUT < VIN < VOUT

Low

Medium

– Limited IOUT range

– No inductors

Buck

VIN > VOUT

Medium

– Chopped IIN

– High side drive

Boost

VIN < VOUT

Medium

– Extra parts needed to isolate

output from input

SEPIC

VOUT < VIN < VOUT

Medium

High

– Smooth IIN

– Multiple outputs

– Two inductors

Buck-Boost

VOUT < VIN < VOUT

Medium

– Single inductor

– Up to four switches

Flyback

Depends on

transformer

Medium

– Transformer can provide electrical

isolation

– Multiple outputs possible

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