LED Lighting Solutions – Dimming Options
Brian Johnson
Abstract: Fairchild Semiconductor’s LED solutions for low-,
mid-, and high-power designs reduce external components to save
space, provide high reliability, and enhance overall system
efficiency. Featuring integrated components in a single IC,
Fairchild utilizes proven, highly efficient, and effective
technologies to power voltage ranges that are 1 W and higher. With
multiple topology offerings that provide energy-efficient solutions,
this paper provides guidelines to help designers select the best
Fairchild LED controller for dimming applications.
I.
FL103, FLS3217, FLS3247, and FL7732. The
higher-power controllers FL6300, FL6961, and
FL7930, also do not have a dimming feature internal to
the controller. There are many other Fairchild parts to
choose from, but let’s focus on the LED lighting
segment PNs only.
Isolation
Is isolation required? If the answer is yes, consider
the flyback topology solution, which many of the
controllers shown in Table 1 support. The buck-boost
or buck topologies become options when isolation is
not required in the LED driver.
When isolation is required, the FL7730 controller is
a primary-side regulated controller and the dimming
input to the controller is also on the primary side. If the
dimming signal is on the secondary side, the dimming
signal requires an isolation circuit. TRIAC dimming is
already configured on the primary side. Using other
controllers in Table 1 require an isolation circuit if the
dimming signal is on the secondary side of the driver.
INTRODUCTION
You’re asked about dimming support... What runs
through your mind? To help you understand the
dimming features available using Fairchild controllers,
let’s start with a couple of basic questions and
categorize the solutions to respond to your needs.
II.
DESIGN DECISIONS
To Dim or Not to Dim
The obvious question is if dimming is needed. Not
all LED lighting solutions require dimming. Table 1
shows the LED controller offerings. Options that do
not have dimming as a feature in the controller are the
TABLE 1. FAIRCHILD LED LIGHTING CONTROLLER PARTS
Topology
PN
FLS0116
FLS3217
FLS3247
FL7701
FL103
FL7730
FL7732
FL6300
FL6961
FL7930
Buck
PSR Flyback or
Buck-Boost
PSR Flyback or
Buck-Boost
Buck
PSR Flyback or
Buck-Boost
PSR Flyback or
Buck-Boost
PSR Flyback or
Buck-Boost
QR Flyback
Flyback or Boost
Flyback or Boost
Reference
Designs
Features
Isolation
Integrated
MOSFET
PFC
TRIAC
Dimming
Analog Dimming
(0-10 V)
PWM
Dimming
RD#
N
Y
Y
N
Y
Y
L032
Y/N
Y
Y
N
N
N
L040/41
Y/N
Y
Y
N
N
N
L042
N
N
Y
N
Y
Y
L030/31
Y/N
N
N
N
N
N
L015
Y/N
N
Y
Y
Y
Y
L020/21
Y/N
N
Y
N
N
N
L025/26/28
Y/N
Y/N
Y/N
N
N
N
Y
Y
Y
External
External
External
External
External
External
External
External
External
L016
L008
L011/12/13
© 2012 Fairchild Semiconductor • Rev. 1.0.0
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START
YES
Dimming
Required?
(PFC Included)
YES
TRIAC
Dimming?
NO
NO
Dimmer
Compatibility
Count ~10 or
less?
YES
YES
Passive Bleeder
PFC Required?
NO
NO
Active Bleeder
Recommend
FL103
YES
Integrated
MOSFET
Required?
NO
YES
YES
Integrated
MOSFET
Required?
YES
NO
Recommend
FL7730 Buckboost
NO
Recommend FLS3217,
FLS3247
Both in Buck-boost OR PSR
Flyback topologies
depending on isolation
requirement
Isolation
Required in the
LED Driver?
Isolation
Required in the
LED Driver?
Recommend
FL7730 PSR
Flyback
Recommend
FLS0116 (nonisolated buck)
Isolation
Required in the
LED Driver?
NO
YES
NO
Recommend
FL7732 Buckboost
Recommend
FL7732 PSR
Flyback
Recommend
FL7730 Buckboost
Recommend
FL7730 PSR
Flyback
In these two branches, the
FL6300, FL6961, and
FL7930 are also
recommended depending on
power level of application
Fig. 1. Part Number Selection Flow Chart
III.



TYPES OF DIMMING
TRIAC or phase-cut where the “rms” voltage
input to the LED driver is proportionally
controlled for the amount of dimming desired.
Analog dimming uses a 0 – 10 V signal or 1 –
10 V signal input where the voltage is
proportional to the amount of dimming.
A PWM signal with the duty cycle proportional
to the amount of dimming output. This can be the
output from a MCU, Digital Adressable Lighting
Interface (DALI), or ZigBee® signal.
© 2012 Fairchild Semiconductor • Rev. 1.0.0

A step method where there are preset dimming
levels selectable by a simple toggle action.
Other dimming methods exist, but the theme with
many methods is dimming control of the LED driver
with an input voltage, an analog signal, a PWM signal,
or a staircase stepped value.
2
TRIAC Dimming
Non-TRIAC Dimming
For TRIAC dimming, the LED driver solution is
based on the FL7730. How many TRIAC dimmers does
the design need for flicker-free operation? If the dimmer
list is less than 10, using a passive bleeder circuit
(simple R-C) is effective to maintain TRIAC holding
currents. Higher dimmer compatibility counts require an
active bleeder. An active bleeder circuit is not
recommended for an LED load less than 8 Watts. The
TRIAC dimmer needs a latching current during firing
and a holding current during turn-on after firing. If these
two currents are not satisfied, the TRIAC dimmer
misfires and the LED light may flicker. The active
bleeder is used for these two current requirements and
the bleeder current is an efficiency loss.
Another consideration for the TRIAC design is at
dimmer firing, a large current spike is induced by the
input line quickly charging the input capacitance of the
driver. Without a resistive damper circuit, the large
current spike creates a line current oscillation, causing
dimmer misfire and damage to the TRIAC dimmer.
While the damper resistor suppresses the spike current,
the power loss in the damper resistor is another
efficiency loss. The damper resistor not only dampens
the spike current, but also handles the input current
from the flyback. For more about bleeder concepts,
reference application note AN-9745 — Design Guide
for TRIAC Dimmable LED Driver Using FL7730.
The non-TRIAC-based dimming methods are
analog signals and the LED driver solutions based on
the FLS0116, FLS0116, and FL7730. These
controllers can use the analog signal directly or need a
translation circuit to operate within the limits of the
controller dimming input I-V characteristics. These
controllers can also accept a PWM input with a filter
applied to convert the PWM signal into an analog
equivalent (average). Step dimming can also use these
controllers, but the interface needs to translate the
sensed input state into an equivalent analog signal for
the controller to accept. The controllers from Table 1
(FL6300, FL6961, and FL7930) do not have embedded
dimming functions and require an external circuit,
regardless of the dimming input.
IV.
CONCLUSION
The main purpose to this paper is to answer basic
dimming questions to help pinpoint an acceptable
controller. There are other considerations in selecting a
controller not discussed; such as integrated MOSFET,
output voltage to input voltage ratio, power factor
correction, transformer versus inductor, etc. Use Table
1 to start the controller part number selection process.
Reference Fig. 1, a flow chart outlining the questions
presented in this paper.
Brian Johnson is the Americas Lighting
Specialist for Fairchild’s LED Lighting Product
Segment. He joined Fairchild after spending 25+
years rotating in Development and Marketing
positions in the Power Electronics Industry. He
graduated from Purdue University with a B.S.E.E.
and M.S.E.E.
REFERENCES
[1] AN-9745 — Design Guide for TRIAC Dimmable LED Driver Using FL7730.
[2] FLS0116 — MOSFET Integrated Smart LED Lamp Driver IC with PFC Function
[3] FL7730 — Dimmable Single-Stage PFC PSR Offline LED Driver
[4] FL6300A — Quasi-Resonant Current Mode PWM Controller for Lighting
[5] FL6961 — Single-Stage Flyback and Boundary Mode PFC Controller for Lighting
[6] FL7930 — Single-Stage Flyback and Boundary Mode PFC Controller for Lighting
[7] FLS3217 — Single-Stage PFC Primary-Side-Regulation Offline LED Driver with Integrated Power MOSFET
[8] FLS3247 — Single-Stage PFC Primary-Side-Regulation Offline LED Driver with Integrated Power MOSFET
[9] FL7701 — Smart Non-Isolated PFC Buck LED Driver
[10] FL103 — Primary-Side-Regulation PWM Controller for LED Illumination
[11] FL7732 — Single-Stage PFC Primary-Side-Regulation Offline LED Driver
© 2012 Fairchild Semiconductor • Rev. 1.0.0
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