LINEAR TECHNOLOGY LT3799 Technical data
Offl ine LED Lighting Simplifi ed: High Power Factor, Isolated
LED Driver Needs No Opto-Isolators and is TRIAC Dimmer
Compatible –
Design Note 490
Wei Gu
Introduction
As environmental concerns over traditional lighting increase and the price of LEDs decreases, high power LEDs
are fast becoming a popular lighting solution for offl ine
applications. In order to meet the requirements of offl ine
lighting— such as high power factor, high effi ciency, isolation and TRIAC dimmer compatibility—prior LED drivers
used many external discrete components, resulting in
cumbersome solutions. The LT
space and performance problems by integrating all the
required functions for offl ine LED lighting.
The LT3799 controls an isolated fl yback converter in
critical conduction (boundary) mode, suitable for LED
a pp li ca ti on s re qu ir i ng 4W to ov er 10 0W of L ED po wer. It s
novel current sensing scheme delivers a well-regulated
output current to the secondary side without using an
90V
TO 270V
L1
33mH
AC
C1
0.068μF
BR1: DIODES, INC. HD06
D1: CENTRAL SEMICONDUCTOR CMR1U-06M
D2, D3: DIODES INC. BAV20W
D4: CENTRAL SEMICONDUCTOR CMR1U-02M
Z1: FAIRCHILD SMBJ170A
Z2: CENTRAL SEMICONDUCTOR CMZ5937B
T1: COILCRAFT JA4429-AL
M1: FAIRCHILD FDPF15N65
®
3799 solves complexity,
L2
750μH
BR1
C3
0.22μF
C2
R18
100k
R1
200Ω
R16
32.4k
100k
NTC
0.1μF
FAULT COMP
R3
499k
R4
499k
R5
3.48k
R9
40.2k
R10
15.4k
R7
100k
R8
100k
D3
V
IN_SENSE
V
REF
CTRL3
CTRL2
CTRL1
FAULT
C6
0.1μF
Figure 1. TRIAC Dimmable 20W Offl ine LED Driver Using the LT3799
opto-coupler. Its unique bleeder circuit makes the LED
driver compa tible with TRIAC dimmers without additional
components. Open- and shor ted-LED protection ensures
long term reliability.
No-Opto Operation
Figure 1 shows a comple te LED driver solution. The LT3799
senses the output current from the primary side switch
current waveform. For a fl yback converter operating in
boundary mode, the equation for the output current is:
= 0.5 • IPK • N • (1 – D)
I
OUT
I
is the peak switch current, N is the primar y to second-
PK
ary turns ratio and D is the duty cycle. The IC regulates
L, LT, LTC, LTM, Linear Technology, the Linear logo and μModule are registered
trademarks of Linear Technology Corporation. All other trademarks are the property of
their respective owners.
R6
D2
20Ω
C4
4.7pF
C5
10μ
F
V
IN
LT3799
SENSE
V
INTVCC
+
CT COMP
C7, 0.1μF
DCM
GATE
GND
R13
2k
R4
100k
FB
R15
4.99k
R16
20Ω
C9
4.7μF
–
4:1:1
C10
560μ
× 2
1A
F
DN490 F01
20W
LED
POWER
Z2
D4
Z1
D1
M1
R
S
0.05Ω
C8
2.2nF
05/11/490
the output current by adjusting the peak switch current
and the duty cycle through a novel feedback control.
Unlike other primary side sensing methods that need to
know input power and output voltage information, this
n ew sc he me pr ov id e s m u ch be t te r o ut pu t cu r re n t r e gu la tion since the accuracy is barely affected by transformer
winding resistance, switch R
, output diode for ward
DS(ON)
voltage drop and LED cable voltage drop.
High Power Factor, Low Harmonics
By forcing the line current to follow the applied sine-wave
voltage, the LT3799 achieves high power factor and
complies with IEC61000-3-2, Class C lighting equipment Harmonics Requirement. A power factor of one
is achieved if the current drawn is proportional to the
input voltage. The LT3799 modulates the peak switch
current with a scaled version of the input voltage. This
technique provides power factors of 0.97 or greater. A
low bandwidth feedback loop keeps the output current
regulated without distorting the input current.
TRIAC Dimmer Compatible
When the TRIAC dimmer is in the off state, it’s not completely off. There is considerable leakage current fl owing
through its internal fi lter to the LED driver. This current
charges up the input capacitor of the LED driver, causing
random switching and LED fl icker. Prior solutions added
a bleeder circuit, including a large, expensive high voltage MOSFET. The LT3799 eliminates the need for this
MOSFET or any other extra components by utilizing the
transformer primary winding and the main switch as the
bleeder circuit. As shown in Figure 2, the MOSFET gate
signal is high and the MOSFET is on when the TRIAC
is off, bleeding off the leakage current and keeping the
input voltage at 0V. As soon as the TRIAC turns on, the
MOSFET seamlessly changes back into a normal power
delivery device.
Open- and Shorted-LED Protection
The LED voltage is constantly monitored through the
transformer third winding. The third winding voltage is
proportional to the output voltage when the main switch
is off and the output diode is conducting current. In the
event of overvoltage or open-LED, the main switch turns
off and the capacitor at the CT pin discharges. The circuit
enters hiccup mode as shown in Figure 3.
In a shorted LED event, the IC runs at minimum frequency
before the V
pin voltage drops below the U VLO threshold
IN
as the third winding can’t provide enough power to the
IC. The IC then enters its start-up sequence as shown
in Figure 4.
CTRL Pins and Analog Dimming
The LT3799’s output can be adjusted through multiple
CTRL pins. For example, the output current would follow
a DC control voltage applied to any CTRL pin for analog
dimming. Overtemperature protection and line brownout
protection can also be easily implemented using these
CTRL pins.
Conclusion
The LT3799 is a complete offl ine LED driver solution
featuring standard TRIAC dimming, active PFC and wellregulated LED current with no opto-coupler. This high
performance and feature-rich IC greatly simplifi es and
shrinks offl ine LED driver solutions.
V
IN
100V/DIV
V
GATE
5V/DIV
2ms/DIV
Figure 2. MOSFET Gate Signal and V
V
OUT
20V/DIV
V
SW
100V/DIV
100ms/DIV
Figure 3. Output Open-Circuit Event
I
OUT
2A/DIV
V
SW
100V/DIV
100ms/DIV
Figure 4. Output Short-Circuit Event
DN490 F02
IN
DN490 F03
DN490 F04
Data Sheet Download
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
For applications help,
call (408) 432-1900, Ext. 3565
dn490f LT/AP 0511 226K • PRINTED IN THE USA
© LINEAR TECHNOLOGY CORPORATION 2011
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