Design Idea DI-131
LinkSwitch-TN
Application
LED Lighting
Device
LNK306DN
®
LED Driver for Halogen Light Bulb Retrofit
Power Output
3W
Input Voltage
85-265 VAC
Output Voltage
Topology
10 V
TYP
(9-15 V), 300 mA Non-isolated Buck
Design Highlights
• Fits inside standard GU10 lamp base
• Constant current (CC) output
• Drive current independent of LED V
F
variations
• Tight line regulation (±5%) for constant light output
• No annoying line frequency flicker
• Universal input voltage range
• A single design can be sold worldwide
• Consistently high efficiency across entire range
• Meets EN55022 B conducted EMI limits
• Complete overvoltage and over temperature protection
• Operates unloaded: can be tested without the LEDs
• Auto-restart: withstands shorted output indefinitely
• Thermal shutdown: protects entire lamp assembly
Diode bridge BR1 rectifies the AC input while capacitors C1
and C2 filter it. Inductor L1 and RF1 attenuate conducted EMI.
In case of a component failure, RF1 will act as a fuse.
The MOSFET within U1, diode D3, inductor L2 and capacitor
C3 form the buck converter stage. The controller within
U1 regulates the output current by enabling and disabling
MOSFET switching cycles.
During normal operation, the output current develops a
voltage drop across R8 and R10, which is impressed across
C6. When the voltage across C6 exceeds the V
BE
of Q1,
Q1 and Q2 both turn on. When Q2 turns on, current is
injected into the FEEDBACK (FB) pin of U1 from C5, via
R3. Whenever the current into the FB pin exceeds 49 mA,
MOSFET switching is disabled. The controller adjusts the
ratio of enabled and disabled cycles to regulate the output
current at 330 mA.
During no-load operation, the output voltage is sensed by
D1, D2 and C2. The voltage across C5 feeds current into
the FB pin of U1 through divider R7 and R1. The FB pin
R7
30 kΩ
Q2
MMST3906
R3
10 kΩ
R1
4.7 kΩ
R9
4.7 kΩ
D1
BAV21WS-7-F
D2
BAV21WS-7-F
Operation
The non-isolated buck converter shown in Figure 1 was
designed around a member of the LinkSwitch-TN IC family,
the LNK306DN (U1). A LNK306 was selected since its
minimum current limit (450 mA) ensures that it will be able
to deliver the required 330 mA of output current.
C5
100 nF
25 V
R6
10 kΩ
R4
10 kΩ
C6
100 nF
25 V
R8
2
Ω
R10
12
Ω
L2
1 mH
C3
2.2
µF
25 V
R11
1 kΩ
Q1
MMST3904
9-15 V,
300 mA
J3
C4
100 nF
25 V
FB
BP
S
D
L
85-265
VAC
N
RF1
10
Ω
2.5 W
BR1
MB6S
600 V
C1
4.7
µF
380 V
L1
1 mH
C2
4.7
µF
380 V
LinkSwitch-TN
U1
LNK306DN
D3
MURS160T3
J4
PI-4575-030707
Figure 1. 3 W LED, Halogen Bulb Replacement using LinkSwitch-TN in Non-Isolated Buck Converter Technology.
DI-131
March 2007
POWERINT [ POWER INTEGRATIONS, INC. ]
