Design Idea DI-92
LinkSwitch®-TN 0.5 W Non-Isolated
Constant Current LED Driver
Application
Device
Power Output
Input Voltage
Output Current
Topology
LED Driver
LNK302P
0.5 W
85-265 VAC
40 mA
Buck-Boost
Design Highlights
in this application, the device will switch and ramp up to the
current limit for each and every cycle.
• Extremely low component count – only 9 components
required
Since the peak current is limited and fixed for each cycle, the
output power is solely determined by the size of the inductor.
It is recommended that this design operate in the discontinuous
conduction mode (DCM). Besides better EMI performance,
this also ensures that a low cost 75 ns reverse recovery diode
such as the UF4005 can be used. For designs that operate in
the continuous conduction mode (CCM), a more expensive
but faster diode (30 ns reverse recovery) like the BYV26C
may be required.
• Universal AC input range – single design worldwide
• Low cost, small size and very lightweight
• Replacement for passive capacitor or resistor droppers
• High efficiency (approximately 70% at 85 VAC)
• Meets EN55022 B EMI limits with >8 dB margin (see
Figure 2)
Operation
Figure 1 shows a simple buck-boost converter, operating in
open loop with no output feedback, being used as a constant
current LED driver. The circuit relies on the internal current
limiting function of the LNK302, which ensures constant
current is supplied to the load. Typical uses include night-
lights, neon sign replacements, emergency exit signs or any
application utilizing LEDs for lighting.
The output will be replenished every switching cycle (66 kHz),
and thus the need for the output filter capacitor is eliminated.
Persistenceofvisionofthehumaneye(typically10ms)ismuch
longer than the switching period, and it thus sees a consistent
light output without flicker.
Select the value of L1 following the LinkSwitch-TN Design
Guide (www.powerint.com/appnotes.htm), or using the PI Xls
design spreadsheet (www.powerint.com/designsoftware.htm).
EntertheoutputvoltageasthevoltageoftheLEDstring,andthe
output current as the total combined LED current. Alternately,
one can calculate the inductance using
TheAC input is rectified and filtered by D1, C1, C2, and RF2.
For safety, resistor RF1 should be a fusible flameproof type,
whereas RF2 can be flameproof only.
LinkSwitch-TN uses current limited ON/OFF control to
regulate the output current. This type of control inherently
rejects any input voltage variations over the entire operating
range. Current greater than 49 µA into the FEEDBACK pin
disables the MOSFET for that switching cycle. Since there
is never any current being fed back into the FEEDBACK pin
1
PO = $ L $ ILIMIT2 $ fS $ h
2
2 $ PO
& L =
ILIMIT2 $ fS $ h
LED 1-12
C3
0.1 uF
50 V
D3
UF4005
D1
1N4007
RF2
47 Ω
13 mA per string
FB
BP
S
(average)
D
RF1
8.2 Ω, 1 W
Fusible
LinkSwitch-TN
U1
LNK302P
C1
2.2 uF
400 V
C2
2.2 uF
400 V
L1
1 mH
85-265
VAC
PI-4087-082605
Figure 1. A 0.5 W, 12.9 V, 40 mA Constant Current LED Driver, Using a Non-Isolated Buck-Boost Topology.
DI-92
September 2005
www.powerint.com
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