Design Idea DI-92

Constant Current LED Driver

Design Highlights

• Extremely low component count – only 9 components

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.

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

in this application, the device will switch and ramp up to the

current limit for each and every cycle.

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.

The output will be replenished every switching cycle (66 kHz),

and thus the need for the output filter capacitor is eliminated.

Persistence of vision of the human eye (typically 10 ms) is much

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

www.powerint.com

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

Application

Device

Power Output

Input Voltage

Output Current

Topology

LED Driver

LNK302P

0.5 W

85-265 VAC

40 mA

Buck-Boost

RF1

8.2

Ω, 1 W

Fusible

D1

1N4007

D3

UF4005

C1

2.2 uF

400 V

C2

2.2 uF

400 V

LED 1-12

13 mA per string

(average)

C3

0.1 uF

50 V

RF2

47

Ω

U1

LNK302P

85-265

VAC

PI-4087-082605

FB

BP

S

D

LinkSwitch-TN

L1

1 mH

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