DER-92

0.5W, 40mA, Open Loop LED Driver

August 11, 2005

4

Circuit Description

This is a very simple circuit, which requires as few as 9 components. An additional input

diode may be placed on the return leg for EMI and surge withstand capability

4.1

Input EMI Filtering

Resistors RF1, RF2 and capacitors C1 and C2 form the EMI filter. C1 C2 and RF2 are

connected to form a low cost resistive

π filter and provide excellent differential mode

filtering and also serve as the stable DC bus voltage. Resistor RF1 is a fusible flameproof

type while RF2 can be only flameproof.

4.2

LinkSwitch-TN

LNK302P is used in open loop mode for this circuit. This particular device in the

LinkSwitch-TN family does not have the auto restart feature, and thus when the

feedback pin is left open, the MOSFET switches every cycle until current limit (or duty

cycle limit) is reached.

4.3

Output Rectification

Since this circuit operates strictly in the discontinuous conduction mode (DCM) a 75 nS

recovery time UF4005 diode is used for output rectification. The buck boost topology also

provides benefits of isolating the output from the input in case of Switch failure

4.4

Circuit Operation

This LED driver operates at 66 kHz and energy is provided to the LED’s every switching

cycle. Since the circuit operates in discontinuous mode, a fixed amount of energy is

stored each cycle in L2, then is completely transferred to the load (i.e. the LEDs). The

current in the LED is a triangular waveform, whose peak is determined by the fixed and

well-controlled current limit of the LNK302. The load current is thus independent of line

voltage. As such the need for output filter capacitor is completely eliminated.

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