MP4068 – HIGH PF, NON-ISOLATED, TRIAC DIMMABLE LED DRIVER

MP4068 Rev. 1.01

www.MonolithicPower.com

16

7/30/2015

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APPLICATION INFORMATION

Component Selection

Inductor

The MP4068 has a minimum off-time limit. The

inductor current ripple at CCM is determined by

the inductor value and the minimum off-time

limit. The current ripple is limited to 80% to get

a tradeoff between the PF and dimming

performance. The inductance value can be

calculated as follows:

OOFF _ MIN

peak

Vt

L

0.8 I

×

=

×

If the inductance value is too large, the

switching frequency will be low, so the EMI

performance will be good, however, the TRIAC

dimming performance will be poor at this

condition. If the inductance value is too small,

the TRIAC dimming performance will be good,

but the system may work in open loop condition,

the current consistency will be bad. So a

tradeoff must be made.

Freewheeling Diode

The diode should have a maximum reverse-

voltage rating, which is greater than the

maximum input voltage. The current rating of

the diode is determined by the output current,

which should be larger than 1.5 to 2 times the

output current.

Slow diodes cause excessive leading edge

current spikes during start-up, which is not

acceptable. Long reverse-recovery time of the

freewheeling diode can also affect the efficiency

and the circuit operation. An ultrafast diode

recommended.

Over-Voltage Protection Point Set

A feedback resistor is used to detect an over-

voltage condition. Fig. 7 shows the feedback

resistor’s connection.

The MP4068 is integrated with over-voltage

protection. The maximum output voltage when

over-voltage protection is triggered can be

calculated with the following equation:

O_ MAX

OVP

D

R2 R3

VV

*

V

R2

+

=

−

voltage drop.

VCC

OVF

GND

L

R2

R3

LED+

MP4068

FIGURE 7. Feedback Resistor Connection

The upper feedback resistor (R3) should be

larger than 100kΩ to avoid an efficiency

reduction in application. A 1% tolerance type is

recommended to achieve accurate protection.

Dummy Load

The dummy load is used to consume the power

transferred to the output capacitor when over-

voltage protection occurs. The IC works in

hiccup mode without any power consumption.

Normally, a dummy load less than 1mA is

recommended, which will not deteriorate the

system efficiency but can guarantee normal

over-voltage protection.

PCB Layout Guidelines

Efficient PCB layout is critical to achieve stable

operation, good EMI, and good thermal

performance, especially in very small sized LED

applications. For best results, refer to Fig. 8 and

follow the guidelines below:

1. Keep the loop formed between the MP4068,

the inductor, the freewheeling diode, and the

output capacitor as small as possible for

better EMI.

2. Place the AC input far away from the

switching nodes to minimize the noise

coupling that may bypass the input filter.

3. The VCC capacitor should be located very

close to the VCC and GND.