CL8800

DS20005357A-page 6

 2015 Microchip Technology Inc.

3.0

APPLICATION INFORMATION

3.1

Overview

Designing a driver to meet particular requirements may

be a difficult task considering the 18 design variables:

tap current (6), number of series-connected LEDs per

segment (6), and the number of parallel-connected

LEDs per segment (6). Manually selecting values will

provide light, but the chosen values may be far from

optimal in regards to efficiency, LED utilization, and line

regulation.

Contact your nearest Microchip Field Applications

Engineer for design assistance.

In addition to configuring the driver, several circuits

may be employed to increase reliability, performance,

and cost. The following sections briefly describe these

circuits.

3.2

Transient Protection

The driver circuits have no need for capacitors that

could otherwise absorb transient energy, nor is there a

need for EMI filters that would block transients. There-

fore, the full burden of transient protection is borne by

the protection circuit. The two-stage approach in the

following schematics provide 2.5kV protection, both

pulse and ring per EN 61000-4-5 and EN 61000-4-12,

six hits each.

FIGURE 3-1:

100 TO 120 VAC

TRANSIENT PROTECTION

FIGURE 3-2:

230VAC TRANSIENT

PROTECTION

3.3

Zener Diode Substitution

Zener diodes may be substituted for LEDs in the bot-

tom stages of the design. The last 1 or 2 stages of

LEDs contribute little to the light output - they are

mainly present to off-load the adjacent upstream regu-

lator at high line voltages to minimize losses. The

advantages of Zener substitution includes minimizing

unlit LEDs at low line for better light uniformity, better

line regulation at high line, fewer LEDs for lower cost

and less PCB area, and fewer board-to-board connec-

tions. Disadvantages include slightly-reduced effi-

ciency at high line, and additional heat load on the

driver board.

3.4

Phase Dimming

As with any light load, the LED lamp might not draw

enough current to ensure proper dimmer operation.

This is especially true for 230VAC dimmers. Triodes for

Alternating Current (TRIAC) used in dimmers require a

minimum latching current when triggered to place the

TRIAC in the latched-on state. Once latched, a mini-

mum holding current is required to maintain the TRIAC

in the on state. Latching current is many times greater

than the holding current, and is the main concern with

dimmer compatibility.

Higher latching current can be provided by a simple

series RC network across the AC line. A short time con-

stant provides a current spike at the turn-on edge.

Less common is inadequate holding current. The mini-

mum dimmer holding current is typically 10-20mA.

Tap1 at 60mA (max) exceeds the minimum.

FIGURE 3-3:

PHASE DIMMING

3.5

Strobing

Twice per AC line cycle the line voltage crosses zero

volts, during which time there is no light output.

The circuit in Figure 3-4 can provide 5-10% valley fill. It

has little effect on input current wave shape (THD, PF)

and efficiency.

This circuit is intended to prevent the output from

reaching zero. It will not significantly reduce output rip-

ple.

22Ω

150VAC

10mm

AC Line

22Ω

33Ω

275VAC

10mm

440VDC

1.5kW

AC Line

Bridge

Rectifier

AC Line

Transient

Protection

500Ω

100 - 200nF