AL1692

Document number: DS39008 Rev. 1 - 2

11 of 15

www.diodes.com

July 2016

© Diodes Incorporated

AL1692

From the buck-boost output current equation, we can get the output current when dimming:

_

_

_

0

_

1

25

()

2

( )

11

2

2

( )

REF

ON

ON

MAX

O

IN

RMS

PEAK

DI M

IN

RMS

O

V

if t

t

R

I

V

Sin

I

d

else

V

Sin

V













 





















Where,

Conduction Angle (deg)

Output Current (%)

100

0

180

Critical Conduction Angle



0

Figure.2 Dimming Curve

Dimmer Compatibility

Passive Bleeder Design

The passive bleeder is designed to supply latching and holding current to eliminate dimmer misfire and flicker.

DB1

C1

L1

R1

FR1

L

N

C2

Passive

Bleeder

Damping

Figure.3 LED Driver Schematic with Passive Bleeder

The passive bleeder includes a capacitor (C2, in hundreds of nF) to provide latching current. A resistor (R1) is necessary to dampen the current

spike. Because a large C2 will affect the PF, THD and efficiency, the value of the capacitor (C2) should be selected accordingly. Generally,

100nF/400V to 330nF/400V is recommended. R1 is used to limit the latching current, If R1 is too large, the latching current is not enough and the

TRIAC dimmer will misfire causing LED flicker. If R1 is too small, it will result in greater power dissipation. Generally speaking

, a 200Ω to 2KΩ

resistor is selected for R1.

Passive Damping Design

FR1 is the damper for reducing the spike current caused by quick charging of C2 at firing. In General, FR1

is selected from 20Ω to 100Ω for low