Diodes AP1694A - Manual (Page 10)

AP1694A
Document number: DS37094 Rev. 1 - 2
10 of 15
www.diodes.com
May 2014
© Diodes Incorporated
AP1694A
A Product Line of
Diodes Incorporated
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Application Information Based on Buck Structure
(Cont.)
In dimmable application, on the condition of the acceptable line regulation, the smaller R1 is selected will be better for dimming performance.
Valley On Control Method
The valley on function can provide low turn-on switching losses for buck converter. The voltage across the power switch is reflected by the
auxiliary winding of the buck transformer. The voltage is sensed by FB pin.
0.1V
1µs
Valley
FB
Figure 2.Valley On Control
According to Figure 2, when the falling edge of 0.1V is sensed by FB pin, the AP1694A will see the toff time is over and delay 1µs to start a new
operating cycle. By this way we can realize valley on function.
Passive Damping and Bleeder Design
The passive bleeder is designed to supply latching and holding current to eliminate misfire and flicker.
F1
DB1
C1
L1
R9
RF
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C2
Passive
Bleeding
Damping
Figure 3. LED Driver Schematic with Passive Bleeder
A passive bleeder is composed of a resister (R9) and a capacitor (C2). C1 is input filter capacitor and RF is damper resistor.
The passive bleeder includes a capacitor (C2, hundreds of nF) to provide latching current. To remove the voltage and current spike, a resistor (R9)
is necessary to dampen the spike.
In dimmable application, because a large C2 will affect the PF, THD and efficiency, the value of the capacitor (C2) should be selected suitable.
Generally, 100nF/400V to 330nF/400V is recommended.
RF is the damper for reducing the spike current caused by quick charging of C2 at firing. RF is selected from 20
Ω to 100Ω for low line application,
and 51 Ω to 200Ω for high line application. If R9 is too small, R9 can’t fully dampen the spike current and ringing current will occur. The ringing
current will cause the TRIAC misfire which will cause LED flicking. Another consideration in R9 selection is power loss, too large R9 will make
more power dissipation. Generally, a 200
Ω to 2KΩ resistor is selected for R9.