Diodes AP1684 - Manual (Page 8)

AP1684
Document number: DS36547 Rev. 3 - 2
8 of 13
www.diodes.com
February 2014
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AP1684
A Product Line of
Diodes Incorporated
Application Information
(Cont.)
The next step is determining
the transformer’s winding turns number, the worst case operation condition of transformer is at the peak voltage area
of sine waveform input voltage where the current of across the inductance is the maximum value. The transformer design should be based on the
worst case operation condition to guarantee that the transformer is not saturated. According to Ferrari's law of electromagnetic induction, the
winding turns number of the buck inductance N
L
is:
8
_
R B A
V L
B A
I L
N
m e
ref cs
m e
pk
L  





Where,
A
e is the core effective area.
B
m
is the maximum magnetic flux density.
The auxiliary winding is power supply for V
CC , the winding turns number N aux is:
d o
cc
L aux
V V
V
N N

 
Where,
V
cc
is the power supply voltage for IC from auxiliary winding.
V
d
is the voltage drop of the freewheel diode.
Setting the Initial On Time
As the AP1684 adopts constant on-time control method, the AP1684 will generate an initial on time to start a working cycle. If the initial on time is
longer than the rated on time, overshoot will happen which could damage the LED. And a good system performance does not permit overshoot, so
the appropriate initial on time should be guaranteed. And initial on time is determined by resister R1 shown in Figure 1.
According to initial on time generation mechanism, the t
on_initial is
s R t
initial on
12
_
10 1 80
   
To guarantee the system with no overshoot phenomenon, the resistor is selected
 


 10
max _ _
10
2 8
25 . 1
1
rms in
U R
L
R
The system operation mode is determined by R2, to guarantee the system working at BCM mode, resistance R2 is generally selected as R2≤R1.
Valley on Control Method
The valley on function can provide low turn-on switching losses for buck converter. The voltage across the collector and emitter of the BJT is
reflected by the auxiliary winding of the buck transformer. The voltage is sensed by the FB pin.
0.1V
1µs
Valley
FB
Figure 2. Valley on Control