Application information, Design parameters, Iv k r – Diodes AP1684 User Manual
Page 7: Rv k i, Ap1684, A product line of diodes incorporated
AP1684
Document number: DS36547 Rev.
3 - 2
7 of 13
February 2014
© Diodes Incorporated
AP1684
A Product Line of
Diodes Incorporated
Application Information
The AP1684 is designed for single voltage application, and it features high power factor correction (PFC), low total harmonic distortion (THD), low
BOM cost and good EMI performance. The device can be widely used in non-dimmable LED application such as GU10, bulb lamps, down lamp,
etc. The AP1684 adopts constant on time control method within one AC cycle to achieve the high power factor and low THD. The control scheme
is very simple, the power factor correction effectiveness is obvious, and the constant current control is also good enough.
T1
OUT
+
VCC
FB
OUT
CS
GND
C
3
R7
R5
R6
R4
D1
C4
R3
Q1
AC
Input
F
1
VR1
DB1
C2
L1
C1
R8
D2
R1
R2
RI
RM
R9
A
P
1
6
8
4
5
7
4
6
3
2
8
Figure 1. Typical Application Circuit
Design Parameters
Setting the Current Sense Resistor R8
As the AP1684 adopts constant on time control method, the current of the inductance will follow the input voltage to get a sinusoidal wave. The
current sense pin CS of the AP1684 will sense the peak current of the inductance by sensing the voltage dropped on the current sense resistor R8,
and the constant current control is realized by controlling the peak current. In buck structure, when the V
o
is higher than V
in
, no energy will be
transferred from input to output which is called dead zone, and considering the dead zone of buck structure, the output current can be calculated
as below:
8
1
_
_
R
V
k
I
ref
cs
mean
o
Where,
V
cs_ref
is the reference of the current sense, and the typical value is 1V.
K is the current modification coefficient, and the value of k is approximate to be 0.7.
So, the current sense resistor R8 is determined:
mean
o
ref
cs
I
V
k
R
_
_
8
Transformer Selection (T1)
The non-isolated buck circuit in Figure 1 is usually selected, and the system is operating at boundary conduction mode.
The system’s operating
frequency does
not keep constant, and considering the limit of the BJT’s operating frequency, the minimum switching frequency at the crest is set
as f
min
, and then the buck inductance value L can be got:
m in
_
_
_
2
8
)
2
(
f
V
V
V
R
V
V
L
rms
in
ref
cs
o
o
rms
in
Where,
V
o
is the output voltage.
V
in_rms
is the RMS value of the input voltage.