Ap1694a, Vn n v v, Sr t – Diodes AP1694A User Manual
Page 9: Lr r u
AP1694A
Document number: DS37094 Rev.
1 - 2
9 of 15
May 2014
© Diodes Incorporated
AP1694A
A Product Line of
Diodes Incorporated
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Application Information Based on Buck Structure
(Cont.)
Design Parameters
Setting the Current Sense Resistor R8
According to the equation of the output current, the current sense resistor R8 is determined:
_
_
8
cs ref
o mean
V
R
k
I
Transformer Selection
The typical non-isolated buck circuit in Figure 1 is usually selected, and the system is operating at boundary conduction mode. The switching
frequency at the crest is set as f
min
, the inductance can be calculated as below:
_
_
_
min
( 2
)
8
2
in rms
o
o
cs ref
in rms
V
V
R V
L
V
V
f
Where,
o
V
is the output voltage.
_
in rms
V
is the RMS value of the input voltage.
According to Ferrari's law of electromagnetic induction, the winding turns number of the buck inductance N
L
is:
_
8
pk
cs ref
L
e
m
e
m
L i
L V
N
A B
A B
R
Where,
e
A
is the core effective area.
m
B
is the maximum magnetic flux density.
The auxiliary winding is power supply for V
CC
, the winding turns number N
aux
is:
cc
aux
L
o
d
V
N
N
V
V
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 AP1694A adopts constant on time control method, the AP1694A 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. The 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 R1 is selected:
10
1
_
_ max
1.25
10
8
2
in rms
L
R
R
U