An368 – Cirrus Logic AN368 User Manual
Page 17
AN368
AN368REV2
17
Calculate the MOSFET ‘OFF’ time T2
CH1
in Mode 1 using Equation 22:
Calculate the MOSFET ‘OFF’ time T2
CH2
in Mode 2 using Equation 23:
f. Calculate the Flyback Primary-side Inductance
Calculate the flyback transformer primary-side inductance L
P
using Equation 24:
g. Calculate Peak Current on the Flyback Primary-side
Calculate peak current I
PK1(FB)
during Mode 1 using Equation 25:
Calculate peak current I
PK2(FB)
during Mode 2 using Equation 26:
Calculate the average current I
MODE1(avg)
during Mode 1 using Equation 27:
Calculate the average current I
MODE2(avg)
during Mode 2 using Equation 28:
Equations 27 and 28 inherently assume that the IC generates the exact value for the FET ‘OFF’ time and the
peak currents. In practice, this is not always true due to the presence of parasitics in the system that cause
differences between the theoretical and measured value of peak current I
PK(FB)
and time T2. Step 5 Optimize
Output Current Regulation on page 24 describes in detail the way the CS1630 can compensate for these
differences.
h. Determine the RMS Current in the Winding
The CS1630 has a minimum required period T1 calculated in Equations 20 and 21 that is dependent on the
leading-edge blanking time T
LEB
. Blanking time T
LEB
is programmable from 150ns to 800ns and is used to
effectively disable the peak current comparator from turning off the gate drive too early due to spurious
switching noise. In applied systems, a good rule of thumb is to target a minimum duty cycle of 10% or greater.
The RMS current in the primary winding I
PRI(RMS)
is calculated using Equation 29:
[Eq. 22]
T2
CH1
TT
CH1
T1
CH1
–
T3
CH1
–
=
[Eq. 23]
T2
CH2
TT
CH2
T1
CH2
–
T3
CH2
–
=
[Eq. 24]
L
P
N
2
V
BST
2
V
MODE1
2
V
BST
N V
MODE1
+
2
I
MODE1
TT F
sw1
2
-----------------------------------------------------------------------------------------------------------------------
=
[Eq. 25]
I
PK1 FB
V
BST
T1
CH1
L
P
----------------
=
[Eq. 26]
I
PK2 FB
V
BST
T1
CH2
L
P
----------------
=
[Eq. 27]
I
MODE1 avg
I
PK1 FB
N
T2
CH1
2 TT
----------------
=
[Eq. 28]
I
MODE2 avg
I
PK2 FB
N
T2
CH2
2 TT
----------------
=
[Eq. 29]
I
PRI RMS
I
PK1 FB
2
D
MODE1
3
--------------------
I
PK2 FB
2
D
MODE2
3
--------------------
+
=