An368 – Cirrus Logic AN368 User Manual
Page 16
AN368
16
AN368REV2
Calculate Mode 1 duty ratio D
MODE1
and Mode 2 duty ratio D
MODE2
using Equations 11 and 12, respectively:
Assuming the resonant times T3
CH1
and T3
CH2
are zero, the remaining relevant flyback stage parameters can
be calculated using the turns ratio N calculated in Equation 4 and the switching frequency F
sw1
selected for
Mode 1. Calculate period TT
CH1
using Equation 13:
Calculate switching frequency F
sw2
using Equation 14:
Calculate period TT
CH2
using Equation 15:
The resonant times T3
CH1
and T3
CH2
are estimated for both channels. Deriving a more accurate value of the
resonant ringing time is discussed in Step 4g Automated Resonant Period Measurements on page 22. To
account for times T3
CH1
and T3
CH2
that were neglected in the first approximation of Equation 13, recalculate
periods TT
CH1
and TT
CH2
using Equations 16 and 17, respectively.
Calculate total period TT using Equation 18:
Calculate switching frequency F
sw
Calculate the MOSFET ‘ON’ time T1
CH1
in Mode 1 using Equation 20:
Calculate the MOSFET ‘ON’ time T1
CH2
in Mode 2 using Equation 21:
[Eq. 11]
D
MODE1
N V
MODE1
V
BST
N V
MODE1
+
----------------------------------------------------------
=
[Eq. 12]
D
MODE2
N V
MODE2
V
BST
N V
MODE2
+
----------------------------------------------------------
=
TT
CH1
T1
CH1
T2
CH1
+
1
F
sw1
-----------
=
=
[Eq. 13]
F
sw2
F
sw1
V
BST
N V
MODE1
+
V
BST
N V
MODE2
+
----------------------------------------------------------
I
MODE1
V
MODE2
I
MODE2
V
MODE1
------------------------------------------
=
[Eq. 14]
TT
CH2
T1
CH2
T2
CH2
+
1
F
sw2
-----------
=
=
[Eq. 15]
TT
CH1
T
= 1
CH1
T2
CH1
T3
CH1
+
+
[Eq. 16]
TT
CH2
T
= 1
CH2
T2
CH2
T3
CH2
+
+
[Eq. 17]
TT
TT
CH1
TT
CH2
+
=
[Eq. 18]
F
sw
1
TT
-------
=
[Eq. 19]
[Eq. 20]
T1
CH1
TT
CH1
T3
CH1
–
D
MODE1
=
[Eq. 21]
T1
CH2
TT
CH2
T3
CH2
–
D
MODE2
=