Step 2) select an appropriate fet, Step 3) determine the buck stage timing, An379 – Cirrus Logic AN379 User Manual
Page 11
AN379
AN379REV2
11
Step 2) Select an Appropriate FET
Determine the FET breakdown voltage V
Breakdown
and margin voltage V
Margin
. The maximum FET drain
voltage V
Drain(max)
is defined when power FET Q
BUCK
is turned off and calculated using Equation 8:
where
V
Diode
= forward voltage across catch diode D
BUCK
V
BST(BOP)
= boost output voltage at the boost overvoltage protection (BOP) threshold
Voltages closely approaching the buck power FET breakdown voltage are acceptable. The FET breakdown
voltage is constrained by cost and performance. A compromise must be reached when partitioning the FET
breakdown voltage V
Breakdown
between voltages V
BST
and V
Margin
.
Operating the buck inductor at near 50% duty cycle optimizes efficiency. Equation 9 is used to balance all
voltages contributing to the FET drain and source voltages:
where
V
Margin
= margin voltage that will prevent a destructive avalanche breakdown
Step 3) Determine the Buck Stage Timing
The buck stage operates in DCM at all times. Use Equations 1 and 2 on page 9 at full brightness frequency to
define the value of critical period (T1
BUCK
+T2
BUCK
Gain factor GAIN
TT
is defined using Equations 11:
where
GAIN
TT
= a constant set to 1.2
Solving Equations 10 and 11 for period T3
BUCK
yields Equation 12:
Step 4) Calculate Peak Current on the Buck Inductor
Calculate the peak current using values that return the highest I
BUCKPK
. Calculate maximum peak
current I
BUCKPK(max)
at full brightness using Equation 13:
where
TT
BUCK(fb)
= switching period at full brightness (full load condition)
T1
BUCK
= period T1
BUCK
at full brightness (full load condition)
T2
BUCK
= period T2
BUCK
at full brightness (full load condition)
V
Drain max
V
BST BOP
V
Diode
+
=
[Eq. 8]
V
Breakdown
V
BST BOP
V
Diode
V
M
in
arg
+
+
=
[Eq. 9]
TT
BUCK
T3
BUCK
–
T1
BUCK
T2
BUCK
+
=
[Eq. 10]
GAIN
TT
TT
BUCK
T1
BUCK
T2
BUCK
+
------------------------------------------------------
=
[Eq. 11]
T3
BUCK
TT
BUCK
GAIN
TT
1
–
GAIN
TT
----------------------------------------------------------------
=
[Eq. 12]
I
BUCKPK max
2 I
OUT
TT
BUCK fb
T1
BUCK fb
T2
BUCK fb
+
------------------------------------------------------------------
2 I
OUT
GAIN
TT
=
=
[Eq. 13]