Cirrus Logic AN372 - Manual (Page 14)

AN372
14 AN372REV1
Step 9) Calculate the Buck Inductance (as Measured Across the N+1 Turns)
Step 10) Calculate R
FBGAIN
(R17)
Use Equation 16 to calculate the second-stage gain resistor R
FBGAIN
(R17).
where
R17 = R
FBGAIN
in 
TT
fb
= Switching period TT at full brightness (full load condition)
T2
fb
= Period T2 at full brightness (full load condition)
Step 11) Determine the RMS Current in the Winding
Determining the RMS current, I
RMS
, is necessary to properly define the wire size. Use Equation 17 to calculate I
RMS
:
where
T
current
= Time when the current is present in the winding
TT = T1+T2+T3 (see Figure 4b)
The factor 3 accounts for the current triangular shape
Step 12) Determine Output Capacitor
The buck output capacitor must provide a low impedance to the switching frequency. When using an
electrolytic capacitor, the choice is based on its ability to carry the ripple current to provide a long service life.
Meeting the ripple, voltage, temperature and lifetime requirements yields a high value capacitor, often greater
than what is required for current handling. In buck applications, the voltage ripple across the output electrolytic
capacitor is determined mostly by the ESR rather than the reactance.
Step 13) Buck Inductor Specification
This step is the first iteration of the inductor design. Due to design constraints, the following requirements must
be met:
• The core must be big enough to fit the windings.
• The number of turns required must fit in the bobbin, along with insulation material.
• The finished inductor must not overheat in the environment.
• Magnetic and geometric constraints may not yield the exact value of L and N calculated.
Once the first iteration of the design is done, calculate the new I
PK(FB)
to maintain the same T1 time as earlier
using the actual value of L, as shown in Equation 18.
Calculate the new T2 value using the new I
PK(FB)
and N, as shown in Equation 19:
L
V
BST
V
OUT
–   T1 
I
PK FB  
----------------------------------------------------- = [Eq. 15]
R
FBGAIN
62.5k 
TT
fb
T2
fb
------------ 2 
 
  1
–
------------------------------------ =
[Eq. 16]
I
RMS
I
PK FB  
T
current
3 TT 
--------------------  = [Eq. 17]
I
PK FB  
V
BST
V
OUT
–   T1 
L
----------------------------------------------------- = [Eq. 18]
T2
N 1 +   L 
N
2
V
O

---------------------------- = [Eq. 19]