Cirrus Logic AN372 - Manual (Page 28)

AN372
28 AN372REV1
Step 13) Buck Inductor Specification
Specifications for the buck inductor L4 can now be compiled to enable suppliers to design within size and cost
constraints.
Step 14) Circuit Adjustments
Now that the inductor has been defined and built, it may need to be adjusted. For more information, see Circuit
Adjustments on page 15.
Step 15) Recalculate R
Sense
Validate that the system meets the operating criteria. This may require adjusting components like R
Sense
and
R
FBGAIN
. For more information, see Recalculate R
Sense
on page 15.
4.2 Boost Stage Design Steps
Step 16) Determine I
PK(BST)
, I
SAT, and
R
IPK
(R13)
Using Equation 23, calculate I
PK(BST)
:
Using Equation 24, calculate R
IPK
:
Step 17) Boost Inductor Specifications
Use Figure 10 in the Boost Inductor Specifications section on page 19 to determine the boost value of the
boost inductance. Choosing a maximum switching frequency of 110kHz, find the intersection with the 230V
maximum switching curve, and get the corresponding power. This is 50 Watt
 mH, the constant (P
IN
 L
BST
),
for this frequency and voltage. Dividing by the input power, obtain the inductor value.
Parameter Value
Output DC Power 10.1W
Converter Topology CRM Buck
Switching Frequency 125kHz
Primary Inductance 3.8 mH

10%
Peak Current in the (N+1) Turn Winding 0.207A
RMS Current in the N-turn Winding 62mA
Turn Ratio N:1 4
RMS Current in the 1-turn Winding 0.511A
Leakage Inductance with 1-turn Winding Shorted <50 H
Auxiliary Winding Turn Ratio (N/N
AUX
) 20
Auxiliary Wire Any convenient gauge
I
PK BST  
3.64 P
IN

V
RMS
---------------------------
3.64 10.1W 
230V
----------------------------------- 160mA = = =
[Eq. 60]
R
IPK
15.625 10
3   V
I
PK BST  
-------------------------------------------
15.625 10
3   V
160mA
------------------------------------------- 97.6k  = = = [Eq. 61]
L
BST
50WmH
P
IN
----------------------
50WmH
10.1W
---------------------- 5mH = = = [Eq. 62]