Cirrus Logic AN368 - Manual (Page 64)

AN368
64 AN368REV2
The PEAK_CUR register at Address 51 is used to store I
PK(code)
. Maximum power output is proportional to
I
PK(code)
. Using Equation 133, calculate I
PK(code)
:
The PEAK_CUR register is set to ‘01000100’.
Current I
SAT
is a constant value of 0.6A and is independent of the power level. I
SAT
is necessary to maintain
the dimmer TRIAC in the conduction mode. I
SAT
is a brief duration that immediately follows the dimmer firing
and has a minimal impact on RMS current and its heating effects. The boost inductor must be capable of
carrying the current I
SAT
without saturating.
Step 9) Boost Inductor Specifications
Use Figure 19 in the Boost Inductor Specifications section on page 39 to determine the boost value of the
boost inductance. Choosing a maximum switching frequency of 100kHz, find the intersection with the 120V
maximum switching curve, and get the corresponding power. This is 14.3 Watt
 mH, the constant
(P
IN
 L
BST
), for this frequency and voltage. Dividing by the input power, obtain the inductor value.
Boost inductor RMS current I
RMS
depends on the AC line RMS current, the triangular shape, and the stepped
envelope. As a first approximation, consider the inductor RMS current to be equal to 1.25 times the AC line
RMS current.
Calculate the auxiliary winding turn ratio using Equation 136.
In summary, the inductor specifications are:
• Primary inductance = 1.6mH
• Saturation current = 0.273A @ 3000Gauss
• RMS current = 104mA
• Turn ratio 7.2:1
After procuring an inductor sample meeting specifications, perform the tests described in the Boost Inductor
Specifications section on page 40.
Step 10) Determine Boost Output Capacitor
Using a flyback transformer with a 90% efficiency, the boost stage output power, P
BST
, can be determined
using Equation 137.
For a 120V design, boost output capacitor C6 must be greater than 2  8.2 = 16.4F. Taking into account the
tolerance, cost, and life degradation, a 22 F capacitor satisfies the requirements. This capacitor must be rated
at 250V.
Step 11) Determine Boost Input Capacitor
To accommodate a variety of dimmers, capacitor C1 is proportional to input power: ~4nF/W for 230V designs
and ~12nF/W for 120V designs. The CS1630 reference design is designed for a 9W input, therefore C1 is a
0.1
F capacitor.
I
PK code  
I
PK BST  
4.1mA
--------------------
273mA
4.1mA
------------------- 66.6 = = = [Eq. 133]
L
BST
14.3WmH
P
IN
---------------------------
14.3WmH
9W
--------------------------- 1.6mH = = = [Eq. 134]
I
RMS
P
IN
PF
--------
 
  1.25
V
IN
-----------
 
 

9W
0.9
---------
 
  1.25
120V
--------------
 
 
 = 104mA = = [Eq. 135]
N
L3
N
BSTAUX
----------------------
V
BOP
V
C11
-------------
235V
35V
-------------- 6.7 = = = [Eq. 136]
P
BST
P
OUT
0.85
-------------
7.0W
0.85
-------------- 8.2 W = = = [Eq. 137]