Cirrus Logic AN368 - Manual (Page 63)

AN368
AN368REV2 63
The minimum dim setting for the second stage is configured using the S2DIM register at Address 37. The
enforced minimum dim percentage dim
min
is determined by Equation 59:
The required gain value for a particular combination of dim and temperature was obtained using a polynomial
curve fit and the coefficients were derived for polynomial GAIN
DTR
and GAIN
DR
using Equation 60. The
polynomial in two variables, dim and temperature, uses coefficients P30, P20, P10, P03, P02, P01, P21, P12,
P11, and P00.
The polynomial in one variable, dim, uses coefficients Q3, Q2, Q1, and Q0.
4.2 Boost Stage Design
Step 8) Determine I
PK(BST)
, I
SAT
The boost inductor current reaches this value during a substantial portion of the line-cycle, affecting the RMS
value of the inductor and line current. The maximum boost inductor peak current is configured by adjusting the
peak switching current with I
PK(code)
. Using Equation 64, calculate I
PK(BST)
:
Coefficient Register Address Binary Decimal
P30 P30_MSB+P30_LSB 5 and 6 ‘0010111110001000’ 2.970703125
P20 P20_MSB+P20_LSB 7 and 8 ‘1011111001000111’ -4.107666016
P10 P10_MSB+P10_LSB 9 and 10 ‘0010001010101111’ 2.167724609
P03 P03_MSB+P03_LSB 11 and 12 ‘0000000000000000’ 0
P02 P02_MSB+P02_LSB 13 and 14 ‘0000001000111001’ 0.138916016
P01 P01_MSB+P01_LSB 15 and 16 ‘1111000101110111’ -0.908447266
P21 P21_MSB+P21_LSB 17 and 18 ‘0000100010000011’ 0.531982422
P12 P12_MSB+P12_LSB 19 and 20 ‘0000000011011010’ 0.053222656
P11 P11_MSB+P11_LSB 21 and 22 ‘1111010001011100’ -0.727539063
P00 P00_MSB+P00_LSB 23 and 24 ‘0001011111101010’ 1.494628906
Table 11. GAIN
DTR
Coefficients
Coefficient Register Address Binary Decimal
Q3 Q30_MSB+Q30_LSB 25 and 26 ‘0000000011100100’ -1.75
Q2 Q20_MSB+Q20_LSB 27 and 28 ‘1111111001011000’ 5.562011719
Q1 Q10_MSB+Q10_LSB 29 and 30 ‘1111101000101011’ 2.748535156
Q0 Q03_MSB+Q03_LSB 31 and 32 ‘0001011010011011’ -6.307128906
Table 12. GAIN
DR
Coefficients
[Eq. 131]
dim
min
S2DIM[7:0] 16 15 + 
4095
--------------------------------------------------------
 
  100
 =
8 16 15 + 
4095
---------------------------
 
  100 
 
 
=
3.5% =
I
PK BST  
3.64 P
IN

V
RMS
--------------------------
3.64 9W 
120V
--------------------------- 273mA = = = [Eq. 132]