Cirrus Logic AN375 - Manual (Page 21)

AN375
AN375REV4 21
Step 8) Calculate Sense Resistor R
Sense
(R19)
Use Equation 16 on page 13 to solve for sense resistance R
Sense
. See Equation 43:
Resistor R19 is selected to be a 2.61  standard value.
Step 9) Determine R
CTRL1
(R13)
Use Equation 17 on page 13 to solve for resistance R
CTRL1
. See Equation 44:
Resistor R13 is selected to be a 42.2k  standard value.
Step 10) Determine R
CTRL2
(R17)
Use Equation 18 on page 13 to solve for resistance R
CTRL2
. See Equation 45:
Resistor R17 is selected to be a 71.5k  standard value.
Step 11) Determine the Switching Frequency Range
The circuit parameters yield a Frequency Profile A. The minimum switching frequency will be found at the
conduction threshold voltage V
Start
when the reflected voltage V
Reflected
is at its maximum tolerance and
that the maximum switching frequency is found at the peak of the highest line voltage when reflected
voltage V
Reflected
is at its minimum tolerance.
To determine period T1, start by calculating duty cycle

FREQ

which renders either a minimum or maximum
switching frequency at the condition specified by a switching Frequency Profile A. Use Equation 13 on page 11
to calculate the duty cycles

FREQ(min)
and

FREQ(max)
. See Equations 46 and 47:
Use Equation 14 on page 12 to calculate minimum and maximum peak current I
PKFREQ
at the condition
specified by a switching Frequency Profile A. See Equations 48 and 49:
R
Sense
1.4V
I
PK
------------
1.4V
513.6mA
------------------------ 2.73  = = = [Eq. 43]
R
CTRL1
1.4V 4M  
511 I
IN CC  
R
Sense
 
------------------------------------------------------------
1.4V 4M  
511 107mA 2.73   
----------------------------------------------------------- 37.5k  = = = [Eq. 44]
R
CTRL2
1.4V N 4M   
1.25 511  R
Sense
I
OUT
 
------------------------------------------------------------------------
1.4V 5.75 4M   
1.25 511  2.73  250mA  
---------------------------------------------------------------------------- 74k  = = = [Eq. 45]

FREQ min  
1
T3
TT
------- –
 
 
1
V
Start
V
Reflected min  
--------------------------------------- +
 
 
--------------------------------------------------------
1
2 s
17.6 s
------------------ –
 
 
1
30V
165.6V
------------------- +
 
 
---------------------------------- 0.75 = = = [Eq. 46]

FREQ max  
1
T3
TT
------- –
 
 
1
V
INPK min  
V
Reflected max  
---------------------------------------- +
 
 
----------------------------------------------------------
1
2 s
17.6 s
------------------ –
 
 
1
0.9 2  120V 
165.6V
------------------------------------------ +
 
 
---------------------------------------------------------- 0.46 = = = [Eq. 47]
I
PKFREQ min  
2 I
IN CC  


FREQ min  
-------------------------------
2 107mA 
0.75
----------------------------- = 285mA = = [Eq. 48]
I
PKFREQ max  
2 I
IN CC  


FREQ max  
--------------------------------
2 107mA 
0.46
----------------------------- = 465mA = = [Eq. 49]