Cirrus Logic AN375 - Manual (Page 13)

AN375
AN375REV4 13
Step 7) Calculate Primary Winding Inductance
Primary inductance L
P
is calculated to allow sufficient current build up at the 90° point when line voltage V
IN
is at a peak. Calculate inductance L
P
using Equation 15:
Step 8) Calculate Sense Resistor R
Sense
(R19)
Calculate sense resistor R
Sense
using Equation 16:
where
R
Sense
= resistor R19 in 
It is recommended to use a slightly smaller value for resistor R
Sense
to leave some margin for component
tolerances and unaccounted series voltage drops.
Step 9) Determine R
CTRL1
(R13)
Resistor R
CTRL1
connects from pin CTRL1 to GND and is used by the CS1615/16 to regulate the input line
current to a constant current I
IN(CC)
. Resistor R
CTRL1
is determined by Equation 17:
where
I
IN(CC)
= constant current used when designing circuit
R
Sense
= resistor attached to pin FBSENSE
R
CTRL1
= resistor R13 in 
Step 10) Determine R
CTRL2
(R17)
Resistor R
CTRL2
connects from pin CTRL2 to GND and is used by the CS1615/16 to set the output load current
I
OUT
. Resistor R
CTRL1
is determined by Equation 18:
where
N = turns ratio
R
Sense
= resistor attached to pin FBSENSE
I
OUT
= current through LED string at maximum output (full brightness)
R
CTRL2
= resistor R17 in 
Step 11) Determine the Switching Frequency Range
The switching frequency range is determined in the No-dimmer Mode and by the ratio of the instantaneous
input voltage and the reflected voltage. The switching frequency profile changes as ratio Z varies across a
range. Changes to the switching frequency coincide with ratio Z which is defined in Equation 19:
L
P
TT
PFC 90   
T3
PFC 90   
–   2
V
INPK
V
Reflected

V
INPK
V
Reflected
+
-------------------------------------------------
 
 
2

4 TT
PFC 90   
P
IN
 
---------------------------------------------------------------------------------------------------------------------------------------- =
[Eq. 15]
R
Sense
1.4V
I
PK
------------ = [Eq. 16]
R
CTRL1
1.4V 4M  
511 I
IN CC  
R
Sense
 
------------------------------------------------------------ = [Eq. 17]
R
CTRL2
1.4V N 4M   
1.25 511  R
Sense
I
OUT
 
------------------------------------------------------------------------ = [Eq. 18]
Z
V
IN
V
Reflected
-------------------------- =
[Eq. 19]