Cirrus Logic AN379 - Manual (Page 16)

AN379
16 AN379REV2
Step 12) Calculate Boost Sense Resistor
Maximum peak current I
BSTPK(max)
is set by the boost sense resistor. Boost sense resistor R
BST(Sense)
is
designed to hit peak current I
BSTPK(max)
at threshold voltage V
BSTPK(th)
. Once peak current I
BSTPK(max)
is
derived from Equation 25, calculate sense resistor R
BST(Sense)
using Equation 26:
It is recommended to use a slightly smaller value for resistor R
BST(Sense)
to leave some margin for component
tolerances and series voltage drops that were not considered.
Step 13) Determine Target Power
Resistor R
CTRL1
connects from pin CTRL1 to GND and is used by the CS1680 to set the target power for a
particular maximum peak current. Resistor R
CTRL1
is based on the output power required and the current
flowing through sense resistor R
BST(Sense)
. Target power P
Target
for the system is achieved by tuning resistor
R
CTRL1
in addition to setting the maximum peak current with the R
BST(Sense)
resistor. Calculate target power
P
Target
from Equation 27:
where
V
LSB
= bit weight of voltage V
rect
as measured at pin VAC
I
LSB
= bit weight of current flowing through resistor R
BST(Sense)
as measured at pin BSTSENSE
P
Target(code)
= ADC code for target power P
Target
The target power ADC code P
Target(code)
is calculated using Equation 28:
Applying a quadratic curve fit, using resistor R
CTRL1
, a relationship is determined between the target power
ADC code and resistor R
CTRL1
ADC code. Calculate target power code P
Target(code)
from Equation 29:
Resistor R
CTRL1
is determined by deriving ADC code R
CTRL1(code)
from Equation 29 and using Equation 30:
Mode2
The boost inductor current is in continuous conduction mode (CCM) with a constant peak current during the
turn-on time of the transformer. The change in boost inductor current I
BSTPK
is the only variable that changes
for different value inductors whereas the maximum peak current remains the same. The peak current is not
affected by the output power rating as the algorithm executes a constant peak current control and does not
depend on the input current characteristics. Therefore, for different output power ratings, the turn-on time is
the only variable that changes for a particular constant peak current, and the Mode2 algorithm requires the
peak current threshold voltage to be set by the R
BST(Sense)
resistor.
Step 14) Boost Inductor Specifications
The CS1680 controls peak current I
BSTPK
and keeps the boost stage operating in CCM. The instantaneous
frequency changes to meet the peak current I
BSTPK
imposed by the controller and the duty cycle imposed by
[Eq. 26]
R
BST Sense  
V
BSTPK th  
I
BSTPK max  
-------------------------------- =
P
T et arg
4 V
LSB
I
LSB
 P
T et code   arg
  = [Eq. 27]
P
T et code   arg
1
4
---
P
T et arg
V
LSB
I
LSB

------------------------------- 
1
4
---
512 R
BST Sense  

1.4V
-------------------------------------------------
256
I
ref
R
AC
20k  +   
----------------------------------------------------  P
T et arg
  = = [Eq. 28]
P
T et code   arg
2133
300 R
CTRL1 code  
–  
2
75
----------------------------------------------------------- –  [Eq. 29]
R
CTRL1
4M 
R
CTRL1 code  
------------------------------------
4M 
300 75 2133 P
T et code   arg
–    –
-------------------------------------------------------------------------------------------- = = [Eq. 30]