Cirrus Logic AN379 - Manual (Page 25)

AN379
AN379REV2 25
Step 12) Calculate Boost Sense Resistor
Sense resistor R
BST(Sense)
is designed to hit maximum peak current I
BSTPK(max)
at a threshold
voltage V
BSTPK(th)
equal to 0.33V. From Equation 26 on page 16, calculate sense resistor R
BST(Sense)
using
Equation 62:
where
R
BST(Sense)
= resistor R1 and selected as 0.16Ω standard value
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
Target power P
Target
for the system is achieved by tuning resistor R
CTRL1
in addition to setting the maximum
peak current with resistor R
BST(Sense)
. Equation 28 on page 16 is used to define target power ADC code
P
Target(code)
. See Equation 63:
Using Equation 30 on page 16 to solve for resistor R
CTRL1
yields Equation 64:
where
R
CTRL1
= resistor R31 and selected to be a 49.9k  standard value
Step 14) Boost Inductor Specifications
Design for a ripple current at maximum boost output voltage V
BST(max)
and a rectified input voltage V
rect(30 
at a conduction angle of 30°. Calculate boost inductance L
BST
using Equation 65:
where
V
rect(30°)
= instantaneous rectified voltage at a conduction angle of 30° and minimum input voltage
L
BST
= power inductor L1 and selected as 15 H
Using Equations 65 solve for the actual ripple current I
ripple(actual)
:
In summary, the inductor specifications are:
• Inductance = 15
H
• Saturation current = 2.4A
[Eq. 62]
R
BST Sense  
V
BSTPK th  
I
BSTPK max  
--------------------------------
0.33V
2.0A
---------------- 0.165  = = =
P
T et code   arg
1
4
---
512 R
BST Sense  

1.4V
-------------------------------------------------
256
I
ref
R
VAC
20k  +   
-------------------------------------------------------  P
T et arg
  =
[Eq. 63]
1
4
---
512 0.16  
1.4V
--------------------------------
256
64 A 604k  20k +   
-----------------------------------------------------------------  15.45W   =
1449 =
R
CTRL1
4M 
300 75 2133 P
T et code   arg
–    –
--------------------------------------------------------------------------------------------
4M 
300 75 2133 1449 –    –
---------------------------------------------------------------------- 54.4k  = = = [Eq. 64]
L
BST
V
BST max  
V
rect 30 o  
–   0.45s 
I
ripple t et arg  
----------------------------------------------------------------------------------------
32V 7.64 –   0.45s 
0.7A
---------------------------------------------------------- 15.7 H = = = [Eq. 65]
I
ripple actual  
V
BST max  
V
rect 30 o  
–   0.45s 
L
BST
----------------------------------------------------------------------------------------
24.36V 0.45 s 
15 H
-------------------------------------------- 0.73A = = = [Eq. 66]