Owner manual

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
Tetarg
=
[Eq. 63]
1
4
---
512 0.16
1.4V
--------------------------------
256
64A604k 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 etarg
----------------------------------------------------------------------------------------
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]