Cirrus Logic AN379 - Manual (Page 26)

AN379
26 AN379REV2
Step 15) Determine Constant Peak Current for Mode2
Sense resistor R
BST(Sense)
is used to set the maximum peak current for the Mode3 algorithm (see Equation 62
on page 25). The maximum peak current I
BSTPK(Mode2)
in the boost inductor for the Mode2 is held constant at
2.0 A. Using Equation 32 on page 17, the peak current code I
BSTPK(code)
for the Mode2 algorithm is set by
Equation 67:
where
I
BSTPK(Mode2)
= the greater of 2.0A or (1.6A+ ½ I
ripple(actual)
)
Resistor R
CTRL2
is used to configure the constant peak current for Mode2 based on maximum peak current
set by resistor R
BST(Sense)
. Using Equation 34 on page 17, resistor R
CTRL2
is determined by Equation 68:
where
R
CTRL2
= resistor R32 and selected to be a 47k  standard value
Step 16) Boost Output Capacitor Selection
Assuming an 88% efficiency for the buck stage, the buck stage input power P
BST
can be determined using
Equation 69:
For a 12V design, boost output capacitor C
BST
must be greater than or equal to (6W  26F/W = 156F).
Taking into account the tolerance and life degradation, the recommended capacitor value is
(6
 33F/W = 198F). Choosing a 180F capacitor will satisfy the requirements. A capacitor rated at 50V is
recommended.
Step 17) Boost Input Capacitor Selection
Input capacitor C
rect
performs a number of important functions: first, it is the local decoupling capacitor to the
boost converter; second, in the case of a leading-edge dimmer paired with an electronic transformer
capacitor C
rect
provides an additional storage of energy that will supply the required amount of power to the
output.
When pairing a trailing-edge dimmer with an electronic transformer, input capacitor C
rect
needs to be
discharged, which provides the electronic transformer with a low impedance input preventing the transformer
from turning off. Also, the inrush current into the LED driver increases with the largest capacitor. Empirical
results have identified that for output power P
OUT
≤6W use a 0.68 F input capacitor. For output power
P
OUT
>6.5W use a 1 F input capacitor.
An RC damping circuit is connected in parallel to capacitor C
rect
to stabilize the transformer paired with a
dimmer. The leakage inductance introduced by a typical electronic transformer is 2 H. Damping factor d is set
to 1 and damping resistance R
damp
is defined using Equation 70:
where
R
damp
= damping resistor R11 and selected as 1 
I
BSTPK code  
I
BSTPK Mode2  
R
BST Sense  
 512 
1.4V
----------------------------------------------------------------------------------------------
2.0A 0.16   512 
1.4V
--------------------------------------------------- 117 = = = [Eq. 67]
R
CTRL2
4M 
300 333 257 I
BSTPK code  
–    –
-------------------------------------------------------------------------------------------
4M 
300 333 257 117 –    –
------------------------------------------------------------------- 47.6k  = = = [Eq. 68]
P
BST
P
OUT

--------------
5.15W
0.88
----------------- 6 W = = = [Eq. 69]
R
damp
1
2d
-------
L
leak
C
rect
------------- 
1
2
---
2 H
1 F
-----------  0.7  = = = [Eq. 70]