Cirrus Logic AN379 - Manual (Page 17)

AN379
AN379REV2 17
the CCM algorithm. Boost inductance L
BST
does not affect the switching frequency. Changes to the boost
inductor alters only the peak-to-peak ripple of the boost current.
The boost inductor design should support a ripple current at maximum boost output voltage V
BST(max)
and
when the instantaneous rectified input voltage applied to the boost stage inductor is at a 30

conduction angle.
Determine boost inductance L
BST
at maximum boost output voltage V
BST(max)
and at the point when the
current through inductor L
BST
reaches peak current I
BSTPK
. It is recommended to select the target peak-to-
peak ripple current I
ripple(target)
between 0.5A and 0.75A, and then calculate boost inductance L
BST
using
Equation 31:
where
V
rect(30 
= instantaneous rectified voltage at a conduction angle of 30° and minimum input voltage
T2
BST
= a fixed demagnetization period of 450ns at output voltage V
BSTPK(max)
The boost inductor rated saturation current must be greater than peak current I
BSTPK(max)
(see Equation 25
on page 15). A larger value for ripple current I
ripple(target)
may be specified, taking into consideration form factor
and saturation current, but this will require a higher constant peak current setting for Mode2.
Step 15) Determine Constant Peak Current for Mode2
Resistor R
CTRL2
connects from pin CTRL2 to GND and is used by the CS1680 to set the comparator threshold
to reflect the desired boost peak current. Sense resistor R
BST(Sense)
is used to set the maximum peak current
for the Mode3 algorithm. Since Mode2 is a constant peak current algorithm, sense resistor R
BST(Sense)
will
inherently set the peak for Mode2 if not configured.The peak current code I
BSTPK(code)
for the Mode2 algorithm
is set using Equation 32:
where
I
BSTPK(Mode2)
= the greater of 2.0A or (1.6A+ ½ I
ripple(target)
)
Resistor R
CTRL2
is used to configure the constant peak current for Mode2 based on maximum peak current
set by resistor R
BST(Sense)
. Applying a quadratic curve fit using resistor R
CTRL2
, a relationship is determined
between the peak current code and resistor R
CTRL2
ADC code. Calculate peak current code I
BSTPK(code)
using
Equation 33:
Use Equation 33 to derive ADC code R
CTRL2(code)
and define resistor R
CTRL2
from Equation 34:
Step 16) Boost Output Capacitor Selection
The boost stage output capacitor is also the buck stage input capacitor. Proper capacitor sizing is required to
ensure boost-stage and buck-stage loop stability.The boost output capacitor size is determined using the
energy stored at line frequency, especially behind operation with an electronic transformer. In addition to the
storage, the constant power control algorithm monitors the boost output voltage to determine the correct dim
level on the output. The size of the boost output capacitor is critical to optimizing Mode3 dimmer compatibility.
To prevent a significant compromise to dimmer compatibility, the minimum sizing of capacitor C
BST
is
26 F/Watt. For a 12V line input system, capacitor C
BST
must be greater than 26 F/Watt of output power and
for a MR16 design capacitor C
BST
is recommended to be 35 F/Watt
L
BST
V
BST max  
V
rect 30 o  
–   T2
BST

I
ripple t et arg  
--------------------------------------------------------------------------------------
V
BST max  
V
rect 30 o  
–   0.45s 
I
ripple t et arg  
---------------------------------------------------------------------------------------- = = [Eq. 31]
I
BSTPK code  
I
BSTPK Mode2  
R
BST Sense  
 512 
1.4V
---------------------------------------------------------------------------------------------- = [Eq. 32]
I
BSTPK code  
257
300 R
CTRL2 code  
–  
2
333
----------------------------------------------------------- – = [Eq. 33]
R
CTRL2
4M 
R
CTRL2 code  
------------------------------------
4M 
300 333 257 I
BSTPK code  
–    –
------------------------------------------------------------------------------------------- = = [Eq. 34]