Cirrus Logic AN379 - Manual (Page 15)

AN379
AN379REV2 15
power in poor dimmer/transformer combinations. The Mode3 algorithm requires maximum peak current
I
BSTPK(max)
to be set by sense resistor R
BST(Sense)
and target power P
Target
to be set by resistor R
CTRL1
.
3. The Mode2 algorithm requires a minimum average current to equal 1.6A during the turn-on time of the
electronic transformer. The turn-on time of the transformer is estimated based on the output power
requirement for the particular conduction angle by the regulation loop. For different output power ratings,
the turn-on time for the electronic transformer is the only variable that changes for a particular constant
peak current. The Mode2 algorithm requires the peak reference current to be set by resistor R
CTRL2
. The
ripple current is set by the boost inductor L
BST
.
Sense resistor R
BST(Sense)
is used to set the peak current requirement for all three algorithms and is defined
in Mode3. In Mode2 a constant peak current algorithm is executed whereas the Mode3 algorithm requires the
peak current to change based on different ranges of output power. In order to address the two control
algorithms, there are two parameter configurations which require using pins CTRL1 and CTRL2.
Mode1
The system requirements are to produce full rated power at a conduction angle of 135°. Maximum peak current
is obtained when the conduction angle is 135° and the power level is at its highest. Maximum peak current
I
BSTPK(max)
is set by resistor R
BST(Sense)
in Mode3, which is greater than the maximum peak current required
for Mode1. Therefore, there is no need to define any system control resistor for Mode1.
Mode3
The maximum peak current for the Mode3 algorithm is the current attained at the trough of the rectified input
voltage. The peak boost current must be set high enough to achieve dimmer compatibility. If the boost current
is set too high, by resistor R
BST(Sense)
, then the peak current sensed by the controller will be clipped and
produce a false sense of constant current. This false condition restricts changes to the output power when the
input voltage varies. The peak current must be allowed to be modulated by the controller to produce a constant
output power at any power level.
This can be done by designing for the peak current requirement at the worst case condition which yields the
maximum current requirement for the system.
Depending on the output power and current requirement, a suitable peak current I
BSTPK(max)
is chosen. The
required output power P
OUT
is defined from Equation 23:
where
k = is a constant and is approximately 0.5
P
Target
> the actual power due to the ripple current in the boost inductor, limiting I
BSTPK(max)
at low voltage

dim(max)
= dimmer/transformer conduction angle where maximum output current occurs
If output power P
OUT
exceeds the defined maximum output power, the CS1680 will reduce target power P
Target
to avoid overloading the clamp circuit. Solving Equation 23 for target power P
Target
results in Equation 24:
The maximum boost peak current is defined when the boost is in CCM and the instantaneous rectified input
voltage applied to the boost stage inductor is at a 30

conduction angle. Calculate maximum boost peak
current I
BSTPK(mx)
using Equation 25:
where
V
rect(30 
= instantaneous rectified voltage at a conduction angle of 30° and minimum input voltage
P
OUT
k P
T et arg

dim max  
180
o
-------------------------
 
 
 
  = [Eq. 23]
P
T et arg
P
OUT
k
--------------
 
  180
o

dim max  
-------------------------
 
 
 
 = [Eq. 24]
[Eq. 25]
I
BSTPK max  
P
T et arg
V
rect 30 o  
------------------------ =