Cirrus Logic AN379 - Manual (Page 10)

AN379
10 AN379REV2
Dimming Operation
The dimmer conduction time or phase cut information is extracted by the boost stage and supplied to the buck
stage as a digital parameter, dim, ranging from 0.05 to 1 as a measure of the normalized output power to be
delivered. The dim parameter controls both the normalized peak current in the inductor and total period
TT
BUCK
. Reaching a lower peak current requires less time T1
BUCK
and consequently less time T2
BUCK
to
discharge the inductor; at the same time, the idle time T3
BUCK
is increased to keep the total period TT
BUCK
nearly constant. Doing so reduces power, maintaining constant frequency, until the peak current is halved.
Further power reduction is obtained by increasing period TT
BUCK
without reducing the current.
When the current is moderately reduced, time T3
BUCK
is increased just enough before turning on FET Q
BUCK
to maintain a constant switching mode of operation. At deeper dimming levels, reducing the switching
frequency also reduces the capacitor losses.
Basic Design Equations
The DC voltage across any winding must be zero. The buck inductor winding is subject to voltage
(V
BST
- V
OUT
) during time T1
BUCK
and subject to voltage V
OUT
during time T2
BUCK
. The average voltage
across inductor L
BUCK
is calculated using Equation 4:
Solve Equation 4 for ratio (T2
BUCK
/T1
BUCK
), which returns Equation 5:
Solve for critical duty cycle 
C
using Equations 2 and 5 yields Equation 6:
where

C
= critical duty cycle for critical period (T1
BUCK
+T2
BUCK
)
Step 1) Select a Value for Boost Output Voltage
The value of the boost output voltage must be greater than the maximum input AC line voltage peak. The
full-scale boost voltage V
BST(full)
should be as low as possible to minimize the FET voltage rating. This allows
for optimal performance. Nominal boost voltage V
BST
is determined by an internal parameter and scaled to
70% of full-scale boost voltage V
BST(full)
. Voltage V
BST
changes slightly depending on the type of dimmer
detected. Resistor R
BST
connected to the BSTOUT pin sets the system full-scale voltage and determines
boost output voltage V
BST
regulation, boost overvoltage protection, and clamp behavior. Full-scale
voltage V
BST(full)
is calculated using Equation 7:
where
I
ref
= internal full-scale reference
R
BST
= external resistor used to set the full-scale voltage and boost output voltage
Boost output voltage V
BST
is regulated by charging the boost output capacitor to its nominal value each half
line-cycle. For an AC line system, boost output voltage V
BST
droops to its lowest value towards the end of each
half line-cycle until the boosting process starts again in the next half line-cycle.
V
BST
V
OUT
–   T1
BUCK
   V
OUT
T2
BUCK
   – 0 = [Eq. 4]
T2
BUCK
T1
BUCK
---------------------
V
BST
V
OUT
–  
V
OUT
--------------------------------------- = [Eq. 5]

C
T1
BUCK
TT
BUCK
T3
BUCK
–  
------------------------------------------------------ =
V
OUT
V
BST
-------------- = [Eq. 6]
V
BST full  
I
ref
R
BST
20k  +    = [Eq. 7]