Cirrus Logic AN368 - Manual (Page 24)

AN368
24 AN368REV2
Frequency in Region C
In Region C, the switching frequency is kept constant, and the peak current I
PK(FB)
starts to reduce. Period
TT
C
is calculated using Equation 46:
Frequency in Region D
Once the peak current I
PK(FB)
reaches its minimum value, the switching frequency continues to reduce until
the minimum peak current limit is reached, or the minimum switching frequency set by the maximum switching
period register TTMAX at Address 38. The period TT
D
is calculated using Equation 47:
where,
I
PKx(FB)
= I
PK(min)
TT
D
> TT
C
(50 s)
TT
D
< TTMAX
If the operating frequency is below TTMAX, the system stops regulating because the controller is out of range
to control the current I
PKx(FB)
or the frequency.
Step 5) Optimize Output Current Regulation
The CS1630 second-stage control loop achieves correct closed-loop output current regulation based on
measured values of time T1, current I
PK(FB)
, and time T2. In practical systems, accurate measurements are
limited due to circuit parasitics. Examples of circuit parasitics are leakage inductance, inter-winding
capacitance, and diode recoveries. The controller has the ability to compensate for these measurement errors
so that the control loop can maintain a tight current regulation across the corners. The following steps describe
the process of tuning these compensations accurately to minimize conversion error.
a. Definition and Scope of Second-stage Output Current Regulation
In a system where all the CS1630 second-stage compensation features have been tuned correctly and the
color system is disabled (for example, GAIN
DR
and GAIN
DTR
are forced to 1), the output current on the two
channels scales perfectly with the 12-bit dim level provided by the boost stage using Equations 48 and 49.
where,
dim = The 12-bit dim level provided by the boost stage
I
CH1(fb)
= Current at full brightness, when the dim is 4095 (full scale) provided that the color system is disabled
[Eq. 46] TT
C
50 s =
[Eq. 47] TT
D
TT
DCM_2
I
PK min  
2
L
2 dim I 
MODEx
V
MODEx
 
-----------------------------------------------------------------  = =
[Eq. 48] I
CH1
I
CH1 fb  
dim 
4095
--------------------------------- =
I
CH2
I
CH2 fb  
dim 
4095
--------------------------------- = [Eq. 49]