Cirrus Logic AN368 - Manual (Page 37)

AN368
AN368REV2 37
3.4 Boost Stage Design
The design process for the boost stage is outlined below:
1. Determine I
PK(BST)
and a tentative I
PK(BST)
OTP setting
2. Determine Boost Inductor Specifications
3. Calculate Boost Input and Output Capacitors
The boost stage is designed in No-dimmer Mode, which has a considerable degree of freedom in its design
parameters. For the boost stage to operate in dimmer mode and with the largest variety of dimmers, the design
is constrained within a more limited set of parameters. Even in No-dimmer Mode, the several operating states
of the boost stage leave an exclusively formula-based design impractical.
Step 8) Determine I
PK(BST)
, I
SAT
The boost stage peak current has two distinct values. Current I
PK(BST)
is related to input power P
IN
. The boost
inductor current reaches this value during a substantial portion of the line-cycle, affecting the RMS value of the
inductor and line current. The maximum boost inductor peak current is configured by adjusting the peak
switching current with I
PK(code)
. The PEAK_CUR register at Address 51 is used to store I
PK(code)
. Maximum
power output is proportional to I
PK(code)
, as shown in Equation 63:
where,
 = correction term = 0.55
V
RMS(typ)
= nominal operating input RMS voltage
I
PK(BST)
= I
PK(code)

4.1mA
Current I
SAT
is a constant value of 0.6A and is independent of the power level. I
SAT
is necessary to maintain
the dimmer TRIAC in the conduction mode. I
SAT
is a brief duration that immediately follows the dimmer firing
and has a minimal impact on RMS current and its heating effects. The boost inductor must be capable of
carrying the current I
SAT
without saturating.
The factor 3.64 accounts for a factor of 2 due to the triangular waveform, a factor of 1.41 due to voltage
sinusoidal shape, and a factor of 1.29 derived by simulation accounting for the line current envelope profile.
The current envelope profile approximates a sine wave in a stepped fashion according to the following
conditional rules:
• IF V
LINE
< 60V THEN I
PK(BST)
equals 0
• IF V
LINE
> 60V AND Phase > 20° THEN the boost inductor peak current equals 0.75
·
I
PK(BST)
• IF 45° < Phase < 135° AND V
BST
< V
MAX
THEN the inductor current peak equals I
PK(BST)
• IF 45° < Phase < 135° AND V
BST
> V
MAX
THEN the inductor current peak equals 0.75
·
I
PK(BST)
• IF V
LINE
< 60V OR Phase > 160° THEN I
PK(BST)
equals 0
Boost output voltage V
BST
is regulated by controlling the time when the current is held equal to I
PK(BST)
during
the central portion of the period between 45° and 135°.
Figure 18 shows the inductor peak current envelope and the AC line current waveform at nominal voltage in
No-dimmer Mode.
P
IN max  
 I
PK BST  
V
RMS typ  
  
2
--------------------------------------------------------- = [Eq. 63]
I
PK BST  
3.64 P
IN

V
RMS
------------------------ = [Eq. 64]