Cirrus Logic AN364 - Manual (Page 13)

AN364
AN364REV3 13
Step 15) Determine I
PK(BST)
, I
SAT
, and R
IPK
(R13)
The boost stage peak current has two distinct values:
• 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.
• 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 7 shows the inductor peak current envelope and the AC line current waveform at nominal voltage in No-
dimmer Mode.
I
PK BST  
3.64 P
IN

V
RMS
--------------------------- = [Eq. 19]