Cirrus Logic AN372 - Manual (Page 17)

AN372
AN372REV1 17
3.4 Boost Stage Design
The design process for the boost stage is outlined below:
1. Determine I
PK(BST)
and a tentative resistor value, R
IPK
(R13)
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 make an exclusively formula-based design impractical.
Step 16) 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 the 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 the 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 9 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. 23]