Cirrus Logic AN372 - Manual (Page 29)

AN372
AN372REV1 29
Boost inductor RMS current I
RMS
depends on the AC line RMS current, the triangular shape, and the stepped
envelope. As a first approximation, consider the inductor RMS current to be equal to 1.25 times the AC line
RMS current.
Calculate the auxiliary winding turn ratio using Equation 64.
In summary, the inductor specifications are:
• Primary inductance = 5mH
• Saturation current = 0.6A @ 3000Gauss
• RMS current = 61mA
• Turn ratio 18.4:1
After procuring an inductor sample meeting specifications, perform the tests described in the Boost Inductor
Specifications section on page 20.
Step 18) Determine Boost Output Capacitor
Assuming a 90% efficiency for the boost stage, the boost stage input power P
BST
can be determined using
Equation 65:
For a 230V design, boost output capacitor C4 must be greater than (0.5  11.2 = 5.6F). Taking into account
the tolerance and life degradation, choosing a 6.8 F capacitor will satisfy the requirements. This capacitor
must be rated at 450V.
Step 19) Determine Boost Input Capacitor
To accommodate a variety of dimmers, capacitor C3 is proportional to input power: ~4nF/W for 230V designs
and ~12nF/W for 120V designs.
4.3 Final Design Steps
Whether the design is a normal buck or a tapped buck, an auxiliary winding is required to interface with the
CS1612/13 FBAUX pin. The FBAUX pin is used to detect the end of time T2 and implement the overvoltage
protection.
Figure 4a shows the drain-to-source voltage and the voltage across the (N+1) winding when the blue dashed
line is used as the V
ZERO
axis. Figure 4a illustrates that the voltage across any winding, in particular across
the AUX winding, crosses zero ¼ of the ringing cycle after time T2 ends.
Pin FBAUX senses when the auxiliary winding voltage crosses zero and a new switching cycle is initiated after
a short delay that is controlled by FB
Gain
. To be precise, the threshold of pin FBAUX is 200mV; therefore, the
signal present on the auxiliary winding needs to be much larger, for example on the order of 10V. Indicating
with Z the normalized turns of the auxiliary winding yields Equation 66:
resulting in Z = 10/24. The turn ratio is not critical when designing the inductor, and the turn ratio can be
adjusted to fit in one layer. In this case, Z = 0.4 was chosen.
The FBAUX pin is also used to detect overvoltage protection (OVP) events. An OVP fault occurs when the
voltage at pin FBAUX reaches the OVP threshold of +1.25V. The maximum load voltage is 25.2V; therefore,
the OVP trip point must be set to approximately 29V, which includes a 1V forward drop for diode D3. The
auxiliary winding voltage is ((29 x 0.4) = +11.6V) when the output load voltage approaches the OVP trip point.
I
RMS
P
IN
PF
---------
 
  1.25
V
IN
-----------
 
 

10.1W
0.9
-----------------
 
  1.25
230V
--------------
 
 
 = 61mA = = [Eq. 63]
N
P
N
AUX
--------------
405
22
---------- 18.4 = = [Eq. 64]
P
BST
P
OUT

--------------
10.1W
0.9
----------------- 11.2 W = = = [Eq. 65]
Z:1 10:24 = [Eq. 66]