Cirrus Logic AN372 - Manual (Page 27)

AN372
AN372REV1 27
Choosing a 6.49Ω standard value will assure margin against resistor tolerance. To prevent false triggering by
the comparator, pin FBSENSE has an internal blanking time of 550ns. To reduce switching spikes, it is
recommended to add an additional RC filter circuit using a 1k
 resistor followed by a 100pF capacitor.
Step 9) Calculate the Buck Inductance (as Measured Across the N+1 Turns)
The buck inductance L is the inductance of the entire (N+1) winding. At the minimum boost voltage, the current
through inductor L must reach peak current I
PK(FB)
during period T1, which yields Equation 55:
The inductor L must not saturate up to a peak current of 0.22A, maintaining some margin against the
calculated peak current 0.207A.
Step 10) Calculate R
FBGAIN
(R17)
Using Equation 16, solve for R
FBGAIN
(R17):
Step 11) Determine the RMS Current in the Winding
The two windings carry substantially different RMS currents; therefore, they can be wound with a different wire
gauge. Current I1 flows into the (N+1) winding and into FET Q4 (see Figure 3). Current I1 has a triangular
shape of amplitude I
PK(FB)
circulating for a fraction of the period equal to 
C
.
The 1-turn winding is subject to the full waveform (I1+ I2 = I3).
Step 12) Determine Output Capacitor
The output capacitor C5 ripple current, I
Ripple(RMS)
, is the vectorial difference between the RMS current in the
1-turn winding of L4 and the DC load current. See Equation 59:
L V
BST min  
T1
I
PK FB  
------------------ 
405V 40.5V –   2.16s 
0.207A
------------------------------------------------------------------ 3804 H = = = [Eq. 55]
R
FBGAIN
62.5k 
TT
fb
T2
fb
------------ 2 
 
  1
–
------------------------------------ =
62.5k 
8 s
5.84 s
------------------ 2 
 
  1
–
----------------------------------------- = 37.6k  = [Eq. 56]
I1
RMS
I
PK FB  

C
3
-------  207mA
0.27
3
-----------  62mA = = = [Eq. 57]
I3
RMS
I
PK FB  
N 1 +  
2
1 
C
–   
3
-------------------------------------------------  207mA
4 1 +  
2
1 0.27 –   
3
----------------------------------------------------  511mA = = = [Eq. 58]
I
Ripple RMS  
0.511
2
0.4
2
–  A 0.318A = = [Eq. 59]