Cirrus Logic AN379 - Manual (Page 24)

AN379
24 AN379REV2
Step 10) Zero-current Detection
Zero-crossings are detected by AC coupling the drain voltage of FET Q
BUCK
to pin BUCKZCD. Capacitor C2
is sized to monitor the zero crossing of a switching drain node. Typical resonant ringing frequencies in the
CRD1680 are less than 5MHz. Implying that period T
resonant
is greater than 200ns.
Series resistor R5 is added to suppress high frequency ringing on the drain node. The RC circuit connected to
the drain of FET Q
BUCK
is required to have a time delay that is less than resonant period T
resonant
. Capacitor
C2 is 27pF and is sized to reduce power dissipation. Power loss P
C2(loss)
in capacitor C2 is calculated from
Equation 58:
where
F
SW
= nominal switching frequency equal to 175kHz
Selecting a larger capacitor will produce more losses. Resistor R5 is selected to hold the RC time constant ,

to less than period T
resonant
. Resistor R5 relationship is defined from Equation 59:
Since the typical stray capacitance on pin BUCKZCD can vary from 1pF to 3pF, resistor R5 is selected to be
6.65k
.
A resistor divider is inserted at the BUCKZCD input to limit input current I
BUCKZCD
to less than or equal to
1.5mA and reduce the zero-crossing switching signal to less than 3V. Resistor R26 is selected to be 2k  to
avoid adding delay to RC time constant

and assist in limiting input current I
BUCKZCD
to be less than 1.5mA.
The voltage across capacitor C2 is calculated at maximum boost output voltage V
BST(max)
and is
approximately ±19V. Resistor R6 is selected to be 35.7k .
4.2 Boost Stage Design Steps
Step 11) Determine Boost Peak Current
Assuming maximum dim is reached at 120
 and allowing for a factor of approximately 2 (k~0.5) for
second stage losses plus boost inductor current ripple. Using Equation 24 on page 15, solve for target
power P
Target
. See Equation 60:
Using Equation 25 on page 15, calculate boost peak current I
BSTPK(max)
. See Equation 61:
where
V
rect(30 
= instantaneous rectified voltage at a conduction angle of 30° and minimum input voltage
P
C2 loss  
C2 V
BST
2
F
SW
  27pF 28V   2
175kHz   3.7mW = = = [Eq. 58]
R5

C2
--------
200ns
27pF
----------------- 7.4k  = =  [Eq. 59]
P
T et arg
P
OUT
k
--------------
 
  180
o

dim max  
-------------------------
 
 
 

5.15W
0.5
-----------------
 
  180
o
120
o
------------
 
 
 
 15.45W = = =
[Eq. 60]
I
BSTPK max  
P
T et arg
V
rect 30 o  
------------------------
15.45W
7.64V
--------------------- 2.0A = = = [Eq. 61]