Cirrus Logic AN368 - Manual (Page 29)

AN368
AN368REV2 29
Times T2
CH1OFF
and T2
CH2OFF
are offset delays that assist in achieving the desired linear performance on the
output currents across dim on the second stage (see Figure 11).
Bits CH1_OFF[2:0] and bits CH2_OFF[2:0] in register Config62 at Address 94 are used for offsetting the errors
caused in the measurement of time T2
CHxOFF
using the ZCD comparator signal when each channel is
activated. Time T2
CHxOFF
is calculated using Equation 53:
FBAUX
COMPARATOR
V
Drain
V
t
200mV
V
BST
0V
T
CHxOFF
T
CHxZCD( Delay)
T
RES
4
V
Drain
= V
BST
Comparator trips
Pin FBAUX = 200mV
Current through the secondary = 0
System is at the valley
Pin FBAUX = 0V
Figure 11. T2 Delay Measurement
T2
CHxOFF
CHx_OFF[2:0] 50ns  = [Eq. 53]