ICL8001G / ICLS8082G
Design Guidelines
Design Parameters
Application Note
12
Version 2.0, 2011-04-14
4.4
Foldback Correction
When the line input voltage increases, the comparator threshold for switching off the power switch is reached in a
shorter time and the resulting increase in operation frequency would lead to a strong increase in the LED driver
power. To limit the input voltage dependence above a certain limit
Vinth
, the foldback correction reduces the
threshold
Vcsmax(Icz)
in accordance with the current detected at the ZCD pin as expressed here:
(19)
The characteristic curve
Vcsmax(Izc)
implemented in the IC shows the following steps:
Figure 4
Vcsmax(lzc) characteristic curve
For the adjusted setting Vinth = Vinp and selection of the foldback operation point Vcs-max = 0.75 V / Icz = 1 mA,
using the characteristic curve the detection resistance is calculated to be Rzc1 ≈ 15 kΩ according to
(20)
From the characterisitic curve it can be seen that the Vin dependence of
Vcsmax
is stronger in the range of higher
threshold values. This means that if a strong impact of the foldback correction is desired, the
Vcs
should reach
rather high values at
Vinp
. For increased power constancy under input voltage voltage variations, higher values
of R4 and R19 should be chosen.
np
na
Rcz
Vin
Vin
Icz
1
)
(
np
na
Icz
th
Vin
Rcz
1
