Datasheet

NCL30081
http://onsemi.com
24
Line Feed−forward
Because of internal and external propagation delays, the
MOSFET is not turned−off immediately when the current
set−point is reached. As a result, the primary peak current is
slightly higher than expected resulting in a small output
current error which can be compensated for during
component selection.
Normally this error would increase if the input line
voltage increased because the slew rate through the primary
inductance would increase. To compensate the peak current
increase brought by the variation, a positive voltage
proportional to the line voltage is added to the current sense
signal. The amount of offset voltage can be adjusted using
the R
LFF
resistor as shown in Figure 51. The offset voltage
is applied only during the MOSFET on−time and when I
out
is above 6% of the nominal output current.
Bulk rail
VIN
CS
Q_drv
Offset_OK
Figure 51. Line Feed−forward Schematic
V
DD
I
CS(offset)
R
LFF
R
sense
Brown−out
In order to protect the supply against a very low input
voltage, the NCL30081 features a brown−out circuit with a
fixed ON/OFF threshold. The controller is allowed to start
if a voltage higher than 1 V is applied to the VIN pin and
shuts−down if the VIN pin voltage decreases and stays
below 0.9 V for 50 ms nominal. Exiting a brown−out
condition overrides the hiccup on V
CC
(V
CC
does not wait
to reach V
CC(off)
) and the IC immediately goes into startup
mode (I
CC
= I
CC(start)
).
+
−
Vbulk
VIN
BO_NOK
50−ms blanking time
1 V if BONOK high
0.9 V if BONOK low
Figure 52. Brown−out Circuit