Datasheet

L6591 Application information
Doc ID 14821 Rev 6 31/41
6.8 Line sensing function
This function basically stops the IC as the input voltage to the converter falls below the
specified range and lets it restart as the voltage goes back within the range. The sensed
voltage can be either the rectified and filtered mains voltage, in which case the function acts
as a brownout protection, or, in systems with a PFC pre-regulator front-end, the output
voltage of the PFC stage, in which case the function serves as a power-on and power-off
sequencing.
L6591 shutdown upon input undervoltage is accomplished by means of an internal
comparator, as shown in the block diagram of
Figure 52 , whose non-inverting input is
available at the LINE pin (#1). The comparator is internally referenced to 1.25 V and
disables the IC if the voltage applied at the LINE pin is below the internal reference. Under
these conditions the soft-start discharges, the pin PFC_STOP is open and the consumption
of the IC is reduced. PWM operation is re-enabled as the voltage on the pin is above the
reference. The comparator is provided with current hysteresis instead of a more usual
voltage hysteresis: an internal 15 µA current sink is ON as long as the voltage applied at the
LINE pin is below the reference and is OFF if the voltage is above the reference.
Figure 50. Line sensing function: internal block diagram and timing diagram
This approach provides an additional degree of freedom: it is possible to set the ON
threshold and the OFF threshold separately by properly choosing the resistors of the
external divider (see below). With voltage hysteresis, instead, fixing one threshold
automatically fixes the other one depending on the built-in hysteresis of the comparator.
With reference to
Figure 52 , the following relationships can be established for the ON
(V
inON
) and OFF (V
inOFF
) thresholds of the input voltage:
AM13266v1
+
-
L6591
V
in O
K
H
V
Input bu
s
3
V
L
I
N
E
V
c
c
1
9
1.25
V
15µ
A
R
H
H
V
Inpu
t
b
u
s
V
in
O
K
V
in
O
N
V
in
O
F
F
L
V
G,
H
V
G
V
o
ut
L
I
N
E
I
H
Y
S
t
t
t
t
t
t
t
R
L
1.25
V
1
5
µ
A
V
c
c