Datasheet

Application information L6591
30/41 Doc ID 14821 Rev 6
Figure 48. Oscillator waveforms and their relationship with gate-driving signals
6.7 Adaptive UVLO
A major problem when optimizing a converter for minimum no load consumption is that the
voltage generated by the self-supply system under these conditions falls considerably as
compared even to a few mA load. This very often causes the IC supply voltage Vcc to drop
and go below the UVLO threshold of the controller so that the operation becomes
intermittent, which is undesired. A low UVLO threshold would be helpful but it may be an
issue when driving the MOSFETS with a sufficient gate-drive voltage at heavy load during
power-off.
To help overcome this problem, the L6591, besides reducing its own consumption during
burst mode operation, also features a proprietary adaptive UVLO function. It consists of
shifting the UVLO threshold downwards at light load, namely when the voltage at the COMP
pin falls below a threshold V
COMPL
internally fixed, so as to provide more headroom. To
prevent any malfunctioning during transients from minimum to maximum load, the normal
(higher) UVLO threshold is re-established when the voltage at the COMP pin exceeds
V
COMPL
(with some mV hysteresis) and Vcc has exceeded the normal UVLO threshold (see
Figure 51 ). The normal UVLO threshold ensures that under heavy load conditions the
MOSFET is driven with an appropriate gate voltage.
Figure 49. Adaptive UVLO block
AM13264v1
OSC
HVG
t
t
LVG
t
Clock
t
T
sw
T
d
T
on
T
d
T
d
AM13265v1
+
-
UVLO
Vcc
L6591
COMP
R
S Q
+
-
+
-
S W
9
7
V
COMPL
PFC_STOP
logic
PFC_STO P
8
VCO MP
Vcc
Q
t
t
t
(*) V
CC O F F 2
< V
CC O F F 1
is selected when Q is high
V
CC OFF1
V
CC OFF2
(*)
V
COMPO
V
COMPL
V
CC O FF 1
(*) V
CC O FF 2