L6566B View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6566B' PDF : 51 Pages View PDF
L6566B
Application information
Figure 12. Addition of an offset to the current sense lowers the burst-mode
operation threshold
Vcso
=
Vref
R
R
+ Rc
Vref
10
4
L6566B
37
Rc
R
Rs
5.4
Adaptive UVLO
A major problem when optimizing a converter for minimum no-load consumption is that the
voltage generated by the auxiliary winding under these conditions falls considerably as
compared even to a few mA load. This very often causes the supply voltage Vcc of the
control IC to drop and go below the UVLO threshold so that the operation becomes
intermittent, which is undesired. Furthermore, this must be traded off against the need of
generating a voltage not exceeding the maximum allowed by the control IC at full load.
To help the designer overcome this problem, the device, 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 pin COMP falls below a threshold VCOMPO internally fixed, so as to have more
headroom. To prevent any malfunction during transients from minimum to maximum load
the normal (higher) UVLO threshold is re-established when the voltage at pin COMP
exceeds VCOMPL and Vcc has exceeded the normal UVLO threshold (see Figure 13). The
normal UVLO threshold ensures that at full load the MOSFET will be driven with a proper
gate-to-source voltage.
Figure 13. Adaptive UVLO block
Vcc
5
VCOMP
(pin 9)
VCOMPL
VCOMPO
Vcc
t
COMP 9
R
(pin 5)
S
Q
UVLO
VCOMPL
VCOMPO
SW
VccOFF1
VccOFF2
VccOFF1
VccOFF2
(*)
Q
t
L6566B
(*) VccOFF2 < VccOFF1 is selected when Q is high
t
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