ALTAIR04-900 Datasheet, PDF (19/29 Pages) STMicroelectronics – Off-line all-primary-sensing switching regulator

ALTAIR04-900
Application information
diagrams of Figure 17 along with the others previously described, is noise-free since the
peak current is low
Figure 17. Load-dependent operating modes: timing diagrams
COMP
VCOMPL
65 mV
hys ter.
IDS
Normal-mode
TSTA R T
TS TA RT
TS TA R T
Burst-mode
TST AR T
Normal-mode
5.8
Soft-start and starter block
The soft start feature is automatically implemented by the constant current block, as the
primary peak current is limited from the voltage on the C_REF capacitor.
During start-up, as the output voltage is zero, the IC starts in CC mode with no high peak
current operations. In this way the voltage on the output capacitor increases slowly and the
soft-start feature is ensured.
Actually the CREF value is not important to define the soft-start time, as its duration
depends on others circuit parameters, like transformer ratio, sense resistor, output
capacitors and load. The user can define the best appropriate value by experiments.
Doc ID 18211 Rev 2
19/29