ALTAIR05T-800 Datasheet, PDF (20/28 Pages) STMicroelectronics – Off-line all-primary-sensing switching regulator

Application information
ALTAIR05T-800
5.8
Burst-mode operation at no load or very light load
When the voltage at the COMP pin falls 65 mV below a threshold fixed internally at a value,
V_COMPBM, the IC is disabled with the MOSFET kept in OFF state and its consumption
reduced at a lower value to minimize Vcc capacitor discharge.
In this condition the converter operates in burst-mode (one pulse train every T_START=500
µs), with minimum energy transfer.
As a result of the energy delivery stop, the output voltage decreases: after 500 µs the
controller switches-on the MOSFET again and the sampled voltage on the ZCD pin is
compared with the internal reference. If the voltage on the EA output, as a result of the
comparison, exceeds the V_COMPL threshold, the device restarts switching, otherwise it stays
OFF for another 500 µs period.
In this way the converter works in burst-mode with a nearly constant peak current defined by
the internal disable level. Then a load decrease causes a frequency reduction, which can go
down even to few hundred hertz, thus minimizing all frequency-related losses and making it
easier to comply with energy saving regulations. This kind of operation, shown in the timing
diagrams of Figure 18 along with the others previously described, is noise-free since the
peak current is low
Figure 18. Load-dependent operating modes: timing diagrams
COMP
VCOMPL
65 mV
hyster.
IDS
5.9
Normal-mode
TSTART
TSTART
TSTART
Burst-mode
TSTART
Normal-mode
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 C_REF 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.
20/28
Doc ID 17957 Rev 1