AN1344 STMicroelectronics, AN1344 Datasheet - Page 7

• Manufacturer Part Number: AN1344
• Description: VIPOWER: 108W POWER SUPPLY USING VIPER100A
• Manufacturer: STMicroelectronics

AN1344 - APPLICATION NOTE
Figure 6: Bad Burst Mode
At Lighter load, the Vdd voltage droops below the under voltage threshold, the start up circuit is reset,
and the Vdd capacitor charges back up to the high threshold of 11V through the start up current source.
As shown in the picture the reoccurrence of this cycle is about 300ms. The worse output voltage swing is
2.4V, which occurs at 20mA. At no load condition, the output voltage swing becomes negligible (45mV).
This mode of operation leads to the following drawbacks:
1) Because the start up current source is turned on to supply the capacitor from a high voltage rail, the
efficiency is dramatically reduced.
2) The recurring period leads to as much as 13% variation on the output voltage. For this audio
application it does not matter, but the designer should review all aspects of operation.
3) Below the minimum current of 40mA, the dynamic behavior is very poor which is typical of all power
supplies. If the demand of current occurs during the recharging phase, the output capacitor will be
discharged and normal operation will return only at the next starting phase.
In conclusion for this design a 40mA minimum load is needed, 0.6% of maximum load, to keep the unit in
optimal performance. However, below this range, the output voltage is still under control and no stresses
are applied to the unit.
STAND-BY INPUT POWER
The transformer was optimized for the current scheme and not for Blue Angel.
7/10
90 Vac
115 Vac
132 Vac
Input Voltage
Input Wattage at No Load
0.85 W
1.1 W
1.3 W
Input Wattage at 40 mA
1.77 W
1.86 W
1.8 W