AN2950 datasheet(13/37 Pages) STMICROELECTRONICS

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AN2950

Testing the board

Doc ID 15494 Rev 1

13/37

2.3

Burst mode and output voltage ripple

When the load is so low that the voltage at the FB pin falls below the internal threshold

VFBbm (0.6 V typical), the VIPer28 is disabled. At this point, the feedback’s reaction to the

stop of energy delivery makes the FB voltage increase again, and when it goes 100 mV

above the VFBbm threshold the device restarts switching. This results in a controlled on/off

operation referred to as "burst mode" and is shown in the figures below, which report the

output voltage ripple and the feedback voltage when the converter is not or only lightly

loaded and supplied with 115 VAC and 230 VAC respectively. This mode of operation keeps

the frequency-related losses low when the load is very light or disconnected and makes it

easier to comply with energy-saving regulations.

Figure 13.

Output voltage ripple 115 VIN_AC full

load

Figure 14.

Output voltage ripple 230 VIN_AC full

load

Figure 15.

Output voltage ripple at 115 VIN_AC

no load

Figure 16.

Output voltage ripple at 230 VIN_AC

no load

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