AN3329 datasheet(23/47 Pages) STMICROELECTRONICS

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AN3329

Functional check

Doc ID 18376 Rev 1

23/47

Dynamic load operation and output voltage regulation

Figure 42 and 43 show the output voltage regulation in the case of load transients on both

the resonant stage outputs. The waveforms have been captured applying a load transient

from 0 to full load to one output while the other is delivering full load. The period of load

steps has been selected very long (1 s), to allow the output voltage to reach the steady-state

condition and output voltage waveforms have been captured using the DC coupling of the

scope to avoid waveform distortions caused by the AC coupling. A suitable offset has been

also added to waveforms in order to get the maximum resolution.

Figure 42 reports the output voltage regulation with a dynamic load on 24 V output while

the 12 V output is delivering the 2 A rated load. It can be noted that the 24 V output voltage

has a tight variation, because even considering the spikes at the current edges, it is within

+/- 4 %.

In the same way, in Figure 43 it is possible to see the output voltage regulation with a

dynamic load on the 12 V output, while the 24 V output is delivering the 6 A rated load. It can

be noted that the 12 V output has a very tight variation - within +/- 3 %.

Cross regulation

Figure 44 and 45 show the output voltage cross regulation similar to previous tests but at

300 Hz load step frequency on one output, with the other delivering the rated load.

Figure 44 shows the simulation of the backlight and audio amplifier connected to the 24 V

output. Load is varying from minimum to maximum and vice versa, as typically happens in a

flat-TV because of backlight dimming and audio power amplifiers. Even in this condition, the

24 V output has a maximum deviation of

± 4 %, mainly due to the series filter inductor L4.

The 12 V output variation due to the dynamic load on 24 V (cross-regulation) is

± 2 %,

therefore very tight and suitable to power properly the internal logic of the LCD panel.

In Figure 45 cross regulation between the 12 V and 24 V has been measured. In this image

the 12 V load is changing from 1 to 2 A, as may happen when powering the LCD panel, and

the variation of 24 V at rated load is measured. It can be noted in the image that the 12 V

output has a deviation less than

± 1.2 %, while the 24 V output variation due to the dynamic

load on 12 V (cross-regulation) is

± 0.5 %.

Figure 42.

12 V - 2 A; 24 V 0 ÷ 6 A transition at

115 Vac - 60 Hz

Figure 43.

24 V - 6 A; 12 V 0 ÷ 2 A transition at

115 Vac - 60 Hz

CH2: +24 V O/P current

CH1: +24 V O/P voltage

CH1: +12 V O/P voltage CH2: +12 V O/P current

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