CS51220 datasheet(13/16 Pages) ONSEMI

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CS51220

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Program Oscillator Frequency

CS51220 requires an external capacitor to program the

oscillator

frequency.

The

internally

trimmed

charge/discharge current determines the maximum duty

cycle. The capacitor for a required switching frequency fS

can be calculated by:

CT + 13400

* 95

where:

CT = Timing capacitance is in pF

fS = Switching frequency is in kHz

Figure 16 shows the relationship of CT and fS.

200

CT (pF)

Figure 16. Operating Frequency

600

550

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200

150

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400

500

600

Synchronized Dual Converters with Soft Hiccup and

Feed Forward

The circuits shown in Figures 17 and 18 illustrate typical

applications for a dual output supply using independent but

synchronized converters. These circuits demonstrate the use

of the soft hiccup, feed forward, volt−second control and

synchronization features of the CS51220.

In Figure 17, the feed forward circuit has a volt−second

constant of 82 V/μs. This would limit the duty factor to 0.51

at 48 V input. With a turns ratio of 4:1 on the power

transformer and 48 V input, a duty factor of 0.46 is required

for 5.0 V output. This converter serves as the master

synchronization generator. The voltage on the VSD pin

establishes the delay as it is compared to the ramp generated

on the CT pin.

Adjustable synchronization allows the conduction time

for the two converters to be adjusted so that they are not on

at the same time. This greatly reduces the ripple current from

the 48 V source.

In Figure 18, the feed forward circuit has a volt−second

constant of 63 V/μs. This would limit the duty factor to 0.39

at 48 V input. With a turns ratio of 4:1 on the power

transformer and 48 V input, a duty factor of 0.33 is required

for 3.3V output.

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