UCC3809PWTR-1 datasheet(7/23 Pages) TI1

Part #	UCC3809PWTR-1

Description	Economy Primary Side Controller

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Manufacturer	TI1 [Texas Instruments]

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7

UCC1809-1/-2

UCC2809-1/-2

UCC3809-1/-2

3

4

UCC3809

OSCILLATOR

RT2

CT

RT1

1k

2N2222A

424

SYNC

PULSE

OPTION I

3

4

UCC3809

OSCILLATOR

RT2

CT

RT1

424

SYNC

PULSE

0.1µF

2N2222A

24

+5V

OPTION II

Figure 5. UCC3809 synchronization options.

UDG-99006

Synchronization

Both of the synchronization schemes shown in Figure 5

can be successfully implemented with the internal oscilla-

tor of the UCC3809. Both schemes allow access to the

timing ramp needed for slope compensation and have

minimal impact on the programmed maximum duty cycle.

In the absence of a sync pulse, the PWM controller will

run independently at the frequency set by RT1, RT2, and

CT. This free running frequency must be approximately

15 to 20% lower than the sync pulse frequency to insure

the free running oscillator does not cross the comparator

threshold before the desired sync pulse.

Option I uses the synchronization pulse to pull pin 3 low,

triggering the internal 1.67V comparator to reset the RS

latch and initiate a charging cycle. The valley voltage of

the CT waveform is higher when synchronized using this

configuration, decreasing the ramp charge and discharge

times, thereby increasing the operating frequency; other-

wise the overall shape of the CT voltage waveform is un-

changed.

Option II uses the synchronization pulse to superimpose

the sync voltage onto the peak of the CT waveform. This

triggers the internal 3.33V comparator, initiating a dis-

charge cycle. The sync pulse is summed with the free

running oscillator waveform at the CT node, resulting in a

spike on top of the CT peak voltage.

ADDITIONAL INFORMATION

Please refer to the following Unitrode application topics

for additional information.

[1] Application Note U-165, Design Review: Isolated 50W

Flyback Converter with the UCC3809 Primary Side Con-

troller by Lisa Dinwoodie.

[2]

Design

Note

DN-89,

Comparing

the

UC3842,

UCC3802, and UCC3809 Primary Side PWM Controllers

by Lisa Dinwoodie.

APPLICATION INFORMATION (cont.)

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