UCC2974

UCC3974

SLUS485A – JULY 2001 – REVISED NOVEMBER 2001

9

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APPLICATION INFORMATION

low-frequency dimming oscillator

The oscillator for the LFD section of the device is an R-C relaxation oscillator with programmable upslope and

downslope on its timing ramp. Figure 3 shows a simplified LFD oscillator diagram that illustrates the principle.

a 4.2-V source through RC. This time is:

tc + 1.126

R

C

C

T

to GND. This time is:

t

d +

1.792

R

D

C

T

The period of the LFD oscillator is simply the sum of the charge and discharge times, or

T

+ C

T

1.126

R

C )

1.792

R

D

The minimum duty cycle of the LFD PWM when operating in this mode is:

d

min +

t

d

T

UDG–01025

6

7

+

+

SQ

Q

R

4.3

0.5

3.0

RC

RD

RC

RD

CT

4.3

3.0

0.5

0.0

T

CHARGE

DISCHARGE

Figure 3. LFD Oscillator

(1)

(2)

(3)

(4)