C u

V

R

SR

(1)

R t

0.2 V

f

SW

Q

(2)

SETTING THE SWITCHING FREQUENCY (PROGRAMMING THE CLOCK OSCILLATOR)

R

1

f

17.82

* 23

kW

(3)

100

1.5

0.5

1000

200

300

400 500

600

700

800 900

1.0

3.0

2.0

2.5

4.5

3.5

4.0

5.0

MINIMUM OUTPUT VOLTAGE

vs

FREQUENCY

SWITCHING FREQUENCY

vs

TIMING RESISTANCE

0

100

0

200

400

600

800

1000

200

300

400

500

600

PROGRAMMING THE RAMP GENERATOR CIRCUIT AND UVLO

TPS40070

TPS40071

where

• V

• f

• Q

• I

The TPS4007x has independent clock oscillator and PWM ramp generator circuits. The clock oscillator serves as

the master clock to the ramp generator circuit. Connecting a single resistor from RT to ground sets the switching

Figure 2.

Figure 3.

The ramp generator circuit provides the actual ramp used by the PWM comparator. The ramp generator provides

voltage feed-forward control by varying the PWM ramp slope with line voltage, while maintaining a constant ramp

magnitude. Varying the PWM ramp directly with line voltage provides excellent response to line variations since

the PWM does not have to wait for loop delays before changing the duty cycle. (See Figure 9).

The PWM ramp must reach approximately 1 V in amplitude during a clock cycle, or the PWM is not allowed to

8

Submit Documentation Feedback

Copyright © 2003–2009, Texas Instruments Incorporated

Product Folder Link(s): TPS40070 TPS40071