TPS40050Q1

TPS40051Q1

TPS40053Q1

SGLS219 − NOVEMBER 2003

www.ti.com

9

APPLICATION INFORMATION

PROGRAMMING THE RAMP GENERATOR CIRCUIT

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 1).

RAMP

COMP

SW

VIN

UDG−02131

VIN

SW

COMP

RAMP

VPEAK

VVALLEY

T2

tON1 >tON2 and d1 >d2

tON2

tON1

d

+

t

ON

T

T1

Figure 1. Voltage Feed-Forward Effect on PWM Duty Cycle

The PWM ramp must be faster than the master clock frequency or the PWM is prevented from starting. The

R

58.14

R

where:

at 25

°C.

(2)