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TPS40051PWPRG4 Datasheet(PDF) 8 Page - Texas Instruments |
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TPS40051PWPRG4 Datasheet(HTML) 8 Page - Texas Instruments |
8 / 37 page TPS40050 TPS40051 TPS40053 SLUS540F − DECEMBER 2002 − REVISED JUNE 2004 www.ti.com 8 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 PWM ramp time is programmed via a single resistor (RKFF) pulled up to VIN. RKFF is related to RT, and the minimum input voltage, VIN(min) through the following: R KFF + VIN (min) * 3.5 58.14 R T ) 1340 W where: D VIN(min) is the ensured minimum start-up voltage. The actual start-up voltage is nominally about 10% lower at 25 °C. D RT is the timing resistance in kΩ The curve showing the RKFF required for a given switching frequency, fSW, is shown in Figure 3. For low input voltage and high duty cycle applications, the voltage feed-forward may limit the duty cycle prematurely. This does not occur for most applications. The voltage control loop controls the duty cycle and regulates the output voltage. For more information on large duty cycle operation, refer to Application Note (SLUA310). (2) |
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