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TPS40057 Datasheet(PDF) 7 Page - Texas Instruments |
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TPS40057 Datasheet(HTML) 7 Page - Texas Instruments |
7 / 37 page ( ) ( ) KFF IN(min) KFF T R V V 58.14 R 1340 = - ´ ´ + W VPEAK VVALLEY tON1 > tON2 and D1 > D2 t2 tON2 t1 tON1 VIN RAMP COMP SW D = tON t UDG-08119 R T + 1 f SW 17.82 10*6 * 17 kW TPS40054 TPS40055 TPS40057 www.ti.com SLUS593H – DECEMBER 2003 – REVISED JULY 2012 APPLICATION INFORMATION The TPS40054/55/57 family of devices allows the user to optimize the PWM controller to the specific application. The TPS40057 is safe for pre-biased outputs, not turning on the synchronous rectifier until the high-side FET has already started switching. The TPS40054 operates in one quadrant and sources output current only, allowing for paralleling of converters and ensures that one converter does not sink current from another converter. This controller also emulates a non-synchronous buck converter at light loads where the inductor current goes discontinuous. At continuous output inductor currents the controller operates as a synchronous buck converter to optimize efficiency. The TPS40055 operates in two quadrants, sourcing and sinking output current. SETTING THE SWITCHING FREQUENCY (PROGRAMMING THE CLOCK OSCILLATOR) The TPS4005x has independent clock oscillator and ramp generator circuits. The clock oscillator serves as the master clock to the ramp generator circuit. The switching frequency, fSW in kHz, of the clock oscillator is set by a single resistor (RT) to ground. The clock frequency is related to RT, in kΩ by Equation 1 and the relationship is charted in Figure 2. (1) 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). 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: where • VIN(min) is the ensured minimum start-up voltage (the actual start-up voltage is nominally about 10% lower at 25°C) • RT is the timing resistance in kΩ Copyright © 2003–2012, Texas Instruments Incorporated Submit Documentation Feedback 7 Product Folder Link(s): TPS40054 TPS40055 TPS40057 |
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