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## UCC28251 Datasheet(PDF) 17 Page - Texas Instruments

 Part No. UCC28251 Description UCC28251 Advanced PWM Controller With Prebias Operation Download 55 Pages Scroll/Zoom 100% Manufacturer TI [Texas Instruments] Direct Link http://www.ti.com Logo

## UCC28251 Datasheet(HTML) 17 Page - Texas Instruments

 17 / 55 page501502503504505506507508509501050115012501350050100150200RT (k TDps=40nsTDps=100nsC00-´=D(SP)SWT1t2 fR66.4pFUCC28251www.ti.comSLUSBD8E – FEBRUARY 2013 – REVISED DECEMBER 2014Feature Description (continued)8.3.4 RT (Oscillator Frequency Set and Synchronization) (15/2)The UCC28251 oscillator frequency is set by an external resistor connected between the RT pin and ground. Theoscillator frequency can be set to any value between 50 kHz to 1.4 MHz, which is equivalent to an 25-kHz to700-kHz switching frequency. Switching frequency selection is a trade-off between efficiency and componentsize. Based on the selected switching frequency, the programming resistor value can be calculated as:(3)In this equation, fSW is the switching frequency and TD(sp) is the dead time between synchronous rectifier turn-offto primary switch turn-on. TD(sp) is set by an external resistor between the SP pin and ground (refer to the SP pindescription).Each output (OUTA, OUTB, SRA, SRB) switches at half the oscillator frequency (fSW = ½ x fOSC). Figure 24shows the relationship between RT and fOSC at certain TD(sp) and can be used to program oscillator frequencyaccordingly.Figure 24. Oscillator Frequency FOSC vs External Resistance of Rt at TD(ps) = 40 ns and 100 nsThe UCC28251 can be synchronized to an external clock by applying an external clock source to the RT pin.Synchronization helps with parallel operation and/or preventing beat frequency noise. The UCC28251synchronizes its internal oscillator to an external frequency source ranging from 84 kHz to 1.89 MHz, which isequivalent to an 42-kHz to 0.945-MHz switching frequency. The internal oscillator frequency is clamped to 84kHz during synchronization if the external source frequency drops below 84 kHz.The UCC28251 aligns the turn-on of primary outputs OUTA and OUTB to the falling edge of the synchronizingsignal, as shown in Figure 25. If the frequency source is from the gate outputs of another half bridge controller,interleaving can be achieved. The interleaving angle is determined by the frequency source’s duty cycle. When a50% duty cycle is applied, optimal interleaving is achieved, and EMI filters can be minimized.Copyright © 2013–2014, Texas Instruments IncorporatedSubmit Documentation Feedback17Product Folder Links: UCC28251

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