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UCC1895 Datasheet(PDF) 12 Page - Texas Instruments |
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UCC1895 Datasheet(HTML) 12 Page - Texas Instruments |
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12 / 25 page  UCC1895 UCC2895 UCC3895 SLUS157L − DECEMBER 1999 − REVISED APRIL 2008 12 www.ti.com APPLICATION INFORMATION Programming DELAB, DELCD and the Adaptive Delay Set The UCC2895N allows the user to set the delay between switch commands within each leg of the full-bridge power circuit according to equations: t DELAY + (25 10*12) R DEL V DEL ) 25 ns From this equation VDEL is determined in conjunction with the desire to use (or not) the adaptive delay set feature from the following formula: V DEL + 0.75 V CS * VADS ) 0.5 V The following diagram illustrates the resistors needed to program the delay periods and the adaptive delay set function. 9 11 10 12 DELCD DELAB ADS CS RDELAB RDELCD UCC3895 Figure 4. Programming Adaptive Delay Set The adaptive delay set feature (ADS) allows the user to vary the delay times between switch commands within each of the converter’s two legs. The delay-time modulation is implemented by connecting ADS (pin 11) to CS, GND, or a resistive divider from CS through ADS to GND to set VADS as shown in Figure 4. From equation (6) for VDEL, if ADS is tied to GND then VDEL rises in direct proportion to VCS, causing a decrease in tDELAY as the load increases. In this condition, the maximum value of VDEL is 2 V. If ADS is connected to a resistive divider between CS and GND, the term (VCS−VADS) becomes smaller, reducing the level of VDEL. This decreases the amount of delay modulation. In the limit of ADS tied to CS, VDEL = 0.5 V and no delay modulation occurs. Figure 5 graphically shows the delay time vs. load for varying adaptive delay set feature voltages (VADS). In the case of maximum delay modulation (ADS=GND), when the circuit goes from light load to heavy load, the variation of VDEL is from 0.5 V to 2 V. This causes the delay times to vary by a 4:1 ratio as the load is changed. The ability to program an adaptive delay is a desirable feature because the optimum delay time is a function of the current flowing in the primary winding of the transformer, and can change by a factor of 10:1 or more as circuit loading changes. Reference[5] describes the many interrelated factors for choosing the optimum delay times for the most efficient power conversion, and illustrates an external circuit to enable adaptive delay set using the UC3879. Implementing this adaptive feature is simplified in the UCC3895 controller, giving the user the ability to tailor the delay times to suit a particular application with a minimum of external parts. (5) (6) |
Similar Part No. - UCC1895_08 |
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Similar Description - UCC1895_08 |
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