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UCC3585 Datasheet(PDF) 10 Page - Texas Instruments |
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UCC3585 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 20 page UCC2585 UCC3585 SLUS304F − JULY 1999 − REVISED JANUARY 2005 10 www.ti.com APPLICATION INFORMATION A solution to this minimum on-time is a counter and time-out circuit. As described in the SD pin description, a capacitor on SD enables the time-out circuit. An internal digital counter is used to count the overcurrent events at the current-sense comparator output. When seven overcurrent conditions are reached, both MOSFET switches are turned off, the SS capacitor is discharged, and an 11 µA (typical) internal current sink discharges the SD capacitor. During this discharge time, both MOSFETs are held off, and the inductor current decays to zero. When the SD capacitor voltage reaches 0.55 V (typical), a softstart cycle restarts the converter. During softstart, the 7-cycle counter is disabled. However, the peak current limit comparator is enabled. When the SS voltage reaches the threshold equal to (VIN − 0.5 V), the 7-cycle counter is enabled. By sizing the SS capacitor relative to the SD capacitor, the amount of time spent switching the MOSFETs can be reduced when the output is overloaded. If the timeout mode is used, the relative capacitance values for CSS and CSD must fall into the following relationship: C SD v 20 C SS This equation also states that, if the time-out mode is used, a softstart capacitor must be used. Figure 4 shows the waveforms when the converter is operated into a short circuit. V(PDRV) V(SD) 0.55 V VIN−0.5 V VIN V(SS) time 0 V VIN t SHUT Figure 4. Converter Operated Into Short Circuit UCC3585 Not Recommended for New Designs |
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