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LMZ14203 bảng dữ liệu(PDF) 11 Page - Texas Instruments |
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LMZ14203 bảng dữ liệu(HTML) 11 Page - Texas Instruments |
11 / 29 page LMZ14203 www.ti.com SNVS632R – DECEMBER 2009 – REVISED OCTOBER 2015 Feature Description (continued) 7.3.4 Thermal Protection The junction temperature of the LMZ14203 should not be allowed to exceed its maximum ratings. Thermal protection is implemented by an internal Thermal Shutdown circuit which activates at 165 °C (typical) causing the device to enter a low power standby state. In this state the main MOSFET remains off causing VO to fall, and additionally the CSS capacitor is discharged to ground. Thermal protection helps prevent catastrophic failures for accidental device overheating. When the junction temperature falls back below 145°C (typical hysteresis = 20 °C) the SS pin is released, VO rises smoothly, and normal operation resumes. Applications requiring maximum output current especially those at high input voltage may require application derating at elevated temperatures. 7.3.5 Zero Coil Current Detection The current of the lower (synchronous) MOSFET is monitored by a zero coil current detection circuit which inhibits the synchronous MOSFET when its current reaches zero until the next ON-time. This circuit enables the DCM operating mode, which improves efficiency at light loads. 7.3.6 Prebiased Start-Up The LMZ14203 will properly start up into a prebiased output. This start-up situation is common in multiple rail logic applications where current paths may exist between different power rails during the start-up sequence. Figure 23 is a scope capture that shows proper behavior during this event. Figure 23. Prebiased Start-Up 7.4 Device Functional Modes 7.4.1 Discontinuous Conduction and Continuous Conduction Modes At light-load, the regulator operates in discontinuous conduction mode (DCM). With load currents above the critical conduction point, it operates in continuous conduction mode (CCM). When operating in DCM the switching cycle begins at zero amps inductor current; increases up to a peak value, and then recedes back to zero before the end of the OFF-time. During the period of time that inductor current is zero, all load current is supplied by the output capacitor. The next ON-time period starts when the voltage on the FB pin falls below the internal reference. The switching frequency is lower in DCM and varies more with load current as compared to CCM. Conversion efficiency in DCM is maintained because conduction and switching losses are reduced with the smaller load and lower switching frequency. Copyright © 2009–2015, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LMZ14203 |
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