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LM2854MH-500 Datasheet(PDF) 9 Page - Texas Instruments |
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LM2854MH-500 Datasheet(HTML) 9 Page - Texas Instruments |
9 / 33 page Ramp and Clock Generator AVIN EN PVIN Gate Drive SW PGND + - + - SS FB Error Amplifier PWM Comparator Oscillator 0.8V Reference UVLO Zc2 AGND Current Limit Reference Selector 1.23V + - 9 LM2854 www.ti.com SNVS560D – MARCH 2008 – REVISED JANUARY 2016 Product Folder Links: LM2854 Submit Documentation Feedback Copyright © 2008–2016, Texas Instruments Incorporated 7 Detailed Description 7.1 Overview The LM2854 PowerWise synchronous DC-DC buck regulator belongs to the Texas Instruments SIMPLE SWITCHER family of switching regulators. Integration of the power MOSFETs and associated drivers, compensation component network, and the PWM controller reduces the number of external components necessary for a complete power supply design, without sacrificing performance. 7.2 Functional Block Diagram 7.3 Feature Description 7.3.1 Switching Frequency The LM2854 is available in two switching frequency options, 500 kHz and 1 MHz. Generally, a higher switching frequency allows for faster transient response and a reduction in the footprint area and volume of the external power stage components, while a lower switching frequency affords better efficiency. These factors should be considered when selecting the appropriate switching frequency for a given application. 7.3.2 Enable The LM2854 features a enable (EN) pin and associated comparator to allow the user to easily sequence the LM2854 from an external voltage rail, or to manually set the input UVLO threshold. The turnon or rising threshold and hysteresis for this comparator are typically 1.23 V and 0.15 V, respectively. The precise reference for the enable comparator allows the user to ensure that the LM2854 will be disabled when the system demands it to be. 7.3.3 Soft-Start The LM2854 begins to operate when both the AVIN and EN voltages exceed the rising UVLO and enable thresholds, respectively. A controlled soft-start eliminates inrush currents during start-up and allows the user more control and flexibility when sequencing the LM2854 with other power supplies. An external soft-start capacitor is used to control the LM2854 start-up time. During soft-start, the voltage on the feedback pin is connected internally to the non-inverting input of the error amplifier. The soft-start period lasts until the voltage on the soft-start pin exceeds the LM2854 reference voltage of 0.8 V. At this point, the reference voltage takes over at the non-inverting amplifier input. |
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