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LM3678 Datasheet(PDF) 9 Page - Texas Instruments |
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LM3678 Datasheet(HTML) 9 Page - Texas Instruments |
9 / 18 page LM3678 www.ti.com SNVS464B – APRIL 2008 – REVISED JANUARY 2009 OPERATION DESCRIPTION DEVICE INFORMATION The LM3678, a high-efficiency step-down DC-DC switching buck converter, delivers a constant voltage from a single Li-Ion battery and input voltage rails from 2.5 V to 5.5 V to portable devices such as cell phones and PDAs. Using a voltage mode architecture with synchronous rectification, the LM3678 has the ability to deliver up to 1.5 A, depending on the input voltage, output voltage, ambient temperature and the inductor chosen. Additional features include soft start, undervoltage protection, current overload protection, and thermal shutdown protection. As shown in Figure 1, only three external power components are required for implementation. The part uses an internal reference voltage of 0.5 V. It is recommended to keep the part in shutdown until the input voltage is 2.5 V or higher. CIRCUIT OPERATION During the first portion of each switching cycle, the control block in the LM3678 turns on the internal PFET switch. This allows current to flow from the input through the inductor to the output filter capacitor and load. The inductor limits the current to a ramp with a slope of (VIN–VOUT)/L, by storing energy in a magnetic field. During the second portion of each cycle, the controller turns the PFET switch off, blocking current flow from the input, and then turns the NFET synchronous rectifier on. The inductor draws current from ground through the NFET to the output filter capacitor and load, which ramps the inductor current down with a slope of - VOUT/L. The output filter stores charge when the inductor current is high, and releases it when inductor current is low, smoothing the voltage across the load. The output voltage is regulated by modulating the PFET switch on time to control the average current sent to the load. The effect is identical to sending a duty-cycle modulated rectangular wave formed by the switch and synchronous rectifier at the SW pin to a low-pass filter formed by the inductor and output filter capacitor. The output voltage is equal to the average voltage at the SW pin. PWM OPERATION During device operation the converter operates as a voltage-mode controller with input voltage feed forward. This allows the converter to achieve good load and line regulation. The DC gain of the power stage is proportional to the input voltage. To eliminate this dependence, feed forward inversely proportional to the input voltage is introduced. The output voltage is regulated by switching at a constant frequency and then modulating the energy per cycle to control power to the load. At the beginning of each clock cycle the PFET switch is turned on and the inductor current ramps up until the comparator trips and the control logic turns off the switch. The current limit comparator can also turn off the switch in case the current limit of the PFET is exceeded. Then the NFET switch is turned on and the inductor current ramps down. The next cycle is initiated by the clock turning off the NFET and turning on the PFET. Figure 25. Typical PWM Operation Copyright © 2008–2009, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM3678 |
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