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LM3676 데이터시트(PDF) 9 Page - Texas Instruments |
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LM3676 데이터시트(HTML) 9 Page - Texas Instruments |
9 / 23 page VOUT TIME (100 Ps/DIV) EN VSW 2V/DIV 500 mV/DIV VIN = 3.6V VOUT = 1.5V IOUT = 1 mA 2V/DIV LM3676 www.ti.com SNVS426B – NOVEMBER 2006 – REVISED MARCH 2007 Typical Performance Characteristics (continued) Circuit of LM3676SD, VIN= 3.6V, VOUT= 1.5V, TA= 25°C, unless otherwise noted. Start Up into PFM Mode VOUT = 1.5V (Output Current= 1mA) Operation Description DEVICE INFORMATION The LM3676, 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 2.9V to 5.5V to portable devices such as cell phones and PDAs. Using a voltage mode architecture with synchronous rectification, the LM3676 has the ability to deliver up to 600 mA depending on the input voltage, output voltage, ambient temperature and the inductor chosen. There are three modes of operation depending on the current required and Mode pin - PWM (Pulse Width Modulation), PFM (Pulse Frequency Modulation), and shutdown. The device operates in PWM mode if the load current > 80 mA or when the Mode pin is set high. When the mode pin is set low, Auto mode, lighter load current causes the device to automatically switch into PFM for reduced current consumption (IQ = 16 µA typ) and prolong battery life . Shutdown mode turns off the device, offering the lowest current consumption (ISHUTDOWN = 0.01 µA typ). Additional features include soft-start, under voltage 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 0.5V. It is recommended to keep the part in shutdown until the input voltage is 2.9V or higher. CIRCUIT OPERATION During the first portion of each switching cycle, the control block in the LM3676 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. MODE PIN Setting the Mode pin low (<0.4V) places the LM3676 in Auto mode. During Auto mode the device automatically switches between PFM-PWM depending on the load. Setting Mode high (>1.0V) places the part in Forced PWM. The part is in forced PWM regardless of the load. Do not leave the Mode pin floating. Copyright © 2006–2007, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM3676 |
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