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LM3670 Folha de dados(PDF) 10 Page - National Semiconductor (TI) |
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LM3670 Folha de dados(HTML) 10 Page - National Semiconductor (TI) |
10 / 17 page Typical Performance Characteristics (unless otherwise stated: V IN=3.6V, VOUT=1.8V) (Continued) PFM Mode V SW,VOUT,IINDUCTOR vs. Time PWM Mode V SW,VOUT,IINDUCTOR vs. Time 20075822 20075823 Soft Start V IN,VOUT,IINDUCTOR vs. Time (I LOAD = 350mA) 20075824 Operation Description DEVICE INFORMATION The LM3670, a high efficiency step down DC-DC switching buck converter, delivers a constant voltage from either a single Li-Ion or three cell NiMH/NiCd battery to portable devices such as cell phones and PDAs. Using a voltage mode architecture with synchronous rectification, the LM3670 has the ability to deliver up to 350 mA depending on the input voltage and output voltage (voltage head room), and the inductor chosen (maximum current capability). There are three modes of operation depending on the cur- rent required - PWM, PFM, and shutdown. PWM mode handles current loads of approximately 70 mA or higher. Lighter output current loads cause the device to automati- cally switch into PFM for reduced current consumption (I Q = 15 µA typ) and a longer battery life. Shutdown mode turns off the device, offering the lowest current consumption (I Q, SHUTDOWN = 0.1 µA typ). The LM3670 can operate up to a 100% duty cycle (PMOS switch always on) for low drop out control of the output voltage. In this way the output voltage will be controlled down to the lowest possible input voltage. Additional features include soft-start, under voltage lock out, current overload protection, and thermal overload protection. As shown in Figure 1, only three external power components are required for implementation. CIRCUIT OPERATION The LM3670 operates as follows. During the first portion of each switching cycle, the control block in the LM3670 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 by storing energy in a magnetic field. During the second portion of each cycle, the controller turns the PFET switch www.national.com 10 |
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