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LM3671MF-ADJ 数据表(PDF) 10 Page - National Semiconductor (TI) |
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LM3671MF-ADJ 数据表(HTML) 10 Page - National Semiconductor (TI) |
10 / 16 page Operation Description DEVICE INFORMATION The LM3671, 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 LM3671 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 cur- rent required - PWM, PFM, and shutdown. The device oper- ates in PWM mode at load currents of approximately 80 mA or higher, having voltage tolerance of ±4% with 90% effi- ciency or better. Lighter load currents cause the device to automatically switch into PFM for reduced current consump- tion (I Q = 16 µA typ) and a longer battery life. Shutdown mode turns off the device, offering the lowest current con- sumption (I Q, SHUTDOWN = 0.01 µA typ). Additional features include soft-start, under voltage protec- tion, current overload protection, and thermal shutdown pro- tection. 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 2.8V or higher. CIRCUIT OPERATION The LM3671 operates as follows. During the first portion of each switching cycle, the control block in the LM3671 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 (V IN–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 - V OUT/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 PWM 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 in- versely proportional to the input voltage is introduced. While in PWM (Pulse Width Modulation) mode, 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. Internal Synchronous Rectification While in PWM mode, the LM3671 uses an internal NFET as a synchronous rectifier to reduce rectifier forward voltage drop and associated power loss. Synchronous rectification provides a significant improvement in efficiency whenever the output voltage is relatively low compared to the voltage drop across an ordinary rectifier diode. Current Limiting A current limit feature allows the LM3671 to protect itself and external components during overload conditions. PWM mode implements current limiting using an internal compara- tor that trips at 1020 mA (typ). If the output is shorted to ground the device enters a timed current limit mode where the NFET is turned on for a longer duration until the inductor current falls below a low threshold, ensuring inductor current has more time to decay, thereby preventing runaway. PFM OPERATION At very light loads, the converter enters PFM mode and operates with reduced switching frequency and supply cur- rent to maintain high efficiency. The part will automatically transition into PFM mode when either of two conditions occurs for a duration of 32 or more clock cycles: A. The inductor current becomes discontinuous. B. The peak PMOS switch current drops below the I MODE level, (Typically I MODE < 30mA + VIN/42 Ω ). 20108423 FIGURE 5. Typical PWM Operation www.national.com 10 |
类似零件编号 - LM3671MF-ADJ |
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类似说明 - LM3671MF-ADJ |
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