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LM3670MFX-1.6 Fiches technique(PDF) 11 Page - Texas Instruments |
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LM3670MFX-1.6 Fiches technique(HTML) 11 Page - Texas Instruments |
11 / 25 page -VOUT L VIN-VOUT L LM3670 www.ti.com SNVS250E – NOVEMBER 2004 – REVISED FEBRUARY 2013 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 current required - PWM (Pulse Width Modulation), PFM (Pulse Frequency Modulation), and shutdown. PWM mode handles current loads of approximately 70 mA or higher. Lighter output current loads cause the device to automatically switch into PFM for reduced current consumption (IQ = 15 µA typ) and a longer battery life. Shutdown mode turns off the device, offering the lowest current consumption (ISHUTDOWN = 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 (1) 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 (2) The output filter stores charge when the inductor current is high, and releases it when low, smoothing the voltage across the load. PWM Operation During PWM operation the converter operates as a voltage-mode controller with input voltage feed forward. This allows the converter to achieve excellent 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. Internal Synchronous Rectification While in PWM mode, the LM3670 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 LM3670 to protect itself and external components during overload conditions PWM mode implements cycle-by-cycle current limiting using an internal comparator that trips at 620 mA (typ). Copyright © 2004–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM3670 |
Numéro de pièce similaire - LM3670MFX-1.6 |
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Description similaire - LM3670MFX-1.6 |
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