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LP3905 Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LP3905 Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 15 page Functional Description (Continued) DC/DC BUCK REGULATORS The LP3905 Buck regulators are high efficiency step down DC-DC switching converters used for delivering 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 rectifi- cation, the Buck Regulators have 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 standard device operates in PWM mode at load currents of approxi- mately 80 mA or higher, having voltage tolerance of ±4% with 90% efficiency or better. Lighter load currents cause the device to automatically switch into PFM for reduced current consumption and a longer battery life. Shutdown mode turns off the device, offering the lowest current consumption . A fixed mode device is also available which is fixed in PWM mode for both low and high load currents. An adjustable voltage version is also avalable for which the output voltage can be selected by using two external resis- tors at each of the two buck outputs. Additional features include soft-start, under voltage protec- tion, current overload protection, and thermal shutdown pro- tection. The part uses an internal reference voltage of 0.5V. It is recommended to keep the part in shutdown until the input voltage is 3V or higher. BUCK CONVERTER BLOCK DIAGRAM CIRCUIT OPERATION The LP3905 Buck regulators operate as follows. During the first portion of each switching cycle, the control block in the LP3905 turns on the internal PFET switch. This allows cur- rent 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 20152918 FIGURE 7. Simplified Functional Diagram www.national.com 8 |
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