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LM2772 データシート(PDF) 8 Page - Texas Instruments |
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LM2772 データシート(HTML) 8 Page - Texas Instruments |
8 / 17 page LM2772 SNVS486B – DECEMBER 2006 – REVISED MAY 2013 www.ti.com OPERATION DESCRIPTION Overview The LM2772 is a switched capacitor converter that produces a regulated, low voltage output. The core of the part is a highly efficient charge pump that utilizes fixed frequency pre-regulation and Pulse Frequency Modulation to minimize ripple and power losses over wide input voltage and output current ranges. A description of the principal operational characteristics of the LM2772 is detailed in the Circuit Description, and Efficiency Performance sections. These sections refer to details in the Block Diagram. Circuit Description The core of the LM2772 is a two-phase charge pump controlled by an internally generated non-overlapping clock. The charge pump operates by using external flying capacitors C1, C2, and C3 to transfer charge from the input to the output. At input voltages below 3.5V (typ.) the LM2772 operates in a 1/2x Gain, with the input current being equal to 1/2 of the load current. At input voltages between 3.5V to 4.6V(typ.) the part utilizes a gain of 2/5x, resulting in an input current equal to 2/5 times the load current. At input voltages above 4.6V (typ.), the part is in a gain of 1/3, with the input current being 1/3 of the load current. The two phases of the switched capacitor switching cycle will be referred to as the "charge phase" and the "discharge phase". During the charge phase, the flying capacitor is charged by the input supply. After half of the switching cycle [ t = 1/(2×FSW) ], the LM2772 switches to the discharge phase. In this configuration, the charge that was stored on the flying capacitors in the charge phase is transferred to the output. The LM2772 uses fixed frequency pre-regulation to regulate the output voltage to 1.2V during moderate to high load currents. The input and output connections of the flying capacitors are made with internal MOS switches. Pre-regulation limits the gate drive of the MOS switch connected between the voltage input and the flying capacitors. Controlling the on resistance of this switch limits the amount of charge transferred into and out of each flying capacitor during the charge and discharge phases, and in turn helps to keep the output ripple very low. When output currents are low (<40mA typ.), the LM2772 automatically switches to a low-ripple Pulse Frequency Modulation (PFM) form of regulation. In PFM mode, the flying capacitors stay in the discharge phase until the output voltage drops below a predetermined trip point. When this occurs, the flying capacitors switch back to the charge phase. After being charged, the flying capacitors repeat the process of staying in the discharge phase and switching to the charge phase when necessary. Efficiency Performance Charge-pump efficiency is derived in the following two ideal equations (supply current and other losses are neglected for simplicity): IIN = G × IOUT (1) E = (VOUT × IOUT) ÷ (VIN × IIN) = VOUT ÷ (G × VIN) (2) In the equations, G represents the charge pump gain. Efficiency is at its highest as G×VIN approaches VOUT. Refer to the efficiency graph in the Typical Performance Characteristics section for detailed efficiency data. The transition between gains of 1/2, 2/5, and 1/3 are clearly distinguished by the sharp discontinuity in the efficiency curve. Shutdown The LM2772 is in shutdown mode when the voltage on the enable pin (EN) is logic-low. In shutdown, the LM2772 draws virtually no supply current. When in shutdown, the output of the LM2772 is completely disconnected from the input. Internal feedback resistors pull the output voltage down to 0V during shutdown. Soft Start The LM2772 employs soft start circuitry to prevent excessive input inrush currents during startup. At startup, the output voltage gradually rises from 0V to the nominal output voltage. This occurs in 150µs (typ.). Soft-start is engaged when the part is enabled, including situations where voltage is established simultaneously on the VIN and EN pins. 8 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated Product Folder Links: LM2772 |
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同様の説明 - LM2772 |
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