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LM2757 Fiches technique(PDF) 9 Page - Texas Instruments |
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LM2757 Fiches technique(HTML) 9 Page - Texas Instruments |
9 / 20 page LM2757 www.ti.com SNVS536D – OCTOBER 2007 – REVISED AUGUST 2009 Typical Performance Characteristics (continued) Unless otherwise specified: VIN = 3.6V, V(M0) = 0V, V(M1) = VIN, C1 = C2 = 0.47µF, CIN = COUT = 1.0µF, TA = 25ºC. Capacitors are low-ESR multi-layer ceramic capacitors (MLCC's). Load Step with a Li-Ion Battery, 10mA to 200mA Load Step with a Li-Ion Battery 200mA to 10mA VBATT = 4V, VOUT = 5V Mode VBATT = 4V, VOUT = 5V Mode CH1: VOUT; Scale: 50mV/Div, AC Coupled CH1: VOUT; Scale: 50mV/Div, AC Coupled CH4: IOUT; Scale: 100mA/Div, DC Coupled CH4: IOUT; Scale: 100mA/Div, DC Coupled Time scale: 10µs/Div Time scale: 10µs/Div Operation Description OVERVIEW The LM2757 is a switched capacitor converter that produces a regulated output voltage of either 5V, 4.5V or 4.1V depending on the mode selected. The core of the part is a highly efficient charge pump that utilizes fixed frequency pre-regulation to minimize ripple and power losses over wide input voltage and output current ranges. A description of the principal operational characteristics of the LM2757 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 LM2757 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 to transfer charge from the input to the output. At input voltages below 3.9V (typ.) for the 5V mode, the LM2757 operates in a 2x gain, with the input current being equal to 2x the load current. At input voltages above 3.9V (typ.) for the 5V mode, the part utilizes a gain of 3/2x, resulting in an input current equal to 3/2 times the load current. For the 4.5V mode, the LM2757 operates in a 2x gain when the input voltage is below 3.35V (typ.) and transitions to a 3/2x gain when the input voltage is above 3.35V (typ.). For the 4.1V mode, the device utilizes the 3/2x gain for the entire input voltage range. The two phases of the switched capacitor switching cycle will be referred to as the "phase one" and the "phase two". During phase one, one flying capacitor is charged by the input supply while the other flying capacitor is connected to the output and delivers charge to the load . After half of the switching cycle [ t = 1/(2×FSW) ], the LM2757 switches to phase two. In this configuration, the capacitor that supplied charge to the load in phase one is connected to the input to be recharged while the capacitor that had been charged in the previous phase is connected to the output to deliver charge. With this topology, output ripple is reduced by delivering charge to the output in every phase. The LM2757 uses fixed frequency pre-regulation to regulate the output voltage. 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. 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 E = (VOUT × IOUT) ÷ (VIN × IIN) = VOUT ÷ (G × VIN) (1) Copyright © 2007–2009, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM2757 |
Numéro de pièce similaire - LM2757_14 |
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Description similaire - LM2757_14 |
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