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LM2770 Folha de dados(PDF) 10 Page - National Semiconductor (TI) |
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LM2770 Folha de dados(HTML) 10 Page - National Semiconductor (TI) |
10 / 14 page Operation Description OVERVIEW The LM2770 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 multiple fractional gains and pulse-frequency-modulated (PFM) switching to minimize power losses over wide input voltage and output current ranges. A description of the principal operational characteristics of the LM2770 is broken up into the following sections: PFM Regulation, Fractional Multi-Gain Charge Pump, and Multi-Gain Efficiency Performance. Each of these sections refers to the Block Diagram. PFM REGULATION The LM2770 achieves tightly regulated output voltages with pulse-frequency-modulated (PFM) regulation. PFM simply means the part only pumps when charge needs to be deliv- ered to the output in order to keep the output voltage in regulation. When the output voltage is above the target regulation voltage, the part idles and consumes minimal supply-current. In this state, the load current is supplied solely by the charge stored on the output capacitor. As this capacitor discharges and the output voltage falls below the target regulation voltage, the charge pump activates, and charge is delivered to the output. This charge supplies the load current and boosts the voltage on the output capacitor. The primary benefit of PFM regulation is when output cur- rents are light and the part is predominantly in the low- supply-current idle state. Net supply current is minimal be- cause the part only occasionally needs to recharge the output capacitor by activating the charge pump. With PFM regulation, input and output ripple frequencies vary signifi- cantly, and are dependent on output current, input voltage, and, to a lesser degree, other factors such as temperature and internal switch characteristics. FRACTIONAL MULTI-GAIN CHARGE PUMP The core of the LM2770 is a two-phase charge pump con- trolled by an internally generated non-overlapping clock. The charge pump operates by using the external flying capaci- tors, C1 and C2, to transfer charge from the input to the output. The two phases of the switching cycle will be referred to as the "charge phase" and the "hold/rest phase". During the charge phase, the flying capacitors are charged by the input supply. After charging the flying capacitors for half of a switching cycle[t= 1/(2xF SW) ], the LM2770 switches to the hold/rest phase. In this configuration, the charge that was stored on the flying capacitors in the charge phase is trans- ferred to the output. If the voltage on the output is below the target regulation voltage at completion of the switching cycle, the charge pump will switch back to the charge phase. But if the output voltage is above the target regulation volt- age at the end of the switching cycle, the charge pump will remain in the hold/rest state. It will idle in this mode until the output voltage drops below the target regulation voltage. When this finally occurs, the LM2770 will switch back to the charge phase. Input, output, and intermediary connections of the flying capacitors are made with internal MOS switches. The LM2770 utilizes two flying capacitors and a versatile switch network to achieve three distinct fractional voltage gains: 1⁄3, 1 ⁄2, and 2⁄3. With this gain-switching ability, it is as if the LM2770 is three-charge-pumps-in-one. The "active" charge pump at any given time is the one that yields the highest efficiency based on the input and output conditions present. MULTI-GAIN EFFICIENCY PERFORMANCE The ability to switch gains based on input and output condi- tions results in optimal efficiency throughout the operating ranges of the LM2770. Charge-pump efficiency is derived in the following two ideal equations (supply current and other losses are neglected for simplicity): I IN =GxIOUT E=(V OUT xIOUT)÷(VIN xIIN)=VOUT ÷(GXVIN) In the equations, G represents the charge pump gain. Effi- ciency is at its highest as GxV IN approaches VOUT. Refer to the efficiency graphs in the Typical Perfromance Charac- teristics section for detailed efficiency data. The gain re- gions are clearly distinguished by the sharp discontinuities in the efficiency curves and are identified at the bottom of each graph (G = 2⁄3,G= 1⁄2,andG= 1⁄3). DYNAMIC OUTPUT VOLTAGE SELECTION The output voltage of the LM2770 can be dynamically ad- justed for the purpose of improving system efficiency. Each LM2770 version contains two built-in output voltage options: a high level and a low level (1.5V and 1.2V, for example). With the simple V SEL logic input pin, the output voltage can be switched between these two voltages. Dynamic voltage selection can be used to improve overall system efficiency. When comparing system efficiency be- tween two different output voltages, evaluating power con- sumption often lends more insight than actually comparing converter efficiencies. An application powered with a Li-Ion battery is a good example to illustrate this. Referring to the LM2770 efficiency curves (see Typical Performance Char- actersitics), all LM2770 output voltage options operate with G= 1⁄2 over the core Li-Ion battery voltage range (3.5V - 3.9V). Thus, the LM2770 circuit will draw an input current that is approximately half the output current in the core Li-Ion voltage range, regardless of the output voltage (I IN =Gx I OUT). While varying the LM2770 output voltage does not directly improve system efficiency, it can have a secondary effect. Different output voltages often will result in different LM2770 load currents. This is where system efficiency can benefit from dynamic output voltage selection: the LM2770 load circuit can run at lower currents. This reduces LM2770 input current and improves overall system efficiency. SLEEP MODE BYPASS LDO The LM2770 offers a bypass low-dropout linear regulator (LDO) for low-noise performance under light loads. Capable of delivering up to 20mA of output current, this LDO has low ground pin current and is ideal for stand-by operation. The LDO is activated with the SLEEP logic input pin. When SLEEP is active, the charge pump is disabled and the LDO supplies all load current. SHUTDOWN The LM2770 is in shutdown mode when the voltage on the enable pin (EN) is logic-low. In shutdown, the LM2770 draws virtually no supply current. When in shutdown, the output of the LM2770 is completely disconnected from the input. The internal feedback resistors will pull the output voltage down to 0V (unless the output is driven by an outside source). In some applications, it may be desired to disable the LM2770 and drive the output pin with another voltage www.national.com 10 |
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