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LTC3221EDC Datasheet(PDF) 7 Page - Linear Technology |
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LTC3221EDC Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page LTC3221/ LTC3221-3.3/LTC3221-5 7 3221f Power Efficiency TheinputcurrentofadoublingchargepumpliketheLTC3221 family is always twice that of the output current. This is true regardless of whether the output voltage is unregulated or regulated or of the regulation method used. In an ideal unregulated doubling charge pump, conservation of energy implies that the input current has to be twice that of the output current in order to obtain an output voltage twice that of the input voltage. In a regulated charge pump like the LTC3221, the regulation of VOUT is similar to that of a linear regulator, with the voltage difference between 2 • VIN (Input voltage plus the voltage across a fully charged flying capacitor) and VOUT being absorbed in an internal pass transistor. In the LTC3221, the controlled current ISWactsas a pass transistor. So the input current of an ideal regulated doubling charge pump is the same as an unregulated one, which is equal to twice the output current. The efficiency (n) of an ideal regulated doubler is therefore given by: η= = = P P VI VI V V OUT IN OUT OUT IN OUT OUT IN • •2 2 At moderate to high output power, the switching losses and quiescent current of the LTC3221 family are negligible and the expression is valid. For example, an LTC3221-5 with VIN = 3V, IOUT = 60mA and VOUT regulating to 5V, has a mea- sured efficiency of 82% which is in close agreement with the theoretical 83.3% calculation. The LTC3221 product family continues to maintain good efficiency even at fairly light loads because of its inherently low power design. Maximum Available Output Current For the adjustable LTC3221, the maximum available output current and voltage can be calculated from the effective open-loop output resistance, ROL, and effective output voltage, 2VIN(MIN). From Figure 1 the available current is given by: I VV R OUT IN OUT OL = 2– Effective Open-Loop Output Resistance (ROL) The effective open-loop output resistance(ROL) of a charge pump is a very important parameter which determines the strength of the charge pump. The value of this parameter APPLICATIO S I FOR ATIO OPERATIO (Refer to Block Diagrams) The LTC3221 has a FB pin in place of the ⎯S⎯H⎯D⎯N pin. This allows the output voltage to be programmed using an external resistive divider. Burst Mode Operation The LTC3221 family regulates the output voltage throughout the full 60mA load range using Burst Mode control. This keeps the quiescent current low at light load and improves the efficiency at full load by reducing the switching losses. All the internal circuitry except the comparator is kept off if the output voltage is high and the flying capacitor has been fully charged. These circuits are turned on only if VOUT drops below the comparator lower threshold. At light load, VOUT stays above this lower threshold for a long period of time, this result in a very low average input current. Soft-Start and Short-Circuit Protection The LTC3221 family uses a controlled current, ISW to deliver current to the output. This helps to limit the input and output current during start-up and output short-circuit condition. During start up ISWisusedtochargeuptheflying capacitor and output capacitor, this limits the input current to approximately 240mA. During short-circuit condition, the output current is delivered through ISW and this limits the output current to approximately 120mA. This prevents excessive self-heating that causes damage to the part. Figure 1. Equivalent Open-Loop Circuit + – + – 2VIN IOUT VOUT 3221 F01 ROL |
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