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LTC3204 Datasheet(PDF) 8 Page - Linear Technology |
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LTC3204 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page LTC3204-3.3/LTC3204-5/ LTC3204B-3.3/LTC3204B-5 8 3204fa Power Efficiency The power efficiency (η) of the LTC3204-3.3/LTC3204-5/ LTC3204B-3.3/LTC3204B-5 is similar to that of a linear regulator with an effective input voltage of twice the actual input voltage. This occurs because the input current for a voltage doubling charge pump is approximately twice the output current. In an ideal regulating voltage doubler the power efficiency would be given by: η = = = P P V I V I V V OUT IN OUT OUT IN OUT OUT IN • • 2 2 At moderate to high output power, the switching losses and the quiescent current of the LTC3204-3.3/LTC3204-5/ LTC3204B-3.3/LTC3204B-5 are negligible and the expres- sion above is valid. For example, with VIN = 3V, IOUT = 100mA and VOUT regulating to 5V, the measured efficiency is 81.8% which is in close agreement with the theoretical 83.3% calculation. Maximum Available Output Current For the LTC3204-3.3/LTC3204-5/LTC3204B-3.3/ LTC3204B-5,the maximum available output current and voltage can be calculated from the effective open-loop output resistance, ROL, and the effective input voltage, 2VIN(MIN). (fOSC), value of the flying capacitor (CFLY), the nonoverlap time, the internal switch resistances (RS), and the ESR of the external capacitors. A first order approximation for ROL is given below: R R f C OL S OSC FLY ≅ ∑ + 2 1 • Typical ROL values as a function of temperature are shown in Figure 2. Figure 1. Equivalent Open-Loop Circuit From Fig. 1, the available current is given by: I V V R OUT IN OUT OL = 2 – Effective Open Loop Output Resistance (ROL) Theeffectiveopenloopoutputresistance(ROL)ofacharge pump is a very important parameter which determines the strength of the charge pump. The value of this parameter depends on many factors such as the oscillator frequency Figure 2. Typical ROL vs Temperature VIN, VOUT Capacitor Selection ThestyleandvalueofcapacitorsusedwiththeLTC3204-3.3/ LTC3204-5/LTC3204B-3.3/LTC3204B-5determineseveral important parameters such as regulator control loop sta- bility, output ripple, charge pump strength and minimum start-up time. To reduce noise and ripple, it is recommended that low ESR (<0.1Ω) ceramic capacitors be used for both CINand COUT. These capacitors should be 1µF or greater. Tantalum and aluminum capacitors are not recommended because of their high ESR. The value of COUT directly controls the amount of output ripple for a given load current. Increasing the size of COUT will reduce the output ripple at the expense of higher minimum turn-on time. The peak-to-peak output ripple is approximately given by the expression: V I f C RIPPLE P P OUT OSC OUT ( ) • − ≅ 2 +– ROL IOUT VOUT 2VIN 3204 F01 + – S=1 TO 4 APPLICATIO S I FOR ATIO 3204 F02 TEMPERATURE ( °C) 100 0 50 VIN = 2.7V VOUT = 4.5V –50 8 7 6 5 4 |
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