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LTC32403.3 数据表(PDF) 8 Page  Linear Technology 

LTC32403.3 数据表(HTML) 8 Page  Linear Technology 
8 / 12 page LTC32403.3/LTC32402.5 8 3240fb Power Efﬁciency During LDO operation, the power efﬁciency ( η) of the LTC3240 is given by: η= = = P P VI VI V V OUT IN OUT OUT IN OUT OUT IN • • At moderate to high output power, the quiescent cur rent of the LTC3240 is negligible and the expression above is valid. For example, the measured efﬁciency of LTC32403.3, with VIN = 3.7V, IOUT = 100mA and VOUT regulating to 3.3V is 87% which is in close agreement with the theoretical value of 89%. During charge pump operation, the power efﬁciency ( η) of the LTC3240 is similar to that of a linear regulator with an effective input voltage of twice the actual input volt age. 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 is 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 the quiescent current of the LTC3240 are negligible and the expression above is valid. For example, the mea sured efﬁciency of LTC32403.3 with VIN = 2.5V, IOUT = 100mA and VOUT regulating to 3.3V is 64% which is in close agreement with the theoretical value of 66%. Effective OpenLoop Output Resistance (ROL) The effective openloop 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 depends on many factors such as the oscillator frequency (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: RR fC LS STO OSC FLY 0 14 2 1 ≅ ∑ + = • For the LTC3240 in charge pump mode, the maximum available output current and voltage can be calculated from the effective openloop output resistance, ROL, and the effective output voltage, 2VIN(MIN). From Figure 1, the available current is given by: I VV R OUT IN OUT OL = 2– Typical ROL values as a function of temperature are shown in Figure 2. APPLICATIO S I FOR ATIO Figure 1. Equivalent OpenLoop Circuit + – ROL IOUT VOUT 2VIN 3240 F01 + – Figure 2. Typical ROL vs Temperature VIN, VOUT Capacitor Selection The style and value of capacitors used with the LTC3240 determine several important parameters such as regulator control loop stability, output ripple, charge pump strength and minimum startup time. To reduce noise and ripple, it is recommended that low ESR (<0.1 Ω) ceramic capacitors be used for both CIN and COUT. CIN should be 1µF or greater while COUT should be 4.7µF or greater. Tantalum and aluminum capacitors are not recommended because of their high ESR. TEMPERATURE ( °C) –40 8 9 10 60 3240 G07 7 6 5 –15 10 35 85 VIN = 1.8V VOUT = 3V 
