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LT1054IN8 Datasheet(PDF) 7 Page - Linear Technology |
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LT1054IN8 Datasheet(HTML) 7 Page - Linear Technology |
7 / 16 page 7 LT1054/LT1054L 1054lfe the oscillator frequency. During the time that CIN is charg- ing, the peak supply current will be approximately equal to 2.2 times the output current. During the time that CIN is delivering charge to COUT the supply current drops to approximately 0.2 times the output current. An input supply bypass capacitor will supply part of the peak input current drawn by the LT1054 and average out the current drawn from the supply. A minimum input supply bypass capacitor of 2 µF, preferably tantalum or some other low ESR type is recommended. A larger capacitor may be desirable in some cases, for example, when the actual input supply is connected to the LT1054 through long leads, or when the pulse current drawn by the LT1054 might affect other circuitry through supply coupling. PIN FUNCTIONS APPLICATIONS INFORMATION Theory of Operation To understand the theory of operation of the LT1054, a review of a basic switched-capacitor building block is helpful. In Figure 3 when the switch is in the left position, capacitor C1 will charge to voltage V1. The total charge on C1 will be q1 = C1V1. The switch then moves to the right, discharging C1 to voltage V2. After this discharge time the charge on C1 is q2 = C1V2. Note that charge has been transferred from the source V1 to the output V2. The amount of charge transferred is: ∆q = q1 – q2 = C1(V1 – V2) If the switch is cycled f times per second, the charge transfer per unit time (i.e., current) is: I = (f)( ∆q) = (f)[C1(V1 – V2)] To obtain an equivalent resistance for the switched-capaci- tor network we can rewrite this equation in terms of voltage and impedance equivalence: I = = V1 – V2 (1/fC1) V1 – V2 REQUIV A new variable REQUIV is defined such that REQUIV = 1/fC1. Thus the equivalent circuit for the switched-capacitor network is as shown in Figure 4. The LT1054 has the same switching action as the basic switched-capacitor building block. Even though this simplification doesn’t include finite switch on-resistance and output voltage ripple, it provides an intuitive feel for how the device works. These simplified circuits explain voltage loss as a function of frequency (see Typical Performance Characteristics). As frequency is decreased, the output impedance will eventu- f C1 C2 RL V2 LT1054 • F03 V1 Figure 3. Switched-Capacitor Building Block C2 RL REQUIV REQUIV = V2 LT1054 • F04 V1 1 fC1 Figure 4. Switched-Capacitor Equivalent Circuit ally be dominated by the 1/fC1 term and voltage losses will rise. Note that losses also rise as frequency increases. This is caused by internal switching losses which occur due to some finite charge being lost on each switching cycle. This charge loss per-unit-cycle, when multiplied by the switch- ing frequency, becomes a current loss. At high frequency this loss becomes significant and voltage losses again rise. The oscillator of the LT1054 is designed to run in the frequency band where voltage losses are at a minimum. Regulation The error amplifier of the LT1054 servos the drive to the PNP switch to control the voltage across the input capaci- tor (CIN) which in turn will determine the output voltage. Using the reference and error amplifier of the LT1054, an external resistive divider is all that is needed to set the regulated output voltage. Figure 5 shows the basic regu- lator configuration and the formula for calculating the appropriate resistor values. R1 should be chosen to be |
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