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CAT661EZD8 Datasheet(PDF) 7 Page - Catalyst Semiconductor |
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CAT661EZD8 Datasheet(HTML) 7 Page - Catalyst Semiconductor |
7 / 16 page CAT661 7 Doc. No. 5003, Rev. J The 1/FC1 term can be modeled as an equivalent impedance REQ. A simple equivalent circuit is shown in figure 4. This circuit does not include the switch resistance nor does it include output voltage ripple. It does allow one to understand the switch-capacitor topology and make prudent engineering tradeoffs. For example, power conversion efficiency is set by the output impedance, which consists of REQ and switch resistance. As switching frequency is decreased, REQ, the 1/FC1 term, will dominate the output impedance, causing higher voltage losses and decreased efficiency. As the frequency is increased quiescent current increases. At high frequency this current becomes significant and the power efficiency degrades. The oscillator is designed to operate where voltage losses are a minimum. With external 150 µF capacitors, the internal switch resistances and the Equivalent Series Resistance (ESR) of the external capacitors determine the effective output impedance. A block diagram of the CAT661 is shown in figure 5. Figure 3. Switched-Capacitor Building Block Figure 4. Switched-Capacitor Equivalent Circuit APPLICATION INFORMATION Circuit Description and Operating Theory The CAT661 switches capacitors to invert or double an input voltage. Figure 3 shows a simple switch capacitor circuit. In position 1 capacitor C1 is charged to voltage V1. The total charge on C1 is Q1 = C1V1. When the switch moves to position 2, the input capacitor C1 is discharged to voltage V2. After discharge, the charge on C1 is Q2 = C1V2. The charge transferred is: ∆Q = Q1 - Q2 = C1 × (V1 - V2) If the switch is cycled "F" times per second, the current (charge transfer per unit time) is: I = F × ∆Q = F × C1 (V1 - V2) Rearranging in terms of impedance: I= (V1-V2) V1-V2 (1/FC1) REQ = V1 C1 C2 RL V2 V1 C2 RL V2 REQ REQ = 1 FC1 |
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