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LTC1046C Datasheet(PDF) 5 Page - Linear Technology |
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LTC1046C Datasheet(HTML) 5 Page - Linear Technology |
5 / 12 page 5 LTC1046 S APPLICATI I FOR ATIO Rewriting in terms of voltage and impedance equivalence, I VV fC VV REQUIV = () = 12 11 12 – / – . A new variable, REQUIV, has been defined such that REQUIV = 1/fC1. Thus, the equivalent circuit for the switched capacitor network is as shown in Figure 3. As frequency is decreased, the output impedance will eventually be dominated by the 1/fC1 term and power efficiency will drop. The typical curves for power effi- ciency versus frequency show this effect for various capaci- tor values. Note also that power efficiency decreases as frequency goes up. 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 switching frequency, becomes a current loss. At high frequency this loss becomes significant and the power efficiency starts to decrease. LV (Pin 6) The internal logic of the LTC1046 runs between V+ and LV (Pin 6). For V+ greater than or equal to 3V, an internal switch shorts LV to GND (Pin 3). For V+ less than 3V, the LV pin should be tied to ground. For V+ greater than or equal to 3V, the LV pin can be tied to ground or left floating. OSC (Pin 7) and BOOST (Pin 1) The switching frequency can be raised, lowered or driven from an external source. Figure 5 shows a functional diagram of the oscillator circuit. Figure 3. Switched Capacitor Equivalent Circuit Figure 4. LTC1046 Switched Capacitor Voltage Converter Block Diagram Figure 5. Oscillator Examination of Figure 4 shows that the LTC1046 has the same switching action as the basic switched capacitor building block. With the addition of finite switch ON resistance and output voltage ripple, the simple theory, although not exact, provides an intuitive feel for how the device works. For example, if you examine power conversion efficiency as a function of frequency (see typical curve), this simple theory will explain how the LTC1046 behaves. The loss, and hence the efficiency, is set by the output impedance. C2 REQUIV = 1046 F03 V2 V1 RL REQUIV 1 fC1 1046 F04 CAP+ (2) CAP– (4) GND (3) VOUT (5) V+ (8) LV (6) 3x (1) OSC (7) OSC +2 CLOSED WHEN V+ > 3.0V C1 C2 BOOST SW1 SW2 φ φ + OSC (7) 1046 F05 LV (6) BOOST (1) ∼14pF I 2I I 2I V+ SCHMITT TRIGGER |
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