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ADM660ARUZ-REEL7 Datasheet(PDF) 7 Page - Analog Devices |
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ADM660ARUZ-REEL7 Datasheet(HTML) 7 Page - Analog Devices |
7 / 11 page REV. ADM660/ADM8660 –7– TEMPERATURE – C –40 100 –20 0 20 406080 160 0 140 80 60 40 20 120 100 LV = GND FC = V+ C1, C2 = 2.2 F TPC 13. Charge-Pump Frequency vs. Temperature TEMPERATURE – C 60 0 –40 100 –20 0 20 406080 50 40 30 20 10 V+ = +1.5V V+ = +3V V+ = +5V TPC 14. Output Resistance vs. Temperature GENERAL INFORMATION The ADM660/ADM8660 is a switched capacitor voltage con- verter that can be used to invert the input supply voltage. The ADM660 can also be used in a voltage doubling mode. The voltage conversion task is achieved using a switched capacitor technique using two external charge storage capacitors. An on- board oscillator and switching network transfers charge between the charge storage capacitors. The basic principle behind the voltage conversion scheme is illustrated in Figures 1 and 2. + + V+ S1 S2 S3 S4 CAP+ CAP– C1 C2 OUT = –V+ Φ1 Φ2 + 2 OSCILLATOR Figure 1. Voltage Inversion Principle + + V+ S1 S2 S3 S4 CAP+ CAP– C1 C2 VOUT = 2V+ Φ1 Φ2 + 2 OSCILLATOR V+ Figure 2. Voltage Doubling Principle Figure 1 shows the voltage inverting configuration, while Figure 2 shows the configuration for voltage doubling. An oscillator generating antiphase signals φ1 and φ2 controls switches S1, S2, and S3, S4. During φ1, switches S1 and S2 are closed charging C1 up to the voltage at V+. During φ2, S1 and S2 open and S3 and S4 close. With the voltage inverter configuration during φ2, the positive terminal of C1 is connected to GND via S3 and the negative terminal of C1 connects to VOUT via S4. The net result is voltage inversion at VOUT wrt GND. Charge on C1 is trans- ferred to C2 during φ2. Capacitor C2 maintains this voltage during φ1. The charge transfer efficiency depends on the on- resistance of the switches, the frequency at which they are being switched, and also on the equivalent series resistance (ESR) of the external capacitors. The reason for this is explained in the following section. For maximum efficiency, capacitors with low ESR are, therefore, recommended. The voltage doubling configuration reverses some of the con- nections, but the same principle applies. Switched Capacitor Theory of Operation As already described, the charge pump on the ADM660/ADM8660 uses a switched capacitor technique in order to invert or double the input supply voltage. Basic switched capacitor theory is discussed below. A switched capacitor building block is illustrated in Figure 3. With the switch in position A, capacitor C1 will charge to voltage V1. The total charge stored on C1 is q1 = C1V1. The switch is then flipped to position B discharging C1 to voltage V2. The charge remaining on C1 is q2 = C1V2. The charge transferred to the output V2 is, therefore, the difference between q1 and q2, so ∆q = q1–q2 = C1 (V1–V2). V1 AB C1 C2 RL V2 Figure 3. Switched Capacitor Building Block As the switch is toggled between A and B at a frequency f, the charge transfer per unit time or current is: I = f (∆q) = f (C1)(V1– V 2) Therefore, I = (V1– V 2)/(1 / fC1) = (V1– V 2)/(R EQ ) where REQ = 1/fC1 The switched capacitor may, therefore, be replaced by an equivalent resistance whose value is dependent on both the capacitor size and the switching frequency. This explains why lower capacitor values may be used with higher switching frequencies. It should be remembered that as the switching frequency is increased the power consumption will increase due to some charge being lost at each switching cycle. As a result, at high frequencies, the power efficiency starts decreasing. Other losses include the resistance of the internal switches and the equivalent series resistance (ESR) of the charge storage capacitors. V1 REQ REQ = 1/fC1 V2 C2 RL Figure 4. Switched Capacitor Equivalent Circuit C |
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