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SP829 Datasheet(PDF) 11 Page - Sipex Corporation |
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SP829 Datasheet(HTML) 11 Page - Sipex Corporation |
11 / 16 page 11 SP828DS/03 SP828/829 High Efficiency Voltage Inverters © Copyright 1999 Sipex Corporation Negative Voltage Converter The typical operating circuit for the SP828/829 devices is a negative voltage converter. Refer to Figure 19. This circuit is used to obtain the Typical Performance Characteristics found in Figures 1 to 18 (unless otherwise noted). Voltage Inverter with the Load from V OUT to VIN A designer can find the most common application for the SP828/829 devices in Figure 20 as a voltage inverter. The only external components needed are 3 capacitors: the flying capacitor, C1, the output capacitor, C2, and the bypass capacitor, C3 (if necessary). Driving Excessive Loads The output should never be pulled above ground. A designer should implement a Schottky diode (1N5817) from OUT to GND when driving heavy loads where a higher supply is sourcing current into OUT. Refer to Figure 23 for this circuit connection. APPLICATION INFORMATION For the following applications, C1 = C2 = 10 µF for the SP828 and C1 = C2 = 3.3 µF for the SP829. Capacitor Selection Low ESR capacitors are needed to obtain low output resistance. Refer to Table 1 for some suggested low ESR capacitors. The output resistance of the SP828/829 devices is a function of the ESR of C1 and C2. This output resistance can be determined by the equation previously provided in the Efficiency section: R OUT ≈ 4 x (2 x RSWITCHES + ESRC1) + ESR C2 + 1 fOSC x C1 , where R OUT is the circuit output resistance, R SWITCHES is the internal resistance of the MOSFET switches, ESR C1 and ESRC2 are the ESR of their respective capacitors, and f OSC is the oscillator frequency. This term with f OSC is derived from an ideal switched-capacitor circuit as seen in Figure 21. Minimizing the ESR of C1 and C2 will minimize the total output resistance and will improve the efficiency. Flying Capacitor Decreasing flying capacitor, C1, values will increase the output resistance of the SP828/829 devices while increasing C1 will reduce the output resistance. There is a point where increasing C1 will have a negligible effect on the output resistance due to the the domination of the output resistance by the internal MOSFET switch resistance and the total capacitor ESR. Output Capacitor Increasing output capacitor, C2, values will decrease the output ripple voltage. Reducing the ESR of C2 will reduce both output ripple voltage and output resistance. If higher output ripple can be tolerated in designs, smaller capacitance values for C2 should be used with light loads. The following equation can be used to calculate the peak-to-peak ripple voltage: VRIPPLE = 2 x IOUT x ESRC2 + IOUT fOSC x C2 . Input Bypass Capacitor The bypass capacitor at the input pin will reduce AC impedance and the impact of any of the SP828/829 devices' switching noise. It is recommended that for heavy loads a bypass capacitor approximately equal to the flying capacitor, C1, be used. For light loads, the value of the bypass capacitor can be reduced. When loading the SP828/829 devices from IN to OUT, the input current remains constant (disregarding any spikes due to internal switching). Implementing a 0.1 µF bypass capacitor should be sufficient. When loading the SP828/829 devices from OUT to GND, the current from the supply will flow into the input for half of the cycle and will be zero for the other half of the cycle. Designers should implement a large bypass capacitor (C3 = C1) if the supply has a high AC impedance. |
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