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MAX868 Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX868 Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 12 page Regulated, Adjustable -2x Inverting Charge Pump 8 ___________________________________ __________Applications Information Low-Output-Voltage Operation Since the difference between the voltage of the series- connected flying capacitors and the output voltage must be dissipated within the device, the MAX868’s efficiency is very similar to that of a linear regulator. Estimate efficiency using the following equation: where k is a constant equal to 2 for the standard con- figuration of Figure 5 and equal to 1 for the circuit of Figure 6. This equation’s denominator is the voltage resulting from the series connection of the flying capac- itors (-2 x VIN, as shown in Figure 3b), while its numera- tor is simply the regulated output voltage. For applications in which the output voltage will not be more negative than - |VIN|, the efficiency can be doubled using the circuit of Figure 6, as compared to the circuit of Figure 5. In Figure 6, a single flying capacitor is con- nected between C2+ and C1-, with C2- and C1+ left unconnected. Furthermore, doubling the flying capaci- tor to provide the same flying capacitance as the stan- dard configuration (i.e., setting CF = C1 + C2) provides the same load-current capability as the standard con- figuration and reduces the MAX868’s open-loop output resistance by a factor of two, due to the reduction in the number of switches in the current path. Layout and Grounding Proper layout is important to obtain optimal perfor- mance. Connect GND to PGND together using the shortest trace possible, and similarly connect these pins to the ground plane. Mount all capacitors as close to the MAX868 as possible, keeping traces short to minimize parasitics. Keep all connections to the FB pin as short as possible. Specifically, locate R1 and R2 next to FB (Figures 7 and 8). Should it become neces- sary in the final layout, leave room to parallel a feed- forward capacitor across R1. η V k x V | | OUT IN = MAX868 C1+ IN PGND SHDN GND FB R2 500k R1 750k OUT 0.1 µF 1 µF 10 µF VOUT = -7.5V VIN = 5V 0.1 µF C1- C2+ C2- Figure 5. Standard Configuration for Generating an Output Voltage up to -2 x VIN MAX868 C2+ IN PGND SHDN GND FB OUT CF = 0.2µF * * 1 µF 10 µF VOUT = -3.3V AT 20mA VIN = 5V C2- C1+ C1- *C1+ AND C2- MUST BE LEFT UNCONNECTED. R2 500k R1 330k Figure 6. Alternative Configuration for |VOUT| ≤ VIN Chip Information TRANSISTOR COUNT: 96 SUBSTRATE CONNECTED TO IN |
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Similar Description - MAX868 |
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