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MAX1044 Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX1044 Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 12 page a function of oscillator frequency and the capacitor value. Oscillator frequency, in turn, is influenced by temperature and supply voltage. For example, with a 5V input voltage and 10µF charge-pump capacitors, the output resistance is typically 50 Ω. Thus, the output voltage is about -5V under light loads, and decreases to about -4.5V with a 10mA load current. Minor supply voltage variations that are inconsequential to digital circuits can affect some analog circuits. Therefore, when using the MAX1044/ICL7660 for powering sensitive analog circuits, the power-supply rejection ratio of those circuits must be considered. The output ripple and output drop increase under heavy loads. If necessary, the MAX1044/ICL7660 out- put impedance can be reduced by paralleling devices, increasing the capacitance of C1 and C2, or connect- ing the MAX1044’s BOOST pin to V+ to increase the oscillator frequency. Inrush Current and EMI Considerations During start-up, pump capacitors C1 and C2 must be charged. Consequently, the MAX1044/ICL7660 devel- op inrush currents during start-up. While operating, short bursts of current are drawn from the supply to C1, and then from C1 to C2 to replenish the charge drawn by the load during each charge-pump cycle. If the voltage converters are being powered by a high- impedance source, the supply voltage may drop too low during the current bursts for them to function prop- erly. Furthermore, if the supply or ground impedance is too high, or if the traces between the converter IC and charge-pump capacitors are long or have large loops, switching noise and EMI may be generated. To reduce these effects: 1) Power the MAX1044/ICL7600 from a low-impedance source. 2) Add a power-supply bypass capacitor with low effective series resistance (ESR) close to the IC between the V+ and ground pins. 3) Shorten traces between the IC and the charge-pump capacitors. 4) Arrange the components to keep the ground pins of the capacitors and the IC as close as possible. 5) Leave extra copper on the board around the voltage converter as power and ground planes. This is easily done on a double-sided PC board. Efficiency, Output Ripple, and Output Impedance The power efficiency of a switched-capacitor voltage converter is affected by the internal losses in the con- verter IC, resistive losses of the pump capacitors, and conversion losses during charge transfer between the capacitors. The total power loss is: The internal losses are associated with the IC’s internal functions such as driving the switches, oscillator, etc. These losses are affected by operating conditions such as input voltage, temperature, frequency, and connec- tions to the LV, BOOST, and OSC pins. The next two losses are associated with the output resistance of the voltage converter circuit. Switch losses occur because of the on-resistances of the MOSFET switches in the IC. Charge-pump capacitor losses occur because of their ESR. The relationship between these losses and the output resistance is as follows: where: and fOSC is the oscillator frequency. R 1 (f / 2) x C1 4 2R ESR ESR OUT OSC SWITCHES C1 C2 ≅+ + ()+ P P I x R OUT 2 OUT += ∑ P = P +P +P +P Switched-Capacitor Voltage Converters 8 _______________________________________________________________________________________ MAX1044 ICL7660 4 3 10µF VOUT = -(V+) 10µF V+ V+ CMOS or TTL GATE 10k Ω REQUIRED FOR TTL 2 1 5 6 7 8 Figure 7. External Clocking LOSS INTERNAL LOSSES SWITCH LOSSES PUMP CAPACITOR LOSSES CONVERSION LOSSES PUMP CAPACITOR LOSSES SWITCH LOSSES |
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