These analog supply rails can drive precision op amps, such as the MAX400 and OP-07, whose input common-mode-rejection and output-range specifications are 2 to 3V within the supply rails. Thus, the rails are good enough if the -5V output is less than -3V and the 10V output is more than 8V.
Accordingly, the component choices in Figure 1, such as the lossy RC output filters and silicon signal diodes in place of Schottky diodes, provide for minimal cost and ripple rather than maximum regulation. The 4.7-mF capacitors, C4 and C7, can be high-ESR, commodity, multilayer-ceramic types with 16V ratings, a 1206 case, and a Y5V dielectric, such as the 1206YG475ZAT2A from AVX Corp (www.avxcorp.com).
The output ripple varies with the supply voltage and output load. Operating with an input voltage of 1.8V, the circuit produces ripple amplitudes over the load of 2 to 10 mV p-p for the 10V rail and 15 to 30 mV p-p for the -5V rail. By increasing C5 and C8 to 2.2 mF, you can reduce these ripple levels to 1 and 5 mV, respectively.
With no load on the auxiliary rails, the 5V output’s maximum available load current rises with input supply voltage (Figure 2a). You can increase this available output power by replacing D1 with a lower loss Schottky diode. At an input of 1.8V, the output power available for the three rails (loaded with 10 mA at 5V, 5 mA at 10V, and 5 mA at -5V) is somewhat less than 125 mA; with a 5-mA load, the 10V and -5V outputs are approximately 9.75 and -3.7V, respectively (Figure 2b). A 2.7V input based on three flat cells yields around 275 mW.
The MAX858 operates with peak inductor currents of 125 mA. If you need more current, you can replace this IC with related parts that have 500 mA and 1A ratings. Note that these changes require different passive components; the inductor and main output diode ratings must match the inductor’s peak current. The charge pumps can remain the same if their output currents don’t change much.
You can also retain the cheap, common, commodity dual diodes D1, D2, and D3, but detail specifications vary, so look carefully at data sheets for the part you actually use. For example, the BAV70’s dc forward current, IF, and peak forward surge current, IFSM for 1 msec, differ among manufacturers. For the Motorola (www.motorola.com) part, IF=200 mA, and IFSM=500 mA. For National Semiconductor (www.national.com), IF=600 mA, and IFSM=2A. For Philips (www.philips.com), IF=125 mA, and IFSM=4A, and for Vishay-Siliconix (www.siliconix.com), IF=250 mA, and IFSM=4.5A. This caution is advisable in all second-source considerations. (DI #2200).
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