Your everyday 7805 regulator is not the best choice for powering accurate measurement circuits and A/D converters, mainly because it generates too much noise, and exhibits spurious behavior at power on. Taking our Universal Precision Measuring Interface as an example we have a type MCP3421 A/D converter with a resolution of 18 bits. To be able to exploit the high resolution to the last bit, the supply rail must be absolutely stable and free of noise. In addition, the supply voltage should rise slowly when turned on, allowing the components in the measuring circuit to stabilize in terms of bias voltages and temperature. Of course, that can also be accomplished by using a software timer, but doing so has an effect on a couple of components only.
The circuit described here meets all conditions mentioned and can easily serve as a replacement for an ordinary 7805, because the circuit board has about the same size, and the connections are identical. That does mean however that everything got designed in SMD technology due to limited space.
The regulator used is a MIC2941 from Micrel. It’s is a low-dropout regulator in which the output voltage is set using a resistance divider, just as with an LM317. The design is simple but effective. The supply voltage is set by (R1 + R2)/R3, resulting in 5 V here. Diode D4 serves as polarity protection. Furthermore, a bunch of capacitors is present for decoupling and noise suppression. At the output, an EMI filter is included (FL1). The DSS6NE52A222Q55B is a 3-pin component from Murata, containing two coils separated by a capacitor to ground. The delayed appearance of the supply voltage is accomplished by capacitor C3. When the supply is switched on, initially the voltage at junction R2/R3 remains at virtually 0 V. Next, the capacitor is charged via R3 charged in an about 20 ms, causing the output voltage to rise slowly (see screen dump). Diodes D1 and D2 prevent negative voltage ending up at the regulation input, causing the capacitor to be discharged via R2.
The circuit can deliver an output current of at least 1 A. With no cooling, however, a few tens of mA are possible at an input voltage of 12 V. The PCB artwork is available as a free download of the Elektor website .