VDD capacitors

Owen

As i have been pouring through all the different propellar boards that are available i noticed that almost none of them use the same value capacitors. The propellar demo board uses .1uf before the voltage regulator and 10uf capacitors after the voltage regulator, which as i can figure is to smooth out the power suply, the .1uf being mostly for high frequency from a wall wort right? and then it has a 0.1uf at the VDD at the chip, which it to reduce noise right? but when I look at the DLP-prop it also has .1uf capacitor before the voltage regulator but then has a 10uf capacitor after the regulator and then 0.1uf capacitors at all the VDD inputs to the propellar chip. and the propstick uses 0.1uf before the regulator and then a 10uf after then 2 more 0.1uf capacitors after but no 0.1uf at the chips VDD. and lastly the ...... uses 22uf capacitors before and after the voltage regulators and no .01uf capacitors at VDD.

how importient are the capacitors at VDD? are they necesary if using battery power? and what dictates the use of a capacitor before the voltage regulator.

Thanks in advance for helping someone new to electronics through this.

Owen Hooker

Mike Green

The 0.1uF capacitors ideally should be present between Vdd and Vss at every "significant" chip in any digital logic. For small chips (logic chips as opposed to high current drivers) they could be provided for every pair or quad of chips. The important thing is that they have to be physically very close to the chip. They act as a power source for a few nanoseconds when the impedance of the wiring (traces on the printed circuit board) prevents the power from the power supply from actually getting to the chip.

The larger electrolytic capacitors are generally required by the voltage regulators for stability and their minimum value depends partly on the particular voltage regulator being used. Another factor is the question of what's being powered. If high current devices are to be powered (like servos), a high value electrolytic capacitor is needed to prevent the initial power demands of the motor from pulling down the regulated voltage and causing the microcontroller to reset.

The capacitor at the input of the voltage regulator is partly dependent on the voltage regulator and is partly dependent on whether the regulator is to be at a distance from the unregulated power source (a couple of feet maybe).

Paul Baker

Voltage regulator datasheets provide typical circuits, many of the newer ones specify 0.1u on the input and 10u on the output, the ESR of the output must also be within an acceptable range otherwize the output will be unstable. Some people kick up the value of the output capacitor to provide general output smoothing for the board, but the newer regulators ussually don't like to see more than 10x the reference design. All moderate to large current spiking devices (chip's whose current draw rapidly changes) should have local bypass capacitors very close to thier power supply pins and typically 0.1 to 10u depending on how "spikey" they are.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Paul Baker
Propeller Applications Engineer

Parallax, Inc.