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123 lines
5.1 KiB
Markdown
123 lines
5.1 KiB
Markdown
## Capacitor Usage
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So far, we already encountered capacitors for many different usages:
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### Load Capacitors
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We have seen load capacitors used with the 2 crystals in the
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discussion about [CPU][1].
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A quartz crystal always provides both series and parallel resonance,
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the series resonance being a few kilohertz lower than the parallel
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one.
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Crystals below 30 MHz like ours are generally operated between series
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and parallel resonance, which means that the crystal appears as an
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inductive reactance in operation, this inductance forming a
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**parallel resonant circuit** with externally connected parallel
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"load" capacitance. Any small additional capacitance added in parallel
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with the crystal pulls the frequency lower in the range between the
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series and parallel resonance frequencies, insuring crystal startup
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and stable operation.
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For modern circuits, these load capacitors have a typical small value
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< 20 pF.
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### Bulk Capacitors
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Bulk capacitors are used to prevent a power supply from dropping too
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far during the periods when current is not available. At the same
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time, they help to reduce the power supply voltage ripples by
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smoothing their output voltage.
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Many such capacitors are used at both the input and output of the
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numerous linear and switched mode power supplies in the [PMIC
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discussion][2].
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The main bulk capacitor value is generally high (some µF), but there
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may be smaller parallel capacitors added for stability.
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### Coupling Capacitors
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As you probably know, capacitors are made of 2 parallel conductive
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electrodes separated by a (thin) isolating dielectric material (even
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if these electrodes are rolled or layered to reduce the component
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size). Thus by construction, no DC (Direct Current) can flow from one
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electrode to the other, but by influence using the electric field, AC
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(Alternative Current) still can go through. This is how coupling
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capacitors are used to link 2 circuits while removing any DC bias
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voltage on one side or the other of the capacitor.
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We use such a coupling capacitor in the [Audio schematic
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description][3] for feeding the audio power amplifier from the CPU
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audio output.
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### Filter Capacitors
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We have seen many examples where capacitors are used within passive
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filter circuits along with resistors or inductors, mainly to remove
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unwanted frequencies from a power supply or a signal.
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### Decoupling (Bypass) Capacitors
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We use some decoupling capacitors in the [buttons circuit][4].
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Active components such as transistors and chips are connected to their
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power supplies through conductors featuring a (small) common impedance
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made up of complex (resistive, capacitive and inductive)
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value. Because of these parasitic components, a device that suddenly
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draws some current in spikes will generate a drop in its voltage power
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supply. If many devices are sharing the same power supply and
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impedance, the state of one device will be coupled to the other ones
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through the common impedance of the power supply conductors and may
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affect their operation.
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In order to decouple the devices, capacitors placed as close as
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possible to the device power supply input pins are used, which act as
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local energy storage. These capacitors are also named "bypass
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capacitors" as they shunt transient energy from the power supplies
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past the device to be decoupled, right to the GND return path.
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There may be different capacitors values placed on the same power
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supply pins in order to filter transients at different frequencies:
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the bigger the capacitor value, the lower the frequency. A typical
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value is 100 nF, and values from 1 µF to 10 µF are used for lower
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frequencies and / or higher current draws, while lower values of a few
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nF are used for filtering higher frequencies.
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In essence, decoupling capacitors are not very different in their
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function from bulk capacitors: the only difference is one of scale,
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both of current and of transient duration. Bulk capacitors deal with
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large currents and periods of 10s of ms, whereas decoupling capacitors
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are used for much lower currents and much briefer periods (typically
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10s of ns for TTL or CMOS devices) .
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## Schematics
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The last part of the FunKey schematics merely contains only decoupling
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capacitors:
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{.lightbox}
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One exception is the Allwinner V3s CPU HPR/HPL circuit which features
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an RC-to-ground circuit between the amplifier and the preamplifier
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input with the resistor **R27** and capacitors **C79** and **C81**, as
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recommended in the [V3s hardware design guide][5].
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The only other remarkable point left in this schematic is the resistor
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divider **R25**/**R28** which provides a reference voltage at half the
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DRAM power supply voltage level, which is used for the integrated DDR2
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DRAM merged drivers and dynamic on-chip termination already discussed
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at the end of the previous [CPU schematic description][6].
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[1]: /developers/hardware/cpu
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[2]: /developers/hardware/power/pmic
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[3]: /developers/hardware/audio
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[4]: /developers/hardware/buttons
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[5]: https://github.com/Squonk42/V3s_Documentation/blob/master/V3s%20hardware%20design%20guide%20V1.0_20150519%20EN%20Non%20Official.pdf
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[6]: /developers/hardware/cpu
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--8<--
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includes/glossary.md
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--8<--
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