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50 lines
2.0 KiB
Markdown
50 lines
2.0 KiB
Markdown
A separate [Sylergy SY8088][1] Buck DC/DC SMPS chip is used to provide
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the DDR2 +1V8 DDR2 DRAM power.
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This is because the AXP20x is originally the PMU (Power Management
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Unit) used by most Allwinner SoCs (A10, A13 and A20), which do not
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integrate SDRAM, so the board designer has a wide choice of memory
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option: DDR2, DDR3, DDR3L, LPDDR3, LPDDR4 with various voltage
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requirements.
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But no specific PMIC was created for the Allwinner V3s used in the
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FunKey device which however integrates a fixed SiP (System In Package)
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512Mbit (64MB) DDR2 SDRAM.
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We thus have to design a separate SMPS (DC-DC) power supply for
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providing the +1.8V 1A required for the DDR2 DRAM power supply.
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For this purpose, we followed closely the [Allwinner Reference
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Design][2].
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Here is the corresponding DRAM Power schematics:
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Nothing very fancy here: the SMPS chip **U4** has its required input
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filter capacitor **C37** and output capacitors **C65** and **C73**.
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The low-profile ferrite-core power inductor **L6** (rated with a
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saturation current of 1.76A and low < 0.1 Ω resistance) provides the
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DC-DC energy storage element.
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The **R20**/**R23** precision voltage divider provides the required
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+0.6V feedback voltage from the +1.8V output voltage by having a 1/3
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resistor ratio.
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The last component is a pull-up resistor **R19** which ties the SMPS
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chip enable input to its active level permanently. The pull-up voltage
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is +3.0V (just as in the original reference design), probably as it is
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the next higher voltage available, in order to limit the current in it
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to its lowest possible value.
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[1]: https://github.com/FunKey-Project/FunKey-S-Hardware/blob/master/Datasheets/C79313_SY8088AAC_2017-03-29.PDF
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[2]: https://github.com/Squonk42/V3s_Documentation/blob/master/V3S_CDR_STD_V1_0_20150514.pdf
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[3]: https://datasheet.lcsc.com/szlcsc/Silergy-Corp-SY8088AAC_C79313.pdf
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[4]: https://datasheet.lcsc.com/szlcsc/1901241230_LOWPOWER-LP3220S-AB5F_C324565.pdf
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[5]: https://www.diodes.com/assets/Datasheets/AP3418.pdf
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--8<--
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includes/glossary.md
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--8<--
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