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VREF regulator for DDR memory
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technix:
How do I generate a stable and strong VREF for a point to point DDR3 bus, from an Allwinner A13 to a Chinese-made DDR3 chip?

The A13 reference schematics used two separate resistor dividers for that, but since I am using a fairly new DRAM brand their current draw can overwhelm those 1k resistor dividers, and being limited to 0603 or bigger SMD parts and can not use either blind vias or microvias means I don't get a lot of real estate among all the vias and bypass capacitors. What I have in mind now is a single VREF regulator placed somewhere in the middle of the bus, probably on the back side, to generate a strong enough VREF for both the DRAM chip and A13 to share: the reference voltage is generated by dividing VDDMEM using two 10k resistors, and an op amp like LMV321 is used to push the current. Questions:

1) Is this overkill?
2) Do I need a push-pull helper stage at the output of the op amp? (BAT54S + MMBT8050 + MMBT8550 - I am using transistors supporting up to 1.5A here.)
Benta:
There are plenty of DDR VREF ICs out there, even combined with termination. TI would be a good candidate.
technix:

--- Quote from: Benta on October 25, 2018, 05:13:11 pm ---There are plenty of DDR VREF ICs out there, even combined with termination. TI would be a good candidate.

--- End quote ---
Being a point-to-point connection (a 16-bit controller with up to 512MB space matched to a 16-bit 512MB chip) I think using a VREF/VTT regulator would be overkill and expensive. LMV321 and MMBT8050/MMBT8550 are all dime a dozen cheap parts.
technix:

--- Quote from: blueskull on October 26, 2018, 02:53:07 am ---Are you looking at VTT or VREF? VREF is a low current load. Regardless how immature the DDR is, as long as it complies with JEDEC standard, its VREF should be able to be supplied by resistors. Alternatively, just use a standard shunt regulator with a bias resistor.

--- End quote ---
I couldn't find shunt regulators but LMV321 is cheap enough.


--- Quote from: blueskull on October 26, 2018, 02:53:07 am ---You don't need VTT as for point-to-point topology. DDR3 allows termination-less operation.

--- End quote ---
I am not using VTT at all.


--- Quote from: blueskull on October 26, 2018, 02:53:07 am ---OT: where did you get the presumably Unis RAM chips?

--- End quote ---
I asked them for samples. They will need to know what kind of controller you are using (usually the main chip's make and model) and what general market sector you are targeting.
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