zashi
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Specific Switching Regulator Used in Model B+?

Mon Nov 10, 2014 5:36 pm

I have not been able to determine what switching regulator is used on the Model B+. Looking at the chips near the power connector and googling the markings proved unfruitful.

I would like to know so I can check the tolerances for source voltage.

Does anyone have the datasheet for the regulator or in the very least the BoM so I can find it myself?

plugwash
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Re: Specific Switching Regulator Used in Model B+?

Mon Nov 10, 2014 6:28 pm

Theres a part number on the partial schematic that was released for the B+.

However from the presense of various zero ohm/not mounted components on the schematic I suspect that they have designed the board to be usable with one or more substitues to minimise their exposure to supply fluctuations.

zashi
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Re: Specific Switching Regulator Used in Model B+?

Mon Nov 10, 2014 6:50 pm

This partial schematic?

https://cdn.shopify.com/s/files/1/0315/ ... tralia.pdf

I do not see a switching reg in there. I see a mosfet (DMG2305UXP) and a 2-in-1 package pnp transistor pack (DMMT5401).

Am I not looking at the right schematics or the right spot?

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Re: Specific Switching Regulator Used in Model B+?

Mon Nov 10, 2014 7:06 pm

Putting the numbers read off the top of the package into Google (plus some keywords) often works wonders.

Note that if you do this, the answer you get will not be canonical. The B+ was designed with the ability to substitute parts in mind - hence the unpopulated resistor sites around the switching regulator for a device that could potentially require a different feedback divider.
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mahjongg
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Re: Specific Switching Regulator Used in Model B+?

Mon Nov 10, 2014 7:22 pm

U3 is a PAM2306AYPKE

plugwash
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Re: Specific Switching Regulator Used in Model B+?

Mon Nov 10, 2014 7:22 pm

Hmm, it seems my memory was faulty, the partial schematic shows the switching reguator (it's top-center in the partial schematic) but doesn't give a part number for it.

I'm sure I found a datasheet somehow but I don't remember exactly how. Maybe I just looked at the chip and either googled for the code (note: small SMT parts often have a package marking code that differs from the main part number for the device) or looked it up on the IC manufacturers website..

zashi
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Re: Specific Switching Regulator Used in Model B+?

Mon Nov 10, 2014 7:43 pm

Thanks everyone. I see PAM2306AYPKE can take up to 5.5 v. So it looks like I will be throwing together my own regulator board.

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TimG
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Re: Specific Switching Regulator Used in Model B+?

Mon Nov 10, 2014 8:01 pm

LadyAda made a guess that the regulator is an RT8020A: https://learn.adafruit.com/introducing- ... wer-supply

That regulator has a low dropout mode, where the voltage drop is determined just by the resistance of the regulator P-MOSFET (0.28 Ohms). My model-B draws 400mA under load with no peripherals. Assuming the B+ draws the same current (when the input voltage is so low that the regulator is no longer switching), the dropout should be about 100mV.

In other words, a B+ ought to run with just 3.4V on the "5V" power input. (To be tested when my B+ arrives)

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mahjongg
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Re: Specific Switching Regulator Used in Model B+?

Mon Nov 10, 2014 9:04 pm

yes, the dual switcher chip works with voltages as low as 3.5V, but do note that the Broadcom SoC has its own integral switching downconverter that takes 5V input, and down converts it to whatever the current core voltage is at that moment (around 1.2 to 1.5V if memory servers me well).

Only think I know about it is that it is supposedly designed to be fed from batteries (say 3 x 1.5V = 4.5V).

ask Gert van Loo, perhaps he can tel you more.

plugwash
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Re: Specific Switching Regulator Used in Model B+?

Mon Nov 10, 2014 9:12 pm

The compute module design guide gives a voltage range of 2.3-5V for vbat it also gives a "tolerance" of 5%. If we interpret the range as a range of nominal voltages and apply the tolerance to either end of that range we would get an overall range of 2.185V to 5.25V

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