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I wish to interface SSD1309 which is an OLED driver.

It's rather straightforward, however I do have board which interfaces this driver (just it doesn't fit to my needs and I wish to create a custom one).

SSD1309's datasheet has clear requirements for power:

enter image description here

My current setup provides 3.3V to this board, so it must do something to pump it up to at least 7V for VCC.

The most interesting part on this interface board is this (please not my red cable is supplying 3.3V to this board):

enter image description here

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Those 662K-s are voltage regulators based on a YouTube guy:

enter image description here

And here comes "what-the-heck" feeling #1: paralleling two regulators? Why?

Then "what-the-heck" feeling #2: continues here, as SG4KL Google has no idea about. What is this chip?

Okay, so here's what I found:

  • top-right corner of this board seems a "huge" powerplane (supplied from my red wire)
  • U3 and U4 are 662K step-down regulators (datasheet)
  • U3 and U4 are paralleled, to achieve higher current in order to supply VDD when input is higher than 3.3V. This doesn't makes too much sense as based on datasheet SSD1309 can draw maximum of 110uA from VDD and 662K can provide 200mA (!) - why are these paralleled?

Investigating a bit further:

  • 662Ks' output seems creating a 3.3V plane aroung C1.
  • Guess: SG4KL must be a step-up converter to provide 7V to SSD1309.
  • Conclusion: SSD1309 draws maximum 580uA from VCC, so reason for paralleling 662Ks are still unknown.

Finally I found "SG4xx", here is its datasheet:

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And its typical application:

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D1 on my board must be this D1 Schottky diode, which makes sense.

L1 however recommended as 10uH, and the marking on my board's inductance is a bit blurry, but starts with "22": might be 220 or 228 (very unlikely, as it's about 3x3mm).

So basically I have two questions:

  1. why do they applied 2x 662K voltage regulator if one would have been perfectly enough?
  2. why do they used 22uH inductor instead of 10uH? SSD1309's current draw is way lower than this converter can supply, so I can't see any reason behind increasing L to 22uH.
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  • \$\begingroup\$ Did you measure voltages on different components to verify your assumptions how the components are connected? Also note that your topic simply says interfacing the SSD1309 chip, but what you are really doing is reverse-engineering a random carrier board bought from random e-commerce site which lets you interface the display with the IC. \$\endgroup\$
    – Justme
    Commented May 3, 2023 at 16:25
  • \$\begingroup\$ I measured the connections, yes. \$\endgroup\$
    – Daniel
    Commented May 3, 2023 at 16:33
  • \$\begingroup\$ What about the voltages? \$\endgroup\$
    – Justme
    Commented May 3, 2023 at 16:47
  • \$\begingroup\$ 12V of the boost, 3.3V of the buck. \$\endgroup\$
    – Daniel
    Commented May 3, 2023 at 17:59

2 Answers 2

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On page 8 of the SG4KL datasheet, it explains the behavior of this chip. It is a switching converter, basically it turns on and off using the inductor current as a way to know when to switch. This current value is a default of 400mA, but raising the inductance is one way to lower this current value. My best guess is they used 22uH instead of 10uH to lower that current limit down to somewhere between 300-350mA, and they use two 662K chips because they need to provide that much current.

So to sum up, the SG4KL draws power and the current through the inductor rises over time as the feedback voltage rises. Once it hits the current limit set by the Peak Current Control equation on Page 8 of the datasheet, it switches off until the feedback voltage falls back below the 1.227 threshold, at which point it switches on again and the whole thing repeats like that. The 622K chips provide that current, which is why there are two of them.

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The U3 and U4 are 3.3V LDO linear regulators, capable of up to 200mA current per regulator.

So it requires more than 3.3V in to make 3.3V output, up to about 4V to work properly.

The 3.3V output regulator do not only drive the LCD 3.3V supply.

The regulators drive the input of the boost regulator too.

The boost converter takes in current in bursts that go at least to 400mA, and even more depending on input voltage and inductor value.

The inductor value is not fixed to 10uH, it is just an example. There are formulas in the data sheet to calculate a good inductor value, based on input and output voltage and other parameters that the inductor must have to properly work in the circuit. The converter can operate with inductors between 2.2uH and 47uH.

The higher the inductor value, the lower peak current it takes. A smaller inductor value lowers the efficiency.

So, two regulators are paralleled to provide the peak currents needed by the boost converter, and the 22uH was found to be a suitable value that works.

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