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TL;DR:

  • Using LDO to drop voltage from Lipo to stable 3V
  • LDO Vout can be adjusted with Voltage Divider
  • Lipo from 4.2V to 3.4V, Vout = 3V
  • Lipo below 3.4V, Vout = 2.3V => How to adjust Vout of LDO with Feedback voltage from Lipo

Problem:

Recently, I am doing a small project to create a Rechargeable Lipo Battery Pack for my old Gameboy. Gameboy only accepts power in range 1.8V - 3.2V (as I researched). So that, in my design, I use a LDO AP2127K-ADJ (datasheet) to drop voltage from a Lipo battery to 3V to use with it.

Application Circuit of AP2127-ADJ

Vout is calculated with the formula:

$$V_{\text{OUT}} = V_{\text{REF}}\left(1 + \frac{R_1}{R_2}\right)$$ with \$V_{\text{REF}} = 0.8\text{ V}\$ (according AP2127-ADJ datasheet).

Besides that, on Gameboy machine, there is one Low Power Indicator Led which will be trigger when voltage input from power pack is below 2.3V. I want to use that LED to announce me whenever Lipo battery is at 15% capacity (Lipo at 3.4V). My idea is switch R2 to another value when Lipo reach 3.4V => Vout will be adjusted to 2.3V => Low Power Led will be triggered.

enter image description here

At this moment, I am thinking about using transistors as switchs like below image but don't know how to do it probally (drive Q1, Q2 so at time, only one transistor is opened and another is closed):

  • Lipo from 4.2V to 3.4V, Q1 close, Q2 open => V1 = 2.96V
  • Lipo below 3.4V, Q1 open, Q2 close => V1 = 2.29

Transistors

Could you give me some hint/solution for my problem? Your support is greatly appreciated!

Null
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Minh Bui
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  • 1) what **current** does the Gameboy need? 2) The AP2127 will drop only 0.17 V while the load current is 300 mA, I would simply set the output voltage to 3.2 V(or lower), that's enough for the Gamboy and will work at Vbat = 3.4 V assuming the load current is 300 mA or less. This looks to me like you're trying to solve a problem that isn's actually a problem. – Bimpelrekkie Aug 27 '21 at 10:54
  • Why not use a 1.8 volt output device? – Andy aka Aug 27 '21 at 10:54
  • @Bimpelrekkie my Gameboy need average 200-250mA. Because I use some extra modification like: IPS screen, Sound Amp,.... I just want to drop the output to 2.3V (when Lipo is at 3.4V) to trigger Low Power Led on Gameboy – Minh Bui Aug 27 '21 at 11:02
  • @Andyaka if you 1.8v, the Low Power Indicator LED is always on. I just want it turn on when Lipo reach 3.4V or below – Minh Bui Aug 27 '21 at 11:02
  • *I just want to drop the output to 2.3V* **WHY?** "Wanting it" is not a good reason. Give a technical reason why that would be needed. I can give a reason why it would **NOT** be a good idea: if the Gameboy (or any of the other devices) has a DCDC-converter build in that will start to **draw more current** when you lower the input voltage. Then you would be wasting more power in your LDO. – Bimpelrekkie Aug 27 '21 at 12:58
  • *I just want it turn on when Lipo reach 3.4V or below* OK, so you want to use this trick to get a low battery indicator? **Why didn't you explain this in your question?**. OK, now it is there but it was not. – Bimpelrekkie Aug 27 '21 at 12:59
  • It would be **much simpler** to use a zenerdiode (or active zener circuit) to drop 1.1 V constantly. Then when the battery is 4.2 V the devices get 3.1 V and when the battery is 3.4 V the devices get 2.3 V. Done. – Bimpelrekkie Aug 27 '21 at 13:03
  • Oh god, thank you @Bimpelrekkie. Your solution is so simple that I have never though about it. I will research more about zenerdiode, specially one with 1.1V voltage drop – Minh Bui Aug 27 '21 at 14:08
  • If you cannot find a suitable zener, consider a couple of Schotky diodes in series or a "Vbe multiplier circuit", (which is easy to tune to the right voltage using a potmeter/trimmer) see the extensive answer by Jonk in this question: https://electronics.stackexchange.com/questions/449914/vbe-multiplier-with-emitter-resistance-cancellation – Bimpelrekkie Aug 27 '21 at 14:12

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