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Could you believe it? Affordable charger circuits on the market still don’t come with load sharing components built in these day and age. So I had to build my own. I’m somewhat new to this so bear with me.

Components:

  • Charging IC - TP4056
  • P-MOSFET - DMP1045UQ-7
  • Schottky diode - PMEG6010ELRX

Here’s a schematic of how the load-sharing circuit would look, albeit wired based on the TP4056 rather than the MCP73872:

enter image description here

Load-sharing circuit schematic

When researching which pull-down resistor I should purchase, I ran into an issue due to the PMEG6010ELRX Schottky diode that I have.

This diode is rated with a reverse leakage current(Ir) of 5nA@Tj 25°C; 5V, which is pretty low.

According to a design guide posted here, it is recommended to keep the Rpull resistor at no more than 100kΩ and when selecting the Rpull resistor we should use the formula;

Rpull = Vtarget * RD / (VBATmax – Vtarget) where, RD= VBATmax / IR.

In this case Vtarget is a range from 1-3V, IR=5nA and VBATmax=4.2V. In most cases including in the example they had in the guide, IR is in the 100-200uA range which yields something in 6-100kΩ range.

However, PMEG6010ELRX has an incredibly low leakage current in the single digit nano Ampere range.

How would that affect my choice of selecting a resistor to pull down the voltage so the P-MOSFET will function normally?

Otherwise I’d get something in the MΩ or even GΩ region which is much more than the recommended 100kΩ. I’m under the impression that this resistor’s rating doesn’t have to be near the actual value obtained from the formula. So long as it doesn’t exceed (<100kΩ) regardless of how small the leakage current of D1 is, will the PMOSFET still work correctly?

JRE
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ElectronicCupid
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    What guide are you referring to? – Kartman Feb 25 '21 at 00:51
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    ElectronicCupid - Hi, Can you add a schematic of what you are trying to create? The question where you originally added yours, included a diagram (from a Microchip app note) with a different Li-Ion charger (the MCP73833). You refer to "the guide" in your question - do you mean that Microchip app note or something else? If you can edit your question to include your schematic, and include a link to the guide you mention, that will help readers. Thanks! – SamGibson Feb 25 '21 at 00:51
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    ElectronicCupid - Hi, I see that you have edited the question, but have not answered the points that I raised. Unless you clarify the question, it may get put on hold as "needs more details", which would be a shame, as I realise you want to get help with your question. It's just unclear what you mean due to the missing info I explained before. Please consider editing the question to clarify those points. Thanks! (Also, even though you reversed most of my edit, please consider splitting the long paragraph. That would help readability, and so increase your chance of getting replies. Thanks.) – SamGibson Feb 25 '21 at 02:13
  • I’ve added links to the guide from Zakkemble.net that I’ve been using along with schematic from Microchip. @Kartman That’s the guide I was referring to. – ElectronicCupid Feb 25 '21 at 03:28
  • We’re only getting half the picture. A precise question gets a precise answer. Why are you getting so hung up on a resistor value that is non critical and resistors cost 10 to the penny. If anything, doing some experimentation can only help your skills. Start with 100k. – Kartman Feb 25 '21 at 05:17
  • QUESTION: Could you believe it?; ANSWER: yes. – Andy aka Feb 25 '21 at 08:39
  • Thanks for the input. Indeed resistors are cheap, but it costs $20 to ship it and if it doesn’t work. I’d have to spend an extra $20 to ship another set that costs a penny. Just not very economical. So that’s why I’d like to get it right the first time. I’ll try with the 100k and hopefully it’s enough to get the P-MOSFET working as intended. – ElectronicCupid Feb 25 '21 at 17:24

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