Design Review: 5V-to-8.4V (2S Li-ion) Charger + MOSFET Protection Logic. - Page 1
Forum section: electronics / projects

author: KarlMcCree500
author_rank: Newbie
timestamp: October 05, 2025, 06:46:10 pm
content: I am a hobbyist who with a knack for power electronics, and a critique of my design would be most welcomed. I am assembling a circuit to charge a 2-cell (2S) Lithium battery pack (max 8.4V) on a standard 5V USB Micro connector and provides simple FET-based protection, too.I understand VIN must be connected to VOUT through a boost topology, so I am including the MCP1 symbol as a proxy to an appropriate 2S boost charger IC in place which I will choose sometime. The logic and relationship between the protection and sensing elements is my primary issue at the moment, even though I will not commit to a particular boost IC.Will you mind looking at my FET connections? Below I have described how I have wired them.I am protecting with the NDS8434A dual N-channel MOSFET, with what I guess would be the SENSE (input current monitoring) and DRV (output control) connection of a higher level charger/protection IC (MCP1 placeholder).The connectivity I am attempting to achieve is shown here:Charging Path (VBUS to BAT+):C11 (10.U F ) filters the VBUS input.It is transmitted by MOSFET 1 (S1,D1) of the NDS8434A.MOSFET 1 Source (S1,S2) is to VBUS directly (through C11).MOSFET 1 Drain (D1,D2) is attached to the VDD input (pin 3) of the placehold MCP1 IC.The SENSE pin 8 of the MCP1 placeholder drives MOSFET 1 Gate (G), through a 100k pull-down (R7). Is this how to control input current by driving a power FET?Battery Path (VBAT to BAT+):The VBAT output (pin 4) of MCP1 IC is connected to the BAT+ of the battery pack.Control and Safety:I have added D5 (1N4001) and R8 (100k ohm) to regulate the EN (Enable) pin and attempted to set the charger to go to the ON state only when VBUS is present.I added C3 (10μF) on VBAT line and C10 (0.1μF) on VSS (GND?).My Key Questions:FET Placement: Is it customary to place the NDS8434A on the VBUS input line (in front of the charger IC) or are the protection FETs generally placed in series across the BAT - line?Protection Logic: Does the SENSE and DRV pins of a charger IC directly drive the gates of protection FETs in this manner, or quite frankly do I need a separate protection IC (such as a DW01) to detect over-voltage, under-voltage, and over-current and then drive the FETs?2S Balancing: I have the BM (Battery Midpoint) circuit on my connector, but nothing on the circuit. Is a balancing circuit required a 2S pack or can I get away with a low-power application?Any suggestions or advice on what to do to correct this would be of great benefit as I complete the design!Thanks in advance

author: jwet
author_rank: Frequent Contributor
timestamp: October 06, 2025, 04:14:18 pm
content: Quote from: KarlMcCree500 on October 05, 2025, 06:46:10 pm

author: KarlMcCree500
author_rank: Newbie
timestamp: October 07, 2025, 09:15:42 am
content: I am really grateful to your highly valuable and straight forward feedback. This was just what I required to go on!I totally agree with the point that the first step is a choice of the IC, and now I understand that the whole circuit topology depends on this choice as I require a 5V-to-8.4v boost charger.These are the points I deconstructed and a couple of questions that I had to ask you:On the battery side. Choose a charger IC and follow their guidelines.>>Yes, I require a certain 2S boost charger IC. I am currently considering such components as MCP738444.Follow-up Question: Will a charger IC with built-in cell balancing and protection features be preferable to a simpler charger IC with a distinct specialized 2S protection IC (such as a special device marked as S-8209) in order to drive the NDS8434A MOSFETs?Choose a charger. A protection circuit is recommended.>>Your argument about balancing being important to a 2S pack is absolutely correct, particularly because I added pin BMS to the connector.Good luck- step zero would be to select a battery charger IC and follow the DS guidelines. Also 1N4001 is a slow rectifier diode- a poor choice here.>>I like the fact that the 1N4001 was corrected--I had not known of the problem of its slow speed and high voltage drop.Follow-Up Question: To provide basic reverse polarity protection on the 5V (VBUS) input, would an {SS14} (a standard Schottky diode you suggested) be an effective substitute, or can a simple P-MOSFET with a control circuit be a much better and lower-loss solution to this input stage? with all this correctios made it is sure to work when i start building my PCB?And again, thank you, as you guided me on the right path! Thanks to you I ended up without many headaches and a board that was dead.

author: jwet
author_rank: Frequent Contributor
timestamp: October 07, 2025, 02:54:03 pm
content: The MCP73844x series looks like a good target device. Before you invest too much time, make sure you can get it. I would then read the data sheet about 3 times and make sure you understand it.>>Follow-up Question: Will a charger IC with built-in cell balancing and protection features be preferable to a simpler charger IC with a distinct specialized 2S protection IC (such as a special device marked >>as S-8209) in order to drive the NDS8434A MOSFETs?The more integrated parts like the MCP73xx are a good pick. Simplicity is good.>>Follow-Up Question: To provide basic reverse polarity protection on the 5V (VBUS) input, would an {SS14} (a standard Schottky diode you suggested) be an effective substitute, or can a simple P-MOSFET >>with a control circuit be a much better and lower-loss solution to this input stage? with all this correctios made it is sure to work when i start building my PCB?The Schottky reccomended in the DS is is the MA2Q705, this is recently discontinued but you can find something similar. It has a Vf of .37v at 1 amp which is a pretty small loss (< .5W). If its a desk charger, you probably don't care, if its in a small box with your gadget, it might matter. PMOS will have near zero Vdrop and power loss. It seems like a .4 watt on a 8W charger is relatively small.Good luck.