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i'm trying to design a battery charger for 5Ah li-ion battery. Due to cost management problems, i am not able to use advanced 5A charger controller ICs; instead, i am going to use a 1A controller with an amplification stage. Power FET configured as common drain seems a good solution; however, the problem is that, as you know, voltage tolerance of li-ion battery is quite low, about 30mV for my case. Vgs voltage of mosfet leads to voltage loss at the output more than that value. Is there a way to amplify DC current of controller IC by keeping output voltage in specified limits?

thank you.

ps: it is my first question in the site, please let me know if anything is wrong:)

Edit:

schematic

simulate this circuit – Schematic created using CircuitLab

the output of the controller is directly connected to the input of Nmos. Common drain amplifier is said to be voltage buffer and the DC current amplifier. Assuming 5A will be drawn by the battery during the constant current period and 4.2V will be supplied during constant voltage period, what modifications do I need if the controller supplies 0.5A during CC and 4.2V during CV?

the controller will be something like this: http://www.ti.com/lit/ds/symlink/bq24090.pdf DC current amplifier should be directly connected between Vout and the positive terminal of the battery pack.

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    \$\begingroup\$ Welcome, Negative feedback should cure your voltage offset problem. It would be best to post a schematic of what you are thinking and then ask for comments/ improvements. \$\endgroup\$ – George Herold Nov 1 '14 at 13:27
  • \$\begingroup\$ Does the datasheet (or associated application notes) for the controller you're using show examples for boosting the output current? They often do. \$\endgroup\$ – Dave Tweed Nov 1 '14 at 13:35
  • \$\begingroup\$ I agree with George. I think people will need to see a schematic and datasheet before useful feedback can be given. I am not totally clear on how you are planning to connect the FET to the charger. Usually chargers have current sense and VBAT voltage sensing. If you can find a way to keep use the existing ones, then it may be possible to make your plan work, although there could be stability issues to deal with due to amplification. \$\endgroup\$ – mkeith Nov 2 '14 at 6:13
  • \$\begingroup\$ Take a look at this: ww1.microchip.com/downloads/en/DeviceDoc/adn008.pdf \$\endgroup\$ – mkeith Nov 2 '14 at 6:31
  • \$\begingroup\$ thank you all for your kind responses. i will re-edit the post as recommend. @DaveTweed yes, they often do to arrange how much output current will be drawn within the limit of controller. since the battery requires more current than that limit of the controller that will be used, another DC current boosting stage is required, i think. \$\endgroup\$ – cryptokovski Nov 2 '14 at 18:11

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