In the previous article I talked about the main differences about linear and switching lithium battery charger.

Here I want to explain my project of a linear battery charger for lithium battery based on the IC AAT3681A.
For the first thing I want to share with you the schematic and then discuss every single component

The charger IC  is AAT3681A,these are the main features:

  • progammable current from 15mA to 500mA
  • input voltage from 4V to 7.5V
  • regulated output voltage at 4.2V for the version that I choose
  • Shoutdown current <1uA
  • State of charging Pin
  • Overvoltage and Emergency thermal protection
  • very small board: 17.8 mm x 15.3 mm

This is the AAT3681A pinout:

The breakout board pinout is (J1 connector from top to bottom):

  • BAT: connected directly to the BAT pin of the AAT3681A, you must connect here the + terminal of the lithium battery that you want to charge
  • IN: connected directly to the ADP pin of the AAT3681A, you must connect here the + terminal of the voltage source used for charging, for example the 5V pin of an USB port, the + terminal of a laboratory power supply or a 5V wall adapter.
    This power supply must be able to source at least the charging current of the breakout board to allow the board to works well: so if the charging current is set to 200mA (default) a power supply that can handle 200 mA or more must be choose.(500mA or 1A power supply is recommended) 
  • VIO: you should connect to this pin an external voltage useful to pull up the resistor connected to the stat pin, this is useful to read the state of the STAT pin of the AAT3681A, if you connect for example a 3,3V on VIO you can read the STAT pin with a microcontroller that works with a 3,3V supply
  • STAT: connect directly to the STAT pin of the AAT3681A, from this pin you can read the state of charging
    • Reading a HIGH logic level means that charging is complete or there is NO charging activite at that time
    • Reading a LOW logic level means that battery is in charging.
  • EN: directly connect to the EN pin of the AAT3681A,
    • When EN is pulled to a HIGH logic level, the AAT3681A is enabled.
    • When left open or pulled to a LOW logic level, the AAT3681A will shut down and forced into the sleep state.
  • GND: directly connect to the GND pin of the AAT3681A, connect it to the GND of your entire circuit

AAT3681A needs very few components to works, in this circuit there are:

  • CIN is a 10uF ceramic capacitor, it is a decoupling capacitor that ensure that the input voltage will be good regulated
  • CBAT is a 10uF ceramic capacitor, it ensure circuit stability and this high capacitance allow to connect battery also at more distance from the charger
  • R2 is a pull up resistor useful to read that STAT pin of the charger
  • R1 is 8.06KΩ resistor useful to programming the charging current of the charger,here is the table to select the correct resistor for your desired charging current

in this circuit I choose R1=8.06K that correspond a current of 200 mA.
Why I choose a low current like this if I can handle up to 500mA?

Because the most important parameter when you design a circuit with a linear battery charger is to take care that the power dissipation of the charger stay lower than the maximum power dissipation allowed by the IC

take a look at this graph:

It show that if you provide 5V in ADP pin, at Ta=25 °C and without PCB thermal enhancements, the charger can handle up to about 340 mA.

This is why I choose a standard charging current of 200mA, according to this graph you should be able to charge the battery at 200mA also at Ta=60°C when you provide 5V, given for example from an USB port, to ADP pin.

In a typical condtion of Ta=25°C this charger will work very well at 200mA.

In general, the worst condition is the greatest voltage drop across the charger IC, that is when battery voltage is
charged up to the preconditioning voltage threshold(when the battery voltage is low).

In the next article I’ll show you how to use this breakout board with a microcontroller and in standalone mode.

keep following!