Specifications

Bluegiga Technologies Oy
Page 26 of 64
6 Battery Charger
The battery charger is a constant current / constant voltage charger circuit, and is suitable for lithium
ion/polymer batteries only. It shares a connection to the battery terminal, VDD_BAT, with the switch-mode
regulator. The charger is initially calibrated by Bluegiga Technologies to have V
float
= 4.15V - 4.2 V.
The constant current level can be varied to allow charging of different capacity batteries. WT32i allows a
number of different currents to be used in the battery charger hardware. Values written to PS key 0x039b
CHARGER_CURRENT in the range 1..15 specify the charger current from 40..135mA in even steps. Values
outside the valid 0..15 range result in no change to the charging current. The default charging current (Key =
0) is nominally 40mA. Setting 0 is interpreted as “no-change” so it will be ignored
The charger enters various states of operation as it charges a battery. These are shown below:
 Off: entered when the charger is disconnected.
 Trickle Charge: entered when the battery voltage is below 2.9V.
 Fast Charge - Constant Current: entered when the battery voltage is above 2.9V.
 Fast Charge - Constant Voltage: entered when the battery has reached V
float
, the charger
switches mode to maintain the cell voltage at V
float
voltage by adjusting the constant
charge current.
 Standby: this is the state when the battery is fully charged and no charging takes place.
When a voltage is applied to the charger input terminal VDD_CHG, and the battery is not fully charged, the
charger will operate and a LED connected to the terminal LED0 will illuminate. By default, until the firmware is
running, the LED will pulse at a low-duty cycle to minimize current consumption.
The battery charger circuitry auto-detects the presence of a power source, allowing the firmware to detect
when the charger is powered. Therefore, when the charger supply is not connected to VDD_CHG, the
terminal must be left open circuit. The VDD_CHG pin, when not connected, must be allowed to float and not
be pulled to a power rail. When the battery charger is not enabled, this pin may float to a low undefined
voltage. Any DC connection will increase current consumption of the device. Capacitive components such as
diodes, FETs, and ESD protection, may be connected.
The battery charger is designed to operate with a permanently connected battery. If the application permits
the charger input to be connected while the battery is disconnected, the VDD_BAT pin voltage may become
unstable. This, in turn, may cause damage to the internal switch-mode regulator. Connecting a 470μF
capacitor to VDD_BAT limits these oscillations thus preventing damage.
WARNING:
Use good consideration for battery safety. Do not charge with too much current. Do not charge when the
temperature is above 60°C or below 0°C. WT32i is initially calibrated to stop charging when battery voltage is
at 4.2 V. Do not try to charge batteries above 4.2 V. Do not short circuit the battery or discharge below 1.5 V.