CHARGER — Battery charger

Host software enables charging using register BCHGENABLESET. Battery charging starts when VBUS voltage is present.

When the battery is detected, trickle charging begins. Constant current charging starts when the battery voltage is above V_{TRICKLE_FAST}. After the battery voltage reaches V_TERM, the charger enters constant voltage charging. The battery voltage is maintained while monitoring current flow into the battery. When the current into the battery drops below I_TERM, charging is complete. Charging is disabled using register BCHGENABLECLR.

Figure 1. Charging cycle flow chart

Figure 2. Charging cycle

Battery detection runs automatically when charging starts. Battery UVLO will release when the battery voltage has increased above the UVLO threshold.

UVLO release refers to the overdischarge protection circuit in the battery pack protection circuit. It prevents battery loss when an overdischarge occurs. An undervoltage lockout circuit is not included on the device and must be set up on the battery pack.

CHARGER waits until a battery is detected before charging. A faulty or absent battery is indicated in bit BATTERY.DETECTED.

If the system resets, charging control bits are reset and charging is disabled by default.

V_TERM is configured through TWI according to the battery type in use, see register BCHGVTERM.

For a higher temperature range, a lower termination voltage (V_TERMR) is available and configured separately in register BCHGVTERMR. V_TERM and V_TERMR can be set to the same value.

The charge current limit is set between 32 mA and 800 mA in 2 mA steps. Charging current I_CHG is configured with TWI with a default value of 32 mA.

CHARGER must be disabled before changing the current setting in registers BCHGISETMSB and BCHGISETLSB. The setting takes effect when charging is enabled.

Charge current is configured using a 9-bit value. The following shows how the register value for BCHGISETMSB can be calculated, where I_CHG is the charge current in mA:

The following shows how the register value for BCHGISETLSB can be calculated, where I_CHG is the charge current in mA:

Some example charging currents are found in the following table.

Trickle charging current, I_TRICKLE, is 10% of I_CHG. Trickle charging is active when VBAT < V_{TRICKLE_FAST} (default 2.9 V).

Termination current, I_TERM, is programmable to 10% (default) or 20% of I_CHG. Termination current is active in the constant voltage phase of charging. Charging stops when the charging current reduces to this value.

These parameters are configured in registers BCHGVTRICKLESEL and BCHGITERMSEL.

CHARGER supports three types of NTC thermistors for battery temperature (T_BAT) monitoring. Only one can be enabled at a time.

The host software must select the corresponding setting that matches the battery thermistor before enabling charging in register ADCNTCRSEL. The following thermistor resistors are supported.

If a thermistor is not used, the NTC pin must be tied directly to ground or through a resistor. The functionality must be disabled in register BCHGDISABLESET. This does not impact Charger thermal regulation because the two temperature measurements are independent of each other.

The following battery temperature thresholds can be set: T_COLD ≤ T_COOL ≤ T_WARM ≤ T_HOT.

These limits can be set between −20°C and +60°C, and setting adjacent thresholds to identical values is allowed. For example, setting T_WARM = T_HOT means that there is no warm region. Charging does not happen below T_COLD or above T_HOT. Charging can be paused at T_WARM instead of T_HOT by setting register BCHGCONFIG.

The thresholds are written into corresponding registers. The battery temperature variable, K_NTCTEMP, is calculated using the following equation:

Here, R_T is the thermistor resistance at a desired temperature and R_B (internal bias resistor) equals the thermistor resistance at 25°C. See NTCCOLD, NTCCOOL, NTCWARM, and NTCHOT for more information. Default values in the registers match the JEITA guideline and are intended for the 10 kΩ thermistor defined in Supported thermistor resistors.

The charging current can be reduced by 50% between NTCCOLD and NTCCOOL. The termination voltage can be configured independently between NTCWARM and NTCHOT. Default is I_COOL (50% of I_CHG), but this can be disabled.

Battery temperature is measured by the on-chip System Monitor at regular intervals during charging. The latest result is available in registers ADCNTCRESULTMSB and ADCGP0RESULTLSBS.

When the battery temperature rises over T_WARM or T_HOT, or falls below T_COOL or T_COLD, an interrupt is sent.

Heat dissipation from the linear charger is managed by setting a maximum temperature limit for the die. This limit must not exceed device and PCB temperature requirements.

Die temperature monitoring is active during charging, with a default limit of T_CHGSTOP. Charging stops when the die temperature reaches the limit. It resumes when the die cools down to T_CHGRESUME.

T_CHGSTOP controls the junction temperature rise and limits the temperature rise on the PCB and device mechanics. The device can be configured to send an interrupt when the limit is met.

The die temperature variable, K_DIETEMP, is calculated with the following equation:

Here, T_D represents the die temperature limit in degrees Celsius.

Registers DIETEMPSTOP and DIETEMPSTOPLSB are concatenated to create a 10-bit value that defines the charging stop temperature T_CHGSTOP. Registers DIETEMPRESUME and DIETEMPRESUMELSB are concatenated to create a 10-bit value that defines the charging resume temperature T_CHGRESUME. The host software reads register DIETEMPSTATUS to determine if the die temperature is above T_CHGSTOP.

A CHARGER error condition occurs when one of the following are present:

• Trickle charge timeout, see t_OUTTRICKLE
• Safety timer expires, see t_OUTCHARGE

After an error is detected, CHARGER is disabled. The charging error indication is activated and the charging indication is deactivated. Error conditions are cleared when VBUS is disconnected and reconnected again.

Errors are reported in register BCHGERRREASON and BCHGERRSENSOR. Host software clears errors with register TASKCLEARCHGERR and releases the charger from the error state with register TASKRELEASEERR.

When the safety timer expires, the host must make sure it is safe to charge before resetting register TASKCLEARSAFETYTIMER.

When CHARGER is enabled and the LEDs are configured, the LEDs indicate the charging status.

The LED[n] pins sink 5 mA of current when active. They are high impedance when disabled. This is suitable for driving LEDs or connecting to host GPIOs in a weak pull-up configuration. The LED anode must be connected to a voltage rail that allows forward bias. If a general purpose open drain output is needed, the LED pins can be used with a pull-up resistor connected to a voltage rail. See LEDDRV — LED drivers for more information.

Charging terminates automatically when the battery voltage reaches V_TERM and charging current is less than I_TERM. An interrupt is issued to the host.

Unless disabled through bit DISABLE.RECHARGE, charging restarts automatically when VBAT is less than V_RECHARGE and an interrupt is sent to the host.

CHARGER manages battery current flow to maintain VSYS voltage.

The battery is isolated when VBUS is connected and the battery is fully charged. Under this condition, VSYS supplies VBUS. When VBUS is disconnected, CHARGER supplies VSYS from VBAT.

The system load requirements are prioritized over battery charge current when VBUS is connected and the battery is charging. During charging, if the system current load exceeds IBUS_LIM, the battery charge current decreases to maintain the VSYS voltage. CHARGER reduces the current to maintain an internal voltage of V_CHDROPOUT above the VBAT voltage. If more current is required, CHARGER enters supplement mode to provide current from the battery, up to IBAT_LIM.