BQ24250C_15 Datasheet, PDF (25/46 Pages) Texas Instruments – bq24250C 2A Single Input I2C, Standalone Switch-Mode Li-Ion Battery Charger with Power-Path Management

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bq24250C
SLUSBY7 – JULY 2014
8.4.10 Safety Timer
At the beginning of charging process, the bq24250C starts the safety timer. This timer is active during the entire
charging process. If charging has not terminated before the safety timer expires, the IC enters suspend mode
where charging is disabled. The safety timer time is selectable using the I²C interface. A single 256μs pulse is
sent on the STAT and INT outputs and the FAULT/ bits of the status registers are updated in the I²C. This
function prevents continuous charging of a defective battery if the host fails to reset the safety timer. When
2xTMR_EN bit is set to “1”, the safety timer runs at a rate 2x slower than normal (the timer is extended) under
the following conditions:
• Pre-charge or linear mode (minimum system voltage mode),
• During thermal regulation where the charge current is reduced,
• During TS fault where the charge current is reduced
The safety timer is suspended during OVP, TS fault where charge is disabled, thermal shut down, and sleep
mode.
8.4.11 Watchdog Timer
In addition to the safety timer, the bq24250C contains a 50-second watchdog timer that monitors the host
through the I²C interface. Once a write is performed on the I²C interface, a watchdog timer is reset and started.
The watchdog timer can be disabled by writing “0” on WD_EN bit of register #1. Writing “1” on that bit enables it
and reset the timer.
If the watchdog timer expires, the IC enters DEFAULT mode where the default charge parameters are loaded
and charging continues. The I²C may be accessed again to re-initialize the desired values and restart the
watchdog timer as long as the safety timer has not expired. Once the safety timer expires, charging is disabled.
8.4.12 Thermal Regulation and Thermal Shutdown
During the charging process, to prevent overheat of the chip, bq24250C monitors the junction temperature, T_J, of
the die and begins to taper down the charge current once T_J reaches the thermal regulation threshold, TREG.
The charge current is reduced when the junction temperature increases above TREG. Once the charge current is
reduced, the system current is reduced while the battery supplements the load to supply the system. This may
cause a thermal shutdown of the IC if the die temperature rises too. At any state, if T_J exceeds TSHTDWN,
bq24250C suspends charging and disables the buck converter. During thermal shutdown mode, PWM is turned
off, all safety timers are suspended, and a single 256μs pulse is sent on the STAT and INT outputs and the
FAULT/STAT bits of the status registers are updated in the I²C. A new charging cycle begins when T_J falls below
TSHTDWN by approximately 10°C.
8.4.13 Fault Modes
The bq24250C includes several hardware fault detections. This allows for specific conditions that could cause a
safety concern to be detected. With this feature, the host can be alleviated from monitoring unsafe charging
conditions and also allows for a “fail-safe” if the host is not present. The table below summarizes the faults that
are detected and the resulting behavior.
FAULT CONDITION
Input OVP
Input UVLO
Sleep (VIN < VBAT)
TS Fault (Batter Over Temp)
Thermal Shutdown
Timer Fault
No Battery
ISET Short
Input Fault & LDO Low
CHARGER BEHAVIOR
VSYS and ICHG Disabled
VSYS and ICHG Disabled
VSYS and ICHG Disabled
VSYS Active and ICHG Disabled
VSYS and ICHG Disabled
VSYS Active and ICHG Disabled
VSYS Active and ICHG Disabled
VSYS Active and ICHG Disabled
VSYS and ICHG Disabled
SAFETY TIMER BEHAVIOR
Suspended
Reset
Suspended
Suspended
Suspended
Reset
Suspended
Suspended
Suspended
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