BQ25120A Datasheet, PDF (25/67 Pages) Texas Instruments – Low IQ Highly Integrated Battery Charge Management Solution for Wearables and IoT

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bq25120A
SLUSD08 – MAY 2017
9.3.15 Safety Timer and Watchdog Timer
At the beginning of the charge cycle, the device starts the safety timer. If charging has not terminated before the
programmed safety time, t_MAXCHG, expires, the device enters idle mode and charging is disabled. The pre-charge
safety time, t_PRECHG, is 10% of t_MAXCHG. When a safety timer fault occurs, a single 128 µs pulse is sent on the INT
pin and the STAT and FAULT bits of the status registers are updated over I²C. The CD pin or power must be
toggled in order to clear the safety timer fault. The safety timer duration is programmable using the TMR bits.
When the safety timer is active, changing the safety timer duration resets the safety timer. The device also
contains a 2X_TIMER bit that enables the 2X timer function to prevent premature safety timer expiration when
the charge current is reduced by a load on PMID, SYS, LS/LDO or a NTC condition. When t_{2X_TIMER} function is
enabled, the timer is allowed to run at half speed when any loop is active other than CC or CV.
In addition to the safety timer, the device contains a 50-second watchdog timer that monitors the host through
the I²C interface. Only after an I²C transaction is performed on the I²C interface, will the watchdog timer start. In
the case where the device is set to operate in High Impedance Mode, the watchdog timer is automatically
disabled and can only be re-started after the device exits the High Impedance Mode and a subsequent I²C
transaction is performed. The watchdog timer is reset by any transaction by the host using the I²C interface. If
the watchdog timer expires without a reset from the I²C interface, all registers except MRRESET_VIN and
MRREC are reset to the default values.
9.3.16 External NTC Monitoring (TS)
The I²C interface allows the user to easily implement the JEITA standard for systems where the battery pack
thermistor is monitored by the host. Additionally, the device provides a flexible voltage based TS input for
monitoring the battery pack NTC thermistor. The voltage at TS is monitored to determine that the battery is at a
safe temperature during charging.
To satisfy the JEITA requirements, four temperature thresholds are monitored: the cold battery threshold, the
cool battery threshold, the warm battery threshold, and the hot battery threshold. These temperatures correspond
to the V_(COLD), V_(COOL), V_(WARM), and V_(HOT) threshold in the Electrical Characteristics. Charging and timers are
suspended when V_(TS) < V_(HOT) or > V_(COLD). When V_(COOL) < V_(TS) < V_(COLD), the charging current is reduced to
half of the programmed charge current. When V_(HOT) < V_(TS) < V_(WARM), the battery regulation voltage is reduced
by 140 mV (minimum V_BATREG under this condition is 3.6V).
The TS function is voltage based for maximum flexibility. Connect a resistor divider from V_IN to GND with TS
connected to the center tap to set the threshold. The connections are shown in Figure 18. The resistor values are
calculated using Equation 1 and Equation 2. To disable the TS function, pull TS above TS_OFF threshold.
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