BQ25120_16 Datasheet, PDF (23/68 Pages) Texas Instruments – Highly Integrated Battery Charge Management Solution

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BQ25120, BQ25121
SLUSBZ9B – AUGUST 2015 – REVISED MAY 2016
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. Once any I²C transaction is performed on the I²C interface, a watchdog timer is started. 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 the programmed charge current.
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 16. 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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