BQ25302 datasheet(16/38 Pages) TI

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8.3.2.5 Charging Safety Timer

The device has built-in safety timer to prevent extended charging cycle due to abnormal battery conditions. The

safety timer is 20 hours when the battery voltage is above VBAT_LOWV threshold and 2 hours below VBAT_LOWV

threshold. When the safety timer expires, charge is suspended until the safety timer is reset. Safety timer is reset

and charge starts under one of the following conditions:

• Battery voltage falls below recharge threshold

• VBUS voltage is recycled

• EN pin is toggled

• Battery voltage transits across VBAT_SHORT threshold

• Battery voltage transits across VBAT_LOWV threshold

If the safety timer expires and the battery voltage is above recharge threshold, the charger is suspended and the

STAT pin is open. If the safety timer expires and the battery voltage is below the recharge threshold, the charger

is suspended and the STAT pin blinks to indicate a fault. The safety timer fault is cleared with safety timer reset.

During input current regulation, thermal regulation, the safety timer counts at half the original clock frequency

and the safety timer is doubled. During TS fault, VBUS_OVP, VBAT_OVP, ICHG pin open and short, and IC thermal

shutdown faults, the safety timer is suspended. Once the fault(s) is clear, the safety timer resumes to count.

8.3.2.6 Thermistor Temperature Monitoring

The charger device provides a single thermistor input TS pin for battery temperature monitor. RT1 and RT2

programs the cold temperature T1 and hot temperature T3. In the equations, R NTC,T1 is NTC thermistor

resistance value at temperature T1 and R NTC,T3 is NTC thermistor resistance values at temperature T3.

Assuming RHOT = 0, select 0°C to 45°C for battery charge temperature range, then NTC thermistor 103AT-2

resistance RNTC,T1 = 27.28 kΩ ( at 0°C) and RNTC,T3 = 4.91 kΩ (at 45°C), from the Equation 1 and Equation 2,

RT1 and RT2 are derived as:

• RT1 = 4.527 kΩ

• RT2 = 23.26 kΩ

On top of the calculation results, adding RHOT resisitor can shift HOT temperature T3 up and only slightly shift

up COLD temperature T1. The actual temperature T3 can be looked up in a NTC resistance table based on (R

NTC,T3 - RHOT) and T1 can be looked up in a NTC resistance table based on (R NTC,T1 - RHOT). Because R

NTC,T1 is much higher than R NTC,T3, RHOT can adjust HOT temperature significantly with mimimal affect on

COLD temperature. RHOT is optional.

RT1

REGN

RT2

RHOT

RTH

103AT

Figure 8-2. Battery Temperature Sensing Circuit

BQ25302

SLUSCZ3 – OCTOBER 2020

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