Other Parts Discussed in Thread: BQ25185
TS pin operation
The BQ2518x uses an internal 38uA current source on the TS pin to bias the NTC thermistor, and the resulting TS voltage is compared against internal reference voltage thresholds to determine which temperature range the battery is in.
External NTC monitoring (TS) is only available in Charger/Adapter mode (when a valid input source is connected to VIN). The BQ2518x does not monitor battery temperature during discharging.
Default NTC and temperature thresholds
The internal reference voltage thresholds are tuned for use with an NTC thermistor with a nominal resistance of 10kΩ at 25°C and a beta value of β(25/85) = 3435K. This is a very common and widely available type of NTC. If using this type of NTC, such as a 103AT-2, then the NTC can simply be connected from the TS pin to GND for the following default battery temperature window during charging:
- HOT: 60°C
- WARM: 45°C
- COOL: 10°C
- COLD: 0°C
For the I2C-controlled BQ2518x devices, the HOT and COLD thresholds are configurable via I2C, as shown in the register map in the JEITA temperature support section below.
Using a different NTC
If a different NTC is being used, then external compensating resistors in series and parallel with the NTC, as shown below, can be used to adjust the resistance of the TS network across temperature.
The following equations can be used to calculate the needed series and parallel resistor values:
Where:
- VC is the COLD temperature TS threshold
- VH is the HOT temperature TS threshold
- RC is the resistance of the NTC thermistor at TC
- RH is the resistance of the NTC thermistor at TH
- ITS_BIAS is the constant current from the internal current source on the TS pin (38uA typical)
JEITA temperature support
The I2C-controlled BQ2518x devices support JEITA battery temperature monitoring during charging, providing four temperature thresholds: HOT, WARM, COOL, and COLD.
The TS_HOT and TS_COLD bits select the reference voltage thresholds that are used for these ranges. These thresholds are listed in the Battery NTC Monitor section of the Electrical Characteristics table in the BQ2518x datasheet.
If the battery is HOT or COLD, charging is suspended if the TS_EN bit is 1. The CHG_DIS bit does not change when charging is suspended. Charging is automatically resumed when VTS reenters the normal battery temperature range.
If the battery is COOL, the charge current is reduced by either 1/2 or 1/5, depending on the setting of the TS_ICHG bit in REG0xB.
If the battery is WARM, the target battery regulation voltage is reduced by either 100mV or 200mV, depending on the setting of the TS_VRCG bit in REG0xB.
The WARM and COOL thresholds can each be disabled in REG0xB.
The HOT and COLD thresholds are configurable in REG0xB.
The BQ25185 (standalone, non-I2C) only supports HOT and COLD battery temperature thresholds.
Using TS with a battery fuel gauge
If a battery fuel gauge is used in the design, it's recommended to use the fuel gauge's external NTC thermistor monitoring pin instead of the TS pin of the charger. The fuel gauge's NTC monitoring is highly accurate (it often uses polynomial approximations of the R-T curve of the NTC) and thermistor coefficients can be modified in RAM. The MCU can read the battery temperature of the fuel gauge and disable charging via I2C or by pulling the /CE pin low. The temperature values read by the fuel gauge can be used to determine whether to allow charging, either via I2C or by pulling the /CE pin high, depending on the BQ2518x device.
If the battery fuel gauge and battery charger are both used for redundant external NTC thermistor monitoring, a separate NTC thermistor should be used for each device. If the fuel gauge and charger share the same NTC, each device will bias the NTC, resulting in a voltage across the NTC which neither device will interpret as the actual temperature of the NTC.
Disabling TS
If the TS function of the charger is not required, tie a 10kΩ resistor from the TS pin to GND. This maintains a valid voltage on this pin for charging. This value is in the normal temperature range, so the charger always detects TS_NORMAL.
For the I2C-controlled BQ2518x devices, the TS feature can be disabled by the TS_EN bit in REG0x7[7]. If TS is disabled by setting this bit to 0, the TS current source will remain active and the charger will still report the TS_STAT bit in REG0x1[7], TS_OPEN_STAT bit in REG0x0[7] and set the TS_FAULT flag in REG0x2[7] when TS_STAT changes, but charging will not be affected by the TS pin. The TS current source cannot be disabled in Charger/Adapter mode, but it can be disabled in battery-only mode by setting the EN_PUSH bit to 0 in REG0x9[0].
Interrupts
When TS_STAT or TS_OPEN changes, an interrupt is generated on the INT pin if the TS_INT_MASK is not set in REG0xC[7].
Tools
There is an external NTC monitoring (TS) calculator available on the TI Charger GUI page for the BQ2518x.