BQ24765_15 Datasheet, PDF (24/43 Pages) Texas Instruments – SMBus-Controlled Multi-Chemistry Battery Charger With Integrated Power MOSFETs

bq24765
SLUS999A – NOVEMBER 2009 – REVISED NOVEMBER 2015
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To set the charge current, use the SMBus to write a 16bit ChargeCurrent() command using the data format listed
in Table 4. The ChargeCurrent() command uses the Write-Word protocol (see Figure 14). The command code for
ChargeCurrent() is 0x14 (0b00010100). When using a 10-mΩ sense resistor, the bq24765 provides a charge
current range of 128 mA to 8.064 A, with 128 mA resolution. Set ChargeCurrent() to 0 to terminate charging.
Setting ChargeCurrent() below 128 mA, or above 8.064 A clears DAC and terminates charge
As charging goes on, the power loss on the switching fets causes the junction temperature to rise. The bq24765
provides a thermal regulation loop to throttle back the maximum charge current when the maximum junction
temperature limit is reached. Once the device junction temperature exceeds thermal regulation limit (typical
120°C), the thermal regulator reduces the charging current to keep the junction temperature at 120°C. When the
junction temperature rises to 125°C, the charging current decreases down close to 0 A.
The bq24765 includes a foldback current limit when the battery voltage is low. If the battery voltage is less than
3.6 V but above 2.5 V, any charge current limit above 3 A will be clamped at 3 A. If the battery voltage is less
than 2.5 V, the charge current is set to 220 mA until that voltage rises above 2.7 V. The ChargeCurrent() register
is preserved and becomes active again when the battery voltage is higher than 2.7 V. This function effectively
provides a fold-back current limit, which protects the charger during short circuit and overload.
Upon reset, the ChargeVoltage() and ChargeCurrent() values are cleared (0) and the charger remains off until
both the ChargeVoltage() and the ChargeCurrent() command are sent. During reset, both high side and low side
fets remain off until the charger gets started.
8.5.4 Input Adapter Current Regulation
The total Input Current from an AC adapter or other DC sources is a function of the system supply current and
the battery charging current. System current normally fluctuates as portions of the system are powered up or
down. Without Dynamic Power Management (DPM), the source must be able to supply the maximum system
current and the maximum charger input current simultaneously. By using DPM, the input current regulator
reduces the charging current to keep the input current from exceeding the limit set by the Input Current SMBus 6
bit DAC register. With the high accuracy limiting, the current capability of the AC adaptor can be lowered,
reducing system cost.
The CSSP and CSSN pins are used to sense R_AC with default value of 10 mΩ. However, resistors of other
values can also be used. For a larger the sense resistor, you get a larger sense voltage, and a higher regulation
accuracy; but, at the expense of higher conduction loss.
The total input current, from a wall cube or other DC source, is the sum of the system supply current and the
current required by the charger. When the input current exceeds the set input current limit, the bq24765
decreases the charge current to provide priority to system load current. As the system supply rises, the available
charge current drops linearly to zero.
where η is the efficiency of the DC-DC converter (typically 85% to 95%).
ƪ ƫ I_INPUT + I_LOAD )
I_LOAD V_BATTERY
V_IN h
) I_BIAS
(2)
To set the input current limit, use the SMBus to write a 16-bit InputCurrent() command using the data format
listed in Table 5. The InputCurrent() command uses the Write-Word protocol (see Figure 14). The command
code for InputCurrent() is 0x3F (0b00111111). When using a 10-mΩ sense resistor, the bq24765 provides an
input-current limit range of 256 mA to 11.004 A, with 256 mA resolution. InputCurrent() settings from 1 mA to 256
mA clears DAC and terminates charge. Upon reset the input current limit is 256 mA.
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