BQ24721 Datasheet, PDF (30/48 Pages) Texas Instruments – ADVANCED MULTI-CHEMISTRY AND MULTI-CELL SYNCHRONOUS SWITCH-MODE CHARGER AND SYSTEM POWER SELECTOR

bq24721
SLUS683 – NOVEMBER 2005
www.ti.com
Automatic Internal Soft-Start Charger Current
The charger automatically soft-starts the charger regulation current every time the charger is enabled, to ensure
there is no overshoot or stress on the output capacitors or the power converter. The soft-start consists of
stepping-up the charge regulation current into eight evenly divided steps up to the programmed charge current of
register (0x14). Each steps last around 1 ms, for a typical rise time of 8 ms. No external components are needed
for this function.
High Accuracy IOUT Input and Charge Current Sense Amplifiers (CSA)
Industry standard, high accuracy current sense amplifiers (CSA) can be used to monitor the input current or the
charge current by the host or some discrete logic through the analog voltage output of the IOUT pin. The current
sense amplifier from the input current and the current sense amplifier of the charge current (voltage across
SRP-SRN pins) amplifies the input sensed voltage by 20x, through the Iout pin. The IOUT output is selectable
between the input current or the charge current, through a multiplexor that is controlled by the Charge Mode
(0x12) control register, IOUT Select bit (b3). The default setting is LO selecting the input current CSA.
Programming a HI selects the charger current CSA.
The IOUT output is a voltage source 20 times the input differential voltage. If the user wants to lower the voltage,
they could use a sense resistor from IOUT to AGND, and still achieve accuracy overtemperature as the resistors
can be matched their thermal coefficients.
A 0.1-µF capacitor connected on the output is recommended for decoupling high-frequency noise. An additional
RC filter is optional if additional filtering is desired. Note that adding filtering also adds additional response delay.
Charger Overcurrent Protection
The charger has a secondary overcurrent protection function that monitors the charge current, and prevents it
from exceeding 200% of the programmed charge current. The high-side gate drive turns off and automatically
resume when the current falls below the overcurrent threshold
Current Regulation Down to Zero Battery Voltage
The bq24721 charger regulates charge current and input current down to zero volts on the battery BAT voltage.
If there is a drop below 2 V, then the converter immediately turns off both high-side and low-side FETs to stop
current flow, then resumes regulating the current. This ensures there is no overcurrent surge that could cause
damage to the battery, charger, or system.
Battery Overvoltage Protection
As an extra level of protection, the bq24721 has an overvoltage protection function, in which the converter is
turned off when the BAT voltage is detected to be above 110% of the programmed battery regulation threshold.
During this overvoltage condition, a 4-mA current sink from the BAT pin to AGND pin turns on to reduce the
response time to bring down the battery voltage during otherwise no-load conditions such as battery removal or
battery disconnect for pulse-charge schemes. This reduces the battery node voltage overshoot and reduce the
battery voltage regulation response time, with low or no load conditions.
The converter stays off and the 4-mA current sink remains on until the battery voltage falls below 102% of the
programmed battery regulation threshold. Afterward, the converter reenables and continues regulating the BAT
voltage as normal.
Thermal Shutdown Protection
The QFN package has low thermal impedance which provides good thermal conduction from the silicon to the
ambient, to keep junctions temperatures low. As added level of protection, the charger converter turns off and
self-protects whenever the junction temperature exceeds the TSHUT threshold of 145°C. The charger stays off
until the junction temperature falls below 130°C.
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