BQ24751B Datasheet, PDF (23/38 Pages) Texas Instruments – Host-Controlled Multi-Chemistry Battery Charger with Low Iq and System Power Selector

Not Recommended For New Designs
bq24751B
www.ti.com .......................................................................................................................................................... SLUS835A – JULY 2008 – REVISED MARCH 2009
Battery Overvoltage Protection
The converter stops switching when BAT voltage goes above 104% of the regulation voltage. The converter will
not allow the high-side FET to turn on until the BAT voltage goes below 102% of the regulation voltage. This
allows one-cycle response to an overvoltage condition, such as when the load is removed or the battery is
disconnected. A 10-mA current sink from BAT to PGND is on only during charge, and allows discharging the
stored output-inductor energy into the output capacitors.
Battery Shorted (Battery Undervoltage) Protection
The bq24751B has a BAT SHORT comparator monitoring the output battery voltage (BAT). If the voltage falls
below 2.9 V per cell (5.8 V for 2 cells, 8.7 V for 3 cells, 11.6 V for 4 cells), a battery-short status is detected.
Below the BAT_SHORT threshold, the charger reduces the charge current to 1/8th of the programmed charging
current (0.1×SRSET/VDAC)/8 = C/8 down to zero volts on BAT pin.. This lower current is used as a pre-charge
current for over-discharged battery packs. Above the BAT_SHORT threshold (plus hysteresis, the charge current
resumes at the programmed value (0.1×SRSET/VDAC).
The BAT_SHORT comparator also serves as a depleted-battery alarm during a LEARN cycle. If the selector is in
a LEARN cycle, and the battery voltage falls bellow the BAT_SHORT threshold, the selector disconnects the
battery from the system and connects the adapter to the system in order to protect the battery pack. If battery
voltage increases, and LEARN is still logic high, then the selector disconnects the adapter from the system and
reconnects the battery to the system.
Charge Overcurrent Protection
The charger has a secondary overcurrent protection feature. It monitors the charge current, and prevents the
current from exceeding 145% of regulated charge current. The high-side gate drive turns off when the
overcurrent is detected, and automatically resumes when the current falls below the overcurrent threshold.
Thermal Shutdown Protection
The QFN package has low thermal impedance, which provides good thermal conduction from the silicon to the
ambient, to keep junction temperatures low. As an added level of protection, the charger converter turns off and
self-protects when the junction temperature exceeds the TSHUT threshold of 155°C. The charger stays off until
the junction temperature falls below 135°C.
Adapter Detected Status Register (ACGOOD Pin)
One status output is available, and it requires an external pullup resistor to pull the pin to the system digital rail
for a high level.
ACGOOD goes low when ACDET is above 2.4 V and the 700-ms delay time is over. It indicates that the adapter
voltage is high enough for normal operation.
Input Overpower Protection (ACOP)
The ACOC/ACOP circuit provides a reliable layer of safety protection that can complement other safety
measures. ACOC/ACOP helps to protect from input current surge due to various conditions including:
• Adapter insertion and system selector connecting adapter to system where system capacitors need to charge
• Learn mode exit when adapter is reconnected to the system; system load overcurrent surge
• System shorted to ground
• Battery shorted to ground
• Phase shorted to ground
• High-side FET shorted from drain to source (SYSTEM shorted to PH)
• BATFET shorted from drain to source (SYSTEM shorted to BAT)
Several examples of the circuit protecting from these fault conditions are shown below.
For designs using the selector functions, an input overcurrent (ACOC) and input overpower protection function
(ACOP) is provided. The threshold is set by an external capacitor from the ACOP pin to AGND. After the adapter
Copyright © 2008–2009, Texas Instruments Incorporated
Product Folder Link(s) :bq24751B
Submit Documentation Feedback
23