BQ24735_17 Datasheet, PDF (18/46 Pages) Texas Instruments – 1- to 4-Cell Li+ Battery SMBus Charge Controller for Supporting Turbo Boost Mode With N-Channel Power MOSFET Selector

bq24735
SLUSAK9B – SEPTEMBER 2011 – REVISED APRIL 2015
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9.3.11 Inductor Short, MOSFET Short Protection
The bq24735 has a unique short-circuit protection feature. Its cycle-by-cycle current monitoring feature is
achieved through monitoring the voltage drop across R_DS(on) of the MOSFETs after a certain amount of blanking
time. In case of MOSFET short or inductor short circuit, the overcurrent condition is sensed by two comparators
and two counters will be triggered. After seven times of short circuit events, the charger will be latched off and
ACFET and RBFET are turned off to disconnect adapter from system. BATFET is turned on to connect battery
pack to system. To reset the charger from latch-off status, the IC VCC pin must be pulled below UVLO or the
ACDET pin must be pulled below 0.6 V. This can be achieved by removing the adapter and shutting down the
operation system. The low-side MOSFET short circuit voltage drop threshold can be adjusted through SMBus
command. ChargeOption() bit [7] = 0, 1 sets the low-side threshold to 135 mV and 230 mV, respectively. The
high-side MOSFET short circuit voltage drop threshold can be adjusted through SMBus command.
ChargeOption() bit [8] = 0, 1 disables the function and sets the threshold to 750 mV, respectively. During boost
function, if the low-side MOSFET short-circuit protection threshold is used for cycle-by-cycle current limiting, the
charger will not latch up.
Due to the certain amount of blanking time to prevent noise when MOSFET just turns on, the cycle-by-cycle
charge overcurrent protection may detect high current and turn off MOSFET first before the short circuit
protection circuit can detect short condition because the blanking time has not finished. In such a case, the
charger may not be able to detect short circuit and counter may not be able to count to seven then latch off.
Instead, the charger may continuously keep switching with very narrow duty cycle to limit the cycle-by-cycle
current peak value. However, the charger should still be safe and will not cause failure because the duty cycle is
limited to a very short of time and MOSFET should be still inside the safety operation area. During a soft start
period, it may take a long time instead of just seven switching cycles to detect short circuit based on the same
blanking time reason.
9.4 Device Functional Modes
9.4.1 Enable and Disable Charging
In Charge mode, the following conditions have to be valid to start charge:
• Charge is enabled through SMBus (ChargeOption() bit [0] = 0, default is 0, charge enabled).
• ILIM pin voltage is higher than 105 mV.
• All three regulation limit DACs have valid value programmed.
• ACOK is valid (see Adapter Detect and ACOK Output for details).
• ACFET and RBFET turns on and gate voltage is high enough (see System Power Selection for details).
• V_SRN does not exceed BATOVP threshold.
• IC Temperature does not exceed TSHUT threshold.
• Not in ACOC condition (see Input Overcurrent Protection (ACOC) for details).
One of the following conditions will stop ongoing charging:
• Charge is inhibited through SMBus (ChargeOption() bit [0] = 1).
• ILIM pin voltage lower than 75 mV.
• One of three regulation limit DACs is set to 0 or out of range.
• ACOK is pulled low (see Adapter Detect and ACOK Output for details).
• ACFET turns off.
• V_SRN exceeds BATOVP threshold.
• TSHUT IC temperature threshold is reached.
• ACOC is detected (see Input Overcurrent Protection (ACOC) for details).
• Short circuit is detected (see Inductor Short, MOSFET Short Protection for details).
• Watchdog timer expires if watchdog timer is enabled (see Charge Time-out for details).
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