BQ24735_17 Datasheet, PDF (15/46 Pages) Texas Instruments – 1- to 4-Cell Li+ Battery SMBus Charge Controller for Supporting Turbo Boost Mode With N-Channel Power MOSFET Selector

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9.3 Feature Description
bq24735
SLUSAK9B – SEPTEMBER 2011 – REVISED APRIL 2015
9.3.1 Adapter Detect and ACOK Output
The bq24735 uses an ACOK comparator to determine the source of power on VCC pin, either from the battery or
adapter. An external resistor voltage divider attenuates the adapter voltage before it goes to ACDET. The
adapter detect threshold should typically be programmed to a value greater than the maximum battery voltage,
but lower than the maximum allowed adapter voltage.
The open-drain ACOK output requires external pullup resistor to system digital rail for a high level. It can be
pulled to external rail under the following conditions:
• V_VCC > UVLO
• 2.4 V < V_ACDET < 3.15 V (not in ACOVP condition, nor in low input voltage condition)
• V_VCC – V_SRN > 275 mV (not in sleep mode)
The first time after IC POR always gives 150-ms ACOK rising edge delay no matter what the ChargeOption
register value is. Only after the ACDET pin voltage is pulled below 2.4 V (but not below 0.6 V, which resets the
IC and forces the next ACOK rising edge deglitch time to be 1.3 s) and the ACFET has been turned off at least
one time, the 1.3 s (or 150 ms) delay time is effective for the next time the ACDET pin voltage goes above 2.4 V.
To change this option, the VCC pin voltage must above UVLO, and the ACDET pin voltage must be above 0.6 V
which enables the IC SMBus communication and sets ChargeOption() bit [15] to 0 which sets the next ACOK
rising deglitch time to be 150 ms. The purpose of the default 1.3 s rising edge deglitch time is to turn off the
ACFET long enough when the ACDET pin is pulled below 2.4 V by excessive system current, such as
overcurrent or short circuit.
9.3.2 Adapter Overvoltage (ACOVP)
When the ACDET pin voltage is higher than 3.15 V, it is considered as adapter overvoltage. ACOK will be pulled
low, and charge will be disabled. ACFET will be turned off to disconnect the high voltage adapter to system
during ACOVP. BATFET will be turned on if turnon conditions are valid. See System Power Selection for details.
When ACDET pin voltage falls below 3.15 V and above 2.4 V, it is considered as adapter voltage returns back to
normal voltage. ACOK will be pulled high by external pullup resistor. BATFET will be turned off and ACFET and
RBFET will be turned on to power the system from adapter. The charge can be resumed if enable charge
conditions are valid. See Enable and Disable Charging for details.
9.3.3 System Power Selection
The bq24735 automatically switches adapter or battery power to system. The battery is connected to system at
POR if battery exists. The battery is disconnected from system and the adapter is connected to system after
default 150 ms delay (first time, the next time default is 1.3 s and can be changed to 150 ms) if ACOK goes
HIGH. An automatic break-before-make logic prevents shoot-through currents when the selectors switch.
The ACDRV drives a pair of common-source (CMSRC) N-channel power MOSFETs (ACFET and RBFET)
between adapter and ACP (see Figure 16 for details). The ACFET separates adapter from battery or system, and
provides a limited DI/DT when plugging in adapter by controlling the ACFET turnon time. Meanwhile it protects
adapter when system or battery is shorted. The RBFET provides negative input voltage protection and battery
discharge protection when adapter is shorted to ground, and minimizes system power dissipation with its low
R_DS(on) compared to a Schottky diode.
When the adapter is not present, ACDRV is pulled to CMSRC to keep ACFET and RBFET off, disconnecting
adapter from system. BATDRV stays at V_SRN + 6 V to connect battery to system if all the following conditions are
valid:
• V_VCC > UVLO
• V_SRN > UVLO
• V_ACN < 200 mV above V_SRN (ACN_SRN comparator)
Approximately 150 ms (first time; the next time default is 1.3 s and can be changed to 150 ms) after the adapter
is detected (ACDET pin voltage from 2.4 V to 3.15 V), the system power source begins to switch from battery to
adapter if all the following conditions are valid:
• Not in LEARN mode or in LEARN mode and V_SRN is lower than battery depletion threshold
• ACOK high
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