BQ24735 Datasheet, PDF (22/42 Pages) Texas Instruments – 1-4 Cell Li+ Battery SMBus Charge Controller for Supporting Turbo Boost Mode with N-Channel Power MOSFET Selector

bq24735
SLUSAK9 – SEPTEMBER 2011
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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 pull up resistor to system digital rail for a high level. It can be
pulled to external rail under the following conditions:
• V_VCC > UVLO;
• 2.4V < V_ACDET < 3.15V (not in ACOVP condition, nor in low input voltage condition);
• V_VCC–V_SRN > 275mV (not in sleep mode);
The first time after IC POR always gives 150ms ACOK rising edge delay no matter what the ChargeOption
register value is. Only after the ACDET pin voltage is pulled below 2.4V (but not below 0.6V, which resets the IC
and forces the next ACOK rising edge deglitch time to be 1.3s) and the ACFET has been turned off at least one
time, the 1.3s (or 150ms) delay time is effective for the next time the ACDET pin voltage goes above 2.4V. To
change this option, the VCC pin voltage must above UVLO, and the ACDET pin voltage must be above 0.6V
which enables the IC SMBus communication and sets ChargeOption() bit[15] to 0 which sets the next ACOK
rising deglitch time to be 150ms. The purpose of the default 1.3s rising edge deglitch time is to turn off the
ACFET long enough when the ACDET pin is pulled below 2.4V by excessive system current, such as over
current or short circuit.
Adapter Over Voltage (ACOVP)
When the ACDET pin voltage is higher than 3.15V, it is considered as adapter over voltage. 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 turns on conditions are valid. See the System Power Selection
section for details.
When ACDET pin voltage falls below 3.15V and above 2.4V, it is considered as adapter voltage returns back to
normal voltage. ACOK will be pulled high by external pull up 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 the Enable and Disable Charging section for details.
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 150ms delay (first time, the next time default is 1.3s and can be changed to 150ms) 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 1 for details). The ACFET separates adapter from battery or system, and
provides a limited di/dt when plugging in adapter by controlling the ACFET turn-on 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 + 6V to connect battery to system if all the following conditions are
valid:
• V_VCC > UVLO;
• V_SRN > UVLO;
• V_ACN < 200mV above V_SRN (ACN_SRN comparator);
Approximately 150ms (first time; the next time default is 1.3s and can be changed to 150ms) after the adapter is
detected (ACDET pin voltage between 2.4V and 3.15V), 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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