BQ24735 Datasheet, PDF (24/42 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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Enable and Disable Charging
In Charge mode, the following conditions have to be valid to start charge:
• Charge is enabled via SMBus (ChargeOption() bit [0]=0, default is 0, charge enabled);
• ILIM pin voltage higher than 105mV;
• All three regulation limit DACs have valid value programmed;
• ACOK is valid (See the Adapter Detect and ACOK Output section for details);
• ACFET and RBFET turns on and gate voltage is high enough (See the System Power Selection section for
details);
• V_SRN does not exceed BATOVP threshold;
• IC Temperature does not exceed TSHUT threshold;
• Not in ACOC condition (See the Input Over Current Protection (ACOC) section for details);
One of the following conditions will stop on-going charging:
• Charge is inhibited via SMBus (ChargeOption() bit[0]=1);
• ILIM pin voltage lower than 75mV;
• One of three regulation limit DACs is set to 0 or out of range;
• ACOK is pulled low (See the Adapter Detect and ACOK Output section for details);
• ACFET turns off;
• V_SRN exceeds BATOVP threshold;
• TSHUT IC temperature threshold is reached;
• ACOC is detected (See the Input Over Current Protection (ACOC) section for details);
• Short circuit is detected (See the Inductor Short, MOSFET Short Protection section for details);
• Watchdog timer expires if watchdog timer is enabled (See the Charger Timeout section for details);
Automatic Internal Soft-Start Charger Current
Every time the charge is enabled, the charger automatically applies soft-start on charge current to avoid any
overshoot or stress on the output capacitors or the power converter. The charge current starts at 128mA, and the
step size is 64mA in CCM mode for a 10mΩ current sensing resistor. Each step lasts around 240µs in CCM
mode, till it reaches the programmed charge current limit. No external components are needed for this function.
During DCM mode, the soft start up current step size is larger and each step lasts for longer time period due to
the intrinsic slow response of DCM mode.
High Accuracy Current Sense Amplifier
As an industry standard, high accuracy current sense amplifier (CSA) is used to monitor the input current or the
charge current, selectable via SMBUS (ChargeOption() bit[5]=0 select the input current, bit[5]=1 select the
charge current) by host. The CSA senses voltage across the sense resistor by a factor of 20 through the IOUT
pin. Once VCC is above UVLO and ACDET is above 0.6V, CSA turns on and IOUT output becomes valid. To
lower the voltage on current monitoring, a resistor divider from IOUT to GND can be used and accuracy over
temperature can still be achieved.
A 100pF 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.
Charge Timeout
The bq24735 includes a watchdog timer to terminate charging if the charger does not receive a write
ChargeVoltage() or write ChargeCurrent() command within 175s (adjustable via ChargeOption() command). If a
watchdog timeout occurs all register values keep unchanged but charge is suspended. Write ChargeVoltage() or
write ChargeCurrent() commands must be re-sent to reset watchdog timer and resume charging. The watchdog
timer can be disabled, or set to 44s, 88s or 175s via SMBus command (ChargeOption() bit[14:13]). After
watchdog timeout write ChargeOption() bit[14:13] to disable watchdog timer also resume charging.
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