BQ24705 Datasheet, PDF (16/30 Pages) Texas Instruments – Host-Controlled Multi-Chemistry Battery Charger With Low Input Power Detect

bq24705
SLUS779 – DECEMBER 2007
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I _ADAPTER
+
V_ACSET
V_VREF
0.10
R_AC
(3)
The input voltage range of ACSET is between 0 and VREF, up to 3.3 V.
The ACP and ACN pins are used to sense R_AC with default value of 10mΩ. However, resistors of other values
can also be used. A larger the sense resistor, gives a larger sense voltage, and a higher regulation accuracy;
but, at the expense of higher conduction loss.
ADAPTER DETECT AND POWER UP
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, and lower than
the minimum allowed adapter voltage. The ACDET divider should be placed before the ACFET in order to sense
the true adapter input voltage whether the ACFET is on or off. Before the adapter is detected, BATFET stays on
and ACFET turns off.
If PVCC is below 5 V, the device is disabled, and both ACFET and BATFET turn off.
If ACDET is below 0.6 V but PVCC is above 5 V, part of the bias is enabled, including a crude bandgap
reference, ACFET drive and BATFET drive. IADAPT is disabled and pulled down to GND. The total quiescent
current is less than 10 µA.
Once ACDET rises above 0.6 V and PVCC is above 5 V, all the bias circuits are enabled and REGN output goes
to 6 V and VREF goes to 3.3 V. IADAPT becomes valid to proportionally reflect the adapter current.
When ACDET rises and passes 2.4 V, a valid AC adapter is present. 500 ms later, the following occurs:
• ACGOOD becomes high through external pull-up resistor to the host digital voltage rail;
• Charger turns on if all the conditions are satisfied and STAT becomes valid. (see Enable and Disable
Charging)
ENABLE AND DISABLE CHARGING
The following conditions must be valid before a charge is enabled:
• CHGEN is LOW;
• Adapter is detected;
• Adapter is higher than PVCC-BAT threshold;
• Adapter is not over voltage;
• 500 ms delay is complete after adapter detected;
• REGNGOOD and VREFGOOD are valid;
• Thermal Shut (TSHUT) is not valid;
One of the following conditions will stop on-going charging:
• CHGEN is HIGH;
• Adapter is removed;
• Adapter is less than 250 mV above battery;
• Adapter is over voltage;
• Adapter is over current;
• TSHUT IC temperature threshold is reached (145°C on rising-edge with 15°C hysteresis).
AUTOMATIC INTERNAL SOFT-START CHARGER CURRENT
The charger automatically soft-starts the charger regulation current every time the charger is enabled to ensure
there is no overshoot or stress on the output capacitors or the power converter. The soft-start consists of
stepping-up the charge regulation current into 8 evenly divided steps up to the programmed charge current. Each
step lasts around 1ms, for a typical rise time of 8 ms. No external components are needed for this function.
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