BQ24171_15 Datasheet, PDF (17/39 Pages) Texas Instruments – bq24171 JEITA-Compliant Stand-Alone, Switched-Mode Li-Ion and Li-Polymer Battery Charger With Integrated MOSFETs and Power Path Selector

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bq24171
SLUSAF2C – FEBRUARY 2011 – REVISED APRIL 2015
Feature Description (continued)
Under high ambient temperature, the charge current will fold back to keep IC temperature not exceeding 120°C.
9.3.3 Battery Precharge Current Regulation
On power up, if the battery voltage is below the V_LOWV threshold, the bq24171 applies the precharge current to
the battery. This precharge feature is intended to revive deeply discharged cells. If the V_LOWV threshold is not
reached within 30 minutes of initiating precharge, the charger turns off and a FAULT is indicated on the status
pins.
For bq24171, the precharge current is set as 10% of the fast charge rate set by ISET voltage.
I_PRECHARGE
=
V_ISET
200 ´ R_SR
(3)
9.3.4 Input Current Regulation
The total input current from an AC adapter or other DC sources is a function of the system supply current and
the battery charging current. System current normally fluctuated as portions of the systems are powered up or
down. Without Dynamic Power Management (DPM), the source must be able to supply the maximum system
current and the maximum available charger input current simultaneously. By using DPM, the input current
regulator reduces the charging current when the summation of system power and charge power exceeds the
maximum input power. Therefore, the current capability of the AC adapter can be lowered, reducing system cost.
Input current is set by the voltage on ACSET pin using the following equation:
I_DPM
=
V_ACSET
20 ´ R_AC
(4)
The ACP and ACN pins are used to sense across RAC with default value of 10 mΩ. However, resistors of other
values can also be used. A larger sense resistor will give a larger sense voltage and higher regulation accuracy,
at the expense of higher conduction loss.
9.3.5 Charge Termination, Recharge, and Safety Timers
The charger monitors the charging current during the voltage regulation phase. Termination is detected when the
FB voltage is higher than recharge threshold and the charge current is less than the termination current
threshold, as calculated below:
I_TERM
=
V_ISET
200 ´ R_SR
where
• V_ISET is the voltage on the ISET pin.
• R_SR is the sense resistor.
(5)
There is a 25-ms deglitch time during transition between fast charge and precharge.
As a safety backup, the charger also provides an internal fixed 30-minute precharge safety timer and a
programmable fast charge timer. The fast charge time is programmed by the capacitor connected between the
TTC pin and AGND, and is given by the formula:
t_TTC = C_TTC ´K_TTC
where
• C_TTC is the capacitor connected to TTC.
• K_TTC is the constant multiplier.
(6)
A new charge cycle is initiated when one of the following conditions occurs:
• The battery voltage falls below the recharge threshold
• A power-on-reset (POR) event occurs
• ISET pin toggled below 40 mV (disable charge) and above 120 mV (enable charge)
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