BQ24171 Datasheet, PDF (17/35 Pages) Texas Instruments – JEITA Compliant Stand-Alone Switch-Mode Li-Ion and Li-Polymer Battery Charger with Integrated MOSFETs and Power Path Selector

bq24171
www.ti.com
SLUSAF2A – FEBRUARY 2011 – REVISED MAY 2011
BATTERY PRECHARGE CURRENT REGULATION
On Power-up, if the battery voltage is below the V_LOWV threshold, the bq24171 applies the pre-charge current to
the battery. This pre-charge feature is intended to revive deeply discharged cells. If the VLOWV threshold is not
reached within 30 minutes of initiating pre-charge, the charger turns off and a FAULT is indicated on the status
pins.
For bq24171, the pre-charge current is set as 10% of the fast charge rate set by ISET voltage.
I_PRECHARGE
=
V_ISET
200 ´ R_SR
(3)
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 10mΩ. 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.
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
(5)
where V_ISET is the voltage on the ISET pin and R_SR is the sense resistor. There is a 25ms deglitch time during
transition between fast-charge and pre-charge.
As a safety backup, the charger also provides an internal fixed 30 minutes pre-charge 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
(6)
Where C_TTC is the capacitor connected to TTC and K_TTC is the constant multiplier.
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 40mV (disable charge) and above 120mV (enable charge)
Pull TTC pin to AGND to disable both termination and fast charge safety timer (reset timer). Pull TTC pin to
VREF to disable the safety timer, but allow charge termination.
Copyright © 2011, Texas Instruments Incorporated
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