BQ24070 Datasheet, PDF (18/26 Pages) Texas Instruments – SINGLE-CHIP CHARGE AND SYSTEM POWER-PATH MANAGEMENT IC

bq24070
SLUS694A – MARCH 2006 – REVISED MARCH 2006
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FUNCTIONAL DESCRIPTIONS (continued)
Battery Voltage Regulation
The voltage regulation feedback is through the BAT pin. This input is tied directly to the positive side of the
battery pack. The bq24070 monitors the battery-pack voltage between the BAT and VSS pins. When the battery
voltage rises to the V_O(REG) threshold, the voltage regulation phase begins and the charging current begins to
taper down.
If the battery is absent, the BAT pin cycles between charge done (V_O(REG)) and charging (battery recharge
threshold, ~4.1 V).
See Figure 7 for power up by battery insertion.
As a safety backup, the bq24070 also monitors the charge time in the charge mode. If charge is not terminated
within this time period, t_(CHG), the bq24070 turns off the charger and enunciates FAULT on the STAT1 and
STAT2 pins. See the DPPM operation under Case 1 for information on extending the safety timer during DPPM
operation. See theTimer Fault Recovery section for additional details.
Temperature Regulation and Thermal Protection
In order to maximize charge rate, the bq24070 features a junction temperature regulation loop. If the power
dissipation of the IC results in a junction temperature greater than the T_J(REG) threshold, the bq24070 throttles
back on the charge current in order to maintain a junction temperature around the T_J(REG) threshold. To avoid
false termination, the termination detect function is disabled while in this mode.
The bq24070 also monitors the junction temperature, T_J, of the die and disconnects the OUT pin from the IN
input if T_J exceeds T_(SHTDWN). This operation continues until T_J falls below T_(SHTDWN) by the hysteresis level
specified in the specification table.
The battery supplement mode has no thermal protection. The Q2 FET continues to connect the battery to the
output (system), if input power is not sufficient; however, a short-circuit protection circuit limits the battery
discharge current such that the maximum power dissipation of the part is not exceeded under typical design
conditions.
Charge Timer Operation
As a safety backup, the bq24070 monitors the charge time in the charge mode. If the termination threshold is
not detected within the time period, t_(CHG), the bq24070 turns off the charger and enunciates FAULT on the
STAT1 and STAT2 pins. The resistor connected between the TMR and VSS, R_TMR, determines the timer period.
The K_(TMR) parameter is specified in the specifications table. In order to disable the charge timer, eliminate R_TMR,
connect the TMR pin directly to the V_REF pin. Note that this action eliminates all safety timers, and also clears
any timer fault. TMR pin should not be left floating.
t_(CHG) + K_(TMR) R_(TMR)
(5)
While in the thermal regulation mode or DPPM mode, the bq24070 dynamically adjusts the timer period in order
to provide the additional time needed to fully charge the battery. This proprietary feature is designed to prevent
against early or false termination. The maximum charge time in this mode, t_(CHG-TREG), is calculated by
Equation 6.
t _{(CHG−TREG)}
+
t_(CHG) V_(SET)
V_(SET*REG)
(6)
Note that because this adjustment is dynamic and changes as the ambient temperature changes and the charge
level changes, the timer clock is adjusted. It is difficult to estimate a total safety time without integrating the
above equation over the charge cycle. Therefore, understanding the theory that the safety time is adjusted
inversely proportionately with the charge current and the battery is a current-hour rating, the safety time
dynamically adjusts appropriately.
The V_(SET) parameter is specified in the specifications table. V_(SET-TREG) is the voltage on the ISET pin during the
thermal regulation or DPPM mode and is a function of charge current. (Note that charge current is dynamically
adjusted during the thermal regulation or DPPM mode.)
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