BQ24075-Q1_15 Datasheet, PDF (22/38 Pages) Texas Instruments – SLUSAU3B –FEBRUARY 2012–REVISED MARCH 2012

bq24075-Q1
SLUSAU3B – FEBRUARY 2012 – REVISED MARCH 2012
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BATTERY DETECTION AND RECHARGE
The bq24075-Q1 automatically detects if a battery is connected or removed. Once a charge cycle is complete,
the battery voltage is monitored. When the battery voltage falls below V_RCH, the battery detection routine is run.
During battery detection, current (I_BAT(DET)) is pulled from the battery for a duration t_DET to see if the voltage on
BAT falls below V_LOWV. If not, charging begins. If it does, then it indicates that the battery is missing or the
protector is open. Next, the precharge current is applied for t_DET to close the protector if possible. If V_BAT < V_RCH,
then the protector closed and charging is initiated. If V_BAT > V_RCH, then the battery is determined to be missing
and the detection routine continues.
BATTERY DISCONNECT (SYSOFF Input)
The bq24075 feature a SYSOFF input that allows the user to turn the FET Q2 off and disconnect the battery
from the OUT pin. This is useful for disconnecting the system load from the battery, factory programming where
the battery is not installed or for host side impedance track fuel gauging, such as bq27500, where the battery
open circuit voltage level must be detected before the battery charges or discharges. The /CHG output remains
low when SYSOFF is high. Connect SYSOFF to VSS, to turn Q2 on for normal operation. SYSOFF is internally
pulled to VBAT through ~5 MΩ resistor.
DYNAMIC CHARGE TIMERS (TMR Input)
The bq24075-Q1 device contains internal safety timers for the pre-charge and fast-charge phases to prevent
potential damage to the battery and the system. The timers begin at the start of the respective charge cycles.
The timer values are programmed by connecting a resistor from TMR to VSS. The resistor value is calculated
using the following equation:
t_PRECHG = K_TMR × R_TMR
t_MAXCHG = 10 × K_TMR × R_TMR
Leave TMR unconnected to select the internal default timers. Disable the timers by connecting TMR to VSS.
Note that timers are suspended when the device is in thermal shutdown, and the timers are slowed proportionally
to the charge current when the device enters thermal regulation.
During the fast charge phase, several events increase the timer durations.
1. The system load current activates the DPPM loop which reduces the available charging current
2. The input current is reduced because the input voltage has fallen to V_IN(LOW)
3. The device has entered thermal regulation because the IC junction temperature has exceeded T_J(REG)
During each of these events, the internal timers are slowed down proportionately to the reduction in charging
current. For example, if the charging current is reduced by half for two minutes, the timer clock is reduced to half
the frequency and the counter counts half as fast resulting in only one minute of "counting" time.
If the pre charge timer expires before the battery voltage reaches V_LOWV, the bq24075-Q1 indicates a fault
condition. Additionally, if the battery current does not fall to I_TERM before the fast charge timer expires, a fault is
indicated. The CHG output flashes at approximately 2 Hz to indicate a fault condition. The fault condition is
cleared by toggling CE or the input power, entering/ exiting USB suspend mode, or an OVP event.
STATUS INDICATORS (PGOOD, CHG)
The bq24075-Q1 contains two open-drain outputs that signal its status. The PGOOD output signals when a valid
input source is connected. PGOOD is low when (V_BAT + V_IN(DT)) < V_IN < V_OVP. When the input voltage is outside
of this range, PGOOD is high impedance.
The charge cycle after power-up, CE going low, or exiting OVP is indicated with the CHG pin on (low - LED on),
whereas all refresh (subsequent) charges will result in the CHG pin off (open - LED off). In addition, the CHG
signals timer faults by flashing at approximately 2 Hz.
Input State
V_IN < V_UVLO
V_UVLO < V_IN < V_IN(DT)
V_IN(DT) < V_IN < V_OVP
Table 2. PGOOD STATUS INDICATOR
PGOOD Output
Hi impedance
Hi impedance
Low
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