Datasheet

T32min Active?
Yes
No
Charge
HostShouldReset
T32sec Timer
T32min
Expired?
Start T32min
Timer
ChargeStart
No
TimerFault
Yes
T32secExpired?
Yes
No
ResetCharge
Parameters
AnyI
2
CWrite-
Action?
Yes
Start T32sec
Stop T32min
No
bq24155
SLUS942 –FEBRUARY 2010 www.ti.com
Figure 14. Timer Flow Chart for bq24155 in Charge Mode
USB Friendly Boot-Up Sequence
At power on reset (POR) of VBUS, if the battery voltage is above the weak battery threshold, V
LOWV
, bq24155
operates in a mode dictated by the I
2
C control registers. On the other hand, if the battery voltage is below V
LOWV
and the host control through I
2
C interface is lost (32 minute mode), bq24155 resets all I
2
C registers with default
values and enables the charger with an input current limit dictated by the ISEL pin voltage level until the host
programs the I
2
C registers. During this period, the input current limit is 100 mA when the voltage level of ISEL
pin is Low; while the input current limit is 500 mA when the voltage level of ISEL pin is high. This feature could
quickly revive a deeply discharged cell. The charge process continues after the battery is charged to the
regulation voltage (default at 3.54 V) since termination is disabled by default. In another case, if the battery
voltage is below V
LOWV
but the host control using I
2
C interface is available (32 second mode), bq24155 operates
in a mode dictated by control registers.
Input Current Limiting
To maximize the charge rate of bq24155 without overloading the USB port, the input current for bq24155 can be
limited to 100mA or 500mA which is programmed in the control register or ISEL pin. Once the input current
reaches the input current limiting threshold, the charge current is reduced to prevent the input current from
exceeding the programmed threshold. For bq24155, the default input current limit is controlled by the ISEL pin at
VBUS power on reset when V
(AUXPWR)
is lower than V
(LOWV)
. The input current sensing resistor and control loop
are integrated into bq24155. The input current limit can also be disabled using I
2
C control, see the definition of
control register (01H) for details.
Thermal Regulation and Protection
To prevent overheating the chip during the charging process, the bq24155 monitors the junction temperature, T
J
,
of the die and begins to taper down the charge current once T
J
reaches the thermal regulation threshold, T
CF
.
The charge current is reduced to zero when the junction temperature increases approximately 10°C above T
CF
.
At any state, if T
J
exceeds T
SHTDWN
, bq24155 suspends charging. At thermal shutdown mode, PWM is turned off
and all timers are frozen. Charging resumes when T
J
falls below T
SHTDWN
by approximately 10°C.
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