Datasheet

bq24278
SLUSB04 –JUNE 2012 www.ti.com
Battery Detection
When termination conditions are met, a battery detection cycle is started. During battery detection, I
DETECT
is
pulled from V
BAT
for t
DETECT
to verify there is a battery. If the battery voltage remains above V
DETECT
for the full
duration of t
DETECT
, a battery is determined to be present and the IC enters “Charge Done”. If V
BAT
falls below
V
DETECT
, a “Battery Not Present” fault is signaled and battery detection continues. During the next cycle of battery
detection, the bq24278 turns on I
BATSHORT
for t
DETECT
. If V
BAT
rises to V
DETECT
, the current source is turned off and
after t
DETECT
, the battery detection continues through another current sink cycle. Battery detection continues until
charge is disabled or a battery is detected. Once a battery is detected, the fault status clears and a new charge
cycle begins. Battery detection is disabled when termination is disabled.
Dynamic Power Path Management
The bq24278 features a SYS output that powers the external system load connected to the battery. This output
is active whenever a source is connected to IN or BAT. The following sections discuss the behavior of SYS with
a source connected to the supply or a battery source only.
Input Source Connected
When a source is connected to IN, and the bq24278 is enabled, the buck converter starts up. If charging is
enabled using CE, the charge cycle is initiated. When V
BAT
> 3.5V, the SYS output is connected to V
BAT
. If the
SYS voltage falls to V
SYS(REG)
, it is regulated to that point to maintain the system output even with a deeply
discharged or absent battery. In this mode, the SYS output voltage is regulated by the buck converter and the
battery FET is linearly regulated to regulate the charge current into the battery. The current from the supply is
shared between charging the battery and powering the system load at SYS. The dynamic power path
management (DPPM) circuitry of the bq24278 monitors the SYS voltage continuously and if V
SYS
falls to V
MINSYS
,
adjusts charge current to maintain the load on SYS while preventing the system voltage from crashing. If the
charge current is reduced to zero and the load increases further, the bq24278 enters battery supplement mode.
During supplement mode, the battery FET is turned on and the battery supplements the system load. When the
charge current is reduced by the DPPM regulation loop, the safety timer runs at half speed, so that it is twice a
long. This prevents false safety timer faults. See the Safety Timer section for more details.
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