Datasheet

1
æ ö
ç ÷
è ø
OK2 OK3
O K1
R + R
VBAT_OK_HYST = VBIAS +
R
1
æ ö
ç ÷
è ø
O K2
OK1
R
VBAT_OK_PROG = VBIAS +
R
bq25504
SLUSAH0A –OCTOBER 2011–REVISED SEPTEMBER 2012 www.ti.com
The sum of the resistors should be approximately 10 MΩ that is, R
OV1
+ R
OV2
= 10 MΩ. The overvoltage
threshold when battery voltage is decreasing is given by OV_HYST. It is internal set to the over voltage threshold
minus an internal hysteresis voltage denoted by VBAT_OV_HYST. Once the voltage at the battery exceeds
VBAT_OV threshold, the boost converter is disabled. The charger will start again once the battery voltage falls
below the VBAT_OV_HYST level. When there is excessive input energy, the VBAT pin voltage will ripple
between the VBAT_OV and the VBAT_OV_HYST levels.
CAUTION
If VIN_DC is higher than VSTOR and VSTOR is higher than VBAT_OV, the input
VIN_DC is pulled to ground through a small resistance to stop further charging of the
attached battery or capacitor. It is critical that if this case is expected, the impedance of
the source attached to VIN_DC be higher than 20 Ω and not a low impedance source.
Battery Voltage in Operating Range (VBAT_OK Output)
The IC allows the user to set a programmable voltage independent of the overvoltage and undervoltage settings
to indicate whether the VSTOR voltage (and therefore the VBAT voltage when the PFET between the two pins is
turned on) is at an acceptable level. When the battery voltage is decreasing the threshold is set by Equation 4
(4)
When the battery voltage is increasing, the threshold is set by Equation 5
(5)
The sum of the resistors should be approximately 10 MΩ i.e., R
OK1
+ R
OK2
+ R
OK3
= 10 MΩ. The logic high level
of this signal is equal to the VSTOR voltage and the logic low level is ground. The logic high level has ~20 KΩ
internally in series to limit the available current to prevent MCU damage until it is fully powered. The
VBAT_OK_PROG threshold must be greater than or equal to the UV threshold. For the best operation of the
system, the VBAT_OK should be setup to drive an external PFET between VSTOR and the system load in order
to determine when the load can be applied or removed to optimize the storage element capacity.
Thermal Shutdown
Rechargeable Li-ion batteries need protection from damage due to operation at elevated temperatures. The
application should provide this battery protection and ensure that the ambient temperature is never elevated
greater than the expected operational range of 85°C.
The bq25504 uses an integrated temperature sensor to monitor the junction temperature of the device. If the
OT_PROG pin is tied low, then the temperature threshold for thermal protection is set to TSD_ProtL which is
65°C typically. If the OT_PROG is tied high, then the temperature is set to TSD_ProtH which is 120°C typically.
Once the temperature threshold is exceeded, the boost converter/charger is disabled and charging ceases. Once
the temperature of the device drops below this threshold, the boost converter and or charger can resume
operation. To avoid unstable operation near the overtemp threshold, a built-in hysteresis of approximately 5°C
has been implemented. Care should be taken to not over discharge the battery in this condition since the boost
converter/charger is disabled. However, if the supply voltage drops to the VBAT_UV setting, then the switch
between VBAT and VSTOR will open and protect the battery even if the device is in thermal shutdown.
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