BQ25570_14 Datasheet, PDF (25/41 Pages) Texas Instruments – Ultra Low Power Harvester Power Management IC with Boost Charger, and Nano-Powered Buck Converter

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SLUSBH2C – MARCH 2013 – REVISED JANUARY 2014
Battery Undervoltage Protection
To prevent rechargeable batteries from being deeply discharged and damaged, and to prevent completely
depleting charge from a capacitive storage element, the boost charger has an internally set undervoltage
(VBAT_UV) threshold plus an internal hysteresis voltage (VBAT_UV_HYST). The VBAT_UV threshold voltage
when the battery voltage is decreasing is internally set to 1.95V (typical). The undervoltage threshold when
battery voltage is increasing is given by VBAT_UV plus VBAT_UV_HYST. For most applications, the system load
should be connected to the VSTOR pin while the storage element should be connected to the VBAT pin. Once
the VSTOR pin voltage goes above the VBAT_UV_HYST threshold, the VSTOR pin and the VBAT pins are
shorted. The switch remains closed until the VSTOR pin voltage falls below VBAT_UV. The VBAT_UV threshold
should be considered a fail safe to the system and the system load should be removed or reduced based on the
VBAT_OK signal.
Battery Overvoltage Protection
To prevent rechargeable batteries from being exposed to excessive charging voltages and to prevent over
charging a capacitive storage element, the over-voltage (VBAT_OV) threshold level must be set using external
resistors. This is also the voltage value to which the charger will regulate the VSTOR/VBAT pin when the input
has sufficient power. The VBAT_OV threshold when the battery voltage is rising is given by Equation 2:
VBAT_OV =
3
V
æ
BI AS _ç1
+
R_OV2
ö
÷
2
è ^ROV1 ø
(2)
The sum of the resistors is recommended to be no higher than 13 MΩ that is, R_OV1 + R_OV2 = 13 MΩ. The
overvoltage threshold when battery voltage is decreasing is given by VBAT_OV_HYST. It is internally 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 charger 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. SLUC484 provides help on sizing
and selecting the resistors.
CAUTION
If VIN_DC is higher than VSTOR and VSTOR is equal to 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 within Operating Range (VBAT_OK Output)
The charger 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 3:
æ
VBAT_OK_PROG = VBIAS_ç1 +
R_{O K2}
ö
÷
è ^ROK1 ø
(3)
When the battery voltage is increasing, the threshold is set by Equation 4:
VBAT_OK_HYST =
æ
VBIAS _ç1 +
R_OK2
+ R_OK3
ö
÷
è
R_{O K1}
ø
(4)
The sum of the resistors is recommend to be no higher than approximately i.e., R_OK1 + R_OK2 + R_OK3= 13 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. SLUC484
provides help on sizing and selecting the resistors.
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