BQ25504 Datasheet, PDF (15/24 Pages) Texas Instruments – Ultra Low Power Boost Converter with Battery Management for Energy Harvester Applications

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bq25504
SLUSAH0 – OCTOBER 2011
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 undervoltage (VBAT_UV) threshold must be set using
external resistors. The VBAT_UV threshold voltage when the battery voltage is decreasing is given by
Equation 2:
V BAT _UV
æ
= VBIAS_ç1 +
R_UV2
ö
÷
è ^RUV1 ø
(2)
The sum of the resistors must be 10 MΩ i.e., R_UV1 + R_UV2 = 10 MΩ. The undervoltage threshold when battery
voltage is increasing is given by UV_HYST. It is internal set to the under voltage threshold plus an internal
hysteresis voltage denoted by VBAT_UV_HYST. For proper functioning of the IC and the overall system, the
load must be connected to the VSTOR pin while the storage element must be connected to the VBAT pin. Once
the VSTOR pin voltage goes above the UV_HYST threshold, the VSTOR pin and the VBAT pins are shorted.
The switch remains closed until the VSTOR pin voltage falls below the under voltage threshold. 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 3:
VBAT_OV =
3
V
æ
BI AS _ç1
+
R_OV2
ö
÷
2
è ^ROV1 ø
(3)
The sum of the resistors must be 10 MΩ i.e. 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
It should also be noted that if VIN_DC is higher than VSTOR and VSTOR is higher
than VBAT_OV, the input VIN_DC is shorted to ground to stop further charging of the
attached battery or capacitor. It is critical that if this case is expected, the source
impedance on VIN_DC is made higher than 20 Ω, it must not be 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 battery voltage is at an acceptable level. When the battery voltage is decreasing the
threshold is by Equation 4
æ
VBAT_OK_PROG = VBIAS_ç1 +
R_{O K2}
ö
÷
è ^ROK1 ø
(4)
When the battery voltage is increasing, the threshold is by Equation 5
VBAT_OK_HYST =
æ
VBIAS _ç1 +
R_OK2
+ R_OK3
ö
÷
è
R_{O K1}
ø
(5)
Copyright © 2011, Texas Instruments Incorporated
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