SLUSAH0C – OCTOBER 2011 – REVISED JUNE 2015
Feature Description (continued)
The internal MPPT circuitry and the periodic sampling of VIN_DC can be disabled by tying the VOC_SAMP pin
to VSTOR. An external reference voltage can be fed to the VREF_SAMP pin. The boost converter will then
regulate VIN_DC to the externally provided reference. If input regulation is not desired (i.e. the input source is a
low-impedance output battery or power supply instead of a high impedance output energy harvester),
VREF_SAMP can be tied to GND.
8.3.2 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
V BAT _UV
= VBIASç1 +
è RUV1 ø
The sum of the resistors is recommended to be no higher than 10 MΩ that is, RUV1 + RUV2 = 10 MΩ.
Spreadsheet SLURAQ1 provides help on sizing and selecting the resistors.
The undervoltage threshold when the battery voltage is increasing is VBAT_UV plus an internal hysteresis
voltage denoted by VBAT_UV_HYST. For the VBAT_UV feature to function properly, the load must be
connected to the VSTOR pin while the storage element should be connected to the VBAT pin. Once the VSTOR
pin voltage goes above VBAT_UV plus VBAT_UV_HYST threshold, the VSTOR pin and the VBAT pins are
effectively shorted through an internal PMOS FET. The switch remains closed until the VSTOR pin voltage falls
below the VBAT_UV threshold. The VBAT_UV threshold should be considered a fail safe to the system. The
system load should be removed or reduced based on the VBAT_OK threshold which should be set above the
8.3.3 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:
BI AS ç1
è ROV1 ø
The sum of the resistors is recommended to be no higher 10 MΩ that is, ROV1 + ROV2 = 10 MΩ. Spreadsheet
SLURAQ1 provides help with sizing and selecting the resistors.
The overvoltage threshold when the battery voltage is decreasing is given by VBAT_OV - VBAT_OV_HYST.
Once the voltage at the battery reaches the VBAT_OV threshold, the boost converter is disabled. The charger
will start again once the battery voltage drop by VBAT_OV_HYST. When there is excessive input energy, the
VBAT pin voltage will ripple between the VBAT_OV and the VBAT_OV - VBAT_OV_HYST levels.
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.
8.3.4 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:
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