SLUSAH0C – OCTOBER 2011 – REVISED JUNE 2015
Device Functional Modes (continued)
If the VSTOR pin voltage remains above the internal under voltage threshold (VBAT_UV) for the additional 64 ms
after the VRDIV initialization pulse (following the 45-ms PFET on time), the internal PFET turns back on and the
main boost charger begins to charge the storage element assuming there is sufficient power available from the
harvester at the VIN_DC pin. If VSTOR does not reach the VBAT_UV threshold, then the PFET remains off until
the main boost charger can raise the VSTOR voltage to VBAT_UV. If a system load tied to VSTOR discharges
VSTOR below VSTOR_GEN or below VBAT_UV during the 32 ms initial MPPT reference voltage measurement
or within 110 ms after hot plug, it is recommended to add an external PFET between the system load and
VSTOR. An inverted VBAT_OK signal can be used to drive the gate of this system-isolating, external PFET.
Otherwise, the VSTOR voltage waveform will have incremental pulses as the IC turns on and off the internal
PFET controlled by VBAT_UV or cycles between cold-start and main boost charger operation.
Once VSTOR is above VSTOR_CHGEN, the main boost charger employs pulse frequency modulation (PFM)
mode of control to regulate the voltage at VIN_DC close to the desired reference voltage. The reference voltage
is set by the MPPT control scheme as described in the features section. Input voltage regulation is obtained by
transferring charge from the input to VSTOR only when the input voltage is higher than the voltage on pin
VREF_SAMP. The current through the inductor is controlled through internal current sense circuitry. The peak
current in the inductor is dithered internally to up to three pre-determined levels in order to maintain high
efficiency of the charger across a wide input current range. The charger transfers up to a maximum of 100 mA
average input current (230mA typical peak inductor current). The boost charger is disabled when the voltage on
VSTOR reaches the user set VBAT_OV threshold to protect the battery connected at VBAT from overcharging.
In order for the battery to charge to VBAT_OV, the input power must exceed the power needed for the load on
VSTOR. See the Power Supply Recommendations section for guidance on minimum input power requirements.
Steady state operation for the boost charger is shown in Figure 16. These plots highlight the inductor current, the
VSTOR voltage ripple, input voltage regulation and the LBOOST switching node. The cycle-by-cycle minor
switching frequency is a function of the boost converter's inductor value, peak current limit and voltage levels on
each side of each inductor. Once the VSTOR capacitor, CSTOR, droops below a minimum value, the hysteretic
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.
8.4.3 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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