BQ25504 Datasheet, PDF (13/24 Pages) Texas Instruments – Ultra Low Power Boost Converter with Battery Management for Energy Harvester Applications

www.ti.com
DETAILED PRINCIPLE OF OPERATION
bq25504
SLUSAH0 – OCTOBER 2011
OPERATION
The bq25504 is an ultra low quiescent current, efficient synchronous boost converter/charger. The boost
converter is based on a switching regulator architecture which maximizes efficient operation while minimizing
start-up and operation power. The bq25504 uses pulse frequency mode (PFM) modulation to maintain efficiency,
even under light load conditions. In addition, bq25504 also implements battery protection features so that either
rechargeable batteries or capacitors can be used as energy storage elements. Figure 5 is a high-level functional
block diagram which highlights most of the major functional blocks inside the bq25504.
Boost Converter / Charger
Operation of the boost converter / charger begins when there is sufficient power available at the input pin
(VIN_DC) or available from an attached battery (VBAT) to raise the voltage at pin VSTOR above 1.8 V. The
start-up below 1.8 V on VSTOR of the boost-converter begins with the Cold-Start sub-system. If the VIN_DC is
greater than VSTOR or VBAT then current may flow until the voltage at the input is reduced or the voltage at
VSTOR and VBAT rise. This is considered an abnormal condition and the boost converter/charger does not
operate.
Cold -Start
The cold-start subsystem is used to turn on the device when the voltage present on pin VSTOR is < 1.8 V. Inside
the IC there is a switch (PMOS) between the energy storage capacitor VSTOR and the battery. If a battery is
initially attached to pin VBAT, the PMOS switch is momentary closed and any available charge from the battery
can be dumped onto VSTOR. If the resulting voltage is greater than about 1.8 V, then the bq25504’s biasing and
oscillator circuits can be turned on, and start up of the boost converter will be initiated. However, if there is
insufficient energy available in a connected battery, then the PMOS circuit is opened after ~20 ms, and the
cold-start sequence is initiated via power provided by power at the VIN_DC input pin.
When the voltage at pin VIN_DC exceeds the minimum input voltage with sufficient power, the cold start
subsystem turns on. When the storage capacitor voltage reaches 1.8 V the main boost regulator starts up. The
cold-start circuitry is then turned off after the voltage condition of VSTOR >1.8V and ~32 ms after input power
was applied. The output of the main boost regulator is now compared against battery undervoltage threshold
(VBAT_UV). When the VBAT_UVLO threshold is reached, the PMOS switch is turned on, which allows the
energy storage element attached to VBAT to charge up. Figure 22 shows the key threshold voltages. The battery
management thresholds are explained later is this section. Cold start is not as efficient as the main boost
regulator. If there is not sufficient power available it is possible that the cold start continuously runs and the
VSTOR output does not increase to 1.8 V and start the main boost regulator.
Boost Converter/Charger Operation
The boost converter in bq25504 is used to charge the storage element attached at VBAT with the energy
available from the DC input source. It employs pulse frequency modulation (PFM) mode of control to regulate the
input voltage (VIN_DC) close to the desired reference voltage. The reference voltage is set by the MPPT control
scheme as described in the next 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 set levels to maintain high efficiency of the converter across a wide input current range. The
converter nominally transfers up to a typical peak of 200 mA of input current. The boost converter is disabled
when the voltage on VSTOR reaches the OV condition to protect the battery connected at VBAT from
overcharging.
Maximum Power Point Tracking
Maximum power point tracking (MPPT) is implemented in bq25504 in order to maximize the power extracted
from an energy harvester source. MPPT is performed by periodically sampling a ratio of the open-circuit voltage
of the energy harvester and using that as the reference voltage (VREF_SAMP) to the boost converter. The
sampling ratio can be externally programmed using the resistors R_OC1 and R_OC2. For solar harvesters, the
resistive division ratio can be typically set between 0.7-0.8 and for thermoelectric harvesters; a resistive division
ratio of 0.5 is typically used. The exact ratio for MPPT can be optimized to meet the needs of the input source
being used.
Copyright © 2011, Texas Instruments Incorporated
Product Folder Link(s) :bq25504
Submit Documentation Feedback
13