BQ25570_14 Datasheet, PDF (27/41 Pages) Texas Instruments – Ultra Low Power Harvester Power Management IC with Boost Charger, and Nano-Powered Buck Converter

bq25570
www.ti.com
SLUSBH2C – MARCH 2013 – REVISED JANUARY 2014
duty cycle of COUT's ripple are a function of the VSTOR voltage, VOUT system load, L2 inductance value and
COUT capacitance value. At heavier output loads (larger output current), the time the converter is off is smaller
when compared to light load conditions. Figure 23 and Figure 24 show the major switching frequency versus load
current and VSTOR voltage, respectively, for the buck converter. Figure 25 and Figure 26 show the output
voltage ripple with COUT = 22 µF versus load current and VSTOR voltage, respectively, for the buck converter.
Nano-Power Management and Efficiency
The high efficiency of the bq25570 boost charger and buck converter is achieved via the proprietary Nano-Power
management circuitry and algorithm. This feature essentially samples and holds all references in order to reduce
the average quiescent current. That is, the internal circuitry is only active for a short period of time and then off
for the remaining period of time at the lowest feasible duty cycle. A portion of this feature can be observed in
Figure 35 where the VRDIV node is monitored. Here the VRDIV node provides a connection to the VSTOR
voltage (first pulse) and then generates the reference levels for the VBAT_OV, VBAT_OK and VOUT_SET
resistor dividers for a short period of time. The divided down values at each pin are sampled and held for
comparison against VBIAS as part of the hysteretic control. Since this biases a resistor string, the current
through these resistors is only active when the Nano-Power management circuitry makes the connection—hence
reducing the overall quiescent current due to the resistors. This process repeats every 64 ms.
The bq25570's boost charger efficiency is shown for various input power levels in Figure 7 through Figure 13.
The bq25570's buck converter efficiency versus output current is plotted in Figure 17 and versus input voltage in
Figure 18. All data points were captured by averaging the overall input current. This must be done due to the
periodic biasing scheme implemented via the Nano-Power management circuitry. In order to properly measure
the resulting input current when calculating the output to input efficiency, the input current efficiency data was
gathered using a source meter set to average over at least 50 samples.
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 bq25570 uses an integrated temperature sensor to monitor the junction temperature of the device. Once the
temperature threshold is exceeded, the boost charger and buck converter are disabled. Once the temperature of
the device drops below this threshold, the boost charger and buck converter 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 charger is
disabled. However, if the supply voltage drops to the VBAT_UV setting, the switch between VBAT and VSTOR
will open and protect the battery even if the device is in thermal shutdown.
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