BQ25570_14 Datasheet, PDF (30/41 Pages) Texas Instruments – Ultra Low Power Harvester Power Management IC with Boost Charger, and Nano-Powered Buck Converter

bq25570
SLUSBH2C – MARCH 2013 – REVISED JANUARY 2014
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VOUT Capacitance
The output capacitor is chosen based on transient response behavior and ripple magnitude. The lower the
capacitor value, the larger the ripple will become and the larger the droop will be in the case of a transient
response. It is recommended to use at least a 22 µF output capacitor between VOUT, pin 14 and VSS, pin 15,
for most applications.
Additional Capacitance on VSTOR or VBAT
If there are large, fast system load transients and/or the storage element has high resistance, then the CSTOR
capacitors may momentarily discharge below the VBAT_UV threshold in response to the transient. This causes
the bq25570 to turn off the PFET switch between VSTOR and VBAT and turn on the boost charger. The CSTOR
capacitors may further discharge below the VSTOR_CHGEN threshold and cause the bq25570 to enter Cold
Start. For instance, some Li-ion batteries or thin-film batteries may not have the current capacity to meet the
surge current requirements of an attached low power radio. To prevent VSTOR from drooping, either increasing
the CSTOR capacitance or adding additional capacitance in parallel with the storage element is recommended.
For example, if boost charger is configured to charge the storage element to 4.2 V and a 500 mA load transient
of 50 µs duration infrequently occurs, then, solving I = C x dv/dt for CSTOR gives :
CSTOR ≥ 500 mA x 50 µs/(4.2 V – 1.8 V) = 10.5 µF
(6)
Note that increasing CSTOR is the recommended solution but will cause the boost charger to operate in the less
efficient cold start mode for a longer period at startup compared to using CSTOR = 4.7 µF. If longer cold start run
times are not acceptable, then place the additional capacitance in parallel with the storage element.
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