SLUSAH0C – OCTOBER 2011 – REVISED JUNE 2015
9 Application and Implementation
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
9.1 Application Information
9.1.1 Storage Element Selection
In order for the charge management circuitry to protect the storage element from over-charging or discharging,
the storage element must be connected to VBAT pin and the system load tied to the VSTOR pin. Many types of
elements can be used, such as capacitors, super capacitors or various battery chemistries. A storage element
with 100 uF equivalent capacitance is required to filter the pulse currents of the PFM switching charger. The
equivalent capacitance of a battery can be computed as computed as:
CEQ = 2 x mAHrBAT(CHRGD) x 3600 s/Hr / VBAT(CHRGD)
In order for the storage element to be able to charge VSTOR capacitor (CSTOR) within the tVB_HOT_PLUG (50 ms
typical) window at hot-plug; therefore preventing the IC from entering cold start, the time constant created by the
storage element's series resistance (plus the resistance of the internal PFET switch) and equivalent capacitance
must be less than tVB_HOT_PLUG . For example, a battery's resistance can be computed as:
RBAT = VBAT / IBAT(CONTINUOUS) from the battery specifications.
The storage element must be sized large enough to provide all of the system load during periods when the
harvester is no longer providing power. The harvester is expected to provide at least enough power to fully
charge the storage element while the system is in low power or sleep mode. Assuming no load on VSTOR (i.e.,
the system is in low power or sleep mode), the following equation estimates charge time from voltage VBAT1 to
VBAT2 for given input power is:
Refer to SLUC462 for a design example that sizes the storage element.
PIN × ηEST × tCHRG = 1/2 × CEQ X (VBAT22 - VBAT12)
Note that if there are large load transients or the storage element has significant impedance then it may be
necessary to increase the CSTOR capacitor from the 4.7uF minimum or add additional capacitance to VBAT in
order to prevent a droop in the VSTOR voltage. See below for guidance on sizing capacitors.
9.1.2 Inductor Selection
The boost charger needs an appropriately sized inductor for proper operation. The inductor's saturation current
should be at least 25% higher than the expected peak inductor currents recommended below if system load
transients on VSTOR are expected. Since this device uses hysteretic control, the boost charger is considered
naturally stable systems (single order transfer function).
For the boost charger to operate properly, an inductor of appropriate value must be connected between
LBOOST, pin 20, and VIN_DC, pin 2. The boost charger internal control circuitry is designed to control the
switching behavior with a nominal inductance of 22 µH ± 20%. The inductor must have a peak current capability
of > 300 mA with a low series resistance (DCR) to maintain high efficiency.
A list of inductors recommended for this device is shown in Table 1.
Table 1. Recommended Inductors
(1) See WHAT? concerning recommended third-party products.
Submit Documentation Feedback
Product Folder Links: bq25504
Copyright © 2011–2015, Texas Instruments Incorporated