Introduction
The bq25570 was designed with the flexibility to support a variety of energy storage elements. The
availability of the sources from which harvesters extract their energy can often be sporadic or time-
varying. Systems will typically need some type of energy storage element, such as a re-chargeable
battery, super capacitor, or conventional capacitor. The storage element will make certain constant power
is available when needed for the systems. In general, the storage element also allows the system to
handle any peak currents that can not directly come from the input source. It is important to remember
that batteries and super capacitors can have significant leakage currents that need to be included with
determining the loading on VSTOR.
To prevent damage to a customer’s storage element, both maximum and minimum voltages are monitored
against the internally programmed under-voltage (VBAT_UV) and user programmed over-voltage
(VBAT_OV) levels.
To further assist users in the strict management of their energy budgets, the bq25570 toggles a user
programmable battery good flag (VBAT_OK), checked every 64 ms, to signal the microprocessor when
the voltage on an energy storage element or capacitor has risen above (OK_HYST threshold) or dropped
below (OK_PROG threshold) a pre-set critical level. To prevent the system from entering an undervoltage
condition or if starting up into a depleted storage element, it is highly recommended to isolate the system
load from VSTOR by 1) setting VBAT_OK equal to the buck converter's enable signal VOUT_EN and 2)
using an NFET to invert the BAT_OK signal so that it drives the gate of PFET, which isolates the system
load from VSTOR.
For details, see the bq25570 data sheet (
1.3
Design and Evaluation Considerations
This user's guide is not a replacement for the data sheet. Reading the data sheet first will help in
understanding the operations and features of this IC. In this document, “battery” or "VBAT" will be used
but one could substitute any appropriate storage element.
System Design Tips
Compared to designing systems powered from an AC/DC converter or large battery (for example, low
impedance sources), designing systems powered by HiZ sources requires that the system load-per-unit
time (for example, per day for solar panel) be compared to the expected loading per the same time unit.
Often there is not enough real time input harvested power (for example, at night for a solar panel) to run
the system in full operation. Therefore, the energy harvesting circuit collects more energy than being
drawn by the system when ambient conditions allow and stores that energy in a storage element for later
use to power the system. See
for an example spreadsheet on how to design a real solar-panel-
powered system in three easy steps:
1. Referring the system rail power back to VSTOR
2. Referring the required VSTOR power back to bq255xx input power
3. Computing the minimum solar panel area from the input power requirement
As demonstrated in the spreadsheet, for any boost converter, you must perform a power balance, P
OUT
/
P
IN
= (V
STOR
× I
STOR
) / (V
IN
× I
IN
)=
η
where
η
is the estimated efficiency for the same or very similar
configuration in order to determine the minimum input power needed to supply the desired output power.
This IC is a highly efficient charger for a storage element such as a battery or super capacitor. The main
difference between a battery and a super capacitor is the capacity curve. The battery typically has little or
no capacity below a certain voltage, where as the capacitor does have capacity at lower voltages. Both
can have significant leakage currents that will appear as a DC load on VSTOR/VBAT.
3
SLUUAA7A – July 2013 – Revised August 2014
User's Guide for bq25570 Battery Charger Evaluation Module for Energy
Harvesting
Copyright © 2013–2014, Texas Instruments Incorporated
