Texas Instruments BQ25504 Battery Management Evaluation Board, Ultra Low Power Boost Converter IC | Energy Harvester BQ2 BQ25504EVM-674 数据表
产品代码
BQ25504EVM-674
R1
D1
C
A
BAT+
GND
Introduction
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. Be sure to make note of the capacitor selection
section when designing the EVM. Many of the IC's pin names start with a "V" and this "V" is removed on
the EVM connector's label. The names are interchangeable.
understanding the operations and features of this IC. Be sure to make note of the capacitor selection
section when designing the EVM. Many of the IC's pin names start with a "V" and this "V" is removed on
the EVM connector's label. The names are interchangeable.
This IC is a highly efficient charger for a storage element such as a battery or super capacitor. In this
document,
document,
“
battery
”
will be used but one could substitute any appropriate storage element. 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.
no capacity below a certain voltage, where as the capacitor does have capacity at lower voltages.
In the lab when using a lab power supply rather than an energy harvester, one will have the output of the
lab supply, Vsource, followed by the harvester's impedance (about 20
lab supply, Vsource, followed by the harvester's impedance (about 20
Ω
) and connected to V
IN
of the EVM.
These two signals are separated by the 20
Ω
source impedance which represents the internal impedance
of the source. V
IN
is equal to V
Source
when there is no load (open circuit) and is pulled down to the MPPT
harvester threshold when the charger is able to deliver the maximum power before reaching OV.
The over voltage (OV) setting initially is lower than the programmed value at startup (varies on conditions)
and is updated after the first ~32ms. Subsequent updates are every ~64ms. The OV threshold is the
reference for maximum voltage on VSTOR and the boost converter will stop switching if the voltage on
VSTOR reaches the OV reference. The UV is checked every ~64ms to determine if the BAT FET should
be on or off. The open circuit (OC) input voltage is measured every ~16 seconds which is used to
calculate the Maximum Power Point Tracking (MPPT) threshold (programmed with resistors to 78% at the
factory). This periodic update continually optimizes maximum power delivery based on the harvesting
conditions.
and is updated after the first ~32ms. Subsequent updates are every ~64ms. The OV threshold is the
reference for maximum voltage on VSTOR and the boost converter will stop switching if the voltage on
VSTOR reaches the OV reference. The UV is checked every ~64ms to determine if the BAT FET should
be on or off. The open circuit (OC) input voltage is measured every ~16 seconds which is used to
calculate the Maximum Power Point Tracking (MPPT) threshold (programmed with resistors to 78% at the
factory). This periodic update continually optimizes maximum power delivery based on the harvesting
conditions.
Harvesting ultra low power energy requires a different mind set when designing a system. Often there is
not enough real time input harvested power to run the system in full operation so energy is collected over
a period of time, stored in a battery and then used periodically to power the system.
not enough real time input harvested power to run the system in full operation so energy is collected over
a period of time, stored in a battery and then used periodically to power the system.
The designer needs to define a
“
Battery OK
”
threshold and battery discharged threshold (Not OK) to allow
successful system operation. The BAT_OK high/low threshold are programmed at the factory to 2.8V and
2.4V using resistors R7, R8, and R9. A BAT_OK high signal would typically indicate to the host that the
battery is above 2.8V and ready to use and if low would indicate that the cell is discharged such that the
system load should be reduced or disabled. The BAT_OK signal is checked every 64ms.
2.4V using resistors R7, R8, and R9. A BAT_OK high signal would typically indicate to the host that the
battery is above 2.8V and ready to use and if low would indicate that the cell is discharged such that the
system load should be reduced or disabled. The BAT_OK signal is checked every 64ms.
The quiescent current, which is basically the current from the battery to the IC, can be measured at the
STOR pin. To measure the current the user should connect a 100k
STOR pin. To measure the current the user should connect a 100k
Ω
resistor to J5-2 (STOR) and connect
a 3V supply from the other end of this resistor to the ground of the EVM. A 10M
Ω
meter can be used to
measure the voltage drop across the resistor and calculate the current. No other connections should be
made to the EVM and the measurement should be taken after steady state conditions are reached (may
take a few minutes). The reading should be in the range of 375nA.
made to the EVM and the measurement should be taken after steady state conditions are reached (may
take a few minutes). The reading should be in the range of 375nA.
The battery (storage element) can be replaced with a simulated battery. Often electronic 4 quadrant loads
give erratic results with a
give erratic results with a
“
battery charger
”
due to the charger changing states (fast-charge to termination
and refresh) while the electronic load is changing loads to maintain the
“
battery
”
voltage. The charging and
loading get out of phase and creates a large signal oscillation which is due to the 4 quadrant meter. A
simple circuit can be used to simulate a battery and works well and can quickly be adjusted for voltage. It
consists of load resistor (~10
simple circuit can be used to simulate a battery and works well and can quickly be adjusted for voltage. It
consists of load resistor (~10
Ω
, 2W) to pull the output down to some minimum storage voltage (sinking
current part of battery) and a lab supply connected to the BAT pin via a diode. The lab supply biases up
the battery voltage to the desired level. It may be necessary to add more capacitance across R1.
the battery voltage to the desired level. It may be necessary to add more capacitance across R1.
3
SLUU654A
–
October 2011
–
Revised October 2011
bq25504 EVM
–
Ultra Low Power Boost Converter with Battery Management for
Energy Harvester Applications
Copyright
©
2011, Texas Instruments Incorporated