Texas Instruments | USB charging solution for 2-cell (2S) discrete using BQ25887 with cell-balancing | Application notes | Texas Instruments USB charging solution for 2-cell (2S) discrete using BQ25887 with cell-balancing Application notes

Texas Instruments USB charging solution for 2-cell (2S) discrete using BQ25887 with cell-balancing Application notes
Application Report
SLUA938 – March 2019
USB charging solution for 2-cell (2S) discrete using
BQ25887 with cell-balancing function
Jing Zou
ABSTRACT
As the market continues to demand higher-performance electronics, the need for increased system power
rises as well. A two-cells-in-series (2S) battery configuration is a good solution to this problem. Using a
5-V USB adapter to charge a 2S battery enables you to use the same adapter across products and
increases cost savings. However, in order to maximize the benefits of a 5-V USB and 2S configuration,
cell balancing is required.
Cell-balancing is essential due to the potential differences in cell voltage; this is especially true in
applications with replaceable individual battery cells. In such cases, the higher-voltage cell is at risk of
overcharging and the lower-voltage cell is underutilized.
This application report introduces the BQ25887, an I2C-controlled 2-cell 2-A boost-mode battery charger
with a novel cell-balancing function. This report also provides a design example illustrating an
implementation of the cell-balancing function.
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3
4
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Contents
Introduction ...................................................................................................................
The BQ25887 Cell-Balancing Algorithm ..................................................................................
Design example .............................................................................................................
Summary .....................................................................................................................
References ...................................................................................................................
2
2
4
8
8
List of Figures
1
Simplified Cell-Balancing Diagram and Current Illustration Where VCELL1 > VCELL2 (a); and VCELL1 <
VCELL2 (b) ................................................................................................................... 2
2
BQ25887 Cell-Balancing Timing Diagram – Entering Pre-Qualification and Qualification Modes ................ 3
3
BQ25887 Application Diagram ............................................................................................. 5
4
Charge Cycle with the Bottom Cell Voltage Higher Than the Top Cell Voltage ..................................... 7
5
Charge Cycle with the Top Cell Voltage Higher Than the Bottom Cell Voltage ..................................... 7
List of Tables
1
Cell-Balancing Design Parameters ........................................................................................ 4
2
REG28 Register
6
3
REG29 Register
6
4
5
6
.............................................................................................................
.............................................................................................................
REG2A Register .............................................................................................................
REG2B Register .............................................................................................................
REG2C Register ............................................................................................................
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1
Introduction
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1
Introduction
In 2S battery applications with one or two replaceable cells, the battery cells can have very different
voltages because replacement batteries can come from anywhere, at any state of charge. Traditional
multi-cell chargers only monitor the total voltage of series-connected cells; the charger does not measure
each individual cell voltage. Consequently, even though the total voltage of series-connected cells does
not reach the regulation voltage, the cell with the higher voltage may overcharge due to unbalancing of the
cells. Overcharging battery cells not only shortens battery life but can also cause serious safety concerns.
On the other hand, the cell at the lower voltage cannot fully charge, which means that its full capacity is
not completely utilized. Therefore, cell-balancing becomes essential during battery charging in this type of
2S battery application.
The BQ25887 boost charger has a cell-balancing function that charges 2S battery cells from a 5-V USB
adapter. Unlike traditional pack-side cell-balancing, an integrated cell-balancing function enables the
charger to balance the voltage of the two cells during charging within one charge cycle. With the
integration of input current- and voltage-limiting functionality, along with an input current limit optimizer, the
BQ25887 can detect the current capability of unknown adapters, enabling the device to universally charge
with any 5-V adapter.
2
The BQ25887 Cell-Balancing Algorithm
The BQ25887 balances cells during charging. Charge current to the cell with the higher voltage is reduced
while the charge current to the cell with the lower voltage is kept at the full charge current setting. With
this architecture, the lower voltage cell can charge up without waiting for the cell with the higher voltage to
discharge.
Figure 1 illustrates the implementation of two field-effect transistors (FETs) to switch the current bypass on
and off to each cell. The circuit shown in Figure 1a illustrates the case where VCELL1 > VCELL2, while
the circuit in Figure 1b shows VCELL1 < VCELL2.
ICHG
ICB
ICHG
ICELL1
ICHG
CELL1
ICELL2
CELL2
SW 1
CELL1
SW 1
R
R
ICB
SW 2
CELL2
ICHG
SW 2
GND
GND
a
b
Figure 1. Simplified Cell-Balancing Diagram and Current Illustration
Where VCELL1 > VCELL2 (a); and VCELL1 < VCELL2 (b)
2
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The BQ25887 Cell-Balancing Algorithm
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In Figure 1:
• ICHG is the charge current source.
• ICB is the bypassing current to the higher voltage cell which is limited by resistor R. The bypassing
current is a function of the corresponding cell voltage. For instance, if VCELL1 is greater than VCELL2,
ICB = VCELL1/R. The maximum bypassing current limit is reached when VCELL1 reaches its
regulation voltage.
• ICELLx is the reduced charge current to the cell with the higher voltage. ICELLx = ICHG - ICB.
Therefore, ICELLx is also a function of cell voltage, but inversely. As the cell voltage increases, ICELLx
reduces.
Based on the architecture described above, the BQ25887 enables antonymous cell-balancing by default.
The device incorporates a cell-balancing algorithm illustrated in Figure 2.
VDIFF_END
VDIFF_START
VQUAL_TH
VCBEN
3.7V
ICHG
ITERM
0A
t0
t1
t2 t3
t4 t5
t6
t7
Figure 2. BQ25887 Cell-Balancing Timing Diagram – Entering Pre-Qualification and Qualification Modes
In
•
•
•
•
•
•
•
•
Figure 2:
[t0:t1] – Cell-balancing circuitry is off.
[t1:t2] – Pre-qualification mode. Charge is continuous during cell-balancing measurements.
[t2:t5] – Qualification mode. Charge is suspended during cell-balancing measurements.
[t2:t3] – Total time of battery voltage settlement time and cell voltage measurement time.
[t3:t4] – Internal time between cell voltage measurements in pre-qualification and qualification modes.
[t4:t5] – Total time of battery voltage settlement time and cell voltage measurement time.
[t5:t6] – Internal time between cell voltage measurements in cell-balancing active mode.
[t5:t7] – Cell-balancing active mode where charge current to higher-voltage cell is reduced.
The cell-balancing circuitry turns on when one of the cell voltages reaches 3.7 V. The BQ25887 starts to
compare voltage measurements of the two cells, putting the device into pre-qualification mode.
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Design example
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The cell voltage measurements are taken in a time interval at the same value as [t3:t4], while the charge
current remains continuous. The device compares the cell voltage difference with VQUAL_TH. If the cell’s
voltage difference is greater than VQUAL_TH, then the device enters qualification mode. In qualification mode,
the integrated circuit (IC) suspends charging in the next measurement interval in order to take a more
accurate cell voltage measurement. Due to the characteristics of lithium-based batteries, a settlement time
is implemented to stabilize the battery voltage after suspending charging for cell voltage measurements.
The measurement time interval is kept at TCB_QUAL_INTERVAL.
Once the measured voltage difference between the two cells exceeds the VDIFF_START threshold in
qualification mode, cell-balancing is active. The corresponding FET switch turns on to bypass charge
current to the cell with the higher voltage. During cell-balancing active mode, the cell voltage
measurement is taken in time interval [t5:t6]. Both charge and cell-balancing are turned on for the time
period of [t5:t6] and then turned off for cell voltage measurement. Implementing the battery settlement
time in cell-balancing active mode obtains a more accurate cell voltage measurement result. When the
measured voltage difference between the two cells is less than VDIFF_END, cell-balancing is complete and
the device exits cell-balancing active mode.
In addition to the antonymous cell-balancing routine, the BQ25887 also integrates a 16-bit analog-to-digital
converter (ADC); I2C can enable and read both which both top-cell and bottom-cell voltage ADC
conversion can be enabled and read through I2C. You can use the cell voltage information for simple
estimations of battery capacity.
To enable the ADC, set the ADC_EN bit to 1 in REG0x15[7] and set VCELL_ADC_DIS to 0 in
REG0x16[1]. The ADC value of the top cell voltage is reported in REG0x1F to REG0x20 and the bottom
cell voltage is reported in REG0x26 to REG0x27.
Charge can be enabled during cell-balancing measurements for faster charging times. In this case, setting
CB_CHG_DIS in REG0x2A[7] to 1 achieves continuous charging during cell-balancing active mode.
The BQ25887 also implements a manual cell-balancing mode where you can build an even more
customized cell-balancing program according to your application requirements. In manual cell-balancing
mode, the cell-balancing discharging FETs, QCBH and QCBL, can be enabled through the QCBH_EN and
QCBL_EN bits. Only one of these bits can be selected at a time. QCBH and QCBL can only be enabled when
charge is enabled and CB_EN is set to 0.
Cell-balancing status is reported in the CB_STAT register to indicate whether the device is in cellbalancing active mode. HS_CV_STAT and LS_CV_STAT report if the top cell or bottom cell is in a
constant voltage phase. For fault conditions, HS_OV_STAT and LS_OV_STAT show overvoltage
conditions, and CB_OC_STAT reports the detection of an overcurrent condition at QCBH or QCBL.
3
Design example
For a BQ25887 cell-balancing design example, see the parameters shown in Table 1. The charge current
is set to 800 mA and the cell regulation voltage is set to 4.2 V. For examples of other charge-related
design parameters, see the Typical Application section in the BQ25887 data sheet (SLUSD89).
Table 1. Cell-Balancing Design Parameters
4
Parameter
Value
Maximum cell-balancing current
300 mA
Voltage difference in pre-qualification mode where charge is continuous
100 mV
Time interval for voltage measurement in pre-qualification mode
2 min
Voltage difference to turn on QCBX starting cell-balancing
80 mV
Voltage difference to exit cell-balancing
10 mV
Time interval for voltage measurement in cell-balancing active mode
2 min
Battery voltage settlement time before voltage measurement
1s
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Design example
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3.1
Cell-Balancing Current Limit Resistor Selection
The BQ25887 implements the cell-balancing architecture with a recommended maximum bypassing
current of 400 mA. In addition, the device also implements a remote sense point for more accurate cell
voltage sensing. The high-side cell voltage is sensed between the BAT and MID pins, and the low-side
cell voltage is sensed between the MID pin and GND. Bypassing the charge current through the external
balancing resistor, RCBSET, reduces the charge current to the higher-voltage cell.
Equation 1 calculates the value of RCBSET, where VREG is the cell regulation voltage and ICB_MAX is the
maximum cell-balancing current.
space
(1)
space
BAT
10 F
MID
QCBH
300
*
Top Cell
CBSET
RCBSET
QCBL
Bottom Cell
BQ25887
GND
*Note: The 300-O resistor on the MID pin is used to limit the current in cases where the bottom cell is plugged in reversely
Figure 3. BQ25887 Application Diagram
Using Equation 1, RCBSET can be calculated as:
(2)
The cell-balancing current is limited by the external RCBSET. Select RCBSET with power rating in mind to
ensure that it is sufficient for the maximum cell-balancing current. For a 300-mA cell-balancing current,
consider connecting multiple power resistors in parallel, such as six 82-Ω resistors (1205, 0.25 W).
3.2
Cell-Balancing Register Setup
Set up cell-balancing registers REG0x28 to REB0X2C to follow Table 2 through Table 6 and the design
specifications in Table 1.
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Design example
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Table 2. REG28 Register
Bit
Value
Field
7
VDIFF_END_OFFSET[2:0] = 100
Voltage difference to exit cell-balancing = VDIFF_START VDIFF_END_OFFSET
4
TCB_QAUL_INTERVAL = 0
Time interval for voltage measurement in pre-qualification mode
3
TCB_ACTIVE[1:0] = 10
Time interval for voltage measurement in cell-balancing active mode
TSETTLE[1:0] = 10
Battery voltage settlement time before voltage measurement
6
5
2
1
0
Table 3. REG29 Register
Bit
Value
Field
7
VQUAL_TH[3:0] = 0110
Voltage difference in pre-qualification mode where charge is
continuous
VDIFF_START[3:0] = 0100
Voltage difference to turn on QCBX, starting cell-balancing
6
5
4
3
2
1
0
Table 4. REG2A Register
Bit
Value
Field
7
CB_CHG_DIS = 1
Disable charging for cell voltage measurement
6
CB_AUTO_EN = 1
Enable automatic cell-balancing
5:0
Status registers = read only
Status register indicates cell-balancing status
Table 5. REG2B Register
Bit
Value
Field
7
QCBH_EN = 0
Enables QCBH in manual mode
6
QCBL_EN = 0
Enables QCBL in manual mode
5:0
FLAG registers = read only
LAG register bit is cleared upon read.
Table 6. REG2C Register
6
Bit
Value
Field
7:6
Reserved = 0
Reserved bit always reads 0h
5
CB_MASK = 0
Device sends out INT pulse when entering cell-balancing active
mode
4
HS_CV_MASK = 0
Device sends out INT pulse when the top cell enters CV mode
3
LS_CV_MASK = 0
Device sends out INT pulse when the bottom cell enters CV mode
2
HS_OV_MASK = 0
Device sends out INT pulse when a top cell over-voltage fault occurs
1
LS_CV_MASK = 0
Device sends out INT pulse when a bottom cell over-voltage fault
occurs
0
CB_OC_MASK = 0
Device sends out INT pulse when a QCBX over-current fault occurs.
USB charging solution for 2-cell (2S) discrete using BQ25887 with cellbalancing function
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3.3
Test results
Figure 4 and Figure 5 show the test results.
Figure 4. Charge Cycle with the Bottom Cell Voltage Higher Than the Top Cell Voltage
Figure 5. Charge Cycle with the Top Cell Voltage Higher Than the Bottom Cell Voltage
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Summary
4
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Summary
The BQ25887 integrates a cell-balancing algorithm that automatically starts and completes cell-balancing
when cell voltage conditions are met. The cell-balancing circuit is easy to design; parameters can be
programmed through user registers, enabling the cell-balancing feature to be more flexible for different
application needs.
The 400-mA maximum cell-balancing current enables fast balancing time between two unmatched cells
with integrated metal-oxide semiconductor field-effect transistors. The integrated ADC monitoring each cell
voltage can also be used for simple voltage-based cell capacity estimation. The BQ25887’s cell-balancing
function is powerful yet easy to use for applications with discrete 2S battery cells.
5
References
•
8
BQ25887 Data sheet SLUSD89
USB charging solution for 2-cell (2S) discrete using BQ25887 with cellbalancing function
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