Texas Instruments | Improved Fast Charge Using the bq25120 Adjustible VBATREG, Charge Current, TS, | Application notes | Texas Instruments Improved Fast Charge Using the bq25120 Adjustible VBATREG, Charge Current, TS, Application notes

Texas Instruments Improved Fast Charge Using the bq25120 Adjustible VBATREG, Charge Current, TS, Application notes
Application Report
SLVA806 – August 2016
Fast Charge Improvements Using the bq25120 Adjustable
VBATREG, Charge Current, TS, and Status Indicators
Steve Schnier
......................................................................................... Battery Management Solutions
ABSTRACT
Most wearable devices are meant to be worn at all times, taking data when the user is awake and asleep.
In order to do that, the time between charging must be maximized and the time spent charging must be
minimized. The low Iq BQ25120 provides several features to extend the life of the product when in use.
The device also has several features that can be used during charging to reduce the time spent charging.
This paper describes how to use the BQ25120 features such as adjustable VBATREG, adjustable fast
charge current, TS (NTC monitor), and status indicators to get the most charge in the smallest amount of
time.
1
Basics of Charge (CC-CV)
Like most chargers, the bq25120 supports multiple battery chemistries for single-cell applications.
Charging is done through the internal battery MOSFET, and there are several loops that influence the
charge current: constant current loop (CC), constant voltage loop (CV), input current limit, VDPPM, and
VIN(DPM). During the charging process, all loops are enabled and the one that is dominant takes control.
The charge current is regulated to ICHARGE until the voltage between BAT and GND reaches the
regulation voltage. The voltage between BAT and GND is regulated to VBATREG (DV Mode) while the
charge current naturally tapers down. When termination is enabled, the device monitos the charging
current during the CV mode, and once the charge current tapers down to the termination threshold,
ITERM, and the battery voltage is above the recharge threshold, the device terminates charge, and turns
off the battery charging FET. A standard charging cycle is show in Figure 4.
The bq5120 has several features that can reduce the time spent in the CC phase and get more energy
into the battery in a shorter amount of time.
2
Improved Fast Charge using the bq25120
The bq25120 has an I2C interface that allows the host to change charging parameters and monitor status
during charging. Using these features can reduce the time spent in the CC phase.
The device implements a simple voltage battery monitor which is normally used to determine the depth of
discharge. However, it can also be used to monitor the battery voltage during charge. The VBMON
function is equivalent to a 5-bit ADC, so it is not very accurate. However, for the improved fast charge
implementation, it is good enough for the purpose.
Additionally, the device has the ability to adjust the battery regulation voltage with the VBATREG
parameter in the customer registers. The VBATREG is adjustable from 3.6 V up to 4.65 V in 50-mV steps.
This enables the ability to change the CV voltage dynamically during charging. When using a 4.35-V end
charge voltage battery at a faster charge rate, to avoid degrading the battery, a lower battery voltage
should be used for the faster charge rate, in this case 4.2 V could be used. Consult with your battery
vendor to determine the right fast charge profile for the battery in your application.
The device also allows dynamically changing the fast charge current. To reduce the charge time, it is
possible to increase the charge rate when in the CC stage, and then reduce the charge current for the
final CC stage when the VBATREG is also adjusted to the recommended final voltage, in this example it is
4.35 V.
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SLVA806 – August 2016
Submit Documentation Feedback
Fast Charge Improvements Using the bq25120 Adjustable VBATREG,
Charge Current, TS, and Status Indicators
Copyright © 2016, Texas Instruments Incorporated
1
Improved Fast Charge using the bq25120
www.ti.com
The improved fast charge flow chart is shown in Figure 1.
EXAMPLE SHOWN:
Battery: 41 mAh Li-Ion
Rechargeable
Recommended Charge
Rate: 0.5C (21mA)
Final Battery Charge
Voltage: 4.35V
Bq25120 Powered
or Unpowered
N
Valid VIN
Plugin
Y
N
Battery >
BUVLO?
Improved Fast Charge Using
VBMON, ICHG, and VBATREG
Pre-Charge
Y
Set VBATREG to
4.2V
Set Charge Rate
to 1.5C
Y
Y
TS Fault?
TS Normal?
Set VBATREG to
4.35V
Set Charge Rate
to 0.5C
N
VBMON >
98%?
Y
Set Charge Rate
to 0.5C
Set VBATREG to
4.35V
BQ25120 finishes
charging with
normal CC/CV
process
Figure 1. Improved Fast Charge Flow Chart Using the bq25120 Features
The flow chart in Figure 1 was implemented with a battery and a simple host, in this case an MSP430.
The same battery was used with the recommended 0.5C charge rate for comparison.
Comparing the normal fast charge cycle with the improved fast charge cycle, one can see that the
improved fast charge cycle reached termination at 150 minutes (2 ½ hours), where the normal fast charge
reached termination at 190 minutes (over 3 hours).
2
Fast Charge Improvements Using the bq25120 Adjustable VBATREG,
Charge Current, TS, and Status Indicators
Copyright © 2016, Texas Instruments Incorporated
SLVA806 – August 2016
Submit Documentation Feedback
References
www.ti.com
5
5
0.07
4.5
0.06
0.06
3
Normal Fast Charge VBAT 0.04
Normal Fast Charge IBAT
2.5
0.03
2
0.02
1.5
0.01
1
0.05
Battery Voltage (V)
0.05
3.5
3.5
2.5
New Fast Charge VBAT
0.04
Normal Fast Charge IBAT
0.03
2
0.02
3
1.5
Charge Current (A)
4
Charge Current (A)
4
Battery Voltage (V)
0.07
4.5
0.01
1
0
0.5
0
0
20
40
60
80 100 120 140
Elapsed Time (m)
160
180
0
0.5
-0.01
200
0
0
20
40
D001
Figure 2. Normal Fast Charge Cycle
60
80 100 120 140
Elapsed Time (m)
160
180
-0.01
200
D002
Figure 3. Improved Fast Charge Cycle
Additionally, by plotting the Coulombs entered into the battery during the same time, in this case 30
minutes, it is obvious that more energy was delivered to the battery when the improved fast charge
method was used. This is a scenario where a user only charges the battery for a short amount of time
each day, such as when they are in the shower. In 30 minutes, 46 Coulombs were delivered to the battery
with the improved fast charge method, versus 37 Coulombs with the normal fast charge method. This is
20% more energy delivered to the battery in the same time.
The results show the potential to reduce the charging time for a full charge. Alternately, they show how to
increase the amount of energy delivered to the battery in the same charging time. The bq25120 has
additional features that can be used for other improved fast charge methods. Be sure to work with your
battery vendor to determine a safe improved fast charge method for the battery in your system.
50
Improved Fast Charge
Normal Fast Charge
45
40
Coulombs (As)
35
30
25
20
15
10
5
0
0
5
10
15
20
Elapsed Time (m)
25
30
D003
Figure 4. Comparison of Standard vs Improved Fast Charging
3
References
1. bq25120 700-nA Low IQ Highly Integrated Battery Charge Management Solution, SLUSBZ9
2. Challenges and Solutions in Battery Fuel Gauging, SLYP086
SLVA806 – August 2016
Submit Documentation Feedback
Fast Charge Improvements Using the bq25120 Adjustable VBATREG,
Charge Current, TS, and Status Indicators
Copyright © 2016, Texas Instruments Incorporated
3
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