View detail for Atmel AVR145: USB HID Lithium-Ion Battery Charging via USB with ATmega16/32U4

View detail for Atmel AVR145: USB HID Lithium-Ion Battery Charging via USB with ATmega16/32U4
APPLICATION NOTE
Atmel AVR145: USB HID Lithium-Ion Battery Charging via
USB with ATmega16/32U4
16-/32-bit Atmel Microcontrollers
Features
•
•
•
•
•
Fully functional design for charging Lithium-Ion batteries
High accuracy measurement with 10-bit A/D converter
Modular “C” source code
Analog inputs for reading battery ID and temperature
USB HID class for user interface
Introduction
This application note describes how to use the Atmel® EVK527 evaluation kit to
charge Lithium-Ion (Li-Ion) batteries using USB connection as power supply.
The USB HID class is used to display battery parameters on PC.
This application note is a merge between two application notes:
•
AVR®328: USB Generic HID Implementation
Battery charge parameters are updated on PC with using HID report messages. A
battery task is added in generic HID firmware.
•
AVR146: USB Lithium-Ion Battery Charging via USB with ATmega16/32U4
The battery task added in generic HID firmware comes from this application note.
AVR146 implements the USB CDC class.
8208A−AVR−10/2012
1.
Description
This document describes an application running on the Atmel EVK527 evaluation kit. The EVK527 is dedicated to the
Atmel ATmega16/32U4.
The USB offers a 5V power supply on the VBUS pin. The available current range is from 100mA to 500mA. This is
enough to charge a Li-Ion battery cell.
A Li-Ion cell needs an accurate control of voltage and current during charge.
ATmega16/32U4 offers a USB full speed interface, PWM channels and 10-bit ADC channels. All these features are
used to perform a Li-Ion battery charger via USB.
Figure 1-1. Hardware description.
EVK527
PC
USB Link
Battery
For a user-friendly interface, all charging parameters (charging status, battery voltage, charge current, battery
temperature…) are displayed on the PC without the use of measurement tools.
After the USB enumeration, a USB Human Interface Device appears (see Figure 1-2). The user may launch the
dedicated USB battery charging PC application. Battery charging parameters are updated with HID report messages.
AVR145: USB HID Lithium-Ion Battery Charging via USB [APPLICATION NOTE]
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Figure 1-2. Device Manager window.
Figure 1-3. Properties window.
AVR145: USB HID Lithium-Ion Battery Charging via USB [APPLICATION NOTE]
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2.
Theory of operation
Battery charging is made possible by a reversible chemical reaction that restores energy in a chemical system.
Depending on the chemicals used, the battery will have certain characteristics. A detailed knowledge of these
characteristics is required in order to avoid inflicting damage to the battery.
2.1
Li-Ion battery technology
Lithium-Ion batteries have the highest energy/weight and energy/space ratios of modern rechargeable batteries (See
Reference 1 on page 20). It is currently the fastest growing battery system on the market, with end applications such as
notebook computers, cell phones, portable media players, Personal Digital Assistants (PDA), power tools and medical
devices.
Compared to traditional rechargeable batteries, Li-Ion batteries have low internal resistance, high cycle life, fast charge
time, low self-discharge, low toxicity and no maintenance requirements. For example, Lithium-Ion cells with cobalt
cathodes hold twice the energy of a nickel-based battery and four times that of lead acid. Lithium-Ion is a low
maintenance system, an advantage that most other chemistries cannot claim. There is no memory effect with LithiumIon and the battery does not require scheduled cycling to prolong its life. Lithium-Ion has a low self-discharge and is
environmentally friendly. Disposal causes minimal harm.
Drawbacks of Li-Ion batteries include low tolerance of overcharge and the need for embedded protection circuitry. An
electrical short can result in a large current flow, a temperature rise and thermal runaway in which flaming gases are
vented.
2.1.1
Safety
Lithium-Ion batteries are safe, provided that certain precautions are met when charging and discharging. In addition,
battery manufacturers ensure a high level of reliability by adding three layers of protection, as follows:
1.
2.
3.
The amount of active material is limited to achieve a workable equilibrium of energy density and safety.
Various safety mechanisms are included within each cell.
An electronic protection circuit is added inside the battery pack.
Cell protection devices work as follows:
•
A PTC/NTC (positive/negative temperature coefficient) device acts as a protection to inhibit high current
surges
•
The CID (circuit interrupt device) opens the electrical path if an excessively high charge voltage raises the
internal cell pressure
•
The safety vent allows a controlled release of gas in the event of a rapid increase in cell pressure
The electronic protection circuit works as follows:
•
•
•
A solid-state switch is opened if the charge voltage of any cell reaches a given threshold
A fuse cuts the current flow if the skin temperature of the cell approaches 90°C (194°F)
The current path is cut when cell voltage drops below a given threshold. This is in order to prevent the battery
from over-discharging
Today, Lithium-Ion is one of the most successful and safe battery chemistries available with billions of cells being
produced every year.
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2.2
Charging Li-Ion batteries
There is only one way to charge lithium-based batteries. Manufacturers of Lithium-Ion cells have very strict guidelines in
charge procedures and the packs should be charged as per the manufacturers "typical" charge technique.
Li-Ion batteries are charged using constant voltage (after having reached the nominal charge voltage), with current
limiter to avoid overheating in the initial stage of the charging process. Charging is terminated when the charge current
drops below a threshold set by the manufacturer. Several parameters are monitored during the charge: charge time,
battery temperature… The battery takes damage from overcharging and may explode if overcharged.
2.2.1
Safety
Static electricity or a faulty charger may destroy the battery's protection circuit and turn solid-state switches to a
permanent ON position. This may happen without the user knowing. A battery with a faulty protection circuit may
function normally but does not provide protection against abuse.
Consumer grade Lithium-Ion batteries cannot be charged below 0°C (32°F). If charged at cold temperatures, battery
packs may appear to be charging normally but chemical reactions inside the cells may cause permanent damage and
can compromise the safety of the pack.
The battery will become more vulnerable to failure if subjected to impact, crush or high rate charging.
The battery must remain cool. A battery pack that gets hot during charge should not be used.
2.2.2
Priming and charge intervals
Unlike many other types of rechargeable batteries, Lithium-Ion batteries do not need priming. The first charge of a Li-Ion
battery is no different than the 10th or the 100th charge.
Lithium-Ion batteries may be – and should be – charged often. The battery lasts longer with partial rather than full
discharges. Full discharges should be avoided because of wear.
The battery loses capacity due to aging, whether used or not.
2.2.3
Charge stages
Lithium-Ion battery charge follows three stages:
1.
2.
3.
Prequalification current.
Charging of a Li-Ion battery starts with a test of battery voltage. If the voltage is under a defined threshold
(PREQUAL_VOLTAGE), the charge starts with a fixed low current.
Constant current.
The charge continues with applying constant current to the battery. The size of the charge current is batterydependent and given by the manufacturer. This stage is complete when battery voltage has reached the
threshold given by the manufacturer.
Constant voltage.
After battery threshold voltage has been reached the charger will switch from supplying constant current to
supplying constant voltage. This stage is complete when charge current has dropped below the threshold
given by the manufacturer.
Figure 2-1 illustrates voltage and current of a Lithium-Ion battery during charging.
AVR145: USB HID Lithium-Ion Battery Charging via USB [APPLICATION NOTE]
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Figure 2-1. Charge stages and limits of a VARTA EasyPack 550mAh.
Voltage/Current
OVERCHARGE VOLTAGE LIMIT
Vbatt
4.2V
3V
OVERDISCHARGE VOLTAGE LIMIT
260mA
Ibatt
100mA
10mA
ICUTOFF
t
CONSTANT CURRENT CHARGE
CONSTANT VOLTAGE CHARGE
In Figure 2-1, “Overcharge” is the level at which cell protection circuitry cuts in and opens a solid-state switch and
discontinues the charge current path. After this, battery voltage typically needs to drop several hundred millivolts before
the current path is restored. “Overdischarge” is the level at which the current path is cut in order to prevent the battery
from over-discharging.
2.3
VARTA battery
2.3.1
Typical charge characteristics
Battery specifications should always be verified from manufacturer’s data sheets. Table 2-1 shows a summary of typical
Lithium-Ion battery charge characteristics is to be found in. Actual parameters may vary.
Table 2-1.
Typical charge characteristics.
Parameter
Typical value
Charge time
3 hours
Charge current
1 C (1)
Charge efficiency
99.9%
0.03 C (1)
Charge current threshold
Charge voltage
4.20V
Charge voltage tolerance (per cell)
±0.05V
Temperature range
0 … +45°C
Humidity range
65 ±20 RH
Notes:
1.
C corresponds to the typical rated capacity value (see Table 2-2).
AVR145: USB HID Lithium-Ion Battery Charging via USB [APPLICATION NOTE]
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2.3.2
Typical battery characteristics
Table 2-2 summarises manufacturer’s data for the batteries types used in this application. Other types of batteries may
be used, but may require adjustments to software and/or hardware.
Table 2-2.
Manufacturer’s data for VARTA EasyPack range of Lithium-Ion batteries.
Parameter
Rated capacity (typical)
EZPack S-3.7V
EZPack M-3.7V
EZPack L-3.7V
EZPack XL-3.7V
Unit
550
750
1000
2000
mAh
Nominal voltage
3.70
V
2.75 … 4.20
V
Charge voltage
4.20
V
Charge voltage tolerance
±50
mV
Operating voltage range
Charge current
520
720
955
955
mA
Charge cut-off time
3
3
3
4
hours
Charge cut-off current
10
14
19
38
mA
3.9
6.8
10
24
kΩ
RID
(1)
(resistor ID)
NTC
10
kΩ
B-value (2)
3435
K
Overcharge detection
4.35
V
Overdischarge detection
2.20
V
Notes:
2.3.3
1.
RID: Battery internal resistor identifies the capacity of battery connected.
2.
B value is used in temperature formula.
Electrical pinout
This application uses a particular type of Lithium-Ion batteries and all configurations presented here are based on
manufacturer’s data. Other Lithium-Ion batteries may naturally be used but it is up to the user to look up battery data
from manufacturer’s data sheets and make sure that necessary adjustments are made to firmware and hardware.
Figure 2-2 illustrates connection pads of the Lithium-Ion batteries used in this application.
Figure 2-2. Connection pads of a VARTA EasyPack cell.
AVR145: USB HID Lithium-Ion Battery Charging via USB [APPLICATION NOTE]
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The battery is connected to the battery charger as follows.
Table 2-3.
2.4
Connecting battery to charger.
Battery connector
Charger connector
Note
- (minus)
BATTERY-
NTC
NTC/RID
Battery temperature measurement
ID
SCL
RID, Battery identification resistor
+ (plus)
BATTERY+
VBUS supply voltage
USB powered applications fall into one of the three following categories:
•
Low-Power Bus
The low-power bus powered functions derive all their power from VBUS and must not draw more than one unit
load (100mA) according to the USB standard. It must also be able to work between the VBUS voltage of 4.40V
and 5.25V.
•
High-Power Bus
The high-power bus powered functions derive all their power from VBUS and cannot draw more than 100mA
until it has been configured. Once configured, it can draw up to five unit loads (500mA) by requesting it in its
descriptor. At full load, it must be able to work between the VBUS voltage of 4.75V and 5.25V.
•
Self-Power
Self powered functions can draw up to 100mA from VBUS and the rest from another source.
The current to power the Atmel EVK527 and to charge the battery comes from VBUS. The EVK527 must limit the
charge current if needed.
An easy solution is to modify the I-charge parameter in the lookup table.
For example, a 550mAh battery allows a 260mA charging current. A modification of this parameter to 90mA (for
example) allows connecting the charger on a Low-Power Bus, knowing that the EVK527 consumption with an 8MHz
oscillator is about 10mA. In this case, the prequalification current must also be limited to 90mA.
2.5
EVK527 revision
The EVK527 Rev1.0.0 is provided with several updates:
•
The shunt resistor is connected between PF0 and PF1 to use the differential input mode. PF0 must replace
PD4 and SP6 must be “without solder” (default configuration)
•
•
•
Gate pin and Source pins of Q1A are disconnected
R6 and R7 new values are 13kΩ
R3 new value is 1Ω
These modifications are implemented in schematics given in §6.
2.6
Buck converter
A buck converter is integrated on EVK527 to control the battery voltage and the battery current. The switch is controlled
by the High speed PWM output.
AVR145: USB HID Lithium-Ion Battery Charging via USB [APPLICATION NOTE]
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Figure 2-3. Buck converter schematic.
2.6.2
PWM frequency
The PWM speed for the PWM is programmed to the maximum (64MHz). The source clock is the PLL output (96MHz)
used both by USB and PWM.
A postscaler offers a 1.5 division for the PLL signal: 96MHz/1.5 = 64MHz (see PLLTM1 and PLLTM0 in PLLFREQ
register).
The result on the PWM output signal is a 250kHz frequency:
64MHz / 256 = 250kHz
Where 256 is the size in bit of the in OCR4A compare register used in Timer 4.
Figure 2-4. PWM output signal.
PWM
output
ton
toff
t
The software controls the battery voltage/current in modifying the duty cycle of PWM output. If ton increases, the battery
voltage/current receives a more important energy load.
AVR145: USB HID Lithium-Ion Battery Charging via USB [APPLICATION NOTE]
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3.
Battery charger firmware
3.1
Scheduler
A scheduler is implemented to call indefinitely defined tasks. Before starting this infinite loop, init functions are called.
There are three tasks. Each task is called after the end of the previous one (no pre-emption).
Figure 3-1. Scheduler.
Scheduler Init:
Usb_task_init()
HID_task_init()
Batt_task_init()
Scheduler task:
Usb_task()
HID_task()
Batt_task()
3.2
List of files
The firmware is written in C language using IAR Embedded Workbench®, version 5.10. Since the firmware has been
written entirely in C, it should not be a difficult task to port it to other AVR C-compilers. Some compiler specific details
may, however, need to be rewritten.
In Table 3-1 are listed the files that are relevant to the compiler project.
Table 3-1.
Project files for HID application (see IAR EW workspace file).
File
Type
hid_task.c
C source code
hid_task.h
Header file
main.c
C source code
main.h
Header file
power_drv.c
C source code
power_drv.h
Header file
scheduler.c
C source code
scheduler.h
Header file
start_boot.c
C source code
start_boot.h
Header file
Note
HID task and HID task init functions
Main program / Program entry point
Power management low level driver
Scheduler routines
Boot functions
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File
Type
time.c
C source code
time.h
Header file
usb_descriptor.c
C source code
usb_descriptor.h
Header file
usb_device_task.c
C source code
usb_device_task.h
Header file
usb_drv.c
C source code
usb_drv.h
Header file
usb_standard_request.c
C source code
usb_standard_request.h
Header file
usb_specific_request.c
C source code
usb_specific_request.h
Header file
usb_task.c
C source code
usb_task.h
Header file
Table 3-2.
Note
Functions for timing
USB parameters that identify the application
USB device controller
USB driver routines
USB device enumeration requests
User call-back functions
Usb task and Usb init task functions
Project files for battery module (see IAR EW workspace file).
File
Type
ADC.c
C source code
ADC.h
Header file
Batt_task.c
C source code
Batt_task.h
Header file
battery.c
C source code
battery.h
Header file
chargefunc.c
C source code
chargefunc.h
Header file
LIIONcharge.c
C source code
LIIONcharge.h
Header file
menu.c
C source code
menu.h
Header file
PWM.c
C source code
PWM.h
Header file
statefunc.c
C source code
statefunc.h
Header file
Note
Functions related to A/D converter
Batt task and Batt init task functions
Battery-specific definitions and functions related to battery control and
data acquisition
Charge functions
Charge state function for Li-Ion batteries
State machine definitions
Functions related to generating pulse-width modulated output
Functions related to the states defined in menu file
AVR145: USB HID Lithium-Ion Battery Charging via USB [APPLICATION NOTE]
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3.3
Battery task
A complete description of battery task is available in the AVR146: Lithium-Ion Battery Charging via USB with
ATmega16/32U4 application note. Battery modules are identical in CDC and HID implementations.
3.4
HID task
The AVR328 USB Generic HID reports are modified to add battery charge parameters.
The OUT reports are not used for battery charge (LEDs management for Generic HID). The size of IN reports is the
same (8 bytes).
Table 3-3.
HID IN report description.
HID IN report
Value
Description
BYTE 0
0
Reserved for HID Generic Demo
1
Reserved for HID Generic Demo
Battery Type
BYTE 1
0
No battery detected
1
VARTA EASYPACK 550mA
2
VARTA EASYPACK 750mA
3
VARTA EASYPACK 1000mA
4
VARTA EASYPACK 2000mA
Charge State
BYTE 2
0
No charge
1
Constant current charge
2
Constant voltage charge
3
Full charge detected
Battery voltage (16 bits)
BYTE 3
Value
Batt Voltage LSB
BYTE 4
Value
Batt Voltage MSB
Battery current (16 bits)
BYTE 5
Value
Batt Current LSB
BYTE 6
Value
Batt Current MSB
Battery temperature (8 bits)
BYTE 7
Value
Batt Temperature
AVR145: USB HID Lithium-Ion Battery Charging via USB [APPLICATION NOTE]
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3.5
Memory requirements
The firmware integrates all functions required to charge a Lithium-Ion battery.
Table 3-4.
Memory requirements of firmware (GCC without optimization, -O0).
Build option
Debug
Memory
Approximate value
CODE (Flash)
13272 bytes
DATA (SRAM)
274 bytes
XDATA (EEPROM)
130 bytes
AVR145: USB HID Lithium-Ion Battery Charging via USB [APPLICATION NOTE]
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4.
Battery charger software
4.1
Overview
A PC application is launched and detects automatically the HID device. Battery voltage, current, temperature, battery
type and charge state are displayed.
The actual firmware only works with VARTA EASYPACK batteries. To support another type of battery, the battery
parameters table (Flash memory) needs to be modified (new firmware revision).
Caution:
“Enter DFU” button needs to have a bootloader loaded in Flash memory. If not, it appears as a reset
of the application firmware.
Figure 4-1. PC screen shot.
AVR145: USB HID Lithium-Ion Battery Charging via USB [APPLICATION NOTE]
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5.
Atmel EVK527 Rev1.0.0 schematics
Figure 5-1. Page 1/5.
AVR145: USB HID Lithium-Ion Battery Charging via USB [APPLICATION NOTE]
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Figure 5-2. Page 2/5.
AVR145: USB HID Lithium-Ion Battery Charging via USB [APPLICATION NOTE]
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Figure 5-3. Page 3/5.
AVR145: USB HID Lithium-Ion Battery Charging via USB [APPLICATION NOTE]
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Figure 5-4. Page 4/5.
AVR145: USB HID Lithium-Ion Battery Charging via USB [APPLICATION NOTE]
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Figure 5-5. Page 5/5
AVR145: USB HID Lithium-Ion Battery Charging via USB [APPLICATION NOTE]
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6.
References
1.
“What’s the best battery?”. Retrieved April 3, 2007, from Battery University:
http://www.batteryuniversity.com/partone-3.htm
2.
“Lithium-Ion safety concerns”. Retrieved April 3, 2007, from Battery University:
http://www.batteryuniversity.com/partone-5B.htm
3.
“Charging Lithium-Ion batteries”. Retrieved April 3, 2007, from Battery University:
http://www.batteryuniversity.com/partone-12.htm
4.
“VARTA EasyPack” datasheet:
•
550mAh
http://www.varta-microbattery.com/applications/mb_data/documents/product_information/PI_56445701099.pdf
•
1000mAh
http://www.varta-microbattery.com/applications/mb_data/documents/product_information/PI_56446701099.pdf
•
2000mAh
http://www.varta-microbattery.com/applications/mb_data/documents/product_information/PI_56446702099.pdf
•
5.
750mAh
Not for sale anymore
“ATmega32U4”. Available from Atmel web site:
http://www.atmel.com/products/avr/
AVR145: USB HID Lithium-Ion Battery Charging via USB [APPLICATION NOTE]
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7.
Revision history
Doc. Rev.
Date
Comments
8208A
10/2012
Initial document release
AVR145: USB HID Lithium-Ion Battery Charging via USB [APPLICATION NOTE]
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