Download Z8 Encore! -Based Battery Charger

Application Note
Z8 Encore!®-Based Battery Charger
AN013703-0708
Abstract
Features
This Application Note demonstrates Zilog’s
Z8 Encore!®-based battery charger that charges
various rechargeable batteries in a fast, efficient,
and safe manner.
The features of Z8 Encore! are as follows:
All the important rechargeable battery types,
Sealed Lead Acid (SLA), Nickel Cadmium (NiCd),
Nickel Metal Hydride (NiMH), and Lithium Ion
(Li-Ion), are addressed in this Application Note.
The Z8 Encore!-based charger manages each battery type according to its individual charging profile.
Note: The source code file associated with
this application note, AN0137SC01.zip is available for download at www.zilog.com.
Product Overview
Z8 Encore! products are based on the new 8-bit
eZ8 CPU, and introduce Flash memory to Zilog’s
extensive line of 8-bit microcontrollers unit
(MCU). The Flash in-circuit programming capability allows for faster development time and program
changes in the field. The new eZ8 core maintains
backward compatibility with Zilog’s popular Z8®
MCU.
Featuring Zilog’s high performance register-to-register based architecture (eZ8), the new Z8 Encore!
MCUs combine a fast 20 MHz core, up to 64 KB
of Flash memory, up to 4 KB of linear register
SRAM, and an extensive array of on-chip peripherals. These peripherals make Z8 Encore! suitable
for a variety of applications including motor control, security systems, home appliances, personal
electronic devices, and sensors.
•
New high-performance 20 MHz eZ8 CPU
•
Up to 64 KB Flash memory with in-circuit programming capability
•
Up to 4 KB register SRAM
•
12-channel, 10-bit analog-to-digital converter
(ADC)
•
Two full-duplex UARTs
•
Two Infrared Data Association (IrDA) compliant
endecs
•
SPI and I2C ports
•
Four 16-bit timers with capture, compare, and
PWM capability
•
Watchdog Timer (WDT) with internal RC
oscillator
•
3-channel DMA
•
Up to 60 I/O pins
•
24 interrupts with configurable priority
•
On-Chip Debugger
•
Voltage Brownout protection (VBO)
•
Power-On Reset (POR)
The Z8 Encore! CPU is capable of a nominal
10 MIPs throughput at 20 MHz. The 4 KB SRAM
extends the Z8 Encore!’s reach to a wider range of
applications. The 10-bit sigma/delta ADC provides
high measurement resolution, and the SPI, UART,
and I2C interfaces can be used concurrently. The
versatile DMA controllers can be configured in
many useful combinations to free the CPU from
performing unnecessary data transfer overhead.
Copyright ©2008 by Zilog®, Inc. All rights reserved.
www.zilog.com
Z8 Encore!®-Based Battery Charger
Discussion
A discussion on designing a battery charger is presented in this section. For further details, see Reference on page 8.
Theory of Operation
When designing a battery charger, the following
aspects are considered:
•
Power control techniques to suit different battery
types and capacities.
•
Charging and charge termination techniques to
avoid overcharging, thus facilitating fast charging.
•
Safety techniques to ensure safe operation
throughout the charging process.
Setpoint
(VSET /I SET )
™
+
-
Error
These aspects are discussed in the following
section.
Power Control Techniques
At the core of a battery charger is the DC–DC
converter that acts as a regulated power source.
The charger hardware is capable of regulating its
output in various modes, such as constant voltage,
constant current, or constant voltage with a current
limit. The charger can be viewed as a control system in itself.
In Figure 1, an initial setpoint is a charger output
value chosen by you. In a battery charger, the type
and capacity of the battery is the determinant of the
mode of operation—namely, a constant current
source or a constant voltage source. It also determines the required current and voltage setpoints.
These setpoints can be expressed as ISET or VSET.
Control Signal
(PWM)
Controller
Actuator
(Buck Converter)
Output
(VOUT /I OUT)
Feedback Signal
(VFB /IFB)
Feedback Circuits
Figure 1. Feedback Control System
The feedback circuits displayed in Figure 1
measures actual output. The difference between the
initial setpoint and the actual value (feedback
signal) is called an error. The controller generates a
control signal according to the magnitude and
direction of the error. It minimizes the steady state
error and also responds quickly to transient fluctuations during input or output. Controllers usually
work at lower power levels and therefore require
an external actuator to generate the appropriate
output.
In a battery charger, the actuator is a step-down
DC–DC converter, also known as a buck converter.
The buck converter converts a higher DC voltage
to a lower one depending on the Pulse Width
Modulated (PWM) control signal generated by the
controller. The frequency of the PWM signal is
maintained at a constant while the width of the
pulse or the duty cycle of the signal varies. This
variation is reflected as a change in voltage and/or
current at the output.
Controllers are differentiated according to the way
they handle errors generated during regulation of
AN013703-0708
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Z8 Encore!®-Based Battery Charger
the system output (in the case of a charger, these
errors are either voltage or current errors). In a proportional controller, the actual value and the set
value are compared, and the resulting error value is
used. In such a system, there exists the possibility
of a steady state error, which is a drawback for the
proportional controller. Adding an integral component to the proportional controller eliminates this
steady state error.
where Kp and Ki are the proportional and integral
constants, respectively.
The equation for a proportional plus integral (PI)
controller is:
Charging and Charge Termination
Techniques
υ ( t ) = ( k1 × e ( t ) + k2 × ∫ e ( t ) dt )
To be useful for a microcontroller-based (discrete)
system, the integral is approximated by a running
sum of the error signal. Thus, an equation can be
expressed as follows called Equation 1:
⎛
⎞
k–1
υ [ k ] = ⎜ C1 × e [ k ] + C2 × ∑ e [ j ]⎟
⎜
⎟
j=0
⎝
⎠
where C1 and C2 are constants.
Equation 1 is the position algorithm. A better representation for Equation 1 is described in
Equation 2, as follows:
⎛
⎞
k–2
⎜
υ [ k – 1 ] = ⎜ C1 × e [ k – 1 ] + C2 × ∑ e [ j ]⎟⎟
j=0
⎝
⎠
Subtracting Equation 2 from Equation 1 and rearranging the terms yields Equation 3, as follows:
υ [ k ] – υ [ k – 1 ] = ( Kp × e [ k ] + Ki × e [ k – 1 ] )
AN013703-0708
Equation 3 is the velocity algorithm. It is a convenient expression, as only the incremental change in
the manipulated variable is calculated.
For a detailed discussion on controllers, see Reference on page 8.
Different battery types require different charging
methods. The basic charging methods are the constant current and constant voltage charging. The
NiCd and NiMH batteries are charged using the
constant current method, whereas the SLA and LiIon batteries are charged via the constant voltage
method. An on/off current limiter is required when
performing constant voltage charging. These
charging methods are based on the type of battery
and the present state of charge for that battery.
In a constant current method of charging, fast
charging occurs when the charging current equals
the rated battery capacity, C. Fast charging requires
constant monitoring of battery parameters and
precise termination techniques. It is therefore
important to know when to terminate charging.
The behavior of different batteries near full charge
varies and demands different termination techniques. The most common termination techniques
are the negative ΔV, zero ΔV, and the absolute
battery voltage, all of which are based on battery
types. For more information, see Appendix D—
Battery Technology on page 15.
Safety Techniques
A battery charger must ensure the safety of batteries. Battery safety is implemented by monitoring
the battery terminal voltage and current against the
battery ratings provided by the manufacturer.
When battery ratings are exceeded, the charging
voltage or current is switched off.
Page 3 of 17
Z8 Encore!®-Based Battery Charger
Z8 Encore!-Based Battery
Charger
This section offers an overview of the functional
architecture of the battery charger implementation
using Z8 Encore!.
Hardware Architecture
The Z8 Encore!-based charger features the following hardware blocks. Figure 2 displays the following hardware blocks:
•
Z8 Encore! MCU
•
Step-down DC–DC (buck) converter
•
Feedback section
•
Battery selector (jumper settings)
•
LED status indicators
+
Step-Down (Buck)
Converter
Battery
Z8 Encore!® MCU
ADC Channels
Converter V/I,
Battery Voltage
Feedback
GPIO as Inputs
Batter Selector
(Jumper Settings)
GPIO as Outputs
Status Indicator
(LED Port)
-
Figure 2. Block Diagram of Battery Charger
Hardware
In this application, the Z8 Encore! MCU’s Ports E
and H are used as GPIO; Port B is used as an ADC
input. Timer1 is used in PWM mode and the output
is tapped at the pin PC1/Timer1 out.
AN013703-0708
The feedback section consists of three differential
amplifiers/attenuators. The corresponding parameters are the converter voltage (VOUT), battery voltage (VBATT), and battery current (IBATT). The
battery current and the converter current are the
same.
The battery type is selected by setting one of the
four jumpers provided. The jumper status is read
initially, and the corresponding routine is selected
for charging.
External
Power Source
PWM Output
The step-down DC–DC (buck) converter provides
appropriate voltage or current for the set battery
type and parameters. The buck converter modulates a higher voltage (from the external source)
with a varying pulse width (PWM method) to
generate a lower voltage. The pulse width is controlled by the control algorithm based on the values
obtained from the feedback section. The output of
the external source is preferably set to twice the
value of the converter output voltage (VOUT).
The charger indicates the charger status via LEDs,
which are used to indicate various states such as
successful completion of charging, safety error, no
battery selection, and the specific battery type
undergoing the charging process. Table 1 lists the
status indicators along with a brief description.
Table 1. LED Status Indicators
LED
Status
Description
D4
ON
SLA battery is selected and
charging is ON.
D5
ON
NiCad battery is selected and
charging is ON.
D6
ON
NiMH battery is selected and
charging is ON.
D7
ON
Li-Ion battery is selected and
charging is ON.
D8
ON
No battery is selected.
D9
ON
Safety error—charging is aborted.
D10
ON
Charging is successfully
completed.
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Z8 Encore!®-Based Battery Charger
For the battery charger schematics, see Appendix
B—Schematics on page 10.
Software Implementation
All Z8 Encore! peripherals are initialized to the
required mode of operation. The jumper settings
are read and the battery type is validated. When the
battery type is fixed, the battery parameters are
loaded into the variables. At present, these battery
parameters are defined in the header file.
Initially, based on battery ratings, each module sets
the safety and termination thresholds. Then typedependent settings, such as converter voltage, current outputs, and current limit are calculated. When
these one-time calculations are completed, the
charger software enters an infinite loop, which is
broken only by a successful charge completion or a
safety error.
Inside the loop, the ADC reads the actual values
for the converter output voltage, the battery voltage, and the current. The ADC measures the output
voltage and output current of the DC–DC converter
as a feedback to the controller. It measures the voltage at the battery terminals as an input to determine
the charge termination.
When the actual values are known, they are
checked for safety limit compliance. The safety
routine is responsible for the overall safety features
associated with the battery charger. The charger
ensures safety by comparing the actual converter
voltage and battery voltage with the calculated
thresholds. Crossing these thresholds switches off
the PWM output, which turns off the converter output and terminates charging functions. Such termination protects the batteries in case of a device
failure. The LED status indicator reflects an unsuccessful termination.
fully charged, charging terminates and the LED
indicators are updated. If the battery requires further charging, the controller calculates the required
duty cycle for maintaining the setpoint at the converter output.
The controller implements proportional plus integral (PI) control to derive the PWM output based
on the equations mentioned in the Theory of Operation on page 2. The timer ISR is invoked every
5 ms. The PWM value computed by the controller
is loaded into the PWM generators to be sent out
via the output pin.
The 16-bit timer PWM mode offers a programmable switching frequency based on the reload value.
This flexibility allows you to trade off between
accuracy and frequency of the PWM switching
signal. The higher the frequency, the lesser the
reload value and the lower the resolution in the
pulse width variation; and vice versa.
The timer ISR also updates the charge termination
variables every 10 seconds.
Testing
This section contains a detailed test procedure to
demonstrate the working of the Z8 Encore! battery
charger as described in this Application Note. The
test setup to demonstrate the battery charger using
Z8 Encore! is displayed in Figure 3.
If everything is within limits, the battery is tested
for full charge. Full charge is tested using different
methods for different batteries (see Appendix D—
Battery Technology on page 15). If the battery is
AN013703-0708
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Z8 Encore!®-Based Battery Charger
External DC Power Supply
Oscilloscope
Z8 Encore!®
Evaluation
Board
PWM
DC-DC Step-Down Converter
Battery
Feedback Attenuators
PC-HyperTerminal
Charger Hardware/External Circuits
Figure 3. Battery Charger Test Setup
The test setup consists of an oscilloscope and a PC
running the HyperTerminal application. For testing, the Z8 Encore! Evaluation Board is used with
the DC–DC converter and the feedback circuits.
An external DC source supplies necessary voltage
and current for the various circuits involved.
The external DC power supply provides two different voltages to the charger circuits—the DC–DC
step-down converter and the feedback attenuators.
The operational amplifier based feedback attenuator circuits are fed with a 12 V supply. The DC–
DC converter works on a 8–12 V DC input for the
batteries tested. The control algorithm provides the
necessary line regulation to sustain the voltage
variation at the input.
During testing, HyperTerminal is set at 57600
baud, 8-bit data, no parity, 1 stop bit, and no flow
control.
Table 2 lists the equipment used to test the Z8
Encore!-based battery charger.
Table 2. Z8 Encore! Battery Charger Test Equipment
Z8 Encore! Evaluation Board (Z8ENCORE000ZCO)
External power supply
Oscilloscope (Tektronix TDS 724D; 500 MHz/1 GSps)
Multimeter
PC (The HyperTerminal utility is used via the COM2 port of the PC)
Batteries Used
Make
Type
Ratings
BP–T40
Sony
Sealed Lead Acid
4 V, 500 mAh
BP–T16
Sony
Nickel Cadmium
3.6 V, 270 mAh
CP2010H T–014
Panasonic
Nickel Metal Hydride
3.6 V, 150 mAh
AN013703-0708
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Z8 Encore!®-Based Battery Charger
The circuits are connected as per the schematics in
Appendix B—Schematics on page 10.
6500
300
V out
6000
290
280
5500
Iout
5000
270
260
V batt
250
4500
240
331
361
301
271
211
241
181
151
91
121
61
1
0
31
3500
Time in minutes
The NiCd charging profile displayed in Figure 5
indicates a marked hump towards the full charge,
before dropping down. The software effectively
detects this drop and the charging is terminated.
AN013703-0708
91
81
71
61
51
41
31
21
1
101
160
150
Iout
4800
140
4600
130
4400
120
4200
110
4000
100
1
Figure 4. SLA Charging Profile
5000
Iout in mAmps
20
170
V batt
5200
81
3700
5400
73
40
180
65
3900
60
57
Iout
190
5600
49
80
200
V out
5800
41
4100
6000
33
100
25
120
4300
17
140
4500
9
160
Figure 5. NiCd Charging Profile
Vbatt / Vout in mVolts
Vbatt
Iout mAmps
V batt / V out in mVolts
180
4700
Time in Minutes
The NiMH charging profile displayed in Figure 6,
lacks a significant drop and is thus terminated
using the zero ΔV termination scheme.
200
Vout
230
11
4000
For SLA batteries, initially the current is effectively limited to 200 mA; it continually falls while
battery voltage increases. The charging profiles
also demonstrate the constant voltage output (Vout)
of the DC–DC converter at 4900 mV. See Figure 4.
4900
Iout in mAmps
Vbatt / Vout in mVolts
When the external power supply and the Evaluation Board power supply are switched on, the
PWM waveforms are observed on the oscilloscope.
The battery/converter’s actual values are indicated
in the HyperTerminal window. The LED status
indicators, as displayed in Figure 2, reflect the
charging status during the charging operation.
Figures 4, 5, and 6 display the test results obtained
while charging various types of batteries.
310
Time in minutes
Figure 6. NiMH Charging Profile
The charging profiles for NiCd and NiMH batteries
demonstrate constant current outputs of 270 mA
and 150 mA respectively. These are equal to their
rated battery capacity measured in mAh. The
Page 7 of 17
Z8 Encore!®-Based Battery Charger
charging times for NiCd and NiMH are 1 hour, 45
minutes and 1 hour, 25 minutes, respectively.
Note: Because the SLA and Li-Ion batteries follow similar charging (constant
voltage with limited current) and termination profiles (absolute voltage),
only the SLA battery was charged.
The results are provided in this
document.
Summary
This Application Note demonstrates the use of Z8
Encore! in a battery charger implementation. Ordinary battery chargers can charge batteries of a particular type and of a particular voltage. The Z8
Encore!-based hardware/software provides flexibility such that batteries of different types can be
charged with the same charger.
•
High Frequency Switching Power Supplies: Theory and Design; author: George Chryssis; ISBN:
0-07-010949-4; Publisher: McGraw-Hill Book
Company
•
Digital Control Systems, Volume 1—Fundamentals, Deterministic Control; author: Rolf Isermann; ISBN: 0-387-50266-1; Publisher:
Springer Verlag
•
Yuasa Technical Manual—http://www.yuasabatteries.com/literature.asp
•
Duracell—http://www.duracell.com/batteries
•
Eveready/Energizer—http://data.energizer.com
•
Panasonic Li-Ion battery documents—http://
www.panasonic.com/industrial/battery/oem/
chem/lithion/index.html
•
Sanyo—http://www.sanyo.com/industrial/batteries/index.html
The Z8 Encore! 10-bit ADC ensures accurate
charge termination, facilitating faster recharge.
Such termination avoids overcharging and
prolongs battery life. The flexibility of the PWM
mode allows for accurate DC–DC buck/step-down
converter implementation with excellent line/load
regulation.
The test results clearly demonstrate the charging
and termination mechanisms used by the charger to
successfully charge different battery types.
Reference
The documents associated with Z8 Encore!® available on www.zilog.com and electronics references
are provided below:
•
Z8 Encore!® Flash Microcontroller Development Kit User Manual (UM0146)
•
Power Electronics Design Handbook: Low
Power Components and Applications; author:
Nihal Kularatna; ISBN: 0-7506-7073-8; Publisher: Oxford; Newnes, 1998
AN013703-0708
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Z8 Encore!®-Based Battery Charger
Appendix A—Glossary
Definitions for terms and expansions for abbreviations used in this application note that are not commonly
used are listed in Table 3.
Table 3. Glossary
Term/Abbreviation
Definition/Expansion
ADC
Analog-to-Digital Converter
ISR
Interrupt Service Routine
Li-Ion
Lithium Ion
mAh
milli-Ampere-hour: a unit of battery capacity
NiCd
Nickel Cadmium
NiMH
Nickel Metal Hydride
PI
Proportional plus Integral
PWM
Pulse Width Modulation
SLA
Sealed Lead Acid
AN013703-0708
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Z8 Encore!®-Based Battery Charger
Appendix B—Schematics
This section provides the schematics for the Z8 Encore! battery charger implementation
4
3
2
F1
Z8 Encore! Interface
1
IN
PA4
PA5
5V
3
OUT
D5 RXE160
VDD
GND
VCC
S2G
2
GND
VDD
1
U14
LM7805C/TO220/0.5A
GND
5
D
80
79
78
77
76
75
74
73
72
71
70
69
68
67
66
65
1
3
2
PA1/T0OUT
PA2
PA3/CTS0
GND
VDD
PF7
PC5/MISO
PD3
PD4/RXD1
PD5/TXD1
PC4/MOSI
VDD
GND
PA4/RXD0
PA5/TXD0
PA6/SCL
C
PE4
PE3
GND
PE2
PE1
PE0
GND
VDD
EXTAL
XTAL
PA0/T0IN
PD2
PC2/SS
PF6
RESET
VDD
PF5
PF4
PF3
PE4
PE3
GND
PE2
PE1
PE0
GND
PF2
PF1
PF0
VDD
PD1/T3OUT
PD0/T3IN
EXTAL
XTAL
64
63
62
61
60
59
58
57
56
55
54
53
52
51
50
49
48
47
46
45
44
43
42
41
VCC
3
1
PE5
PE6
PE7
VDD
3.3V
2
VIN VOUT
GND
VDD
VDD
C
R14
680
C24
+ C23
0.1
100/6.3
D6
VDD
GREEN
GND
PC1
R17
U19
1
VCC
3
GND
RESET
2
B
100K
RESET
DS1233A-15
VDD
R3
1M
C1
C2
18pF
18pF
SW4
RESET
PH3
PH2
PB2
PB3
PB1
PB0
PH1
PH0
A
4
C46
C49
C50
0.1uF
0.1uF
0.1uF
0.1uF
GND
0.01
A
Title
Battery Charger using Z8 Encore!
Size
A
Date:
5
C45
C30
PB3_ALG3
PB2_ALG2
PH2_ALG10
PH3_ALG11
18.432MHz
GND
VDD
PH0_ALG8
PH1_ALG9
PB0_ALG0
PB1_ALG1
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
Y1
100/10
LT1086-3.3/TO220
GND
AVDD
PH0/ALG8
PH1/ALG9
PB0/ALG0
PB1/ALG1
PB4/ALG4
PB5/ALG5
PB6/ALG6
PB7/ALG7
PB3/ALG3
PB2/ALG2
PH2/ALG10
PH3/ALG11
VREF
AGND
XTAL
0.1
C22
47uF
U16
GND
B
EXTAL
+
0.1
PA7/SDA
PD6/CTS1
PC3/SCK
PD7/RCOUT
PG0
GND
PG1
PG2
PE5
PE6
PE7
VDD
PG3
PG4
PG5
PG6
VDD
PG7
PC7/T2OUT
PC6/T2IN
DBG
PC1/T1OUT
PC0/T1IN
GND
Z8F
C21
P3
+ C15
1
RESET
VDD
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
D
C17
2
U1
PW R JACK
3
Document Number
<Doc>
Tuesday, January 07, 2003
2
Rev
Sheet
1
of
1
1
Figure 7. Schematic for Z8 Encore! Interface
AN013703-0708
Page 10 of 17
Z8 Encore!®-Based Battery Charger
5
4
3
2
1
Vp
DC-DC Step Down Converter
R3
D
D
1K
V_out(+)
R37
V_out(-)
IRF9540
Q1
18E
D2
R1
L1
Q2
2N2222
PC1/ T1OUT
3.3K
120uH
MBR360
C1
R4
470E
C2
(+)
R2
D3
MBR360
2.2K
100uF
V_batt(+)
R5
79E
100uF
BT1
BATTERY
TO BE CHARGED
D1
LED
V_batt(-)
(-)
C
R6A
10E
R6B
10E
C3
0.1uF
C
Rsense
I_out(+)
3.3 Volts
LED Indicator Port
I_out(-)
R7
560E
B
D4
LED
R8
560E
R9
560E
D5
LED
D6
LED
R10
560E
D7
LED
R11
560E
D8
LED
R12
560E
R13
560E
D9
LED
B
D10
LED
PE1
PE2
PE3
PE4
PE5
PE6
PE7
A
A
Title
Using Z8 Encore! as a Battery Charger
Size
A
Date:
5
4
3
Document Number
<Doc>
Tuesday, January 07, 2003
2
Rev
0.0
Sheet
1
of
2
1
Figure 8. DC–DC Step-Down Converter and LED Indicator Port
AN013703-0708
Page 11 of 17
Z8 Encore!®-Based Battery Charger
5
4
3
2
1
0.1uF
0.1uF
C7
C7
R25
1K
R17
Feedback Circuits
12V
9
R23
10
10K
+
I_out(+)
PB2/ANA2
V_batt(+)
Battery Current
11
R14
2
R15
3
1
R24
1K
R16
1K
C8
10uF
PB1/ANA1
Battery Voltage
10K
4
1K
V_batt(-)
R34
8
D
LM324
U1A
4
11
R22
10K
-
I_out(-)
LM324
U1C
+
1K
-
D
1K
12V
0.1uF
C7
C
VCC
C
R21
1K
11
12V
10K
V_out(-)
V_out(+)
R18
6
7
R19
5
10K
R20
1K
PB3/ANA3
Converter Output Voltage
4
C10
0.1uF
+
C9
0.1uF
-
C11
100uF
LM324
U1B
B
B
3.3 Volts
Jumpers for Battery Selection
R30
10K
A
R31
10K
1
2
PH0
PH1
R32
10K
1
2
PH2
R33
10K
1
2
Note:
1. R14 - R30 all 1% MFR.
2. Signal, Digital, and Power Grounds are connected on
the evaluation board.
1
2
PH3
A
J2
J3
J4
J5
Title
Select
NiCd
Select
NiMH
Select
SLA
Select
Li-Ion
Size
A
Using Z8 Encore! as a Battery Charger
Date:
5
4
3
Document Number
<Doc>
Tuesday, January 07, 2003
2
Rev
0.0
Sheet
2
of
2
1
Figure 9. Feedback Section and Battery Type Selector Jumper Settings
AN013703-0708
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Z8 Encore!®-Based Battery Charger
Appendix C—Flowcharts
The main battery-charging routine is displayed in Figure 10.
Start
Initialize peripherals
Read and verify battery type
Get the battery parameters
Calculate safety limits and thresholds
for charging and termination
Read feedback values for battery voltage,
current, and converter voltage
Within safety limits?
No
Yes
Is the battery charged?
No
Yes
Terminate
Calculate the duty cycle
Figure 10. Flowchart for the Main Routine
AN013703-0708
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Z8 Encore!®-Based Battery Charger
The ISR return routine is displayed in Figure 11.
Start ISR
Reload PWM Value
Update charge ending data every 10 seconds
Return from ISR
Figure 11. Flowchart for the ISR Return Routine
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Z8 Encore!®-Based Battery Charger
Appendix D—Battery Technology
The four mainstream battery chemistries discussed
in this Application Note feature different charging
and discharging characteristics. Long-term battery
life and performance are critically dependent upon
how batteries are charged. Therefore, it is important to charge batteries with a mechanism specific
to their requirement.
It is also important to know when to terminate
charging, because overcharging of a battery invariably results in poor performance and can damage
the battery in extreme cases. Different battery types
behave differently near full charge condition and
thus require specific charge termination techniques. During charging, all batteries exhibit a
marked rise in voltage above the rated battery voltage.
The four major rechargeable battery types—SLA,
NiCd, NiMH, and Li-Ion, are briefly discussed
below. For further details, see Reference on page 8.
Sealed Lead Acid (SLA)
Sealed Lead Acid batteries are most commonly
seen in automobiles. The single cell voltage for
SLA is 2 V. According to their use, several such
cells are connected in series to get higher voltages
such as 12 V/24 V.
SLA batteries are usually charged with a constant
voltage supply of 2.45 V per cell. For this Application Note, 4.90 V is used as the charging voltage
for the 4 V SLA battery.
At the start of charging, depending on their state of
charge, SLA batteries require huge amounts of current. If this current uptake is not controlled, the battery electrolyte may boil, producing gasses inside
the battery. It is therefore necessary to limit the
charging current. When the battery achieves some
charge, the current is limited and constant voltage
charging is enforced.
AN013703-0708
The charge termination mechanism is simple and is
achieved as battery voltage reaches the charging
voltage. At the same time, there is a corresponding
drop in charging current.
Nickel Cadmium (NiCd)
NiCd batteries are used in camcorders, Walkmans,
and other similar consumer portable equipment.
The single-cell voltage for NiCd batteries is 1.2 V.
These batteries are charged using the constant current charging method. While charging, as the voltage crosses the full charge point, it starts dropping.
This drop is approximately 15 mV per cell in the
battery. This drop is recognized as a full charge
condition, and charging is terminated. This termination mechanism is named as –ΔV termination.
During charging, battery voltage rises to 1.65 V
per cell.
The disadvantage of the NiCd battery is that the
battery must be periodically discharged to protect
performance. In battery parlance, this phenomenon
is known as memory effect.
Nickel Metal Hydride (NiMH)
NiMH batteries exhibit higher power density compared to NiCd batteries. The per-cell voltage of the
NiMH battery type is 1.2 V, similar to NiCd batteries.
NiMH batteries are charged via the constant current charging method. While charging, as the voltage crosses the full charge point, the voltage drop
is not as low as for the NiCd batteries. As a consequence, –ΔV charge termination is usually not recommended for these batteries. Instead of the fall in
cell voltage, the battery tends to plateau after a
small drop. This flat region is the preferred indication for full battery charging, rather than the drop.
Consequently, this termination mechanism is
named zero ΔV termination.
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Z8 Encore!®-Based Battery Charger
NiMH batteries do not suffer from memory effect
as do NiCd batteries. As a result, they replace NiCd
battery types in applications such as cell phones
because the increase in price is justified by the
reduction in weight and absence of memory effect.
Lithium Ion (Li-Ion)
Li-Ion batteries are lighter in weight than NiCd and
NiMH batteries. Available with a high voltage rating of 3.7 V, one Li-Ion battery can replace three
NiCd/NiMH battery types. These two features
make Li-Ion high-energy density batteries. They
exhibit flat discharge characteristics and are free
from memory effect.
If the starting voltage of these batteries is initially
too low, a small constant current is applied until the
battery reaches a certain threshold specified by the
manufacturer. The battery is charged with constant
voltage when this threshold is crossed. Charging is
terminated when battery voltage reaches the rated
voltage.
AN013703-0708
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Z8 Encore!®-Based Battery Charger
Warning: DO NOT USE IN LIFE SUPPORT
LIFE SUPPORT POLICY
ZILOG'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE
SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS PRIOR WRITTEN APPROVAL OF
THE PRESIDENT AND GENERAL COUNSEL OF ZILOG CORPORATION.
As used herein
Life support devices or systems are devices which (a) are intended for surgical implant into the body, or (b)
support or sustain life and whose failure to perform when properly used in accordance with instructions for
use provided in the labeling can be reasonably expected to result in a significant injury to the user. A
critical component is any component in a life support device or system whose failure to perform can be
reasonably expected to cause the failure of the life support device or system or to affect its safety or
effectiveness.
Document Disclaimer
©2008 by Zilog, Inc. All rights reserved. Information in this publication concerning the devices,
applications, or technology described is intended to suggest possible uses and may be superseded. ZILOG,
INC. DOES NOT ASSUME LIABILITY FOR OR PROVIDE A REPRESENTATION OF ACCURACY
OF THE INFORMATION, DEVICES, OR TECHNOLOGY DESCRIBED IN THIS DOCUMENT.
Z I L O G A L S O D O E S N O T A S S U M E L I A B I L I T Y F O R I N T E L L E C T U A L P R O P E RT Y
INFRINGEMENT RELATED IN ANY MANNER TO USE OF INFORMATION, DEVICES, OR
TECHNOLOGY DESCRIBED HEREIN OR OTHERWISE. The information contained within this
document has been verified according to the general principles of electrical and mechanical engineering.
eZ8, Z8, Z8 Encore!, and Z8 Encore! XP are trademarks or registered trademarks of Zilog, Inc. All other
product or service names are the property of their respective owners.
AN013703-0708
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