an0137

AbstractThis Application Note demonstrates Zilogs Z8 Encore!-based battery charger that charges various rechargeable batteries in a fast, efficient, and safe manner.

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 bat-tery type according to its individual charging pro-file.

The source code file associated withthis application note, AN0137-SC01.zip is available for down-load at www.zilog.com.

Product OverviewZ8 Encore! products are based on the new 8-bit eZ8 CPU, and introduce Flash memory to Zilogs extensive line of 8-bit microcontrollers unit (MCU). The Flash in-circuit programming capabil-ity allows for faster development time and program changes in the field. The new eZ8 core maintains backward compatibility with Zilogs popular Z8 MCU.

FeaturesThe features of Z8 Encore! are as follows:

New high-performance 20 MHz eZ8 CPU Up to 64 KB Flash memory with in-circuit pro-

gramming 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)

Note:

Application Note

Z8 Encore!-Based Battery Charger

AN013703-0708 Featuring Zilogs high performance register-to-reg-ister 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 extensivals. These peripherals mfor a variety of applicatrol, security systems, helectronic devices, and

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,

e Copyright 2008 by Zilog, Inc. All rights reserved.

sensors.e array of on-chip peripher-ake Z8 Encore! suitable

tions including motor con-ome appliances, personal

and I2C interfaces can be used concurrently. Thversatile DMA controllers can be configured inmany useful combinations to free the CPU fromperforming unnecessary data transfer overhead.www.zilog.com

Z8 Encore!-Based Battery ChargerDiscussionA discussion on designing a battery charger is pre-sented in this section. For further details, see Refer-ence on page 8.

Theory of OperationWhen 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 charg-ing.

Safety techniques to ensure safe operation throughout the charging process.

These aspects are discussed in the following section.

Power Control TechniquesAt the core of a battery charger is the DCDC 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 sys-tem 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 operationnamely, a constant current source or a constant voltage source. It also deter-mines the required current and voltage setpoints. These setpoints can be expressed as ISET or VSET.

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 fluctua-tions during input or output. Controllers usually work at lower power levels and therefore require an external actuator to generate the appropriate

In a battery charger, the actuator is a step-down DCDC 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.

Figure 1. Feedback Control System

Feedback Circuits

ControllerError

Control Signal(PWM) Actuator

(Buck Converter)Setpoint(V /I )SET SET

Feedback Signal(V /I )FB FB

Output(V /I )OUT OUT

+-AN013703-0708 Page 2 of 17

output.Controllers are differentiated according to the way they handle errors generated during regulation of

Z8 Encore!-Based Battery Chargerthe system output (in the case of a charger, these errors are either voltage or current errors). In a pro-portional 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 compo-nent to the proportional controller eliminates this steady state error.

The equation for a proportional plus integral (PI) controller is:

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:

where C1 and C2 are constants.

Equation 1 is the position algorithm. A better rep-resentation for Equation 1 is described in Equation 2, as follows:

Subtracting Equation 2 from Equation 1 and rear-ranging the terms yields Equation 3, as follows:

where Kp and Ki are the proportional and integral constants, respectively.

Equation 3 is the velocity algorithm. It is a conve-nient expression, as only the incremental change in the manipulated variable is calculated.

For a detailed discussion on controllers, see Refer-ence on page 8.

Charging and Charge Termination TechniquesDifferent battery types require different charging methods. The basic charging methods are the con-stant current and constant voltage charging. The NiCd and NiMH batteries are charged using the constant current method, whereas the SLA and Li-Ion 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 tech-niques. The most common termination techniques are the negative V, zero V, and the absolute battery voltage, all of which are based on battery types. For more information, see Appendix DBattery Technology on page 15.

Safety TechniquesA battery charger must ensure the safety of batter-ies. Battery safety is implemented by monitoring

t( ) k1 e t( ) k2 e t( ) td+( )=

k[ ] C1 e k[ ] C2 e j[ ]j 0=

k 1+ =

k 1[ ] C1 e k 1[ ] C2 e j[ ]j 0=

k 2+ =AN013703-0708 Page 3 of 17

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.

k ][ k 1[ ] Kp e k ][ Ki e k 1 ][+( )=

Z8 Encore!-Based Battery ChargerZ8 Encore!-Based Battery ChargerThis section offers an overview of the functional architecture of the battery charger implementation using Z8 Encore!.

Hardware ArchitectureThe Z8 Encore!-based charger features the follow-ing hardware blocks. Figure 2 displays the follow-ing hardware blocks:

Z8 Encore! MCU Step-down DCDC (buck) converter Feedback section Battery selector (jumper settings) LED status indicators

In this application, the Z8 Encore! MCUs Ports E and H are used as GPIO; Port B is used as an ADC

The step-down DCDC (buck) converter provides appropriate voltage or current for the set battery type and parameters. The buck converter modu-lates a higher voltage (from the external source) with a varying pulse width (PWM method) to generate a lower voltage. The pulse width is con-trolled 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 feedback section consists of three differential amplifiers/attenuators. The corresponding parame-ters are the converter voltage (VOUT), battery volt-age (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.

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.

Figure 2. Block Diagram of Battery Charger Hardware

Step-Down (Buck)Converter

Batter Selector(Jumper Settings)

Status Indicator(LED Port)

Converter V/I,Battery Voltage

Feedback

Ba

tte

ry

Z8

En

co

re!

MC

U

ExternalPower Source

-

+

PWM Output

ADC Channels

GPIO as Inputs

GPIO as Outputs

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.

AN013703-0708 Page 4 of 17

input. Timer1 is used in PWM mode and the output is tapped at the pin PC1/Timer1 out.

D9 ON Safety errorcharging is aborted.

D10 ON Charging is successfully completed.

Z8 Encore!-Based Battery ChargerFor the battery charger schematics, see Appendix BSchematics on page 10.

Software ImplementationAll 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 type-dependent settings, such as converter voltage, cur-rent 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 volt-age, and the current. The ADC measures the output voltage and output current of the DCDC converter as a feedback to the controller. It measures the volt-age 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 out-put and terminates charging functions. Such ter-mination protects the batteries in case of a device failure. The LED status indicator reflects an unsuc-cessful termination.

If everything is within limits, the battery is tested

fully charged, charging terminates and the LED indicators are updated. If the battery requires fur-ther charging, the controller calculates the required duty cycle for maintaining the setpoint at the con-verter output.

The controller implements proportional plus inte-gral (PI) control to derive the PWM output based on the equations mentioned in the Theory of Oper-ation 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 programma-ble 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.

TestingThis 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.

AN013703-0708 Page 5 of 17

for full charge. Full charge is tested using different methods for different batteries (see Appendix DBattery Technology on page 15). If the battery is

Z8 Encore!-Based Battery ChargerThe test setup consists of an oscilloscope and a PC running the HyperTerminal application. For test-ing, the Z8 Encore! Evaluation Board is used with the DCDC 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 differ-ent voltages to the charger circuitsthe DCDC step-down converter and the feedback attenuators. The operational amplifier based feedback attenua-tor circuits are fed with a 12 V supply. The DCDC converter works on a 812 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.

Figure 3. Battery Charger Test Setup

External DC Power SupplyOscilloscope

Z8 Encore!Evaluation

Board Battery

PC-HyperTerminal

DC-DC Step-Down Converter

Feedback Attenuators

Charger Hardware/External Circuits

PWM

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

BPT40 Sony Sealed Lead Acid 4 V, 500 mAhAN013703-0708 Page 6 of 17

BPT16 Sony Nickel Cadmium 3.6 V, 270 mAh

CP2010H T014 Panasonic Nickel Metal Hydride 3.6 V, 150 mAh

Z8 Encore!-Based Battery ChargerThe circuits are connected as per the schematics in Appendix BSchematics on page 10.

When the external power supply and the Evalua-tion Board power supply are switched on, the PWM waveforms are observed on the oscilloscope. The battery/converters 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.

For SLA batteries, initially the current is effec-tively limited to 200 mA; it continually falls while battery voltage increases. The charging profiles also demonstrate the constant voltage output (Vout) of the DCDC converter at 4900 mV. See Figure 4.

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.

The NiMH charging profile displayed in Figure 6, lacks a significant drop and is thus terminated using the zero V termination scheme.

The charging profiles for NiCd and NiMH batteries demonstrate constant current outputs of 270 mA

Figure 4. SLA Charging Profile

3500

3700

3900

4100

4300

4500

4700

4900

1 31 61 91 121

151

181

211

241

271

301

331

361

Time in minutes

Vba

tt / V

out i

n m

Volts

020406080100120140160180200

I out m

Am

ps

Vbatt

Vout

Iout

Figure 5. NiCd Charging Profile

Figure 6. NiMH Charging Profile

4000

4500

5000

5500

6000

6500

1 11 21 31 41 51 61 71 81 91 101

Time in MinutesV b

att /

Vou

t in

mVo

lts

230

240

250

260

270

280

290

300

310

I out i

n m

Am

ps

Vbatt

Vout

Iout

4000

4200

4400

4600

4800

5000

5200

5400

5600

5800

6000

1 9 17 25 33 41 49 57 65 73 81

Time in minutes

V bat

t / V

out i

n m

Volts

100

110

120

130

140

150

160

170

180

190

200

I out i

n m

Am

psVbatt

Vout

IoutAN013703-0708 Page 7 of 17

and 150 mA respectively. These are equal to their rated battery capacity measured in mAh. The

Z8 Encore!-Based Battery Chargercharging times for NiCd and NiMH are 1 hour, 45 minutes and 1 hour, 25 minutes, respectively.

Because the SLA and Li-Ion batter-ies follow similar charging (constantvoltage with limited current) and ter-mination profiles (absolute voltage),only the SLA battery was charged.The results are provided in this document.

SummaryThis Application Note demonstrates the use of Z8 Encore! in a battery charger implementation. Ordi-nary battery chargers can charge batteries of a par-ticular type and of a particular voltage. The Z8 Encore!-based hardware/software provides flexi-bility such that batteries of different types can be charged with the same charger.

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 DCDC 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.

ReferenceThe documents associated with Z8 Encore! avail-able on www.zilog.com and electronics references are provided below:

Z8 Encore! Flash Microcontroller Develop-ment Kit User Manual (UM0146)

Power Electronics Design Handbook: Low

High Frequency Switching Power Supplies: The-ory and Design; author: George Chryssis; ISBN: 0-07-010949-4; Publisher: McGraw-Hill Book Company

Digital Control Systems, Volume 1Fundamen-tals, Deterministic Control; author: Rolf Iser-mann; ISBN: 0-387-50266-1; Publisher: Springer Verlag

Yuasa Technical Manualhttp://www.yuasab-atteries.com/literature.asp

Duracellhttp://www.duracell.com/batteries Eveready/Energizerhttp://data.energizer.com Panasonic Li-Ion battery documentshttp://

www.panasonic.com/industrial/battery/oem/chem/lithion/index.html

Sanyohttp://www.sanyo.com/industrial/bat-teries/index.html

Note:AN013703-0708 Page 8 of 17

Power Components and Applications; author: Nihal Kularatna; ISBN: 0-7506-7073-8; Pub-lisher: Oxford; Newnes, 1998

AN013703-0708 Page 9 of 17


Appendix AGlossaryDefinitions 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

AN01 Page 10 of 17


AppThis

1

1

D

C

B

A

5V

RESET

3.3V VDD

VDD

GND

VCC

GND

VDD

Rev

Sheet o f

ore!

1 13

C50

0.1uF

D6GREEN

2

1

4

C45

0.1uF

47805C/TO220/0.5A

OUT 3

G

N

D

2

C17

0.1

+C23

100/6.3

R14

680

C46

0.1uF

+ C15

100/10227uF

C49

0.1uF3703-0708

endix BSchematicssection provides the schematics for the Z8 Encore! battery charger implementation

Figure 7. Schematic for Z8 Encore! Interface

5

5

4

4

3

3

2

2

D

C

B

A

G

N

D

VCC

V

D

D

V

D

D

G

N

D

EXTAL XTAL

RESETVDD

GND

VDD

GND

VDD

GND

VDD

V

D

D

P

H

1

_

A

L

G

9

P

H

0

_

A

L

G

8

P

B

1

_

A

L

G

1

P

B

0

_

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L

G

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_

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L

G

1

0

P

B

3

_

A

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G

3

P

B

2

_

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G

2

P

H

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_

A

L

G

1

1

XTAL

G

N

D

EXTAL

PH3PH2PB2PB3PB1PB0PH1PH0

PC1

PE5PE6PE7

PA5

PE3

PE2PE1PE0

PE4

PA4

Title

Size Document Number

Date:

Battery Charger using Z8 Enc

A

Tuesday, January 07, 200

RESET

Z8 Encore! Interface

U1

Z8F

PA0/T0IN1PD22PC2/SS3PF64RESET5VDD6PF57PF48PF39

PE311PE410

GND12PE213PE114PE015GND16PF217PF118PF019VDD20PD1/T3OUT21PD0/T3IN22EXTAL23XTAL24

P

A

1

/

T

0

O

U

T

8

0

P

A

6

/

S

C

L

6

5

PA7/SDA 64

G

N

D

2

5

A

G

N

D

4

0

GND 41

A

V

D

D

2

6

P

H

0

/

A

L

G

8

2

7

P

H

1

/

A

L

G

9

2

8

P

B

0

/

A

L

G

0

2

9

P

B

1

/

A

L

G

1

3

0

P

B

4

/

A

L

G

4

3

1

P

B

5

/

A

L

G

5

3

2

P

B

6

/

A

L

G

6

3

3

P

B

7

/

A

L

G

7

3

4

P

B

3

/

A

L

G

3

3

5

P

B

2

/

A

L

G

2

3

6

P

H

2

/

A

L

G

1

0

3

7

P

H

3

/

A

L

G

1

1

3

8

V

R

E

F

3

9

PC0/T1IN 42PC1/T1OUT 43

DBG 44PC6/T2IN 45

PC7/T2OUT 46PG7 47VDD 48PG6 49PG5 50PG4 51PG3 52PE7 54PE6 55PE5 56PG2 57PG1 58PG0 60

PD7/RCOUT 61PC3/SCK 62

PD6/CTS1 63

P

A

5

/

T

X

D

0

6

6

P

A

4

/

R

X

D

0

6

7

P

C

4

/

M

O

S

I

7

0

P

D

5

/

T

X

D

1

7

1

P

D

4

/

R

X

D

1

7

2

P

D

3

7

3

P

C

5

/

M

I

S

O

7

4

P

F

7

7

5

P

A

3

/

C

T

S

0

7

8

P

A

2

7

9

G

N

D

7

7

V

D

D

7

6

G

N

D

6

8

V

D

D

6

9

GND 59

VDD 53

P3

PWR JACK

231

R3 1M

C21

0.1

SW4

C2

0.1

U1LMIN1

D5

S2G

U19

DS1233A-15

GND1RESET 2

VCC 3R17

100K

F1

RXE160

C2

18pF

U16

LT1086-3.3/TO220

GND1VIN3 VOUT 2

+ C4

Y1

18.432MHzC30

0.01

C1

18pF

AN01 Page 11 of 17


1

1

D

C

B

A

V_out(-)

V_batt(+)

V_out(+)

I_out(+)

I_out(-)

V_batt(-)

Rev

Sheet o f

0.0

rger

1 2

(+)

(-)

nse

TO BE CHARGEDBATTERY3703-0708

Figure 8. DCDC Step-Down Converter and LED Indicator Port

5

5

4

4

3

3

2

2

D

C

B

A

PC1/ T1OUT

PE7

PE1PE2PE3PE4PE5PE6

Vp

3.3 Volts

Title


Date:

Using Z8 Encore! as a Battery Cha

A


DC-DC Step Down Converter

Rse

LED Indicator Port

D4LED

L1

120uH

D2

MBR360

D10LED

R4470E

R7560E

Q1IRF9540

D1LED

D5LED

D6LED

R13560E

D7LED

R579E

D8LED

R6B 10E

R8560E

D9LED

R9560E

R10560E

R37

18E

R6A 10E

R11560E

R2

2.2K BT1

C1

100uF

Q22N2222

R3

1K

R12560E

D3MBR360

R1

3.3K

C3 0.1uF

C2

100uF

AN01 Page 12 of 17


gs

1

1

D

C

B

A

PB3/ANA3

PB1/ANA1

Rev

Sheet o f

0.0

tery Charger

2 203

rounds are connected on

Converter Output Voltage

Battery Voltage

1AM324

1

0.1uF

1BM324

7

1K

0.1uF

1K3703-0708

Figure 9. Feedback Section and Battery Type Selector Jumper Settin

5

5

4

4

3

3

2

2

D

C

B

A

PB2/ANA2

PH3PH2PH1PH0

12V

12V

12V

VCC

3.3 Volts

I_out(-)I_out(+) V_batt(+)

V_batt(-)

V_out(+)V_out(-)

Title


Date:

Using Z8 Encore! as a Bat

A


Note:1. R14 - R30 all 1% MFR.2. Signal, Digital, and Power G the evaluation board.

Battery Current

Feedback Circuits

Jumpers for Battery Selection

SelectNiCd

SelectNiMH

SelectSLA

SelectLi-Ion

R15

10K

-

+

UL

3

2

4

1

1

R241K

R3110K

R201K

-

+

U1CLM324

10

98

4

1

1

C7

J4

12

-

+

UL

5

6

4

1

1

R3410K

C810uF

R17

C11100uF

R221K

R1810K

R3310K

R161K

J3

12

R23

1K

C90.1uF

R3010K

R19

10K

R1410K

R3210K

J5

12

C70.1uF

R25 1K

J2

12

C100.1uF

C7

R21

Z8 Encore!-Based Battery ChargerAppendix CFlowchartsThe main battery-charging routine is displayed in Figure 10.

Figure 10. Flowchart for the Main Routine

Start

Terminate

Initialize peripherals

Read and verify battery type

Get the battery parameters

Calculate safety limits and thresholdsfor charging and termination

Read feedback values for battery voltage,current, and converter voltage

Within safety limits?

Is the battery charged?

Calculate the duty cycle

No

Yes

Yes

NoAN013703-0708 Page 13 of 17

Z8 Encore!-Based Battery ChargerThe ISR return routine is displayed in Figure 11.

Figure 11. Flowchart for the ISR Return Routine

Start ISR

Return from ISR

Reload PWM Value

Update charge ending data every 10 secondsAN013703-0708 Page 14 of 17

Z8 Encore!-Based Battery ChargerAppendix DBattery TechnologyThe 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 impor-tant 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 invari-ably 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 tech-niques. During charging, all batteries exhibit a marked rise in voltage above the rated battery volt-age.

The four major rechargeable battery typesSLA, 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 Applica-tion 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 cur-rent. If this current uptake is not controlled, the bat-tery electrolyte may boil, producing gasses inside the battery. It is therefore necessary to limit the charging current. When the battery achieves some

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 cur-rent charging method. While charging, as the volt-age 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 termi-nation mechanism is named as V 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 com-pared to NiCd batteries. The per-cell voltage of the NiMH battery type is 1.2 V, similar to NiCd batter-ies.

NiMH batteries are charged via the constant cur-rent charging method. While charging, as the volt-age crosses the full charge point, the voltage drop is not as low as for the NiCd batteries. As a conse-quence, V charge termination is usually not rec-ommended 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 indica-AN013703-0708 Page 15 of 17

charge, the current is limited and constant voltage charging is enforced.

tion for full battery charging, rather than the drop. Consequently, this termination mechanism is named zero V termination.

Z8 Encore!-Based Battery ChargerNiMH 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 rat-ing 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 Page 16 of 17

AN013703-0708 Page 17 of 17


DO NOT USE IN LIFE SUPPORT

LIFE SUPPORT POLICYZILOG'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFESUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS PRIOR WRITTEN APPROVAL OFTHE PRESIDENT AND GENERAL COUNSEL OF ZILOG CORPORATION.

As used hereinLife 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 foruse provided in the labeling can be reasonably expected to result in a significant injury to the user. Acritical component is any component in a life support device or system whose failure to perform can bereasonably expected to cause the failure of the life support device or system or to affect its safety oreffectiveness.

Document Disclaimer2008 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 ACCURACYOF THE INFORMATION, DEVICES, OR TECHNOLOGY DESCRIBED IN THIS DOCUMENT.ZILOG ALSO DOES NOT ASSUME LIABILITY FOR INTELLECTUAL PROPERTYINFRINGEMENT RELATED IN ANY MANNER TO USE OF INFORMATION, DEVICES, ORTECHNOLOGY DESCRIBED HEREIN OR OTHERWISE. The information contained within thisdocument 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 otherproduct or service names are the property of their respective owners.

Warning:

Z8 Encore!-Based Battery ChargerAbstractProduct OverviewFeatures

DiscussionTheory of Operation

Z8 Encore!-Based Battery ChargerHardware ArchitectureSoftware Implementation

TestingSummaryReferenceAppendix A-GlossaryAppendix B-SchematicsAppendix C-FlowchartsAppendix D-Battery Technology

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