Rev. 1.3 7/13 Copyright 2013 by Silicon Laboratories AN137Silicon Laboratories Confidential. Information contained herein is covered under non-disclosure agreement (NDA).

AN137

LITHIUM ION BATTERY CHARGER USING C8051F300

IntroductionDriven by the need for untethered mobility andease of use, many systems rely on rechargable bat-teries as their primary power source. The batterycharging circuitry for these systems is typicallyimplemented using a fixed-function IC to controlthe charging current/voltage profile.

The C8051F30x family provides a flexible alterna-tive to fixed-function battery chargers. This appli-cation note discusses how to use the C8051F30xfamily in Li-Ion battery charger applications. TheLi-Ion charging algorithms can be easily adapted toother battery chemistries, but an understanding ofother battery chemistries is required to ensureproper charging for those chemistries.

The code accompanying this application note wasoriginally written for C8051F30x devices. Thecode can also be ported to other devices in the Sili-con Labs microcontroller range.

Key Points On-chip high-speed, 8-bit ADC provides supe-

rior accuracy in monitoring charge voltage (critical to prevent overcharging in Li-Ion applications), maximizing charge effectiveness and battery life.

On-chip PWM provides means to implement buck converter with a very small external inductor.

On-chip Temp sensor provides an accurate and stable drive voltage for determining battery temperature. An external RTD (resistive tem-perature device) can also be used via the flexi-ble analog input AMUX.

A single C8051F30x platform provides full product range for multi-chemistry chargers, expediting time to market and reducing inven-tory.

V Pos (+)

V Neg (-)

LED

BuckConverter

Sense Resistor

Li-IonCells

8k FLASH, PWM,Temp Sensor,

Precision Time Base

8051F30x

CygnalIntegratedProducts

ResistorDivider

LDO

PWM Out

AIN

Figure 1. Lithium Ion Battery Charge Block Diagram.

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2 Rev. 1.3

Charging BasicsBatteries are exhaustively characterized to deter-mine safe yet time-efficient charging profiles. Theoptimum charging method for a battery is depen-dent on the batterys chemistry (Li-Ion, NiMH,NiCd, SLA, etc.). However, most charging strate-gies implement a 3-phase scheme:

1. Low-current conditioning phase

2. Constant-current phase

3. Constant-voltage phase/charge termination

All batteries are charged by transferring electricalenergy into them (refer to the references at the endof this note for a battery primer). The maximumcharge current for a battery is dependent on the bat-terys rated capacity (C). For example, a batterywith a cell capacity of 1000mAh is referred to asbeing charged at 1C (1 times the battery capacity) ifthe charge current is 1000mA. A battery can becharged at 1/50C (20 mA) or lower if desired.However, this is a common trickle-charge rate andis not practical in fast charge schemes where shortcharge-time is desired.

Most modern chargers utilize both trickle-chargeand rated charge (also referred to as bulk charge)while charging a battery. The trickle-charge currentis usually used in the initial phases of charging tominimize early self heating which can lead to pre-mature charge termination. The bulk charge is usu-ally used in the middle phase where the most of thebatterys energy is restored.

During the final phase of battery charge, whichgenerally takes the majority of the charge time,either the current or voltage or a combination ofboth are monitored to determine when charging iscomplete. Again, the termination scheme dependson the batterys chemistry. For instance, most Lith-ium Ion battery chargers hold the battery voltageconstant, and monitor for minimum current. NiCd

batteries use a rate of change in voltage or tempera-ture to determine when to terminate.

Note that while charging a battery, most of the elec-trical energy is stored in a chemical process, but notall as no system is 100 percent efficient. Some ofthe electrical energy is converter to thermal energy,heating up the battery. This is fine until the batteryreaches full charge at which time all the electricalenergy is converted to thermal energy. In this case,if charging isnt terminated, the battery can bedamaged or destroyed. Fast chargers (chargers thatcharge batteries fully in less than a couple hours)compound this issue, as these chargers use a highcharge current to minimize charge time. As one cansee, monitoring a batterys temperature is critical(especially for Li-Ion as they explode if over-charged). Therefore, the temperature is monitoredduring all phases. Charge is terminated immedi-ately if the temperature rises out of range.

Li-Ion Battery Charger - HardwareCurrently, Li-Ion batteries are the battery chemistryof choice for most applications due to their highenergy/space and energy/weight characteristicswhen compared to other chemistries. Most modernLi-Ion chargers use the tapered charge termination,minimum current (see Figure 2), method to ensurethe battery is fully charged as does the examplecode provided at the end of this note.

Buck Converter

The most economical way to create a tapered ter-mination charger is to use a buck converter. A buckconverter is a switching regulator that uses aninductor and/or a transformer (if isolation isdesired), as an energy storage element to transferenergy from the input to the output in discretepackets (for our example we use an inductor; thecapacitor in Figure 3 is used for ripple reduction).Feedback circuitry regulates the energy transfer viathe transistor, also referred to as the pass switch, tomaintain a constant voltage or constant current

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Rev. 1.3 3

within the load limits of the circuit. See Figure 3for details.

Tapered Charger Using the F30x

Figure 3 illustrates an example buck converterusing the F30x. The pass switch is controlled viathe on-chip 8-bit PWM (Pulse Width Modulator)output of the PCA. When the switch is on, currentwill flow like in Figure 3A. The capacitor ischarged from the input through the inductor. Theinductor is also charged. When the switch isopened (Figure 3B), the inductor will try to main-tain its current flow by inducing a voltage as thecurrent through an inductor cant change instanta-neously. The current then flows through the diodeand the inductor charges the capacitor. Then thecycle repeats itself. On a larger scale, if the dutycycle is decreased (shorter on time), the average

voltage decreases and vice versa. Therefore, con-trolling the duty cycles allows one to regulate thevoltage or the current to within desired limits.

Selecting the Buck Converter Inductor

To size the inductor in the buck converter, one firstassumes a 50 percent duty cycle, as this is wherethe converter operates most efficiently.

Duty cycle is given by Equation 1, where T is theperiod of the PWM (in our example T = 10.5S).

Charge Current

Charge Voltage

TimeConditioningPhase

Current regulation Voltage regulation

Figure 2. Lithium Ion Charge Profile.

Inductor

CapacitorPowerSource Battery

Inductor

Pass Switch Off

CapacitorPowerSource Battery

(A) (B)

Pass Switch On

ShottkyDiode

ShottkyDiode

Figure 3. Buck Converter.

DutyCycle tonT

---------=

Equation 1. Duty Cycle.

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4 Rev. 1.3

With this established, select a PWM switching fre-quency. As Equation 2

shows, the larger the PWM switching frequency,the smaller (and more cost effective) the inductor.Our example code configures the F30xs 8-bithardware PWM to use the internal master clock of24.5MHz divided by 256 to generate a 95.7kHzswitch rate.

Now we can calculate the inductors size. Assum-ing Vi, the charging voltage, is 15V, Vsat, the satu-ration voltage, is 0.5V, the desired output voltage,Vo, is 4.2V, and I0MAX, the maximum output cur-rent, is 1500 mA, the inductor should be at least18H. Note that the capacitor in this circuit is simply aripple reducer. The larger it is the better as ripple isinversely proportional to the size of the cap. Formore details on buck converters, refer to the refer-ences listed at the end of this note.

Li-Ion Battery Charger - SoftwareThe software example that follows demonstrates aLi-Ion battery charger using the C8051F300. TheF300 is designed for high-level languages like Cand includes an 8-bit 8051 based micro-controller,an 8-bit 500 ksps ADC, 8k FLASH, an 8-bit and16-bit PWM, and a 2% accurate oscillator all on-chip. The algorithms discussed are written entirelyin C making them easily portable. Refer to theF300s datasheet for a full description of thedevice.

Calibration

To ensure accurate voltage and current measure-ments, the algorithms use a two-point system cali-bration scheme. In this scheme, the user is expectedto apply two known voltages and two known cur-rents, preferable, one point near ground and theother point near full-scale. The algorithm thentakes these two points, calculates a slope and anoffset for both the current and voltage channels,and stores the results in FLASH. All future conver-sions are scaled relative to these slope and offsetcalculations. Note that if an external amplifier isused for the current channel, it will need to be cali-brated with a similar two-point calibration schemeto ensure maximum accuracy.

Temperature

To monitor the temperature, the algorithms use theon-chip temperature sensor. The sensor is leftuncalibrated, but still provides a sufficiently accu-rate temperature measurement. For more accuratetemperature measurement, one or two-point tem-perature calibration is required.

An external temperature sensor can be used ifdesired. The AMUX can to be reconfigured toaccommodate this additional input voltage.

Current

The charge-current to the battery cells is monitoredby taking a differential voltage reading across asmall but accurate sense resistor. The current isamplified through the on-chip PGA, digitized bythe on-chip 8-bit ADC, and scaled accordingly viathe slope and offset calibration coefficients. Anexternal gain stage may be necessary if more reso-lution is desired for the current measurement.

Voltage

The batterys voltages are divided down and moni-tored via external resistors. Note that this exampleuses the supply voltage as the ADC voltage refer-ence. Any monitored voltage above the referencevoltage must be divided down for accurate moni-

L Vi Vsat Vo ton2Iomax

----------------------------------------------------=

Equation 2. Inductor Size.

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Rev. 1.3 5

toring. If a more accurate reference is required, anexternal voltage reference can be used. Adjustmentto the divide resistors must be made accordingly.

Charging - Phase1

In phase 1, (for description purposes, we assumethe battery is initially discharged), the F30x regu-lates the batterys current to ILOWCURRENT (typi-cally 1/50 C) until the batterys voltage reachesVMINVOLTBULK. Note that the batterys charge cur-rent is current limited to ILOWCURRENT to ensuresafe initial charge and to minimize battery self-heating. If at any time the temperature increases outof limit, charging is halted.

Charging - Phase 2

Once the battery reaches VMINVOLTBULK the char-ger enters phase 2, where the batterys algorithmcontrols the PWM pass switch to ensure the outputvoltage provides a constant charge-current IBULKto the battery (rate or bulk current is usually 1C andis definable in the header file as is ILOWCURRENTand VMINVOLTBULK).

Charging - Phase 3

After the battery reaches VTop (typically 4.2 V insingle cell charger), the charger algorithm entersphase 3, where the PWM feeds back and regulatesthe batterys voltage. In phase 3, the battery contin-ues to charge until the batterys charge currentreaches IMINIBULKl, after which, the battery ischarged for an additional 30 minutes and thencharge terminates. Phase 3 typically takes themajority of the charging time.

Note that in most practical applications, such as aportable PC, the batteries may be in any of the threephases when charging is activated. This doesntreally affect the charger as it simply monitors the

batterys current condition and starts charging fromthat point.

ConclusionThe C8051F300s high level of analog integration,small form-factor, integrated FLASH memory, andlow power consumption makes it ideal for flexiblenext generation battery charging applications. Thisapplication note discussed how to use theC8051F30x family in Lithium Ion battery chargerapplications. Example code is provided as well.

ReferencesMaxim Integrated Product, DC-DC ConverterTutorial.

Martinez, Carlos and Drori, Yossi and Ciancio,Joe, AN126 Smart Battery Primer, Xicor, Octo-ber 1999.

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6 Rev. 1.3

Appendix

Figure 4. 1 Cell Battery Charger Schematic.

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Rev. 1.3 7

Figure 5. 1 Cell Buck Converter Schematic.

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8 Rev. 1.3

main()

Config_F300()

ErrorDetected

?

BULK_charge()

Turn off LED0, Error

Yes

No

No

CalibrateADCforMeasurement()

Enable Interrupts

Clear Termination FlagsClear Charge Status Flags

Yes

LOWCURRENT_charge()

No

Status = BULK?

Status =LOWCURRENT

?

SW0Pressed?

?

ErrorDetected

?

Infinite Loop

Yes/No

Yes

No

Yes

Turn on LED0

InfiniteLoop

Yes/No

Figure 6. main() Flow Chart.

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Rev. 1.3 9

CalibrateADCforMearurement()

END

NoSW0

Pushed?

Setup ADC0's AMUX,Throughput, Gain, for nearzero-scale voltage cal point

Yes

Acquire 16-bitMeasurement

Setup ADC0's AMUX,Throughput, Gain, for nearfull-scale voltage cal point

Calculate Voltage SlopeCoefficient

Calculate Voltage OffsetCoefficient

Erase Memory Page0x1A00

Store Voltage Offset andSlope Coefficients in

FLASH Memory

Acquire16-bitMeasurement

SW0Pushed

?

Setup ADC0's AMUX,Throughput, Gain, for nearzero-scale Current cal point

Yes

Acquire 16-bitMeasurement

Setup ADC0's AMUX,Throughput, Gain, for nearfull-scale Current cal point

Calculate Current SlopeCoefficient

Calculate Current OffsetCoefficient

Store Current Offset andSlope Coefficients in

FLASH Memory

Acquire16-bitMeasurement

No

Figure 7. CalibrateADCforMeasurement() Flow Chart.

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10 Rev. 1.3

Monitor_Battery()

MeasurementType

?

AMUX = Current AMUX = Volt

AV = AV + ADC0 Turn PWM on

Stop PWM

I?

ADC0 Done?

AMUX = Volt

Stop PWM

AMUX = Temperature

Start ADC0

AV = AV/10

END

No

Current Charge Voltage Temperature Battery Voltage

AV = 0I = 0

Yes

No

Yes

Calculate Voltage w/Calibration Coefficients

Calculate Current w/Calibration Coefficients

Calculate Temperature w/Calibration Coefficients

TemperatureCurrentVoltage w/ or w/out PWM

Return Desired Parameter

Figure 8. Monitor_Battery() Flow Chart.

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Rev. 1.3 11

Bulk_Charge()

Start PWM w/ Zero Output

TWithin Limits

?

Yes

Status = const_C

Calculate bulk_finish_time

Green LED On

Regulate Battery Current

Read Charge Voltage

Change Status fromconst_C to const_V

V min_Bulk

?

Status =BULK & No

Error?

Status =const_c

?

ChargeVoltage Within

Limits?

Yes

No

No

ACB D

Yes

No

No

No

Yes

Yes

Set Appropriate Flags

Figure 9. Bulk_Charge() Flow Chart (Part 1).

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12 Rev. 1.3

Status =const_V

?

Yes

Regulate Voltage()

Stop PWM& Flag Error

Stop PWM& Flag Error

Status = const_CStatus = LOWCURRENT

Green LED Off

TimeOverflow

?

Temp.Overflow

?

60 Sec.Over

?

DelayTimeOver

?

Yes

No

No

ACB D

Yes

No

No

Yes

END

Stop PWM

const_V, NOT Delay & Current

Below Threshold?

Calculate bulk_finish_time

Status = Delay

No

No

Yes

Yes

Figure 10. BULKCurrent() Flow Chart (Part 2).

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Rev. 1.3 13

LOWCURRENT_charge()

ResetTimeBase()

Tempwithin Limits

?

ChargeVoltage

Within Limits?

V

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14 Rev. 1.3

Turn_PWM_Off()

END

Increment CEX0Counter

CEX0Counter

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Rev. 1.3 15

measure()

END

i = 0?

Set accumulator andcounter i variables to zero

Yes

accumulator =accumulator + ADC0

Increment i

Clear End of ConversionFlag

ConversionComplete

?

No

Return 16-bitMeasurement

No

Start New Conversion

Yes

Figure 13. Measure() Flow Chart.

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16 Rev. 1.3

Make Duty Cycle Larger

Voltage