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MIC2179 lithium-ion battery charger

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August 1999 1 Application Note 30

Application Note 30 Micrel

Application Note 30

MIC2179 Lithium-Ion Battery Charger

by Jeff Dixon

Introduction

The Micrel MIC2179 is a current-mode, 200kHz, synchro-

nous, buck (step-down) regulator. In this application, the

MIC2179 is configured to provide both constant-current and

constant-voltage for a 2-cell lithium battery charger. The

current is sensed on the high side to avoid ground-bounce

noise and the associated problems usually found in ground

referenced circuits. Voltage is sensed using a simple voltage

controller to reduce the parts count and provide a 0.5%

voltage tolerance. The MIC2179 operates from a 4.5V to

16.5Vinputandhasthefollowingvalue-addedfeatures:dual-

mode (skip-mode, PWM-mode) operation for high efficiency

(up to 96%), low quiescent current (1.0mA in PWM mode,

6005A in skip mode), internal current limit, thermal shutdown,

undervoltage lockout (4.35V), low dropout (100% duty cycle)

and simplified loop compensation (current-mode control).

Lithium-Ion Charge States

The three charge states for a lithium-ion battery are trickle

charge, bulk-charge, and overcharge. Starting with a fully

dischargedbattery,alithiumbatterychargerneedstochange

modes sequentially through these different charge states.

Trickle charge--a constant-current mode used to bring a

battery up to the cutoff voltage (VCUTOFF). The battery could

be in deep discharge (2.5V to 2.7V per cell) for many reasons

such as low state of charge, low ambient temperature,

shorted cells, or high internal leakage. Also, to maintain full

charge.

Bulkcharge--occurswhileinconstant-currentmodeandthe

charger is delivering the maximum allowable current to the

battery (see manufacture's specification). This method re-

places a majority of the battery's charge as quickly as

possible until the overcharge voltage threshold is reached.

Overcharge--is a constant-voltage mode function that oc-

curs consecutively after bulk charge. As the Li-ion battery

nears full capacity, the current decreases, and the battery's

terminal voltage increases until 4.2V per cell is reached.

When the current becomes low enough, less than trickle-

charge normally, the charging cycle is complete. Li-ion cells

should not be float charged. After the charge cycle is com-

plete, the charger should be shut down.

Theory of Operation

The lithium-ion battery charger can be divided in to four

blocks: a constant-current source, constant-voltage source,

a switching regulator and an end-of-charge circuit.

Constant-Current Block

Toanalyzethisblock,assumetheconstant-voltageblock(U3

and supporting circuitry) are inactive. Starting with a dis-

Micrel, Inc. 7 1849 Fortune Drive 7 San Jose, CA 95131 7 USA 7 tel + 1 (408) 944-0800 7 fax + 1 (408) 944-0970 7 http://www.micrel.com

charged battery connected to the charger, the circuit acts like

a constant-current source. An MIC2179 synchronous buck

regulatorprovidestheregulatedpower.Theconstant-current

source's feedback loop consists of R7 (current-sense resis-

tor), U2 (MIC6211 op amp), Q3 (VN2222 N-channel MOS-

FET), and the internal 1.24V reference of U1 (MIC2179

synchronous buck regulator).

First,thelithium-ionbatterychargerstartsupintrickle-charge

mode. In trickle mode, the output of U2 (MIC6211) is on,

biasing Q2 on, allowing the output of the MIC2179 to source

157mA of charging current to the battery. The intelligent

system or microprocessor senses the low logic level at the

output of U4 and places the charger into the bulk-mode

charging state (1.5A). Sequencing from trickle mode to bulk

mode is accomplished by applying a logic high on Q2's gate.

In bulk-mode, current ramps up to 1.50A (for a 2-cell lithium

application) through sense-resistor R7 (100m) creating a

157mV drop.

Initially,theinvertinginputofU2islowerthanthenoninverting

input causing the output to go higher, increasing drive to Q3.

This increases the current through the parallel combination of

R3 and R4 until 1.24V is developed across them. The

MIC2179 feedback pin senses the 1.24V and compares it to

the internal 1.24V bandgap reference and reduces output

duty cycle until 157mV is maintained across resistor R7.

Finally, there is 157mV at both inputs of the MIC6211 (op

amp), completing the negative feedback loop.

Trickle charge and bulk charge is calculated using:

Trickle Charge Current

1.24V

R3

R5

R7

157mA=

=

Bulk Charge Current

1.24V

R3||R4

R5

R7

1.55A=

=

Constant-Voltage Source

Once the battery's terminal reaches the 8.4V (2-cell lithium-

ionbatteryoverchargethreshold),theconstant-currentsource

circuit is biased off. As the output voltage of the charger tries

to go higher than 8.4V, the output of U3 (open-emitter

configuration)biasesD3on.ThefeedbackpinoftheMIC2179

is now pulled-up toward the 8.4V VOUT rail, reducing duty

cycle and maintaining output voltage regulation. The con-

stant voltage feedback loop consists of U3 (LM3420A), D3,

U1 (MIC2179), L1, R7, D2, and Q4.

Because overvoltage conditions greatly reduce the life span

of a lithium battery, an LM3420A was chosen for the voltage

feedback loop to help maintain terminal voltage to 0.5%.

In this charger design when the input supply is disconnected,

diode D2 is reverse biased to prevent battery discharge. Q4

Application Note 30 Micrel

Application Note 30 2 August 1999

is bifunctional: it prevents the battery from being discharged

when the input supply is removed; and it alleviates a race

conditionbetweentheLM3420andthestart-upoftheMIC2179

switching regulator. The LM3420 must be on before the

output of the MIC2179 comes up.

End-of-Charge Circuit

The end-of-charge circuit is used to signal the microproces-

sor or another subsystem when the lithium-ion battery pack

has reached the overcharge threshold. Again, in this 2-cell

lithium-ion application, the overcharge threshold is 8.4V.

Starting with a discharged battery and the charger in bulk-

charge mode, the constant-current is decreasing and the

voltage across the battery is increasing over time. The

change in current versus voltage is a function of the changing

internal impedance of the lithium-ion battery under charge.

U5 is being used as a differential amplifier to monitor the

output current by sensing the I7R drop across sense resistor

R7. The gain of U5 is set to 82.5. The gain of U5 is set to

maintain greater than 1.24V at its output down to a IOUT of

greater than 157mA. U4 compares the output of U5 against

the 1.24V bandgap reference on its noninverting pin gener-

ated by the internal bandgap reference of MIC2179. In bulk-

charge mode, the output of U5 is always higher than U4's

reference voltage, thus maintaining a logic level low at the

end-of-charge pin.

Once R7 has less than a 15mV drop across it, the output of

U5cannolongersustaingreaterthan1.24Vatitsoutput.Now

U4 has 1.24V on both its inputs.

Next, the IOUT drawn by the battery reduces even further due

tocharging.NowthevoltageatU4'sinvertingpinislowerthan

the 1.24V reference producing a logic level high at the output

of U4, signaling an end-of-charge.

The present industry-standard variable used to determine

lithium-ion battery end-of-charge is the current draw at about

90% charge (see Figure 3). The end-of-charge current is

typically about C/10. A simple countdown timer circuit (not

shown) is usually started upon reaching the end-of-charge

state. Based on the individual manufacture specification, this

completes the charge cycle.

End-of-charge output current is calculated using a 1.24V

reference:

Endof ChargeThreshold I R7

R9

R6

1.24VOUT- - W( ) =

1.24V is the reference for U4 pin 3 which is the end-of-

charge comparator.

Note:Thiscircuituses157mAasanend-of-chargethreshold.

At the end of charge, the charging circuit should be shut

down. It is not recommended to float charge Li-ion cells for

long periods of time.

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 10 20 30 40 50 60 70 80 90100

CELLVOLTAGE(V)

RATED CAPACITY (%)

1C C/2

C/3

Figure 1. Typical Li-Ion Voltage

vs. Capacity for C1-C3 Rates2

0

20

40

60

80

100

120

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

CAPACITYRATIO(%)

DISCHARGE RATE (A)

Li-Ion

NiCd

Figure 2. Typical Li-Ion and NiCad Capacity

vs. Discharge Rate2

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 0.5 1.0 1.5 2.0 2.5 3.0

CELLVOLTAGE(V)

CHARGECURRENT(A)

TIME (HRS)

Current

Cell Voltage

Figure 3. Typical Lithium Cell Voltage

vs. Charging Current2

August 1999 3 Application Note 30

Application Note 30 Micrel

C6

0.015F

U3

LM3420M5-8.4

8.4VVoltageController

IN

COMP

GND

2-Cell

Li-Ion

Battery

Q4

VN2222

D2

SS25

1

OUT

MIC6211

U2

3

4

1

5

2

C5

0.015F

R5

100

0.5%

C4

0.015F

Q3

VN2222

BIASSGND

PWRGD

COMP

PGND

FB

SW

VINL1

685H

C2

6.8nF

C3

0.015F

MIC2179

PWM

SYNC

EN

R1

100k

1%

R2

10k

1%

C1

685F

20V

U1

15

6

5

13

89-1214

7

1,2,

19,20

3,4

16,17

D1

MBRS1401.1M

MIC6270

U4

200k

3

4

1

5

2MIC6211

U5

4

3

1

5

R11

75k

R9

82.5k

2

R6

1k

1%

R8

1k

1%

R10

82.5k

1%

R7

0.1

1%

Trickle

Charge

Resistor

(157mA)

R4

887

0.5%

R3

7.87k

0.5%

Fast

Charge

Resistor

(1.5A)

Q2

VN2222

HIGH=FASTCHARGE

LOW=TRICKLECHARGE

Charge

Current

Shutdown

HIGH=SHUTDOWN

LOW=ENABLE

VIN

10Vto16.5V

Q1

VN2222

R12

100k1%

100k

C7

VOUT

8.4V/1.5A

C2

685F

20V

EndCharge

D3

1N4148

1.24V

TTLLogicLevels

IOUT157mA

IOUT157mA

R13

4.2k

R14

10k

1%

Figure4.2-CellLithium-IonBatteryChargerUtilizingtheMIC2179SynchronousBuckRegulator

Application Note 30 Micrel

Application Note 30 4 August 1999

C6

0.015F

U3

LM3420M5-4.2

4.2VVoltageController

IN

COMP

GND

Single

Li-Ion

Cell

Q4

VN2222

D2

SS12

1

OUT

MIC6211

U2

3

4

1

5

2

C5

0.015F

R5

100

0.5%

C4

0.015F

Q3

VN2222

BIASSGND

PWRGD

COMP

PGND

FB

SW

VINL1

685H

C2

6.8nF

C3

0.015F

MIC2179

PWM

SYNC

EN

R1

100k

1%

R2

10k

1%

C1

685F

20V

U1

15

6

5

13

89-1214

7

1,2,

19,20

3,4

16,17

D1

MBRS1401.1M

MIC6270

U4

200k

3

4

1

5

2MIC6211

U5

4

3

1

5

R11

75k

R9

100k

2

R6

976

1%

R8

976

1%

R10

100k

1%

R7

0.1

1%

Trickle

Charge

Resistor

(120mA)

R4

1.15k

0.5%

R3

10.2k

0.5%

Fast

Charge

Resistor

(1.2A)

Q2

VN2222

HIGH=FASTCHARGE

LOW=TRICKLECHARGE

Charge

Current

Shutdown

HIGH=SHUTDOWN

LOW=ENABLE

VIN

6Vto16.5V

Q1

VN2222

R12

100k1%

100k

C7

VOUT

4.2V/1.2A

C2

685F

20V

EndCharge

D3

1N4148

1.24V

TTLLogicLevels

IOUT120mA

IOUT120mA

R13

4.2k

R14

10k

1%

Figure5.Single-CellLithium-IonChargerUtilizingtheMIC2179SynchronousBuckRegulator

August 1999 5 Application Note 30

Application Note 30 Micrel

Lithium-Ion Battery Charging Terms1

Ampere-hours--product of current in amperes multiplied by

time current is flowing through the battery.

Capacity--a designation by the battery manufacturer which

helps identify a particular cell model and also provides an

approximation of capacity. Manufacturers typically define

capacityintermsofC/10asastandardvaluewhenspecifying

capacity.

C rate--discharge or charge rate in amperes typically ex-

pressed in terms of capacity. Example: a 5.0Ah cell can

sustain 500mA discharge rate for 10 hours. This example

used a C rate of C/10.

Self-discharge rate--the spontaneous decomposition of

the internal chemicals of the battery causes the battery to

discharge at some manufacturer's specified rate when not in

use. Determines shelf life.

Deep discharge--when a battery is discharged below the

manufacturer's recommended cutoff voltage. Lithium-ion

ranges from 2.7V discharged, 3.6V nominal, to 4.2V per cell

when fully charged. For example, if a lithium-ion battery has

a voltage between 2.5V and 2.7V, the cell is considered to be

in a deeply discharged state.

Constant-voltagecharger--achargingcircuitusedtomain-

tain a regulated voltage across a battery but allows the

current to vary depending upon the battery's state of charge.

Constant-currentcharger--achargingcircuitusedtomain-

tain a regulated current to a battery's but allows the voltage

to vary depending upon the battery's state of charge.

Trickle charge--a constant-current source used to slowly

bring a battery up to its recommended cutoff voltage (lithium-

ion 2.7V per cell) or to maintain full charge (4.2V per cell). In

a typical trickle-charger lithium-ion battery application, a

current of between C/50 to C/10 is used to maintain the

cell(s).

References

1. Battery Reference Book, second edition (Terms)

2. Typical Lithium-Ion Battery Curves/Charts, Toshiba Battery Corporation

Application Note 30 Micrel

Application Note 30 6 August 1999

August 1999 7 Application Note 30

Application Note 30 Micrel

Application Note 30 Micrel

Application Note 30 8 August 1999

MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131 USA

TEL + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB http://www.micrel.com

This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or

other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc.

) 1999 Micrel Incorporated





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