LINEAR TECHNOLOGY LTC4081 Technical data

LTC4081

500mA Li-Ion Charger

with NTC Input and

FEATURES

Battery Charger:

■

Constant-Current/Constant-Voltage Operation

with Thermal Feedback to Maximize Charge Rate
Without Risk of Overheating

■

Internal 4.5 Hour Safety Timer for Termination

■

Charge Current Programmable Up to 500mA with

5% Accuracy

■

NTC Thermistor Input for Temperature Qualifi ed

Charging

■

C/10 Charge Current Detection Output

■

5μA Supply Current in Shutdown Mode

Switching Regulator:

■

High Effi ciency Synchronous Buck Converter

■

300mA Output Current (Constant-Frequency Mode)

■

2.7V to 4.5V Input Range (Powered from BAT Pin)

■

0.8V to V

■

MODE Pin Selects Fixed (2.25MHz) Constant-Frequency

PWM Mode or Low I

■

2μA BAT Current in Shutdown Mode

■

10-lead, low profi le (0.75 mm) 3mm × 3mm DFN

Output Range

BAT

(23μA) Burst Mode® Operation

CC

package

U

APPLICATIO S

■

Wireless Headsets

■

Bluetooth Applications

■

Portable MP3 Players

■

Multifunction Wristwatches

300mA Synchronous Buck

U

DESCRIPTIO

The LTC4081 is a complete constant-current/constant­voltage linear battery charger for a single-cell 4.2V
lithium-ion/polymer battery with an integrated 300mA
synchronous buck converter. A 3mm × 3mm DFN pack­age and low external component count make the LTC4081
especially suitable for portable applications. Furthermore,
the LTC4081 is specifi cally designed to work within USB
power specifi cations.

⎯C⎯H⎯R⎯

The
dropped to ten percent of its programmed value (C/10).
An internal 4.5 hour timer terminates the charge cycle.
The full-featured LTC4081 battery charger also includes
trickle charge, automatic recharge, soft-start (to limit inrush
current) and an NTC thermistor input used to monitor
battery temperature.

The LTC4081 integrates a synchronous buck converter
that is powered from the BAT pin. It has an adjustable
output voltage and can deliver up to 300mA of load cur­rent. The buck converter also features low-current high­effi ciency Burst Mode operation that can be selected by
the MODE pin.

The LTC4081 is available in a 10-lead, low profi le (0.75
mm) 3mm × 3mm DFN package.

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation. All other
trademarks are the property of their respective owners. Protected by U.S. Patents,
including 6522118.

G pin indicates when charge current has

TYPICAL APPLICATIO

Li-Ion Battery Charger with 1.8V Buck Regulator

510Ω

V

CC

(3.75V

TO 5.5V)

100k

4.7μF

100k

V

CC

EN_BUCK

NTC

EN_CHRG

MODE

T

LTC4081

GND

CHRG

BAT

SW

FB

PROG

1OμH

806Ω

U

10pF

500mA

1M

806k

4081 TA01a

4.7μF

V

OUT

(1.8V/300mA)

4.7μF

+

4.2V
Li-Ion/
POLYMER
BATTERY

Buck Effi ciency vs Load Current

(V

= 1.8V)

OUT

100

EFFICIENCY

80

(Burst)

EFFICIENCY

60

40

EFFICIENCY (%)

20

0

0.01

(PWM)

0.1 10 1001 1000
LOAD CURRENT (mA)

POWER LOSS
(Burst)

POWER
LOSS
(PWM)

V

= 3.8V

BAT

= 1.8V

V

OUT

L = 10μH
C = 4.7μF

1000

100

10

1

0.1

0.01

4081 TA01b

POWER LOSS (mW)

4081f

1

LTC4081

WW

W

U

ABSOLUTE AXI U RATI GS

(Note 1)

VCC, t < 1ms and Duty Cycle < 1% .............. – 0.3V to 7V

Steady State ......................................... – 0.3V to 6V

V

CC

⎯C⎯H⎯R⎯

BAT,

⎯E⎯N⎯_⎯C⎯H⎯R⎯

MODE, EN_BUCK .......................... – 0.3V to V

G .................................................. – 0.3V to 6V

G, PROG, NTC ...................– 0.3V to VCC + 0.3V

+ 0.3V

BAT

FB ............................................................... – 0.3V to 2V

PIN CONFIGURATION

TOP VIEW

BAT

1

V

2

CC

EN_CHRG

PROG

NTC

10-LEAD (3mm × 3mm) PLASTIC DFN

= 110°C, θJA = 43°C/W (NOTE 3)

T

EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB

JMAX

3
4

5

DD PACKAGE

11

10

9
8
7
6

SW
EN_BUCK
MODE
FB
CHRG

BAT Short-Circuit Duration ............................ Continuous

BAT Pin Current ...................................................800mA

PROG Pin Current ....................................................2mA

Junction Temperature .......................................... .125°C

Operating Temperature Range (Note 2) .. – 40°C to 85°C

Storage Temperature Range .................. – 65°C to 125°C

ORDER INFORMATION

LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE

LTC4081EDD#PBF LTC4081EDD#TRPBF LDBX 10-Lead (3mm × 3mm) DFN 0°C to 70°C

Consult LTC Marketing for parts specifi ed with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based fi nish parts.

For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifi cations, go to: http://www.linear.com/tapeandreel/

The ● denotes specifi cations which apply over the full operating tempera-

ELECTRICAL CHARACTERISTICS

ture range, otherwise specifi cations are at TA = 25°C, VCC = 5V, V
(Note 2)

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V

CC

V

BAT

I

CC

I

CC_SD

Battery Charger Supply Voltage (Note 4)

Input Voltage for the Switching
Regulator

Quiescent Supply Current (Charger On,
Switching Regulator Off)

Supply Current in Shutdown (Both
Battery Charger and Switching
Regulator Off)

(Note 5)

V

= 4.5V (Forces I

BAT

V

EN_BUCK

V

⎯E⎯N⎯_⎯C⎯H⎯R⎯

V

⎯E⎯N⎯_⎯C⎯H⎯R⎯

V

(4V)

BAT

= 0

= 5V, V

G

= 4V, V

G

= 3.8V, V

BAT

EN_BUCK
EN_BUCK

and I

BAT

= 0, VCC > V

= 0, V

⎯E⎯N⎯_⎯C⎯H⎯R⎯

PROG

CC

= 0V, V

G

= 0),

BAT

(3.5V) <

= 0V, V

NTC

●

●

●

●

EN_BUCK

3.75 5 5.5 V

2.7 3.8 4.5 V

= V

BAT

110 300 μA

5
2

, V

MODE

10 μA

= 0V.

μA

2

4081f

LTC4081

The ● denotes specifi cations which apply over the full operating tempera-

ELECTRICAL CHARACTERISTICS

ture range, otherwise specifi cations are at TA = 25°C, VCC = 5V, V
(Note 2)

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

I

BAT_SD

Battery Charger

V

FLOAT

I

BAT

V

UVLO_CHRG

V

PROG

V

ASD

t

SS_CHRG

I

TRKL

V

TRKL

V

TRHYS

ΔV

RECHRG

ΔV

UVCL1,

ΔV

UVCL2

t

TIMER

I

C/10

T

LIM

R

ON_CHRG

f

BADBAT

D

BADBAT

I

NTC

V

COLD

V

HOT

V

DIS

f

NTC

D

NTC

Battery Current in Shutdown (Both
Battery Charger and Switching
Regulator Off)

V

Regulated Output Voltage I

BAT

Current Mode Charge Current R

VCC Undervoltage Lockout Voltage VCC Rising

PROG Pin Servo Voltage 0.8k ≤ R

Automatic Shutdown Threshold Voltage (VCC – V

Battery Charger Soft-Start Time 180 μs

Trickle Charge Current V

Trickle Charge Threshold Voltage V

Trickle Charge Threshold Voltage
Hysteresis

Recharge Battery Threshold Voltage V

(VCC – V

) Undervoltage Current

BAT

Limit Threshold Voltage

Charge Termination Timer

Recharge Time

Low-Battery Charge Time V

End of Charge Indication Current Level R

Junction Temperature in Constant­Temperature Mode

Power FET On-Resistance (Between

and BAT)

V

CC

Defective Battery Detection ⎯C⎯H⎯R⎯G Pulse
Frequency

Defective Battery Detection ⎯C⎯H⎯R⎯G Pulse
Frequency Duty Ratio

NTC Pin Current V

Cold Temperature Fault Threshold
Voltage

Hot Temperature Fault Threshold
Voltage

NTC Disable Threshold Voltage Falling Threshold; VCC = 5V

Fault Temperature ⎯C⎯H⎯R⎯G Pulse
Frequency

Fault Temperature ⎯C⎯H⎯R⎯G Pulse
Frequency Duty Ratio

V

⎯E⎯

⎯N⎯_⎯C⎯H⎯R⎯

G

V

⎯E⎯

⎯N⎯_⎯C⎯H⎯R⎯

G

V

(4V)

BAT

= 2mA

BAT

I

= 2mA, 4.3V < VCC < 5.5V

BAT

= 4k; Current Mode; V

PROG

R

= 0.8k; Current Mode; V

PROG

V

Falling

CC

PROG

BAT

(V

– V

CC

BAT

= 2V, R

BAT

Rising

BAT

– V

FLOAT

I

= 0.9 I

BAT

I

= 0.1 I

BAT

= 2.5V

BAT

= 2k (Note 6)

PROG

I

= 350mA, VCC = 4V 700 mΩ

BAT

V

= 2V 2 Hz

BAT

V

= 2V 75 %

BAT

= 2.5V 1 μA

NTC

Rising Voltage Threshold
Hysteresis

Falling Voltage Threshold
Hysteresis

Hysteresis

= 3.8V, V

BAT

= 5V, V

EN_BUCK

= 4V, V

EN_BUCK

≤ 4k

), VCC Low to High
), VCC High to Low

= 0.8k

PROG

⎯E⎯N⎯_⎯C⎯H⎯R⎯

= 0, V
= 0, V

> V

CC

(3.5V) <

CC

EN_BUCK

EN_BUCK

= 0V, V

G

BAT

= 0

= 0

NTC

●

= 0V, V

EN_BUCK

0.6

= V

BAT

2

4.179

4.158

●

●

90

475

●

●

3.5

2.8

●

●

0.98 1.0 1.02 V

60
15

35 50 65 mA

●

2.75 2.9 3.05 V

4.2

4.2

100
500

3.6

3.0

82
32

, V

MODE

5μA

4.221

4.242

110
525

3.7

3.2

100

45

100 150 350 mV

, 0°C < TA < 85°C 70 100 130 mV

BAT

CHG
CHG

180

90

●

3 4.5 6 hrs

●

1.5 2.25 3 hrs

●

0.75 1.125 1.5 hrs

●

0.085 0.1 0.115 mA/mA

300
130

115 °C

0.76 • V

CC

0.015 • V

0.017 • V

0.35 • V

CC

CC

CC

82
50

2Hz

25 %

= 0V.

μA

mA
mA

mV
mV

mV
mV

mV
mV

V
V

V
V

V
V

V
V

4081f

3

LTC4081

The ● denotes specifi cations which apply over the full operating tempera-

ELECTRICAL CHARACTERISTICS

ture range, otherwise specifi cations are at T
(Note 2)

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

Buck Converter

V

FB

I

FB

f

OSC

I

BAT_NL_CF

I

BAT_NL_BM

I

BAT_SLP

V

UVLO_BUCK

R

ON_P

R

ON_N

I

LIM_P

I

LIM_N

I

ZERO_CF

I

PEAK

I

ZERO_BM

t

SS_BUCK

Logic

V

IH

V

IL

V

OL

I

IH

I

IL

R

⎯E⎯N⎯_⎯C⎯H⎯R⎯

G

I

⎯C⎯H⎯R⎯

G

Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.

Note 2: The LTC4081 is guaranteed to meet performance specifi cations
from 0°C to 85°C. Specifi cations over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.

Note 3: Failure to solder the exposed backside of the package to the PC
board ground plane will result in a thermal resistance much higher than
43°C/W.

FB Servo Voltage

FB Pin Input Current V

Switching Frequency

No-Load Battery Current (Continuous
Frequency Mode)

No-Load Battery Current (Burst Mode
Operation)

Battery Current in SLEEP Mode V

Buck Undervoltage Lockout Voltage V

PMOS Switch On-Resistance 0.95

NMOS Switch On-Resistance 0.85

PMOS Switch Current Limit

NMOS Switch Current Limit 700 mA

NMOS Zero Current in Normal Mode 15 mA

Peak Current in Burst Mode Operation MODE = V

Zero Current in Burst Mode Operation MODE = V

Buck Soft-Start Time From the Rising Edge of EN_BUCK to 90%

Input High Voltage

Input Low Voltage

Output Low Voltage (⎯C⎯H⎯R⎯G) I

Input Current High EN_BUCK, MODE Pins at 5.5V, V

Input Current Low

⎯E⎯N⎯_⎯C⎯H⎯R⎯

G Pin Input Resistance V

⎯⎯C⎯H⎯R⎯

G Pin Leakage Current V

= 25°C, VCC = 5V, V

A

= 0.85V –50 50 nA

FB

No-Load for Regulator, V
L = 10μH, C = 4.7μF

No-Load for Regulator, V
MODE = V

⎯E⎯N⎯_⎯C⎯H⎯R⎯

G

> Regulation Voltage

V

OUT

Rising

BAT

Falling

V

BAT

of Buck Regulated Output

⎯E⎯N⎯_⎯C⎯H⎯R⎯

G, EN_BUCK, MODE Pin Low to High

⎯E⎯N⎯_⎯C⎯H⎯R⎯

G, EN_BUCK, MODE Pin High to Low

= 5mA

SINK

⎯E⎯N⎯_⎯C⎯H⎯R⎯

G, EN_BUCK, MODE Pins at GND

⎯E⎯N⎯_⎯C⎯H⎯R⎯

G

= 4.5V, V

BAT

= 3.8V, V

BAT

⎯E⎯N⎯_⎯C⎯H⎯R⎯

, L = 10μH, C = 4.7μF

BAT

⎯E⎯N⎯_⎯C⎯H⎯R⎯

= 5V, MODE = V

BAT

BAT

= 5V

= 5V

⎯C⎯H⎯R⎯

G

Note 4: Although the LTC4081 charger functions properly at 3.75V, full
charge current requires an input voltage greater than the desired fi nal
battery voltage per ΔV

Note 5: The 2.8V maximum buck undervoltage lockout (V
threshold must fi rst be exceeded before the minimum V
applies.

Note 6: I
with indicated PROG resistor.

⎯E⎯N⎯_⎯C⎯H⎯R⎯

BAT

= 0V, V

G

= 5V,

G

= 5V,

G

,

= 0V, V

NTC

●

●

EN_BUCK

0.78 0.80 0.82 V

1.8 2.25 2.75 MHz

= V

BAT

1.9 mA

23 μA

●

10 15 20 μA

●

2.6

●

2.4

375 520 700 mA

2.7

2.5

, V

MODE

2.8

2.6

50 100 150 mA

20 35 50 mA

400 μs

●

●

0.4 V

●

UVCL1

●

–1 1 μA

●

–1 1 μA

1 1.45 3.3 MΩ

●

specifi cation.

= 5V

BAT

is expressed as a fraction of measured full charge current

C/10

60 105 mV

1.2 V

1μA

UVLO_BUCK

specifi cation

BAT

= 0V.

Ω

Ω

) exit

V
V

4

4081f

TYPICAL PERFORMANCE CHARACTERISTICS

(TA = 25°C, VCC = 5V, V
specifi ed)

LTC4081

= 3.8V, unless otherwise

BAT

Battery Regulation (Float) Voltage
vs Charge Current

4.21
R

= 2k

PROG

4.20

4.19

4.18

4.17

4.16

FLOAT VOLTAGE (V)

4.15

4.14

4.13

0

50 100

CHARGE CURRENT (mA)

150

Charge Current vs Temperature
with Thermal Regulation
(Constant-Current Mode)

250

VCC = 6V

= 3V

V

BAT

= 2k

R

PROG

200

150

100

CHARGE CURRENT (mA)

50

0

–50

THERMAL CONTROL
LOOP IN OPERATION

0–25 50

25 75

TEMPERATURE (°C)

⎯E⎯N⎯_⎯C⎯H⎯R⎯

MODE Pin Threshold Voltage
vs Temperature

0.95

0.90

0.85

0.80

0.75

0.70

0.65

THRESHOLD VOLTAGE (V)

0.60

0.55

0.50
–50

200 250

4081 G01

125

100

4081 G04

G, EN_BUCK and

RISING

–10

–30

10 90

TEMPERATURE (°

Battery Regulation (Float) Voltage
vs Temperature

4.210

4.205

4.200

4.195

4.190

4.185

4.180

FLOAT VOLTAGE (V)

4.175

4.170

4.165

4.160
–30 –10 30

–50

10

TEMPERATURE (°C)

PROG Pin Voltage
vs Charge Current

1.0
R

= 2k

PROG

0.8

0.6

(V)

PROG

V

0.4

0.2

0

25

FALLING

30

0

50 70

C)

75

100

CHARGE CURRENT (mA)

4081 G07

50 70 90

12550

150

PULLDOWN RESISTANCE (MΩ)

Battery Regulation (Float) Voltage
vs VCC Supply Voltage

4.25

4.20

4.15

4.10

4.05

4.00

FLOAT VOLTAGE (V)

3.95

3.90

3.85
4

4081 G02

Charger FET On-Resistance
vs Temperature

0.9
VCC = 4V

I

0.8

BAT

0.7

0.6

(Ω)

0.5

0.4

DS(ON)

R

0.3

0.2

0.1

0

175

200

4081 G05

⎯E⎯N⎯_⎯C⎯H⎯R⎯

–50

G Pin Pulldown

Resistance vs Temperature

1.7

1.6

1.5

1.4

1.3

1.2

1.1

1.0
–50

–10

–30

TEMPERATURE (°C)

10 90

30

4.5 5
VCC SUPPLY VOLTAGE (V)

= 350mA

–30 –10 30

10

TEMPERATURE (°C)

50 70

4081 G08

5.5

4081 G03

50 70 90

4081 G06

6

4081f

5

LTC4081

TYPICAL PERFORMANCE CHARACTERISTICS

(TA = 25°C, VCC = 5V, V
specifi ed)

⎯C⎯H⎯R⎯

G Pin Output

Low Voltage vs Temperature

80

I

= 5mA

CHRG

70

60

50

40

30

VOLTAGE (mV)

20

10

Normalized Charge Termination
Time vs Temperature

1.05

1.00

0.95

0.90

NORMALIZED TIMER PERIOD

0.85

Buck Oscillator Frequency
vs Battery Voltage

2.28

2.27

2.26

2.25

2.24

FREQUENCY (MHz)

2.23

= 3.8V, unless otherwise

BAT

0

–50

–30

–10

10 90

TEMPERATURE (°

Buck Oscillator Frequency
vs Temperature

2.4

V

= 3.8V

BAT

2.3

2.2

2.1

2.0

FREQUENCY (MHz)

1.9

1.8
–60

–20 20

–40 0

TEMPERATURE (°C)

Buck Output Voltage
vs Battery Voltage

1.810
I

= 1mA

OUT

SET FOR 1.8V

V

OUT

1.805

1.800

1.795

1.790

BUCK OUTPUT VOLTAGE (V)

1.785

30

V

BAT

50 70

C)

V

= 4.5V

BAT

= 2.7V

60

40

Burst Mode
OPERATION

PWM MODE

2.22

2.5

Buck Effi ciency vs Load Current
(V

OUT

100

EFFICIENCY

80

POWER LOSS (mW)

(BURST)

60

40

EFFICIENCY (%)

20

0

0.01

3.0 3.5 4.5
BATTERY VOLTAGE (V)

4.0

4081 G11

= 1.5V)

EFFICIENCY
(PWM)

POWER LOSS
(BURST)

0.1 10 1001 1000
LOAD CURRENT (mA)

POWER
LOSS
(PWM)

V

= 3.8V

BAT

= 1.5V

V

OUT

L = 10μH
C = 4.7μF

4081 G13a

1000

100

POWER LOSS (mW)

10

1

0.1

0.01

4081 G09

0.80
–50

–30

–10

10 90

30

50 70

TEMPERATURE (°C)

4081 G10

Buck Effi ciency vs Load Current
(V

= 1.8V)

OUT

100

EFFICIENCY

80

(BURST)

60

40

EFFICIENCY (%)

20

100

80

4081 G12

0

0.01

EFFICIENCY
(PWM)

POWER LOSS
(BURST)

V

BAT

V

OUT

L = 10μH
C = 4.7μF

0.1 10 1001 1000
LOAD CURRENT (mA)

POWER
LOSS
(PWM)

= 3.8V
= 1.8V

4081 G13

1000

100

10

1

0.1

0.01

No-Load Buck Input Current
Buck Output Voltage
vs Temperature

1.810
I

= 1mA

OUT

SET FOR 1.8V

V

OUT

1.805

1.800

1.795

1.790

BUCK OUTPUT VOLTAGE (V)

1.785

Burst Mode
OPERATION

PWM MODE

(Burst Mode Operation)
vs Battery Voltage

35

I

= 1mA

OUT

= 1.8V

V

OUT

30

L = 10μH

25

20

15

10

BUCK INPUT CURRENT (μA)

5

6

1.780

2.5

3.0 3.5 4.0 4.5
BATTERY VOLTAGE (V)

4081 G14

1.780
–50

–30 –10

30 70 90

10 50

TEMPERATURE (˚C)

4081 G15

0

2.5

3.0

3.5

BATTERY VOLTAGE (V)

4.0

4.5

4081 G16

4081f

TYPICAL PERFORMANCE CHARACTERISTICS

(TA = 25°C, VCC = 5V, V
specifi ed)

No-Load Buck Input Current
(Burst Mode Operation)
vs Temperature

35

L = 10μH
C = 4.7μF

30

= 1.8V

V

OUT

25

20

15

10

NO LOAD INPUT CURRENT (μA)

5

V

= 4.2V

BAT

V

= 3.8V

BAT

V

= 2.7V

BAT

Buck Main Switch (PMOS)
On-Resistance vs Battery Voltage

1.2

1.0

0.8

0.6

0.4

ON-RESISTANCE (Ω)

0.2

Buck Main Switch (PMOS)
On-Resistance vs Temperature

1.2

1.0

0.8

0.6

0.4

ON-RESISTANCE (Ω)

0.2

LTC4081

= 3.8V, unless otherwise

BAT

0

–50 10 50

–30 –10

TEMPERATURE (˚C)

30 70 90

Buck Synchronous Switch (NMOS)
On-Resistance vs Battery Voltage

1.2

1.0

0.8

0.6

0.4

ON-RESISTANCE (Ω)

0.2

0

2.5

Maximum Output Current
(PWM Mode) vs Battery Voltage

500

L = 10μH

400

300

200

MAXIMUM OUTPUT CURRENT (mA)

100

2.7 4.23 3.6

4081 G18

3.5

3.0
BATTERY VOLTAGE (V)

V

OUT

3.3 3.9 4.5

BATTERY VOLTAGE (V)

4.0

SET FOR 1.8V

0

2.5

4.5 5.0

4081 G21

4081 G23

3.5

3.0
BATTERY VOLTAGE (V)

4.5 5.0

4.0

4081 G19

Buck Synchronous Switch (NMOS)
On-Resistance vs Temperature

1.2

1.0

0.8

0.6

0.4

ON-RESISTANCE (Ω)

0.2

0

–50 10 50

–30 –10

Maximum Output Current (Burst
Mode Operation) vs Battery Voltage

80

L = 10μH

70

60

50

40

30

20

MAXIMUM OUTPUT CURRENT (mA)

10

0

2.7 4.23 3.63.3 3.9 4.5
BATTERY VOLTAGE (V)

0

–50 10 50

–30 –10

TEMPERATURE (°C)

30 70 90

TEMPERATURE (°C)

V

SET FOR 1.8V

OUT

30 70 90

4081 G20

4081 G22

4081 G24

4081f

7

LTC4081

TYPICAL PERFORMANCE CHARACTERISTICS

(TA = 25°C, VCC = 5V, V
specifi ed)

= 3.8V, unless otherwise

BAT

V

OUT

20mV/DIV

AC COUPLED

I

LOAD

250mA/DIV

0mA

Output Voltage Transient
Step Response (PWM Mode)

50μs/DIV

1V/DIV

V

EN_BUCK

5V/DIV

V

OUT

Buck V
(I

0V

0V

LOAD

Soft-Start

OUT

= 50mA)

4081 G25

200μs/DIV

V

OUT

50mV/DIV

AC COUPLED

V

MODE

5V/DIV

Output Voltage Waveform
when Switching Between Burst
and PWM Mode (I

0V

50μs/DIV

4081 G28

LOAD

200mV/DIV

= 10mA)

V

PROG

4081 G27

Charger V

0V

V

OUT

20mV/DIV

AC COUPLED

I

LOAD

50mA/DIV

0mA

PROG

50μs/DIV

Output Voltage Transient
Step Response (Burst Mode
Operation)

50μs/DIV

Soft-Start

4081 G29

4081 G26

8

4081f

PI FU CTIO S

LTC4081

UUU

BAT (Pin 1):
Input. Provides charge current to the battery and regulates
the fi nal fl oat voltage to 4.2V. An internal precision resistor
divider from this pin sets the fl oat voltage and is disconnected
in charger shutdown mode. This pin must be decoupled
with a low ESR capacitor for low-noise buck operation.

VCC (Pin 2): Positive Input Supply Voltage. This pin provides
power to the battery charger. V
to 5.5V. This pin should be bypassed with at least a 1μF
capacitor. When V
voltage, the battery charger enters shutdown mode.

⎯E⎯N⎯_⎯C⎯H⎯R⎯

Pulling this pin above the manual shutdown threshold

) puts the LTC4081 charger in shutdown mode, thus

(V

IH

stopping the charge cycle. In battery charger shutdown
mode, the LTC4081 has less than 10μA supply current and
less than 5μA battery drain current provided the regula­tor is not running. Enable is the default state, but the pin
should be tied to GND if unused.

PROG (Pin 4): Charge Current Program and Charge Cur­rent Monitor Pin. Connecting a 1% resistor, R
ground programs the charge current. When charging in
constant-current mode, this pin servos to 1V. In all modes,
the voltage on this pin can be used to measure the charge
current using the following formula:

Charge Current Output and Buck Regulator

can range from 3.75V

CC

is less than 32mV above the BAT pin

CC

G (Pin 3): Enable Input Pin for the Battery Charger.

, to

PROG

pin below 0.016 • V
approximately 3°C of temperature hysteresis associated
with each of the input comparator’s thresholds.

⎯C⎯H⎯R⎯

G (Pin 6): Open-Drain Charge Status Output. The

charge status indicator pin has three states: pulldown,
high impedance state, and pulsing at 2Hz. This output can
be used as a logic interface or as an LED driver. When the
battery is being charged, the
an internal N-channel MOSFET. When the charge current
drops to 10% of the full-scale current, the
forced to a high impedance state. When the battery volt­age remains below 2.9V for one quarter of the full charge
time, the battery is considered defective, and the
pin pulses at a frequency of 2Hz with 75% duty cycle.
When the NTC pin voltage rises above 0.76 • V
below 0.35 • V
2Hz (25% duty cycle).

FB (Pin 7): Feedback Pin for the Buck Regulator. A resistor
divider from the regulator’s output to the FB pin programs
the output voltage. Servo value for this pin is 0.8V.

MODE (Pin 8): Burst Mode Enable Pin. Tie this pin high
to force the LTC4081 regulator into Burst Mode operation
for all load conditions. Tie this pin low to force constant­frequency mode operation for all load conditions. Do not
fl oat this pin.

CC

disables the NTC feature. There is

CC

⎯C⎯H⎯R⎯

G pin is pulled low by

⎯C⎯H⎯R⎯

G pin is

⎯C⎯H⎯R⎯

G

or drops

CC

, the ⎯C⎯H⎯R⎯G pin pulses at a frequency of

V

I

BAT

NTC (Pin 5): Input to the NTC (negative temperature coef­fi cient) Thermistor Temperature Monitoring Circuit. For
normal operation, connect a thermistor from the NTC pin
to ground and a resistor of equal value from the NTC pin
to V

CC

at hot temperatures or rises above 0.76 • VCC at cold,

V

CC

charging is suspended, the internal timer is frozen and the

⎯C⎯H⎯R⎯

G pin output will start to pulse at 2Hz. Pulling this

PROG

= •400

R

. When the voltage at this pin drops below 0.35 •

EN_BUCK (Pin 9): Enable Input Pin for the Buck Regulator.
Pull this pin high to enable the regulator, pull low to shut
down. Do not fl oat this pin.

SW (Pin 10): Switch Pin for the Buck Regulator. Minimize
the length of the metal trace connected to this pin. Place
the inductor as close to this pin as possible.

GND (Pin 11): Ground. This pin is the back of the Exposed
Pad package and must be soldered to the PCB for electrical
connection and rated thermal performance.

4081f

9

LTC4081

BLOCK DIAGRA

3

EN_CHRG

0.82V

R

EN

6

CHRG

+

2.9V

C2

–

BAT

4

PROG

R

PROG

V

CC

V

CC

R

NOM

5

R

T

NTC

R9

NTC

R10

W

+

C3

–

PROGC10.1V

PULSE
LOGIC

–

C8

+

–

C9

+

CHARGER
SHUTDOWN

–+

1V

0.1V

BADBAT

TOO COLD

TOO HOT

MP3 MP1

D1

CA

–+

MP4

2

V

CC

X1 X400

–+

MA

CHARGER

ENABLE

V

CC

3.6V

R1

R2

+

C4

–

+

C5

+ 80mV

V

BAT

–

SUSPEND

UVLO

CHARGER

OSCILLATOR

D3

D2

VA

–+

1.22V

TA

CHARGE

CONTROL

LOGIC

COUNTER

115°C

–

T

+

DIE

1

BAT

10

9

EN_BUCK

8

MODE

0.82V

0.82V

R11

R12

+

C10

NTC_EN

–

LINEAR BATTERY CHARGER

0.8V

MP2

MN1

SW

4081 BD

L1

10

7

FB

V

OUT

C

R7

PL

C

OUT

R8

+

C6

–

ENABLE BUCK

+

2.25MHz

C7

–

BUCK

OSCILLATOR

SYNCHRONOUS BUCK CONVERTER

11

GND

PWM

CONTROL

AND DRIVE

ERROR

AMP

–

+

Figure 1. LTC4081 Block Diagram

4081f

OPERATIO

LTC4081

U

The LTC4081 is a full-featured linear battery charger with
an integrated synchronous buck converter designed pri­marily for handheld applications. The battery charger is
capable of charging single-cell 4.2V Li-Ion batteries. The
buck converter is powered from the BAT pin and has a
programmable output voltage providing a maximum load
current of 300mA. The converter and the battery charger
can run simultaneously or independently of each other.

BATTERY CHARGER OPERATION

Featuring an internal P-channel power MOSFET, MP1,
the battery charger uses a constant-current/constant­voltage charge algorithm with programmable current.
Charge current can be programmed up to 500mA with a

⎯C⎯H⎯R⎯

fi nal fl oat voltage of 4.2V ±0.5%. The
status output indicates when C/10 has been reached. No
blocking diode or external sense resistor is required; thus,
the basic charger circuit requires only two external com­ponents. An internal charge termination timer adheres to
battery manufacturer safety guidelines. Furthermore, the
LTC4081 battery charger is capable of operating from a
USB power source.

A charge cycle begins when the voltage at the V
rises above 3.6V and approximately 82mV above the BAT
pin voltage, a 1% program resistor is connected from the
PROG pin to ground, and the
the shutdown threshold (V

When the BAT pin approaches the fi nal fl oat voltage of

4.2V, the battery charger enters constant-voltage mode
and the charge current begins to decrease. When the
current drops to 10% of the full-scale charge current, an
internal comparator turns off the N-channel MOSFET driving

⎯C⎯H⎯R⎯

the

An internal thermal limit reduces the programmed charge
current if the die temperature attempts to rise above a
preset value of approximately 115°C. This feature protects
the LTC4081 from excessive temperature and allows the
user to push the limits of the power handling capability
of a given circuit board without the risk of damaging the
LTC4081 or external components. Another benefi t of the
thermal limit is that charge current can be set

G pin, and the pin becomes high impedance.

⎯E⎯N⎯_⎯C⎯H⎯R⎯

).

IL

G open-drain

pin

CC

G pin is pulled below

according

to typical, rather than worst-case, ambient temperatures
for a given application with the assurance that the battery
charger will automatically reduce the current in worst-case
conditions.

An internal timer sets the total charge time, t
cally 4.5 hours). When this time elapses, the charge cycle
terminates and the
state even if C/10 has not yet been reached. To restart
the charge cycle, remove the input voltage and reapply
it or momentarily force the
new charge cycle will automatically restart if the BAT pin
voltage falls below V

Constant-Current / Constant-Voltage /
Constant-Temperature

The LTC4081 battery charger uses a unique architecture
to charge a battery in a constant-current, constant-volt­age and constant-temperature fashion. Figure 1 shows a
Simplifi ed Block Diagram of the LTC4081. Three of the
amplifi er feedback loops shown control the constant-cur­rent, CA, constant-voltage, VA, and constant-temperature,
TA modes. A fourth amplifi er feedback loop, MA, is used to
increase the output impedance of the current source pair,
MP1 and MP3 (note that MP1 is the internal P-channel
power MOSFET). It ensures that the drain current of MP1
is exactly 400 times the drain current of MP3.

Amplifi ers CA and VA are used in separate feedback loops
to force the charger into constant-current or constant­voltage mode, respectively. Diodes D1 and D2 provide
priority to either the constant-current or constant-voltage
loop, whichever is trying to reduce the charge current the
most. The output of the other amplifi er saturates low which
effectively removes its loop from the system. When in
constant-current mode, CA servos the voltage at the PROG
pin to be precisely 1V. VA servos its non-inverting input
to 1.22V when in constant-voltage mode and the internal
resistor divider made up of R1 and R2 ensures that the
battery voltage is maintained at 4.2V. The PROG pin volt­age gives an indication of the charge current anytime in
the charge cycle, as discussed in “Programming Charge
Current” in the Applications Information section.

⎯C⎯H⎯R⎯

G pin assumes a high impedance

RECHRG

⎯E⎯N⎯_⎯C⎯H⎯R⎯

(typically 4.1V).

G pin above VIH. A

TIMER

(typi-

4081f

11

LTC4081

OPERATIO

U

If the die temperature starts to creep up above 115°C
due to internal power dissipation, the transconductance
amplifi er, TA, limits the die temperature to approximately
115°C by reducing the charge current. Diode D3 ensures
that TA does not affect the charge current when the die
temperature is below 115°C. In thermal regulation, the
PROG pin voltage continues to give an indication of the
charge current.

In typical operation, the charge cycle begins in constant­current mode with the current delivered to the battery equal
to 400V/R
results in the junction temperature approaching 115°C, the
amplifi er (TA) will begin decreasing the charge current to
limit the die temperature to approximately 115°C. As the
battery voltage rises, the LTC4081 either returns to full
constant-current mode or enters constant-voltage mode
straight from constant-temperature mode.

Battery Charger Undervoltage Lockout (UVLO)

An internal undervoltage lockout circuit monitors the V
input voltage and keeps the battery charger off
rises above 3.6V and approximately 82mV above the BAT
pin voltage. The 3.6V UVLO circuit has a built-in hysteresis
of approximately 0.6V, and the 82mV automatic shutdown
threshold has a built-in hysteresis of approximately 50mV.
During undervoltage lockout conditions, maximum battery
drain current is 5

Undervoltage Charge Current Limiting (UVCL)

The battery charger in the LTC4081 includes undervoltage
charge current limiting that prevents full charge current
until the input supply voltage reaches approximately 300mV
above the battery voltage (ΔV
larly useful if the LTC4081 is powered from a supply with
long leads (or any relatively high output impedance). See
Applications Information section for further details.

Trickle Charge and Defective Battery Detection

At the beginning of a charge cycle, if the battery volt­age is below 2.9V, the battery charger goes into trickle
charge mode, reducing the charge current to 10% of the
programmed current. If the low battery voltage persists

. If the power dissipation of the LTC4081

PROG

μ

A and maximum supply current is 10μA.

). This feature is particu-

UVCL1

until VCC

for one quarter of the total time (1.125 hr), the battery is
assumed to be defective, the charge cycle terminates and
the
a 75% duty cycle. If, for any reason, the battery voltage
rises above 2.9V, the charge cycle will be restarted. To
restart the charge cycle (i.e., when the dead battery is
replaced with a discharged battery less than 2.9V), the
charger must be reset by removing the input voltage and
reapplying it or temporarily pulling the
the shutdown threshold.

Battery Charger Shutdown Mode

The LTC4081’s battery charger can be disabled by pulling
the
In shutdown mode, the battery drain current is reduced
to about 2μA and the V
provided the regulator is off. When the input voltage is
not present, the battery charger is in shutdown and the
battery drain current is less than 5μA.

CC

⎯C⎯H⎯R⎯

The charge status indicator pin has three states: pulldown,
pulsing at 2Hz (see Trickle Charge and Defective Battery
Detection and Battery Temperature Monitoring) and high
impedance. The pulldown state indicates that the battery
charger is in a charge cycle. A high impedance state indi­cates that the charge current has dropped below 10% of
the full-scale current or the battery charger is disabled.
When the timer runs out (4.5 hrs), the ⎯C⎯H⎯R⎯G pin is also
forced to the high impedance state. If the battery charger
is not in constant-voltage mode when the charge current
is forced to drop below 10% of the full-scale current by
UVCL, ⎯C⎯H⎯R⎯G will stay in the strong pulldown state.

Charge Current Soft-Start

The LTC4081’s battery charger includes a soft-start circuit
to minimize the inrush current at the start of a charge
cycle. When a charge cycle is initiated, the charge cur­rent ramps from zero to full-scale current over a period
of approximately 180μs. This has the effect of minimizing
the transient current load on the power supply during
start-up.

⎯C⎯H⎯R⎯

G pin output pulses at a frequency of 2Hz with

⎯E⎯N⎯_⎯C⎯H⎯R⎯

G Status Output Pin

G pin above the shutdown threshold (VIH).

⎯E⎯N⎯_⎯C⎯H⎯R⎯

supply current to about 5μA

CC

G pin above

12

4081f

OPERATIO

LTC4081

U

Timer and Recharge

The LTC4081’s battery charger has an internal charge
termination timer that starts when the input voltage is
greater than the undervoltage lockout threshold and at
least 82mV above BAT, and the battery charger is leaving
shutdown.

At power-up or when exiting shutdown, the charge time
is set to 4.5 hours. Once the charge cycle terminates, the
battery charger continuously monitors the BAT pin voltage
using a comparator with a 2ms fi lter time. When the aver­age battery voltage falls below 4.1V (which corresponds
to 80%-90% battery capacity), a new charge cycle is initi­ated and a 2.25 hour timer begins. This ensures that the
battery is kept at, or near, a fully charged condition and
eliminates the need for periodic charge cycle initiations.

⎯C⎯H⎯R⎯

The

G output assumes a strong pulldown state dur­ing recharge cycles until C/10 is reached or the recharge
cycle terminates.

Battery Temperature Monitoring via NTC

When the charger is in Hold mode (battery temperature

⎯C⎯H⎯R⎯

is either too hot or too cold) the

G pin pulses in a
2Hz, 25% duty cycle frequency unless the charge task is
fi nished or the battery is assumed to be defective. If the
NTC pin is grounded, the NTC function will be disabled.

SWITCHING REGULATOR OPERATION:

The switching buck regulator in the LTC4081 can be turned
on by pulling the EN_BUCK pin above VIH. It has two user­selectable modes of operation: constant-frequency (PWM)
mode and Burst Mode Operation. The constant-frequency
mode operation offers low noise at the expense of effi ciency
whereas the Burst Mode operation offers higher effi ciency
at light loads at the cost of increased noise, higher output
voltage ripple, and less output current. A detailed descrip­tion of different operating modes and different aspects of
operation follow. Operations can best be understood by
referring to the Block Diagram.

V

CC

The battery temperature is measured by placing a nega­tive temperature coeffi cient (NTC) thermistor close to the
battery pack. The NTC circuitry is shown in Figure 2.

To use this feature, connect the NTC thermistor, R
tween the NTC pin and ground and a resistor, R
the NTC pin to VCC. R

should be a 1% resistor with a

NOM

NTC

NOM

, be-

, from

value equal to the value of the chosen NTC thermistor at
25°C (this value is 10k for a Vishay NTHS0603NO1N1002J
thermistor). The LTC4081 goes into hold mode when the
value of the NTC thermistor drops to 0.53 times the value
of R

, which corresponds to approximately 40°C, and

NOM

when the value of the NTC thermistor increases to 3.26
times the value of R

, which corresponds to approxi-

NOM

mately 0°C. Hold mode freezes the timer and stops the
charge cycle until the thermistor indicates a return to a
valid temperature. For a Vishay NTHS0603NO1N1002J
thermistor, this value is 32.6k which corresponds to ap­proximately 0°C. The hot and cold comparators each have
approximately 3°C of hysteresis to prevent oscillation
about the trip point.

R

NOM

0.76 • V

CC

NTC

6

R

T

NTC

0.35 • V

CC

0.016 • V

CC

Figure 2. NTC Circuit Information

–

+

–

+

+

–

TOO COLD

TOO HOT

NTC_ENABLE

4081 F02

4081f

13

LTC4081

OPERATIO

U

Constant-Frequency (PWM) Mode Operation

The switching regulator operates in constant-frequency
(PWM) mode when the MODE pin is pulled below VIL. In
this mode, it uses a current mode architecture including
an oscillator, an error amplifi er, and a PWM comparator
for excellent line and load regulation. The main switch
MP2 (P-channel MOSFET) turns on to charge the inductor
at the beginning of each clock cycle if the FB pin voltage
is less than the 0.8V reference voltage. The current into
the inductor (and the load) increases until it reaches the
peak current demanded by the error amp. At this point,
the main switch turns off and the synchronous switch
MN1 (N-channel MOSFET) turns on allowing the inductor
current to fl ow from ground to the load until either the
next clock cycle begins or the current reduces to the zero
current (I

Oscillator: In constant-frequency mode, the switching
regulator uses a dedicated oscillator which runs at a fi xed
frequency of 2.25MHz. This frequency is chosen to mini­mize possible interference with the AM radio band.

Error Amplifi er: The error amplifi er is an internally com­pensated transconductance (g
of 65
compared to the voltage at the FB pin to generate a
current signal at the output of the error amplifi er. This cur­rent signal represents the peak inductor current required
to achieve regulation.

PWM Comparator: Lossless current sensing converts
the PMOS switch current signal to a voltage which is
summed with the internal slope compensation signal.
The PWM comparator compares this summed signal to
determine when to turn off the main switch. The switch
current sensing is blanked for ~12ns at the beginning of
each clock cycle to prevent false switch turn-off.

Burst Mode Operation

Burst Mode operation can be selected by pulling the
MODE pin above V
lator is disabled, the error amplifi er is converted into a

μ

) level.

ZERO

) amplifi er with a gm

m

mhos. The internal 0.8V reference voltage is

. In this mode, the internal oscil-

IH

comparator monitoring the FB voltage, and the inductor
current swings between a fi xed I
(35mA) irrespective of the load current as long as the FB
pin voltage is less than or equal to the reference voltage
of 0.8V. Once V
shuts off both switches along with most of the circuitry
and the regulator is said to enter into SLEEP mode. In
SLEEP mode, the regulator only draws about 20
the BAT pin provided that the battery charger is turned
off. When the output voltage droops about 1% from its
nominal value, the regulator wakes up and the inductor
current resumes swinging between I
output capacitor recharges and causes the regulator to
re-enter the SLEEP state if the output load remains light
enough. The frequency of this intermittent burst operation
depends on the load current. That is, as the load current
drops further, the regulator turns on less frequently. Thus
Burst Mode operation increases the effi ciency at light loads
by minimizing the switching and quiescent losses. However,
the output voltage ripple increases to about 2%.

To minimize ripple in the output voltage, the current limits
for both switches in Burst Mode operation are reduced
to about 20% of their values in the constant-frequency
mode. Also the zero current of the synchronous switch
is changed to about 35mA thereby preventing reverse
conduction through the inductor. Consequently, the regu­lator can only deliver approximately 67mA of load current
while in Burst Mode operation. Any attempt to draw more
load cur
regulation.

Current Limit

To prevent inductor current runaway, there are absolute
current limits (I
the NMOS synchronous switch. These limits are internally
set at 520mA and 700mA respectively for PWM mode. If
the peak inductor current demanded by the error amplifi er
ever exceeds the PMOS I
ignored and the inductor current will be limited to PMOS
I

LIM

reduced to 100mA to minimize output voltage ripple.

rent will cause the output voltage to drop out of

. In Burst Mode operation, the PMOS current limit is

is greater than 0.8V, the control logic

FB

) on both the PMOS main switch and

LIM

, the error amplifi er will be

LIM

(~100mA) and I

PEAK

and I

PEAK

μ

A from

ZERO

ZERO

. The

14

4081f

OPERATIO

LTC4081

U

Zero Current Comparator

The zero or reverse current comparator monitors the induc­tor current to the output and shuts off the synchronous
rectifi er when this current reduces to a predetermined
value (I
tive 15mA meaning that the regulator allows the inductor
current to fl ow in the reverse direction (from the output to
ground through the synchronous rectifi er) to a maximum
value of 15mA. This is done to ensure that the regulator
is able to regulate at very light loads without skipping any
cycles thereby keeping output voltage ripple and noise low
at the cost of effi ciency.

However, in Burst Mode operation, I
35mA meaning that the synchronous switch is turned off
as soon as the current through the inductor to the output
decreases to 35mA in the discharge cycle. This preserves
the charge on the output capacitor and increases the overall
effi ciency at light loads.

Soft-Start

The LTC4081 switching regulator provides soft-start in
both modes of operation by slowly charging an internal
capacitor. The voltage on this capacitor, in turn, slowly
ramps the current limits of both switches from a low value
to their respective maximum values over a period of about

μ

s. The soft-start capacitor is discharged completely

400
whenever the regulator is disabled.

). In fi xed frequency mode, this is set to nega-

ZERO

is set to positive

ZERO

is blanked for ~12ns at the beginning of each clock cycle,
inductor current can build up to a dangerously high level
over a number of cycles even if there is a hard current
limit on the main PMOS switch. This is why the switching
regulator in the LTC4081 also monitors current through
the synchronous NMOS switch and imposes a hard limit
on it. If the inductor current through the NMOS switch at
the end of a discharge cycle is not below this limit, the
regulator skips the next charging cycle thereby preventing
inductor current runaway.

Switching Regulator Undervoltage Lockout

Whenever V
out circuit keeps the regulator off, preventing unreliable
operation. However, if the regulator is already running
and the battery voltage is dropping, the undervoltage
comparator does not shut down the regulator until V
drops below 2.5V.

Dropout Operation

When the BAT pin voltage approaches V
of the switching regulator approaches 100%. When V
is approximately equal to V
in dropout. In dropout, the main switch (MP2) stays on
continuously with the output voltage being equal to the
battery voltage minus the voltage drops across the main
switch and the inductor.

is less than 2.7V, an undervoltage lock-

BAT

, the duty cycle

OUT

, the regulator is said to be

OUT

BAT

BAT

Short-Circuit Protection

In the event of a short circuit at the output or during
start-up, V
slope of the inductor current is ~V
current may not get a chance to discharge enough to
avoid a runaway situation. Because the current sensing

will be near zero volts. Since the downward

OUT

/L, the inductor

OUT

Global Thermal Shutdown

The LTC4081 includes a global thermal shutdown which
shuts off the entire device (battery charger and switch­ing regulator) if the
LTC4081 resumes normal
drops approximately 14°C.

die temperature exceeds 160°C. The

operation once the temperature

4081f

15

LTC4081

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APPLICATIO S I FOR ATIO

BATTERY CHARGER

Programming Charge Current

The battery charge current is programmed using a single
resistor from the PROG pin to ground. The charge current
is 400 times the current out of the PROG pin. The program
resistor and the charge current are calculated using the
following equations:

V

R

PROG

==400

1

I

BAT

I

BAT

•, •

400

The charge current out of the BAT pin can be determined
at any time by monitoring the PROG pin voltage and using
the following equation:

V

I

BAT

PROG

= • 400

R

Stability Considerations

The LTC4081 battery charger contains two control loops:
constant-voltage and constant-current. The constant­voltage loop is stable without any compensation when a
battery is connected with low impedance leads. Excessive
lead length, however, may add enough series inductance
to require a bypass capacitor of at least 1
GND. Furthermore, a 4.7μF capacitor with a 0.2Ω to 1Ω
series resistor from BAT to GND is required to keep ripple
voltage low when the battery is disconnected.

V

1

R

PROG

μ

F from BAT to

Average, rather than instantaneous, battery current may be
of interest to the user. For example, when the switching
regulator operating in low-current mode is connected in
parallel with the battery, the average current being pulled
out of the BAT pin is typically of more interest than the
instantaneous current pulses. In such a case, a simple RC
fi lter can be used on the PROG pin to measure the average
battery current as shown in Figure 3. A 10k resistor has
been added between the PROG pin and the fi lter capacitor
to ensure stability.

Undervoltage Charge Current Limiting (UVCL)

USB powered systems tend to have highly variable source
impedances (due primarily to cable quality and length). A
transient load combined with such impedance can easily trip
the UVLO threshold and turn the battery charger off unless
undervoltage charge current limiting is implemented.

Consider a situation where the LTC4081 is operating under
normal conditions and the input supply voltage begins to
sag (e.g. an external load drags the input supply down).
If the input voltage reaches V
above the battery voltage, ΔV

(approximately 300mV

UVCL

), undervoltage charge

UVCL

current limiting will begin to reduce the charge current in
an attempt to maintain ΔV

between VCC and BAT. The

UVCL

LTC4081 will continue to operate at the reduced charge
current until the input supply voltage is increased or volt­age mode reduces the charge current further.

In constant-current mode, the PROG pin voltage is in
the feedback loop, not the battery voltage. Because of
the additional pole created by PROG pin capacitance,
capacitance on this pin must be kept to a minimum. With
no additional capacitance on the PROG pin, the battery
charger is stable with program resistor values as high
as 25k. However, additional capacitance on this node
reduces the maximum allowed program resistor. The pole
frequency at the PROG pin should be kept above 100kHz.
Therefore, if the PROG pin is loaded with a capacitance,

, the following equation should be used to calculate

C

PROG

the maximum resistance value for R

R

PROG

≤

π12 100••

kHz C

PROG

PROG

:

16

LTC4081

PROG

GND

Figure 3. Isolating Capacitive Load
on PROG Pin and Filtering

R

10k

PROG

4081 F03

C

FILTER

CHARGE
CURRENT
MONITOR
CIRCUITRY

4081f

LTC4081

U

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APPLICATIO S I FOR ATIO

Operation from Current Limited Wall Adapter

By using a current limited wall adapter as the input sup­ply, the LTC4081 can dissipate signifi cantly less power
when programmed for a current higher than the limit of
the wall adapter.

Consider a situation where an application requires a 200mA
charge current for a discharged 800mAh Li-Ion battery.
If a typical 5V (non-current limited) input supply is avail­able then the peak power dissipation inside the part can
exceed 300mW.

Now consider the same scenario, but with a 5V input
supply with a 200mA current limit. To take advantage
of the supply, it is necessary to program the LTC4081
to charge at a current greater than 200mA. Assume that
the LTC4081 charger is programmed for 300mA (i.e.,
R

= 1.33kΩ) to ensure that part tolerances maintain

PROG

a programmed current higher than 200mA. Since the
battery charger will demand a charge current higher than
the current limit of the input supply, the supply voltage
will collapse to the battery voltage plus 200mA times the
on-resistance of the internal PFET. The on-resistance of
the battery charger power device is approximately 0.7Ω
with a 5V supply. The actual on-resistance will be slightly
higher due to the fact that the input supply will have col­lapsed to less than 5V. The power dissipated during this
phase of charging is approximately 30mW. That is a ten
times improvement over the non-current limited supply
power dissipation.

USB and Wall Adapter Power

Although the LTC4081 allows charging from a USB port,
a wall adapter can also be used to charge Li-Ion batter­ies. Figure 4 shows an example of how to combine wall
adapter and USB power inputs. A P-channel MOSFET,
MP1, is used to prevent back conducting into the USB
port when a wall adapter is present and Schottky diode,
D1, is used to prevent USB power loss through the 1k
pulldown resistor.

Typically a wall adapter can supply signifi cantly more
current than the current-limited USB port. Therefore, an
N-channel MOSFET, MN1, and an extra program resistor
can be used to increase the charge current when the wall
adapter is present.

Power Dissipation

The conditions that cause the LTC4081 battery charger to
reduce charge current through thermal feedback can be
approximated by considering the total power dissipated
in the IC. For high charge currents, the LTC4081 power
dissipation is approximately:

PVV I P

=−

()

D CC BAT BAT D BUCK

+•

_

Where PD is the total power dissipated within the IC, VCC
is the input supply voltage, V
is the charge current and P
due to the regulator. P

D_BUCK

D_BUCK

⎛

PVI

D BUCK OUT OUT_

•=−

⎜

⎝

is the battery voltage, I

BAT

is the power dissipation

can be calculated as:

⎞

1

1

⎟

⎠

η

BAT

BAT

I

CHG

1

4

+

4k

4081 F04

5V WALL

ADAPTER

(500mA)

USB

POWER

(100mA)

Figure 4. Combining Wall Adapter and USB Power

MP1

D1

LTC4081

2

V

CC

PROG

1k

MN1

1k

SYSTEM
LOAD

Li-Ion
BATTERY

4081f

17

LTC4081

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APPLICATIO S I FOR ATIO

Where V
regulator, I
effi ciency at that particular load.

It is not necessary to perform worst-case power dissipa­tion scenarios because the LTC4081 will automatically
reduce the charge current to maintain the die temperature
at approximately 115°C. However, the approximate ambi­ent temperature at which the thermal feedback begins to
protect the IC is:

= 115°C – P

T

A

TA = 115°C – (VCC – V

is off.

Example: Consider the extreme case when an LTC4081 is
operating from a 6V supply providing 250mA to a 3V Li-Ion
battery and the regulator is off. The ambient temperature
above which the LTC4081 will begin to reduce the 250mA
charge current is approximately:

= 115°C – (6V – 3V) • (250mA) • 43°C/W

T

A

is the regulated output of the switching

OUT

is the regulator load and η is the regulator

OUT

DθJA

BAT

) • I

BAT

• θ

if the regulator

JA

Using the previous example with an ambient temperature
of 85°C, the charge current will be reduced to approxi­mately:

I

BAT

CC

°− °

115 85

=

VV CW

−

63 43

()

°

•/ /

=

30

129

C

°

CA

°

=

2

332 6.mA

Furthermore, the voltage at the PROG pin will change
proportionally with the charge current as discussed in the
Programming Charge Current section.

Bypass Capacitor

V

CC

Many types of capacitors can be used for input bypassing;
however, caution must be exercised when using multi-layer
ceramic capacitors. Because of the self-resonant and high
Q characteristics of some types of ceramic capacitors, high
voltage transients can be generated under some start-up con­ditions, such as connecting the battery charger input to a live

Ω

power source. Adding a 1

series resistor in series with an
X5R ceramic capacitor will minimize start-up voltage transients.
For more information, refer to Application Note 88.

= 115°C – 0.75W • 43°C/W = 115°C – 32.25°C

TA

= 82.75°C

T

A

If there is more power dissipation due to the regulator,
the thermal regulation will begin at a somewhat lower
temperature. In the above circumstances, the LTC4081
can be used above 82.75°C, but the charge current will be
reduced from 250mA. The approximate current at a given
ambient temperature can be calculated:

I

=

BAT

VV

()

CC BAT JA

115

−

CT

°−

A

• θ

Thermistors

The LTC4081 NTC trip points are designed to work with therm­istors whose resistance-temperature characteristics follow
Vishay Dale’s “R-T Curve 1.” The Vishay NTHS0603NO1N1002J
is an example of such a thermistor. However, Vishay Dale
has many thermistor products that follow the “R-T Curve 1”
characteristic in a variety of sizes. Furthermore, any thermis­tor whose ratio of R

COLD

to R

(Vishay Dale R-T Curve 1 shows a ratio of R

is about 5 will also work

HOT

to R

COLD

HOT

of

3.266/0.5325 = 6.13).

18

4081f

LTC4081

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APPLICATIO S I FOR ATIO

Power conscious designs may want to use thermistors whose
room temperature value is greater than 10k. Vishay Dale has a
number of values of thermistor from 10k to 100k that follow
the “R-T Curve 1.” Using different R-T curves, such as Vishay
Dale “R-T Curve 2”, is also possible. This curve, combined with
LTC4081 internal thresholds, gives temperature trip points of
approximately 0°C (falling) and 40°C (rising), a delta of 40°C.
This delta in temperature can be moved in either direction by
changing the value of R

will move both trip points to higher temperatures. To

R

NOM

calculate R

for a shift to lower temperature for example,

NOM

with respect to R

NOM

. Increasing

NTC

use the following equation:

R

R

NOM

where R

COLD

=°

3 266

.

is the resistance ratio of R

COLD

RatC

•

NTC

25

at the desired cold

NTC

temperature trip point. If you want to shift the trip points to
higher temperatures use the following equation:

R

R

NOM

where R

HOT

=°

0 5325

.

is the resistance ratio of R

HOT

RatC

•

NTC

25

at the desired hot

NTC

temperature trip point.

Here is an example using a 100k R-T Curve 2 thermistor
from Vishay Dale. The difference between the trip points is
40°C, from before, and we want the cold trip point to be 0°C,
which would put the hot trip point at 40°C. The R

NOM

needed

is calculated as follows:

R

R

NOM

COLD

=°

3 266

.

2 816

.

=

..

3

RatC

•

NTC

•.

10 8 62kk=

266

25

The nearest 1% value for R
to bias the NTC thermistor to get cold and hot trip points of
approximately 0°C and 40°C respectively. To extend the delta
between the cold and hot trip points, a resistor, R1, can be
added in series with R
resistors are calculated as follows:

where R
R

COLD

points. Continuing the example from before with a desired
trip point of 50°C:

R

NOM

Rk

1

is 8.66k. This is the value used

NOM

(see Figure 5). The values of the

NTC

R

R

NOM

1

RR

=

3 266 0 5325

⎛

=

⎜

⎝

.

3 266

is the value of the bias resistor and R

NOM

are the values of R

RR

COLD HOT

=

3 266 0 5325

..

88 887

=

.,.kkis %.

10

⎛

•

⎜

⎝

=

−

COLD HOT

..

−

.

0 5325

0 5325.

−−

−

⎞
⎟

⎠

at the desired temperature trip

NTC

10 2 816 0

=

−

the nearest value

.

.

326

0 5325

66 0 5325

−

⎞
⎟

⎠

.

•R R R

()

COLD HOT HOT

k

•. .

()

3 266 0 5325

..

1

•. . .

2 816 0 4086 0 4086

()

,%.= 604 604 1Ω is the nearest value

V

CC

R

NOM

8.87k

R1
604Ω

T

R
10k

NTC

6

0.76 • V

NTC

0.35 • V

CC

CC

–

+

–

+

44086

−

and

HOT

−

−

−

−

−

TOO COLD

TOO HOT

0.016 • V

CC

Figure 5. NTC Circuits

+

NTC_ENABLE

–

4081 F05

4081f

19

LTC4081

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APPLICATIO S I FOR ATIO

NTC Trip Point Error

When a 1% resistor is used for R

, the major error in the 40°C

HOT

trip point is determined by the tolerance of the NTC thermistor.
A typical 100k NTC thermistor has ±10% tolerance. By look­ing up the temperature coeffi cient of the thermistor at 40°C,
the tolerance error can be calculated in degrees centigrade.
Consider the Vishay NTHS0603N01N1003J thermistor, which
has a temperature coeffi cient of –4%/°C at 40°C. Dividing
the tolerance by the temperature coeffi cient, ±5%/(4%/°C) =
±1.25°C, gives the temperature error of the hot trip point.

The cold trip point error depends on the tolerance of the NTC
thermistor and the degree to which the ratio of its value at
0°C and its value at 40°C varies from 6.14 to 1. Therefore,
the cold trip point error can be calculated using the tolerance,
TOL, the temperature coeffi cient of the thermistor at 0°C, TC
(in %/°C), the value of the thermistor at 0°C, R
value of the thermistor at 40°C, R

⎛

+1

⎜

.

614

Temperature Error C

()

°=

⎝

. The formula is:

HOT

R

TOL

COLD

•
R

HOT

TC

−−

COLD

⎞

1 100•

⎟
⎠

, and the

For example, the Vishay NTHS0603N01N1003J thermistor
with a tolerance of ±5%, TC of –5%/°C and R

COLD/RHOT

of

6.13, has a cold trip point error of:

.

+

1005

Temperature Error C()

°=

⎛
⎜⎜

⎝

.

614

•.

613 1

−

⎞

• 100

−

⎟

⎠

5

.,.=− ° °095 105CC

SWITCHING REGULATOR

Setting the Buck Converter Output Voltage

The LTC4081 regulator compares the FB pin voltage with
an internal 0.8V reference to generate an error signal at the
output of the error amplifi er. A voltage divider from V
to ground (as shown in the Block Diagram) programs the
output voltage via FB using the formula:

Keeping the current low (<5μA) in these resistors maxi­mizes effi ciency, but making them too low may allow stray
capacitance to cause noise problems and reduce the phase
margin of the error amp loop. To improve the frequency
response, a phase-lead capacitor (C
10pF can be used. Great care should be taken to route the
FB line away from noise sources, such as the inductor or
the SW line.

Inductor Selection

The value of the inductor primarily determines the cur­rent ripple in the inductor. The inductor ripple
current ΔI
increases with higher V

R

7

VV

=+

OUT

ΔI

08 1

L

V

=−

L

fLVV

OSC

⎡

.•

⎢
⎣

decreases with higher inductance and

•

⎛

•1

⎜

⎝

OUT

R

IN

⎤
⎥

8

⎦

or V

OUT

IN

OUT

⎞
⎟

⎠

) of approximately

PL

:

OUT

20

4081f

LTC4081

⎝

⎠

U

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APPLICATIO S I FOR ATIO

Accepting larger values of ΔIL allows the use of low
inductances, but results in higher output voltage ripple,
greater core losses, and lower output current capability. A
reasonable starting point for setting ripple current is ΔI
=0.3 • I

, where I

LIM

is the peak switch current limit.

LIM

The largest ripple current occurs at the maximum input
voltage. To guarantee that the ripple current stays below a
specifi ed maximum, the inductor value should be chosen
according to the following equation:

OUT

Δ

⎛

1••

⎜
⎜

L

IN

V

L

≥−

fIVV

0

For applications with V

⎞

OUT

⎟
⎟

MAX

()

= 1.8V, the above equation

OUT

suggests that an inductor of at least 6.8μH should be used
for proper operation.

Many different sizes and shapes of inductors are
available from numerous manufacturers. To maximize
effi ciency, choose an inductor with a low DC resistance.
Keep in mind that most inductors that are very thin or
have a very small volume typically have much higher core
and DCR losses and will not give the best effi ciency. Also
choose an inductor with a DC current rating at least 1.5
times larger than the peak inductor current limit to ensure
that the inductor does not saturate during normal opera­tion. To minimize radiated noise use a toroid or shielded
pot core inductor in ferrite or permalloy materials. Table
1 shows a list of several inductor manufacturers.

L

Table 1. Recommended Surface Mount Inductor Manufacturers

Coilcraft www.coilcraft.com

Sumida www.sumida.com

Murata www.murata.com

Toko www.tokoam.com

Input and Output Capacitor Selection

Since the input current waveform to a buck converter is a
square wave, it contains very high frequency components.
It is strongly recommended that a low equivalent series
resistance (ESR) multilayer ceramic capacitor be used to
bypass the BAT pin which is the input for the converter.
Tantalum and aluminum capacitors are not recommended
because of their high ESR. The value of the capacitor on
BAT directly controls the amount of input voltage ripple for
a given load current. Increasing the size of this capacitor
will reduce the input ripple.

To prevent large V

voltage steps during transient

OUT

load conditions, it is also recommended that a ceramic
capacitor be used to bypass V

. A typical value for this

OUT

capacitor is 4.7μF.

Multilayer Ceramic Chip Capacitors (MLCC) typically have
exceptional ESR performance. MLCCs combined with a
carefully laid out board with an unbroken ground plane will
yield very good performance and low EMI emissions.

4081f

21

LTC4081

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APPLICATIO S I FOR ATIO

There are several types of ceramic capacitors with consider­ably different characteristics. Y5V ceramic capacitors have
apparently higher packing density but poor performance
over their rated voltage or temperature ranges. Under
given voltage and temperature conditions, X5R and X7R
ceramic capacitors should be compared directly by case
size rather than specifi ed value for a desired minimum
capacitance. Some manufacturers provide excellent data on
their websites about achievable capacitance. Table 2 shows
a list of several ceramic capacitor manufacturers.

Table 2. Recommended Ceramic Capacitor Manufacturers

Taiyo Yuden www.t-yuden.com

AVX www.avxcorp.com

Murata www.murata.com

TDK www.tdk.com

Board Layout Considerations

To be able to deliver maximum charge current under all
conditions, it is critical that the exposed metal pad on the
backside of the LTC4081’s package has a good thermal
contact to the PC board ground. Correctly soldered to a
2500mm2 double-sided 1 oz. copper board, the LTC4081
has a thermal resistance of approximately 43°C/W. Failure
to make thermal contact between the exposed pad on the
backside of the package and the copper board will result
in thermal resistance far greater than 43°C/W.

Furthermore due to its high frequency switching circuitry,
it is imperative that the input capacitor, BAT pin capaci­tor, inductor, and the output capacitor be as close to the
LTC4081 as possible and that there is an unbroken ground
plane under the LTC4081 and all of its high frequency
components.

22

4081f

PACKAGE DESCRIPTIO

LTC4081

U

DD Package

10-Lead Plastic DFN (3mm × 3mm)

(Reference LTC DWG # 05-08-1699)

3.50 ±0.05

0.675 ±0.05

1.65 ±0.05
(2 SIDES)2.15 ±0.05

PACKAGE
OUTLINE

0.25 ± 0.05

0.50
BSC

2.38 ±0.05

(2 SIDES)

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

NOTE:

1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2).
CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT

2. DRAWING NOT TO SCALE

3. ALL DIMENSIONS ARE IN MILLIMETERS

4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE

MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE

5. EXPOSED PAD SHALL BE SOLDER PLATED

6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE

PIN 1

TOP MARK

(SEE NOTE 6)

0.200 REF

3.00 ±0.10

(4 SIDES)

0.75 ±0.05

1.65 ± 0.10

(2 SIDES)

0.00 – 0.05

R = 0.115

TYP

2.38 ±0.10

(2 SIDES)

BOTTOM VIEW—EXPOSED PAD

106

15

0.25 ± 0.05

0.50 BSC

0.38 ± 0.10

(DD) DFN 1103

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
no responsibility is assumed for its use. Linear Technology Corporation makes no representation that
the interconnection of its circuits as described herein will not infringe on existing patent rights.

However,

4081f

23

LTC4081

TYPICAL APPLICATIO

U

V

(3.75V

TO 5.5V)

Li-Ion Battery Charger with 1.5V Buck Regulator

LTC4081

GND

D1

CHRG

PROG

BAT

SW

500mA

4.2V

L1

1OμH

C

R

PROG

806Ω

PL

10pFR1715k

R2
806k

4081 TA02a

FB

C

BAT

4.7μF

V

OUT

(1.5V/300mA)

C

OUT

4.7μF

+

Li-Ion/
POLYMER
BATTERY

R3

510Ω

CC

R

NOM

100k

C

IN

4.7μF

R

NTC

100k

V

CC

EN_BUCK

NTC

EN_CHRG

MODE

T

Buck Effi ciency vs Load Current

(V

= 1.5V)

OUT

100

EFFICIENCY

80

(Burst)

EFFICIENCY

60

40

EFFICIENCY (%)

20

0

0.01

(PWM)

POWER LOSS
(Burst)

V

BAT

V

OUT

L = 10μH
C = 4.7μF

0.1 10 1001 1000
LOAD CURRENT (mA)

POWER
LOSS
(PWM)

= 3.8V

= 1.5V

4081 TA02b

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

Battery Chargers

LTC3550

Dual Input USB/AC Adapter Li-Ion Battery Charger
with Adjustable Output 600mA Buck Converter

LTC3550-1

Dual Input USB/AC Adapter Li-Ion Battery Charger
with 600mA Buck Converter

LTC4054 Standalone Linear Li-Ion Battery Charger with

Integrated Pass Transistor in ThinSOT

TM

LTC4061 Standalone Li-Ion Charger with Thermistor

Interface

LTC4061-4.4 Standalone Li-Ion Charger with Thermistor

Interface

LTC4062 Standalone Linear Li-Ion Battery Charger with

Micropower Comparator

LTC4063 Li-Ion Charger with Linear Regulator Up to 1A Charge Current, 100mA, 125mV LDO, 3mm × 3mm DFN Package

LTC4080 Standalone 500mA Charger with 300mA

Synchronous Buck

Power Management

LTC3405/LTC3405A 300mA (I

), 1.5MHz, Synchronous Step-Down

OUT

DC/DC Converter

LTC3406/LTC3406A 600mA (I

), 1.5MHz, Synchronous Step-Down

OUT

DC/DC Converter

LTC3411 1.25A (I

), 4MHz, Synchronous Step-Down

OUT

DC/DC Converter

LTC3440 600mA (I

), 2MHz, Synchronous Buck-Boost

OUT

DC/DC Converter

LTC4411/LTC4412 Low Loss PowerPath

TM

Controller in ThinSOT Automatic Switching Between DC Sources, Load Sharing, Replaces ORing Diodes

LTC4413 Dual Ideal Diode in DFN 2-Channel Ideal Diode ORing, Low Forward On-Resistance, Low Regulated

ThinSOT and PowerPath are trademarks of Linear Technology Corporation.

Linear Technology Corporation

24

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone]

●

www.linear.com

Synchronous Buck Converter, Effi ciency: 93%, Adjustable Output: 600mA,
Charge Current: 950mA Programmable, USB Compatible, Automatic Input Power
Detection and Selection

Synchronous Buck Converter, Effi ciency: 93%, Output: 1.875V at 600mA,
Charge Current: 950mA Programmable, USB Compatible, Automatic Input Power
Detection and Selection

Thermal Regulation Prevents Overheating, C/10 Termination

4.2V, ±0.35% Float Voltage, Up to 1A Charge Current, 3mm × 3mm
DFN Package

4.4V (Max), ±0.4% Float Voltage, Up to 1A Charge Current, 3mm × 3mm
DFN Package

Up to 1A Charge Current, Charges from USB Port, Thermal Regulation
3mm × 3mm DFN Package

For 1-Cell Li-Ion/Polymer Batteries; Trickle Charge; Timer Termination +C/10;
Thermal Regulation, Buck Output: 0.8V to V

, Buck Input: 2.7V to 5.5V, 3mm ×

BAT

3mm DFN-10 Package

95% Effi ciency, VIN: 2.7V to 6V, V

= 0.8V, IQ = 20μA, ISD < 1μA,

OUT

ThinSOT Package

95% Effi ciency, VIN: 2.5V to 5.5V, V

= 0.6V, IQ = 20μA, ISD < 1μA,

OUT

ThinSOT Package

95% Effi ciency, VIN: 2.5V to 5.5V, V

= 0.8V, IQ = 60μA, ISD < 1μA,

OUT

MS Package

95% Effi ciency, VIN: 2.5V to 5.5V, V

= 2.5V, IQ = 25μA, ISD < 1μA,

OUT

MS Package

Forward Voltage, 2.5V ≤ V

≤ 5.5V

IN

LT 0707 • PRINTED IN USA

© LINEAR TECHNOLOGY CORPORATION 2007

1000

100

POWER LOSS (mW)

10

1

0.1

0.01

4081f