LTC4081
500mA Li-Ion Charger
with NTC Input and
300mA Synchronous Buck
DESCRIPTION
FEATURES
Battery Charger:
nn Constant-Current/Constant-Voltage Operation
with Thermal Feedback to Maximize Charge Rate
without Risk of Overheating
nn Internal 4.5-Hour Safety Timer for Termination
nn Charge Current Programmable Up to 500mA with
5% Accuracy
nn NTC Thermistor Input for Temperature Qualified
Charging
nn C/10 Charge Current Detection Output
nn 5µA Supply Current in Shutdown Mode
Switching Regulator:
nn High Efficiency Synchronous Buck Converter
nn 300mA Output Current (Constant-Frequency Mode)
nn 2.7V to 4.5V Input Range (Powered from BAT Pin)
nn 0.8V to V
BAT Output Range
nn MODE Pin Selects Fixed (2.25MHz) Constant-Frequency
PWM Mode or Low ICC (23µA) Burst Mode® Operation
nn 2µA BAT Current in Shutdown Mode
nn 10-Lead, Low Profile (0.75 mm) 3mm × 3mm DFN Package
APPLICATIONS
The LTC®4081 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 specifically designed to work within USB
power specifications.
The CHRG pin indicates when charge current has dropped
to ten percent of its programmed value (C/10). An internal
4.5-hour timer terminates the charge cycle. The fullfeatured 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 high
efficiency Burst Mode operation that can be selected by
the MODE pin.
The LTC4081 is available in a 10-lead, low profile (0.75mm)
3mm × 3mm DFN package.
Wireless Headsets
nn Bluetooth Applications
nn Portable MP3 Players
nn Multifunction Wristwatches
nn
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks
and ThinSOT and PowerPath are trademarks of Linear Technology Corporation. All other
trademarks are the property of their respective owners. Protected by U.S. Patents, including
6522118.
TYPICAL APPLICATION
Buck Efficiency vs Load Current
(VOUT = 1.8V)
Li-Ion Battery Charger with 1.8V Buck Regulator
100
1000
510Ω
CHRG
500mA
BAT
100k
4.7μF
4.7μF
EN_BUCK
NTC LTC4081
SW
1OμH
10pF
EN_CHRG
T
VOUT
(1.8V/300mA)
MODE GND PROG
806Ω
60
40
1M
FB
100k
4.2V
Li-Ion/
POLYMER
BATTERY
+
806k
4.7μF
4081 TA01a
20
0
0.01
EFFICIENCY
(Burst)
EFFICIENCY
(PWM)
100
POWER
LOSS 10
(PWM)
POWER LOSS
(Burst)
1
POWER LOSS (mW)
VCC
EFFICIENCY (%)
80
VCC
(3.75V
TO 5.5V)
VBAT = 3.8V 0.1
VOUT = 1.8V
L = 10μH
C = 4.7μF
0.01
0.1
1
10
100
1000
LOAD CURRENT (mA)
4081 TA01b
4081fa
For more information www.linear.com/LTC4081
1
LTC4081
ABSOLUTE MAXIMUM RATINGS
(Note 1)
VCC, t < 1ms and Duty Cycle < 1%................ –0.3V to 7V
VCC Steady State........................................... –0.3V to 6V
BAT, CHRG.................................................... –0.3V to 6V
EN_CHRG, PROG, NTC.....................–0.3V to VCC + 0.3V
MODE, EN_BUCK............................ –0.3V to VBAT + 0.3V
FB ................................................................. –0.3V to 2V
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
PIN CONFIGURATION
TOP VIEW
10 SW
BAT
1
VCC
2
EN_CHRG
3
PROG
4
7 FB
NTC
5
6 CHRG
9 EN_BUCK
11
8 MODE
DD PACKAGE
10-LEAD (3mm × 3mm) PLASTIC DFN
TJMAX = 110°C, θJA = 43°C/W (NOTE 3)
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
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 specified with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL
CHARACTERISTICS
The l denotes specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C, VCC = 5V, VBAT = 3.8V, VEN_CHRG = 0V, VNTC = 0V, VEN_BUCK = VBAT, VMODE = 0V. (Note 2)
SYMBOL
PARAMETER
CONDITIONS
VCC
Battery Charger Supply Voltage
(Note 4)
l
VBAT
Input Voltage for the Switching Regulator (Note 5)
l
ICC
Quiescent Supply Current (Charger On,
Switching Regulator Off)
VBAT = 4.5V (Forces IBAT and IPROG = 0), VEN_BUCK = 0
ICC_SD
Supply Current in Shutdown (Both Battery
Charger and Switching Regulator Off)
IBAT_SD
Battery Current in Shutdown (Both Battery
Charger and Switching Regulator Off)
2
MIN
TYP
MAX
UNITS
3.75
5
5.5
V
2.7
3.8
4.5
V
l
110
300
µA
VEN_CHRG = 5V, VEN_BUCK = 0, VCC > VBAT
VEN_CHRG = 4V, VEN_BUCK = 0, VCC (3.5V) < VBAT (4V)
l
5
2
10
µA
µA
VEN_CHRG = 5V, VEN_BUCK = 0, VCC > VBAT
VEN_CHRG = 4V, VEN_BUCK = 0, VCC (3.5V) < VBAT (4V)
l
0.6
2
5
µA
µA
4081fa
For more information www.linear.com/LTC4081
LTC4081
ELECTRICAL
CHARACTERISTICS
The l denotes specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C, VCC = 5V, VBAT = 3.8V, VEN_CHRG = 0V, VNTC = 0V, VEN_BUCK = VBAT, VMODE = 0V. (Note 2)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
V
Battery Charger
IBAT = 2mA
IBAT = 2mA, 4.3V < VCC < 5.5V
l
4.179
4.158
4.2
4.2
4.221
4.242
RPROG = 4k; Current Mode; VEN_BUCK = 0
RPROG = 0.8k; Current Mode; VEN_BUCK = 0
l
l
90
475
100
500
110
525
mA
mA
VUVLO_CHRG VCC Undervoltage Lockout Voltage
VCC Rising
VCC Falling
l
l
3.5
2.8
3.6
3.0
3.7
3.2
V
V
VPROG
PROG Pin Servo Voltage
0.8k ≤ RPROG ≤ 4k
l
0.98
1.0
1.02
V
VASD
Automatic Shutdown Threshold Voltage
(VCC – VBAT), VCC Low to High
(VCC – VBAT), VCC High to Low
60
15
82
32
100
45
mV
mV
tSS_CHRG
Battery Charger Soft-Start Time
ITRKL
Trickle Charge Current
VBAT = 2V, RPROG = 0.8k
VBAT Rising
VFLOAT
IBAT
VBAT Regulated Output Voltage
Current Mode Charge Current
180
35
µs
50
65
mA
2.75
2.9
3.05
V
100
150
350
mV
130
mV
VTRKL
Trickle Charge Threshold Voltage
VTRHYS
Trickle Charge Threshold Voltage Hysteresis
DVRECHRG
Recharge Battery Threshold Voltage
VFLOAT – VBAT, 0°C < TA < 85°C
70
100
DVUVCL1,
DVUVCL2
(VCC – VBAT) Undervoltage Current Limit
Threshold Voltage
IBAT = 0.9 ICHG
IBAT = 0.1 ICHG
180
90
300
130
tTIMER
Charge Termination Timer
l
3
4.5
6
hrs
Recharge Time
l
1.5
2.25
3
hrs
1.5
hrs
l
mV
mV
Low-Battery Charge Time
VBAT = 2.5V
l
0.75
1.125
IC/10
End of Charge Indication Current Level
RPROG = 2k (Note 6)
l
0.085
0.1
0.115 mA/mA
TLIM
Junction Temperature in ConstantTemperature Mode
115
°C
RON_CHRG
Power FET On-Resistance
(Between VCC and BAT)
IBAT = 350mA, VCC = 4V
700
mW
fBADBAT
Defective Battery Detection CHRG
Pulse Frequency
VBAT = 2V
2
Hz
DBADBAT
Defective Battery Detection CHRG
Pulse Frequency Duty Ratio
VBAT = 2V
75
%
VNTC = 2.5V
INTC
NTC Pin Current
VCOLD
Cold Temperature Fault Threshold Voltage Rising Voltage Threshold
Hysteresis
0.76 • VCC
0.015 • VCC
1
µA
V
V
VHOT
Hot Temperature Fault Threshold Voltage
Falling Voltage Threshold
Hysteresis
0.35 • VCC
0.017 • VCC
V
V
VDIS
NTC Disable Threshold Voltage
Falling Threshold; VCC = 5V
Hysteresis
fNTC
DNTC
82
50
mV
mV
Fault Temperature CHRG Pulse Frequency
2
Hz
Fault Temperature CHRG Pulse Frequency
Duty Ratio
25
%
4081fa
For more information www.linear.com/LTC4081
3
LTC4081
ELECTRICAL
CHARACTERISTICS
The l denotes specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C, VCC = 5V, VBAT = 3.8V, VEN_CHRG = 0V, VNTC = 0V, VEN_BUCK = VBAT, VMODE = 0V. (Note 2)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
0.78
0.80
0.82
V
50
nA
Buck Converter
VFB
FB Servo Voltage
IFB
FB Pin Input Current
fOSC
Switching Frequency
IBAT_NL_CF
No-Load Battery Current (Continuous
Frequency Mode)
No-Load for Regulator, VEN_CHRG = 5V,
L = 10µH, C = 4.7µF
1.9
mA
IBAT_NL_BM
No-Load Battery Current (Burst Mode
Operation)
No-Load for Regulator, VEN_CHRG = 5V,
MODE = VBAT, L = 10µH, C = 4.7µF
23
µA
IBAT_SLP
Battery Current in SLEEP Mode
VEN_CHRG = 5V, MODE = VBAT,
VOUT > Regulation Voltage
l
10
15
20
µA
VBAT Rising
VBAT Falling
l
l
2.6
2.4
2.7
2.5
2.8
2.6
V
V
VUVLO_BUCK Buck Undervoltage Lockout Voltage
l
VFB = 0.85V
–50
l
1.8
2.25
2.75
MHz
W
RON_P
PMOS Switch On-Resistance
0.95
RON_N
NMOS Switch On-Resistance
ILIM_P
PMOS Switch Current Limit
ILIM_N
NMOS Switch Current Limit
700
mA
IZERO_CF
NMOS Zero Current in Normal Mode
15
mA
IPEAK
Peak Current in Burst Mode Operation
MODE = VBAT
50
IZERO_BM
Zero Current in Burst Mode Operation
MODE = VBAT
20
tSS_BUCK
Buck Soft-Start Time
From the Rising Edge of EN_BUCK to 90%
of Buck Regulated Output
VIH
Input High Voltage
EN_CHRG, EN_BUCK, MODE Pin Low to High
l
VIL
Input Low Voltage
EN_CHRG, EN_BUCK, MODE Pin High to Low
l
VOL
Output Low Voltage (CHRG)
ISINK = 5mA
l
IIH
Input Current High
EN_BUCK, MODE Pins at 5.5V, VBAT = 5V
l
IIL
Input Current Low
EN_CHRG, EN_BUCK, MODE Pins at GND
l
REN_CHRG
EN_CHRG Pin Input Resistance
VEN_CHRG = 5V
ICHRG
CHRG Pin Leakage Current
VBAT = 4.5V, VEN_CHRG = 5V
W
0.85
375
520
700
mA
100
150
mA
35
50
mA
400
µs
Logic
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 specifications
from 0°C to 85°C. Specifications 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.
4
1.2
V
60
105
mV
–1
1
µA
–1
1
µA
3.3
MW
1
µA
1
l
V
0.4
1.45
Note 4: Although the LTC4081 charger functions properly at 3.75V, full
charge current requires an input voltage greater than the desired final
battery voltage per ∆VUVCL1 specification.
Note 5: The 2.8V maximum buck undervoltage lockout (VUVLO_BUCK) exit
threshold must first be exceeded before the minimum VBAT specification
applies.
Note 6: IC/10 is expressed as a fraction of measured full charge current
with indicated PROG resistor.
4081fa
For more information www.linear.com/LTC4081
LTC4081
TYPICAL PERFORMANCE CHARACTERISTICS
(TA = 25°C, VCC = 5V, VBAT = 3.8V, unless otherwise specified)
Battery Regulation (Float) Voltage
vs Charge Current
4.21
4.210
RPROG = 2k
4.20
4.200
4.18
4.17
4.16
4.15
4.15
4.195
FLOAT VOLTAGE (V)
FLOAT VOLTAGE (V)
4.19
4.190
4.185
4.180
4.175
4.10
4.05
4.00
4.170
3.95
4.14
4.165
3.90
4.13
4.160
– 50 – 30
0
50
200
150
100
CHARGE CURRENT (mA)
250
Battery Regulation (Float) Voltage
vs VCC Supply Voltage
4.25
4.205
4.20
FLOAT VOLTAGE (V)
Battery Regulation (Float) Voltage
vs Temperature
50
30
10
TEMPERATURE (°C)
70
– 10
4081 G01
3.85
90
4
4081 G02
PROG Pin Voltage
vs Charge Current
0.9
THERMAL CONTROL
LOOP IN OPERATION
100
Charger FET On-Resistance
vs Temperature
VCC = 4V
0.8 IBAT = 350mA
RPROG = 2k
0.7
0.6
0.6
RDS(ON) (Ω)
150
5
5.5
VCC SUPPLY VOLTAGE (V)
4081 G03
0.8
VPROG (V)
200
1.0
VCC = 6V
VBAT = 3V
RPROG = 2k
0.4
0.5
0.4
0.3
0.2
50
0.2
0.1
0
–25
0
25
50
75
100
125
25
0
50 75 100 125 150 175 200
CHARGE CURRENT (mA)
TEMPERATURE (°C)
4081 G04
50
30
10
TEMPERATURE (°C)
–10
70
90
4081 G06
EN_CHRG, EN_BUCK and
MODE Pin Threshold Voltage
vs Temperature
EN_CHRG Pin Pull-Down
Resistance vs Temperature
1.7
0.95
0.90
RISING
0.85
0.80
FALLING
0.75
0.70
0.65
0.60
0.55
0.50
–50
0
– 50 – 30
4081 G05
PULLDOWN RESISTANCE (MΩ)
0
–50
THRESHOLD VOLTAGE (V)
CHARGE CURRENT (mA)
250
Charge Current vs Temperature
with Thermal Regulation
(Constant-Current Mode)
6
4.5
–30
–10 10
30
50
TEMPERATURE (°C)
70
90
1.6
1.5
1.4
1.3
1.2
1.1
1.0
–50
–30
4081 G07
–10 10
30
50
TEMPERATURE (°C)
70
90
4081 G08
4081fa
For more information www.linear.com/LTC4081
5
LTC4081
TYPICAL PERFORMANCE CHARACTERISTICS
(TA = 25°C, VCC = 5V, VBAT = 3.8V, unless otherwise specified)
80
CHRG Pin Output
Low Voltage vs Temperature
Normalized Charge Termination
Time vs Temperature
2.28
1.05
ICHRG = 5mA
70
50
40
30
20
2.27
1.00
FREQUENCY (MHz)
NORMALIZED TIMER PERIOD
VOLTAGE (mV)
60
0.95
0.90
0.85
–10 10
30
50
TEMPERATURE (°C)
70
0.80
–50
90
–30
–10
10
30
50
TEMPERATURE (°C)
Buck Oscillator Frequency
vs Temperature
100
EFFICIENCY (%)
2.1
2.0
1.9
60
40
20
1.8
20 40 60
–60 –40 –20 0
TEMPERATURE (°C)
80
0
0.01
100
4081 G12
4.5
1000
100
100
80
POWER
LOSS 10
(PWM)
1
POWER LOSS
(BURST)
VBAT = 3.8V
0.1
VOUT = 1.8V
L = 10μH
C = 4.7μF
0.01
0.1
1
10
100
1000
LOAD CURRENT (mA)
1000
EFFICIENCY
(BURST)
EFFICIENCY
(PWM)
60
40
100
POWER
LOSS 10
(PWM)
POWER LOSS (mW)
EFFICIENCY
(BURST)
EFFICIENCY
(PWM)
80
VBAT = 2.7V
3.5
4.0
BATTERY VOLTAGE (V)
Buck Efficiency vs Load Current
(VOUT = 1.5V)
POWER LOSS (mW)
FREQUENCY (MHz)
VBAT = 4.5V
2.2
3.0
4081 G11
Buck Efficiency vs Load Current
(VOUT = 1.8V)
2.4
2.3
2.24
4081 G10
4081 G09
VBAT = 3.8V
2.25
2.22
2.5
90
70
EFFICIENCY (%)
–30
2.26
2.23
10
0
–50
Buck Oscillator Frequency
vs Battery Voltage
1
POWER LOSS
(BURST)
VBAT = 3.8V
0.1
VOUT = 1.5V
L = 10μH
C = 4.7μF
0.01
0.1
1
10
100
1000
LOAD CURRENT (mA)
4081 G14
20
0
0.01
4081 G13
1.805
IOUT = 1mA
VOUT SET FOR 1.8V
1.800
1.810
Burst Mode
OPERATION
BUCK OUTPUT VOLTAGE (V)
BUCK OUTPUT VOLTAGE (V)
1.810
PWM MODE
1.795
1.790
1.785
1.780
2.5
3.0
3.5
4.0
BATTERY VOLTAGE (V)
4.5
4081 G15
6
1.805
Buck Output Voltage
vs Temperature
IOUT = 1mA
VOUT SET FOR 1.8V
1.800
35
Burst Mode
OPERATION
PWM MODE
1.795
1.790
1.785
1.780
–50 –30
No-Load Buck Input Current
(Burst Mode Operation)
vs Battery Voltage
30
BUCK INPUT CURRENT (μA)
Buck Output Voltage
vs Battery Voltage
IOUT = 1mA
VOUT = 1.8V
L = 10μH
25
20
15
10
5
30
50
–10 10
TEMPERATURE (°C)
70
90
4081 G16
0
2.5
3.5
3.0
4.0
BATTERY VOLTAGE (V)
4.5
4081 G17
4081fa
For more information www.linear.com/LTC4081
LTC4081
TYPICAL PERFORMANCE CHARACTERISTICS
(TA = 25°C, VCC = 5V, VBAT = 3.8V, unless otherwise specified)
1.2
VBAT = 4.2V
Buck Main Switch (PMOS)
On-Resistance vs Temperature
1.2
20
VBAT = 2.7V
15
10
1.0
0.8
ON-RESISTANCE (Ω)
25
0.6
0.4
0.2
5
30
50
–10 10
TEMPERATURE (°C)
70
0
2.5
90
0.8
0.6
0.4
0.2
3.0
3.5
4.5
4.0
BATTERY VOLTAGE (V)
0
–50 –30
5.0
4081 G18
30
50
–10 10
TEMPERATURE (°C)
1.0
1.0
ON-RESISTANCE (Ω)
1.2
0.6
0.4
0.2
0.8
0.6
0.4
0.2
0
2.5
3.0
3.5
4.5
4.0
BATTERY VOLTAGE (V)
0
–50 –30
5.0
30
50
–10 10
TEMPERATURE (°C)
70
4081 G21
Maximum Output Current
(PWM Mode) vs Battery Voltage
Maximum Output Current (Burst
Mode Operation) vs Battery Voltage
L = 10μH
80
VOUT SET FOR 1.8V
400
300
200
100
2.7
3
3.3
3.6
3.9
4.2
90
4081 G22
MAXIMUM OUTPUT CURRENT (mA)
500
90
Buck Synchronous Switch (NMOS)
On-Resistance vs Temperature
1.2
0.8
70
4081 G20
4081 G19
Buck Synchronous Switch (NMOS)
On-Resistance vs Battery Voltage
ON-RESISTANCE (Ω)
0
–50 –30
Buck Main Switch (PMOS)
On-Resistance vs Battery Voltage
1.0
VBAT = 3.8V
ON-RESISTANCE (Ω)
30
L = 10μH
C = 4.7μF
VOUT = 1.8V
MAXIMUM OUTPUT CURRENT (mA)
NO LOAD INPUT CURRENT (μA)
35
No-Load Buck Input Current
(Burst Mode Operation)
vs Temperature
4.5
L = 10μH
70
60
VOUT SET FOR 1.8V
50
40
30
20
10
0
2.7
3
3.3
3.6
3.9
4.2
4.5
BATTERY VOLTAGE (V)
BATTERY VOLTAGE (V)
4081 G23
4081 G24
4081fa
For more information www.linear.com/LTC4081
7
LTC4081
TYPICAL PERFORMANCE CHARACTERISTICS
(TA = 25°C, VCC = 5V, VBAT = 3.8V, unless otherwise specified)
Output Voltage Waveform
when Switching Between Burst
and PWM Mode (ILOAD = 10mA)
Output Voltage Transient
Step Response (PWM Mode)
Output Voltage Transient
Step Response (Burst Mode
Operation)
VOUT
20mV/DIV
AC COUPLED
VOUT
50mV/DIV
AC COUPLED
VOUT
20mV/DIV
AC COUPLED
ILOAD
250mA/DIV
VMODE
5V/DIV
ILOAD
50mA/DIV
0mA
0V
0mA
50μs/DIV
4081 G25
4081 G26
50μs/DIV
Buck VOUT Soft-Start
(ILOAD = 50mA)
50μs/DIV
4081 G27
Charger VPROG Soft-Start
VOUT
1V/DIV
0V
VPROG
200mV/DIV
VEN_BUCK
5V/DIV
0V
0V
200μs/DIV
8
4081 G28
50μs/DIV
4081 G29
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LTC4081
PIN FUNCTIONS
BAT (Pin 1): Charge Current Output and Buck Regulator
Input. Provides charge current to the battery and regulates
the final float voltage to 4.2V. An internal precision resistor
divider from this pin sets the float 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. VCC can range from 3.75V
to 5.5V. This pin should be bypassed with at least a 1µF
capacitor. When VCC is less than 32mV above the BAT
pin voltage, the battery charger enters shutdown mode.
EN_CHRG (Pin 3): Enable Input Pin for the Battery Charger.
Pulling this pin above the manual shutdown threshold
(VIH) puts the LTC4081 charger in shutdown mode, thus
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, RPROG, to
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:
IBAT =
VPROG
•400
RPROG
NTC (Pin 5): Input to the NTC (negative temperature coefficient) 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 VCC. When the voltage at this pin drops below 0.35 •
VCC at hot temperatures or rises above 0.76 • VCC at cold,
charging is suspended, the internal timer is frozen and the
CHRG pin output will start to pulse at 2Hz. Pulling this
pin below 0.016 • VCC disables the NTC feature. There is
approximately 3°C of temperature hysteresis associated
with each of the input comparator’s thresholds.
CHRG (Pin 6): Open-Drain Charge Status Output. The
charge status indicator pin has three states: pull-down,
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 CHRG pin is pulled low by
an internal N-channel MOSFET. When the charge current
drops to 10% of the full-scale current, the CHRG pin is
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 CHRG
pin pulses at a frequency of 2Hz with 75% duty cycle.
When the NTC pin voltage rises above 0.76 • VCC or drops
below 0.35 • VCC, the CHRG pin pulses at a frequency of
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
float this pin.
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 float 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.
4081fa
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9
LTC4081
BLOCK DIAGRAM
2
VCC
+
EN_CHRG
0.82V
CHARGER
SHUTDOWN
C3
–
MP3
MP1
X1
REN
D3
X400
D1
0.1V
+
–
D2
C1
–
CA
TDIE
BAT
MA
R1
+
VA
1.22V
1V
PULSE
LOGIC
+
+
MP4
6
CHRG
115C
1
–
+
PROG
–
TA
–
3
R2
CHARGER
ENABLE
0.1V
+
2.9V
C2
–
BAT
BADBAT
+
VCC
C4
3.6V
4
UVLO
–
PROG
RPROG
+
VCC
VCC
C5
R9
–
RNOM
5
VBAT + 80mV
C8
TOO COLD
SUSPEND
+
NTC
R10
C9
LOGIC
CHARGER
OSCILLATOR
–
T RNTC
CHARGE
CONTROL
–
COUNTER
TOO HOT
+
R11
+
C10
NTC_EN
–
R12
LINEAR BATTERY CHARGER
+
9
EN_BUCK
0.82V
–
MP2
SYNCHRONOUS BUCK CONVERTER
C6
L1
PWM
CONTROL
AND DRIVE
ENABLE BUCK
SW
CPL
MN1
+
0.82V
–
C7
2.25MHz
BUCK
OSCILLATOR
11
GND
10
R7
COUT
7
ERROR
AMP
+
MODE
–
8
VOUT
10
FB
0.8V
R8
4081 BD
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LTC4081
OPERATION
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
final float voltage of 4.2V ±0.5%. The CHRG open-drain
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 VCC pin
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 EN_CHRG pin is pulled
below the shutdown threshold (VIL).
When the BAT pin approaches the final float 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 the
CHRG pin, and the pin becomes high impedance.
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 benefit of the
thermal limit is that charge current can be set 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, tTIMER (typically 4.5 hours). When this time elapses, the charge cycle
terminates and the CHRG pin assumes a high impedance
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 EN_CHRG pin above VIH. A new
charge cycle will automatically restart if the BAT pin voltage falls below VRECHRG (typically 4.1V).
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. Three of the amplifier
feedback loops shown control the constant-current, CA,
constant-voltage, VA, and constant-temperature, TA modes
(see Block Diagram). A fourth amplifier 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.
Amplifiers 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 amplifier 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.
If the die temperature starts to creep up above 115°C
due to internal power dissipation, the transconductance
amplifier, 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
For more information www.linear.com/LTC4081
4081fa
11
LTC4081
OPERATION
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/RPROG. If the power dissipation of the LTC4081
results in the junction temperature approaching 115°C, the
amplifier (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 VCC
input voltage and keeps the battery charger off until VCC
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 5µA and maximum supply current is 10µA.
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 (DVUVCL1). This feature is particularly 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
for one quarter of the total time (1.125 hr), the battery is
assumed to be defective, the charge cycle terminates and
the CHRG pin output pulses at a frequency of 2Hz with
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
12
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 EN_CHRG pin above
the shutdown threshold.
Battery Charger Shutdown Mode
The LTC4081’s battery charger can be disabled by pulling
the EN_CHRG pin above the shutdown threshold (VIH).
In shutdown mode, the battery drain current is reduced
to about 2µA and the VCC supply current to about 5µA
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.
CHRG Status Output Pin
The charge status indicator pin has three states: pull-down,
pulsing at 2Hz (see Trickle Charge and Defective Battery
Detection and Battery Temperature Monitoring) and high
impedance. The pull-down 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 CHRG 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, CHRG will stay in the strong pull-down 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.
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.
4081fa
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LTC4081
OPERATION
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 filter 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.
The CHRG output assumes a strong pull-down state during recharge cycles until C/10 is reached or the recharge
cycle terminates.
Battery Temperature Monitoring via NTC
The battery temperature is measured by placing a negative temperature coefficient (NTC) thermistor close to
the battery pack. The NTC circuitry is shown in Figure 1.
When the charger is in Hold mode (battery temperature
is either too hot or too cold) the CHRG pin pulses in a
2Hz, 25% duty cycle frequency unless the charge task is
finished or the battery is assumed to be defective. If the
NTC pin is grounded, the NTC function will be disabled.
VCC
RNOM
0.76 • VCC
6
NTC
–
TOO COLD
+
SWITCHING REGULATOR OPERATION
–
T RNTC
0.35 • VCC
TOO HOT
+
+
NTC_ENABLE
0.016 • VCC
To use this feature, connect the NTC thermistor, RNTC, between the NTC pin and ground and a resistor, RNOM, from
the NTC pin to VCC. RNOM should be a 1% resistor with a
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 RNOM, which corresponds to approximately 40°C, and
when the value of the NTC thermistor increases to 3.26
times the value of RNOM, which corresponds to approximately 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.
–
4081 F01
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 efficiency
whereas the Burst Mode operation offers higher efficiency
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.
Figure 1. NTC Circuit Information
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13
LTC4081
OPERATION
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 amplifier, 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 flow from ground to the load until either the
next clock cycle begins or the current reduces to the zero
current (IZERO) level.
Oscillator: In constant-frequency mode, the switching
regulator uses a dedicated oscillator which runs at a
fixed frequency of 2.25MHz. This frequency is chosen to
minimize possible interference with the AM radio band.
Error Amplifier: The error amplifier is an internally compensated transconductance (gm) amplifier with a gm of
65 µmhos. The internal 0.8V reference voltage is compared
to the voltage at the FB pin to generate a current signal
at the output of the error amplifier. 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 VIH. In this mode, the internal oscillator is disabled, the error amplifier is converted into a
comparator monitoring the FB voltage, and the inductor
current swings between a fixed IPEAK (~100mA) and IZERO
(35mA) irrespective of the load current as long as the FB
14
pin voltage is less than or equal to the reference voltage
of 0.8V. Once VFB is greater than 0.8V, the control logic
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 20µA from
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 IPEAK and IZERO. The
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 efficiency 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 current
will cause the output voltage to drop out of regulation.
Current Limit
To prevent inductor current runaway, there are absolute
current limits (ILIM) on both the PMOS main switch and
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 amplifier
ever exceeds the PMOS ILIM, the error amplifier will be
ignored and the inductor current will be limited to PMOS
ILIM. In Burst Mode operation, the PMOS current limit
is reduced to 100mA to minimize output voltage ripple.
Zero Current Comparator
The zero or reverse current comparator monitors the inductor current to the output and shuts off the synchronous
rectifier when this current reduces to a predetermined
value (IZERO). In fixed frequency mode, this is set to
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LTC4081
OPERATION
negative 15mA meaning that the regulator allows the
inductor current to flow in the reverse direction (from the
output to ground through the synchronous rectifier) 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 efficiency.
However, in Burst Mode operation, IZERO is set to positive
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
efficiency 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
400µs. The soft-start capacitor is discharged completely
whenever the regulator is disabled.
Short-Circuit Protection
In the event of a short circuit at the output or during
start-up, VOUT will be near zero volts. Since the downward
slope of the inductor current is ~VOUT/L, the inductor
current may not get a chance to discharge enough to
avoid a runaway situation. Because the current sensing
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 VBAT is less than 2.7V, an undervoltage lockout 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 VBAT
drops below 2.5V.
Dropout Operation
When the BAT pin voltage approaches VOUT, the duty cycle
of the switching regulator approaches 100%. When VBAT
is approximately equal to VOUT, the regulator is said to be
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.
Global Thermal Shutdown
The LTC4081 includes a global thermal shutdown which
shuts off the entire device (battery charger and switching regulator) if the die temperature exceeds 160°C. The
LTC4081 resumes normal operation once the temperature
drops approximately 14°C.
4081fa
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15
LTC4081
APPLICATIONS INFORMATION
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:
RPROG = 400 •
1V
IBAT
, IBAT = 400 •
1V
RPROG
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:
IBAT =
PROG
GND
1
2π •100kHz •CPROG
RPROG
CFILTER
CHARGE
CURRENT
MONITOR
CIRCUITRY
Figure 2. Isolating Capacitive Load
on PROG Pin and Filtering
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,
CPROG, the following equation should be used to calculate
the maximum resistance value for RPROG:
RPROG ≤
10k
4081 F02
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 1µF from BAT to
GND. Furthermore, a 4.7µF capacitor with a 0.2W to 1W
series resistor from BAT to GND is required to keep ripple
voltage low when the battery is disconnected.
16
LTC4081
VPROG
•400
RPROG
Stability Considerations
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
filter can be used on the PROG pin to measure the average
battery current as shown in Figure 2. A 10k resistor has
been added between the PROG pin and the filter 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 VUVCL (approximately 300mV
above the battery voltage, DVUVCL), undervoltage charge
current limiting will begin to reduce the charge current in
an attempt to maintain DVUVCL between VCC and BAT. The
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.
Operation from Current Limited Wall Adapter
By using a current limited wall adapter as the input supply, the LTC4081 can dissipate significantly less power
when programmed for a current higher than the limit of
the wall adapter.
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LTC4081
APPLICATIONS INFORMATION
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.
Typically a wall adapter can supply significantly 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.
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., RPROG = 1.33kW)
to ensure that part tolerances maintain 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 0.7W 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.
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 3 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
pull-down resistor.
5V WALL
ADAPTER
(500mA)
USB
POWER
(100mA)
D1
2
MP1
BAT
ICHG
SYSTEM
LOAD
LTC4081
VCC
PROG
MN1 1k
1k
1
+
4
Li-Ion
BATTERY
4k
4081 F03
Figure 3. Combining Wall Adapter and USB Power
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:
PD = ( VCC − VBAT ) •IBAT +PD _ BUCK
Where PD is the total power dissipated within the IC, VCC
is the input supply voltage, VBAT is the battery voltage, IBAT
is the charge current and PD_BUCK is the power dissipation
due to the regulator. PD_BUCK can be calculated as:
1
PD_ BUCK = VOUT •IOUT −1
η
Where VOUT is the regulated output of the switching
regulator, IOUT is the regulator load and η is the regulator
efficiency 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:
TA = 115°C – PDθJA
TA = 115°C – (VCC – VBAT) • IBAT • θJA if the regulator
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:
TA = 115°C – (6V – 3V) • (250mA) • 43°C/W
TA = 115°C – 0.75W • 43°C/W = 115°C – 32.25°C
TA = 82.75°C
4081fa
For more information www.linear.com/LTC4081
17
LTC4081
APPLICATIONS INFORMATION
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:
IBAT =
115°C− TA
( VCC − VBAT ) • θ JA
Using the previous example with an ambient temperature of
85°C, the charge current will be reduced to approximately:
I
BAT
=
115°C− 85°C
30°C
=
= 232.6mA
(6V − 3V ) • 43°C/W 129°C/A
Furthermore, the voltage at the PROG pin will change
proportionally with the charge current as discussed in
the Programming Charge Current section.
VCC Bypass Capacitor
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 power source. Adding a 1W series resistor in series
with an X5R ceramic capacitor will minimize start-up voltage
transients. For more information, refer to Application Note 88.
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 RCOLD to RHOT is about 5 will also work
(Vishay Dale R-T Curve 1 shows a ratio of RCOLD to RHOT of
3.266/0.5325 = 6.13).
18
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 RNOM with respect to RNTC. Increasing
RNOM will move both trip points to higher temperatures. To
calculate RNOM for a shift to lower temperature for example,
use the following equation:
RNOM =
RCOLD
• RNTC at 25°C
3.266
where RCOLD is the resistance ratio of RNTC at the desired cold
temperature trip point. If you want to shift the trip points to
higher temperatures use the following equation:
RNOM =
RHOT
•R
at 25°C
0.5325 NTC
where RHOT is the resistance ratio of RNTC at the desired hot
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 RNOM needed
is calculated as follows:
RCOLD
• RNTC at 25°C
3.266
2.816
=
• 10k = 8.62k
3.266
RNOM =
The nearest 1% value for RNOM is 8.66k. This is the value used
to bias the NTC thermistor to get cold and hot trip points of
approximately 0°C and 40°C respectively. To extend the delta
4081fa
For more information www.linear.com/LTC4081
LTC4081
APPLICATIONS INFORMATION
between the cold and hot trip points, a resistor, R1, can be
added in series with RNTC (see Figure 4). The values of the
resistors are calculated as follows:
RCOLD −RHOT
3.266− 0.5325
0.5325
• (RCOLD −RHOT ) −RHOT
R1 =
3.266− 0.5325
RNOM =
where RNOM is the value of the bias resistor and RHOT and
RCOLD are the values of RNTC at the desired temperature trip
points. Continuing the example from before with a desired
trip point of 50°C:
10k • ( 2.816− 0.4086 )
RCOLD −RHOT
=
3.266− 0.5325
3.266− 0.5325
= 8.8k, 8.87k is the nearest 1% value.
0.5325
• ( 2.816− 0.4086 ) − 0.4086
R1 = 10k •
3.266− 0.5325
= 604W, 604 is the nearest 1% value.
RNOM =
VCC
RNOM
8.87k
0.76 • VCC
6
+
NTC
R1
604Ω
T
–
TOO COLD
–
RNTC
10k
0.35 • VCC
+
TOO HOT
When a 1% resistor is used for RHOT, the major error in the
40°C trip point is determined by the tolerance of the NTC
thermistor. A typical 100k NTC thermistor has ±10% tolerance.
By looking up the temperature coefficient of the thermistor
at 40°C, the tolerance error can be calculated in degrees
centigrade. Consider the Vishay NTHS0603N01N1003J
thermistor, which has a temperature coefficient of –4%/°C at
40°C. Dividing the tolerance by the temperature coefficient,
±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 coefficient of the thermistor at 0°C, TC
(in %/°C), the value of the thermistor at 0°C, RCOLD, and the
value of the thermistor at 40°C, RHOT. The formula is:
1+ TOL R
COLD −1 • 100
•
6.14
RHOT
Temperature Error(°C) =
TC
For example, the Vishay NTHS0603N01N1003J thermistor
with a tolerance of ±5%, TC of –5%/°C and RCOLD/RHOT of
6.13, has a cold trip point error of:
1+0.05
6.14 • 6.13−1 • 100
Temperature Error(°C) =
−5
= −0.95°C, 1.05°C
SWITCHING REGULATOR
+
NTC_ENABLE
0.016 • VCC
NTC Trip Point Error
–
4081 F04
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 amplifier. A voltage divider from
Figure 4. NTC Circuits
4081fa
For more information www.linear.com/LTC4081
19
LTC4081
APPLICATIONS INFORMATION
VOUT to ground (as shown in the Block Diagram) programs
the output voltage via FB using the formula:
R7
VOUT = 0.8V • 1+
R8
Keeping the current low (
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