Final Electrical Specifications
LTC1731-4.2
Lithium-Ion Linear
Battery Charger Controller
December 1999
FEATURES
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DESCRIPTION
The LTC
®
1731-4.2 is a complete constant-current/con-
stant-voltage linear charge controller for lithium-ion bat-
teries. Nickel-cadmium (NiCd) and nickel-metal-hydride
(NiMH) batteries can also be charged with constant
current using external termination. The external sense
resistor sets the charge current with 5% accuracy. An
internal resistor divider and precision reference set the
final float potential with 1% accuracy. The output float
voltage is set internally to 4.2V.
When the input supply is removed, the LTC1731-4.2
automatically enters a low current sleep mode, dropping
the battery drain current to 5µA. An internal comparator
detects the end-of-charge (C/10) condition while a pro-
grammable timer, using an external capacitor, sets the
total charge time. Fully discharged cells are automatically
trickle charged at 10% of the programmed current until
cell voltage exceeds 2.5V.
The LTC1731-4.2 is available in the 8-pin MSOP and SO
packages.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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Complete Linear Charger Controller for
1-Cell Lithium-Ion Batteries
1% Voltage Accuracy
Programmable Charge Current
C/10 Charge Current Detection Output
Programmable Charge Termination Timer
Space Saving 8-Pin MSOP Package
Automatic Sleep Mode When Input Supply
is Removed (Only 7µA Battery Drain)
Automatic Trickle Charging of Low Voltage Cells
Programmable for Constant-Current-Only Mode
APPLICATIONS
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Cellular Phones
Handheld Computers
Charging Docks and Cradles
Programmable Current Source
TYPICAL APPLICATION
500mA Li-Ion Charger
V
IN
5V TO 15V
MBRM120T3
7
1k
V
CC
SENSE
2
CHRG
DRV
8
6
R
SENSE
0.2Ω
Q1
Si9430DY
I
BAT
= 500mA
LTC1731-4.2
3
C
TIMER
0.1µF
TIMER
BAT
PROG
GND
4
1
5
R
PROG
*
19.6k
+
Li-ION
CELL
1731 TA01
*SHUTDOWN INVOKED BY FLOATING THE PROG PIN
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
U
U
U
1µF
10µF
1
LTC1731-4.2
ABSOLUTE
MAXIMUM
RATINGS
Input Supply Voltage (V
CC
) ................................... 13.2V
Input Voltage (SENSE, DRV, BAT,
TIMER, PROG) ....................... – 0.3V to (V
CC
+ 0.3V)
Output Voltage (CHRG) ......................... – 0.3V to 13.2V
PACKAGE/ORDER INFORMATION
TOP VIEW
BAT
CHRG
TIMER
GND
1
2
3
4
8
7
6
5
SENSE
V
CC
DRV
PROG
ORDER PART
NUMBER
BAT 1
LTC1731EMS8-4.2
MS8 PART MARKING
LTKQ
MS8 PACKAGE
8-LEAD PLASTIC MSOP
T
JMAX
= 150°C,
θ
JA
= 180°C/W
Consult factory for Industrial and Military grade parts.
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
CC
= 6V unless otherwise noted.
SYMBOL
V
CC
I
CC
PARAMETER
Input Supply Voltage
Input Supply Current
Charger On, Current Mode
Shutdown Mode
Sleep Mode (Battery Drain Current)
LTC1731-4.2 (5V
≤
V
CC
≤
12V)
R
PROG
= 19.6k, R
SENSE
= 0.2Ω
R
PROG
= 19.6k, R
SENSE
= 0.2Ω
R
PROG
= 97.6k, R
SENSE
= 0.2Ω
V
BAT
= 2V, R
PROG
= 19.6k, I
TRIKL
= (V
CC
– V
SENSE
)/0.2Ω
From Low to High
From Low to High
PROG Pin Low to High
PROG Pin High to Low
CONDITIONS
q
q
q
q
q
q
q
q
ELECTRICAL CHARACTERISTICS
V
BAT
I
BAT
Regulated Output Voltage
Current Mode Charge Current
I
TRIKL
V
TRIKL
V
UV
∆V
UV
V
MSD
Trickle Charge Current
Trickle Charge Threshold Voltage
V
CC
Undervoltage Lockout Voltage
V
CC
Undervoltage Lockout Hysteresis
Manual Shutdown Threshold Voltage
2
U
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W
W W
U
W
(Note 1)
Operating Temperature Range (Note 2) .....– 40° to 85°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
TOP VIEW
8
7
6
5
SENSE
V
CC
DRV
PROG
ORDER PART
NUMBER
LTC1731ES8-4.2
S8 PART MARKING
173142
CHRG 2
TIMER 3
GND 4
S8 PACKAGE
8-LEAD PLASTIC SO
T
JMAX
= 150°C,
θ
JA
= 125°C/W
MIN
4.5
TYP
1
1
7
MAX
12
3
2
20
4.242
535
585
130
95
2.55
4.5
UNITS
V
mA
mA
µA
V
mA
mA
mA
mA
V
V
mV
V
V
4.158
465
415
70
30
2.4
4.2
500
100
50
2.457
4.1
200
2.457
2.446
LTC1731-4.2
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
CC
= 6V unless otherwise noted.
SYMBOL
V
ASD
V
DIS
I
PROG
V
PROG
I
CHRG
V
CHRG
I
C/10
t
TIMER
V
CLAMP
PARAMETER
Automatic Shutdown Threshold Voltage
Voltage Mode Disable Threshold Voltage
PROG Pin Current
PROG Pin Load Regulation
PROG Pin Voltage
CHRG Pin Weak Pull-Down Current
CHRG Pin Output Low Voltage
End of Charge Indication Current Level
TIMER Accuracy
DRV Pin Clamp Voltage
CONDITIONS
(V
CC
– V
BAT
) High to Low
(V
CC
– V
BAT
) Low to High
V
TIMER
Internal Pull-Up Current, No R
PROG
PROG Pin Source Current,
∆V
PROG
≤
5mV
R
PROG
=19.6k
V
CHRG
= 1V
I
CHRG
= 5mA
R
PROG
= 19.6k, R
SENSE
= 0.2Ω
C
TIMER
= 0.1µF
V
CLAMP
= V
CC
– V
DRV
, I
DRIVE
= 50µA
q
q
q
ELECTRICAL CHARACTERISTICS
MIN
30
40
5.6
TYP
54
69
2.5
MAX
90
100
UNITS
mV
mV
V
µA
µA
V
300
2.457
70
25
100
0.6
50
10
6.5
130
1.2
100
µA
V
mA
%
V
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LTC1731-4.2 is guaranteed to meet performance
specifications from 0°C to 70°C. Specifications over the – 40°C to 85°C
operating temperature range are assured by design, characterization and
correlation with statistical process controls. Guaranteed I grade parts are
available, consult factory.
PIN FUNCTIONS
BAT (Pin 1):
Battery Sense Input. A bypass capacitor of at
least 10µF is required to keep the loop stable when the
battery is not connected. A precision internal resistor
divider sets the final float potential on this pin. The resistor
divider is disconnected in sleep mode.
CHRG (Pin 2):
Open-Drain Charge Status Output. 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 for at least 0.32
seconds, the N-channel MOSFET turns off and a 100µA
current source is connected from the CHRG pin to GND.
When the timer runs out or the input supply is removed,
the current source will be disconnected and the CHRG pin
is forced into a high impedance state.
TIMER (Pin 3):
Timer Capacitor and Constant-Voltage
Mode Disable Input Pin. The timer period is set by placing
a capacitor, C
TIMER
, to GND. The timer period is t
TIMER
=
(C
TIMER
• 3 hours)/(0.1µF). When the TIMER pin is
connected to V
CC
, the constant-voltage mode is disabled
and the chip will operate in constant-current mode only.
Short the TIMER pin to GND to disable the internal timer
function.
GND (Pin 4):
Ground.
PROG (Pin 5):
Charge Current Program and Shutdown
Input Pin. The charge current is programmed by connect-
ing a resistor, R
PROG
to ground. The charge current is I
BAT
= (V
PROG
• 800Ω)/(R
PROG
• R
SENSE
). The IC can be forced
into shutdown by floating the PROG pin and allowing the
internal 2.5µA current source to pull the pin above the
2.457V shutdown threshold voltage.
DRV (Pin 6):
Drive Output Pin for the P-Channel MOSFET
or PNP Transistor. The impedance is high at this pin,
therefore, a high gain PNP pass transistor should be used.
The DRV pin is internally clamped to 8V (max) below V
CC
.
V
CC
(Pin 7):
Positive Input Supply Voltage. When V
BAT
is
within 54mV of V
CC
, the LTC1731-4.2 is forced into sleep
mode, dropping I
CC
to 7µA. V
CC
ranges from 4.5V to 12V.
Bypass this pin with a 1µF capacitor.
SENSE (Pin 8):
Current Sense Input. A sense resistor,
R
SENSE
, must be connected from V
CC
to the SENSE pin.
This resistor is chosen using the following equation:
R
SENSE
= (V
PROG
• 800Ω)/(R
PROG
• I
BAT
)
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3
LTC1731-4.2
BLOCK DIAGRA
2
CHRG
C/10
STOP
C/10
100µA
SHDN
LOGIC
SLP
720Ω
3
OSCILLATOR
V
REF
COUNTER
+
C3
+
A1
–
–
2.5µA
V
REF
2.457V
CHARGE
PROG
R
PROG
GND
5
BATTERY CURRENT I
BAT
= (2.457V • 800Ω)/(R
PROG
• R
SENSE
)
4
+
V
CC
VA
–
+
TIMER
LBO
C2
–
+
–
W
V
CC
7
R
SENSE
+
C1
80Ω
800Ω
SENSE
8
–
C4
+
–
+
–
CA
54mV
DRV
BAT
6
1
4
1731 BD
LTC1731-4.2
OPERATIO
The LTC1731-4.2 is a linear battery charger controller. The
charge current is programmed by the combination of a
program resistor (R
PROG
) from the PROG pin to ground
and a sense resistor (R
SENSE
) between the V
CC
and SENSE
pins. R
PROG
sets a program current through an internal
trimmed 800Ω resistor setting up a voltage drop from V
CC
to the input of the current amplifier (CA). The current
amplifier servos the gate of the external P-channel MOSFET
to force the same voltage drop across R
SENSE
which sets
the charge current. When the potential at the BAT pin
approaches the preset float voltage, the voltage amplifier
(VA) will start sinking current which shrinks the voltage
drop across R
SENSE
, thus reducing the charge current.
Charging begins when the potential at V
CC
pin rises above
the UVLO level and a program resistor is connected from
the PROG pin to ground. At the beginning of the charge
cycle, if the battery voltage is below 2.457V per cell, the
charger goes into trickle charge mode. The trickle charge
current is 10% of the full-scale current. If the cell voltage
stays low for one quarter of the total charge time, the
charge sequence will be terminated immediately.
The charger goes into the fast charge constant-current
mode after the voltage on the BAT pin rises above 2.457V
per cell. In constant-current mode, the charge current is
set by the combination of R
SENSE
and R
PROG
.
APPLICATIONS INFORMATION
Charge Termination
The charger is off when any of the following conditions
exist: the voltage at the V
CC
pin is below 4.1V, the voltage
at the V
CC
pin is higher than 4.1V but is less than 54mV
above V
BAT
, or the PROG pin is floating. The DRV pin will
be pulled to V
CC
and the internal resistor divider is discon-
nected to reduce the current drain on the battery.
Undervoltage Lockout (UVLO)
An internal undervoltage lockout circuit monitors the input
voltage and keeps the charger in shutdown mode until V
CC
rises above 4.1V. To prevent oscillation around
V
CC
= 4.1V, the UVLO circuit has built-in hysteresis.
Shutdown
The LTC1731-4.2 can be forced into shutdown by float-
ing the PROG pin and allowing the internal 2.5µA current
source to pull the pin above the 2.457V shutdown
Trickle Charge and Defective Battery Detection
At the beginning of the charging sequence, if the battery
voltage is low (below 2.457V per cell) the charger goes
into trickle mode. The charge current is dropped to 10% of
the full-scale current. If the low cell voltage persists for one
quarter of the total charging time, the battery is considered
defective, the charging will be terminated and the CHRG
pin output is forced to a high impedance state.
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When the battery approaches the final float voltage, the
charge current will begin to decrease. When the current
drops to 10% of the full-scale charge current, an internal
comparator will turn off the pull-down N-channel MOSFET
at the CHRG pin and connect a weak current source to
ground to indicate an end-of-charge (C/10) condition.
An external capacitor on the TIMER pin sets the total
charge time. After a time-out occurs, the charging will be
terminated immediately and the CHRG pin is forced to a
high impedance state. To restart the charge cycle, simply
remove the input voltage and reapply it, or float the PROG
pin momentarily.
For batteries like lithium-ion that require accurate final
float potential, the internal 2.457V reference, voltage
amplifier and the resistor divider provide regulation with
±1%
(max) accuracy. For NiMH and NiCd batteries, the
LTC1731-4.2 can be turned into a current source by
pulling the TIMER pin to V
CC
. When in the constant-
current only mode, the voltage amplifier, timer and the
trickle charge function are all disabled.
When the input voltage is not present, the charger goes
into a sleep mode, dropping I
CC
to 5µA. This greatly
reduces the current drain on the battery and increases the
standby time. The charger can be shut down by floating the
PROG pin. An internal current source will pull it high and
clamp at 3.5V.
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