1
LTC1734
Lithium-Ion Linear
Battery Charger in ThinSOT
The LTC
1734 is a low cost, single cell, constant-current/
constant-voltage Li-Ion battery charger controller. When
combined with a few external components, the SOT-23
package forms a very small, low cost charger for single cell
lithium-ion batteries.
The LTC1734 is available in 4.1V and 4.2V versions with
1% accuracy. Constant current is programmed using a
single external resistor between the PROG pin and ground.
Manual shutdown is accomplished by floating the pro-
gram resistor while removing input power automatically
puts the LTC1734 into a sleep mode. Both the shutdown
and sleep modes drain near zero current from the battery.
Charge current can be monitored via the voltage on the
PROG pin allowing a microcontroller or ADC to read the
current and determine when to terminate the charge cycle.
The output driver is both current limited and thermally
protected to prevent the LTC1734 from operating outside
of safe limits. No external blocking diode is required.
The LTC1734 can also function as a general purpose
current source or as a current source for charging nickel-
cadmium (NiCd) and nickel-metal-hydride (NiMH) batter-
ies using external termination.
s
Low Profile (1mm) ThinSOT
TM
Package
s
No Blocking Diode Required
s
No Sense Resistor Required
s
1% Accurate Preset Voltages: 4.1V or 4.2V
s
Charge Current Monitor Output
for Charge Termination
s
Programmable Charge Current: 200mA to 700mA
s
Automatic Sleep Mode with Input Supply Removal
s
Manual Shutdown
s
Negligible Battery Drain Current in Shutdown
s
Undervoltage Lockout
s
Self Protection for Overcurrent/Overtemperature
s
Cellular Telephones
s
Handheld Computers
s
Digital Cameras
s
Charging Docks and Cradles
s
Low Cost and Small Size Chargers
s
Programmable Current Sources
, LTC and LT are registered trademarks of Linear Technology Corporation.
300mA Li-Ion Battery Charger
PROG Pin Indicates Charge Status
FEATURES
DESCRIPTIO
U
APPLICATIO S
U
TYPICAL APPLICATIO
U
V
IN
5V
I
BAT
= 300mA
1734 TA01
SINGLE
Li-Ion
BATTERY
10
F
+
GND
PROG
DRIVE
BAT
2
4
6
5
LTC1734
V
CC
I
SENSE
3
1
1
F
R
PROG
5k
FMMT549
CHARGING
BEGINS
CHARGING
COMPLETE
1734 TA01b
5V
4V
3V
2V
1V
0V
1.5V
V
BAT
CONSTANT
CURRENT
V
BAT
(V)
V
PROG
(V)
CONSTANT
VOLTAGE
V
PROG
ThinSOT is a trademark of Linear Technology Corporation.
2
LTC1734
ORDER PART
NUMBER
(Note 1)
Supply Voltage (V
CC
) ................................... 0.3V to 9V
Input Voltage (BAT, PROG) ........ 0.3V to (V
CC
+ 0.3V)
Output Voltage (DRIVE) .............. 0.3V to (V
CC
+ 0.3V)
Output Current (I
SENSE
) ................................... 900mA
Short-Circuit Duration (DRIVE) ...................... Indefinite
Junction Temperature .......................................... 125
C
Operating Ambient Temperature Range
(Note 2) ...............................................40
C to 85
C
Operating Junction Temperature (Note 2) ............ 100
C
Storage Temperature Range ................. 65
C to 150
C
Lead Temperature (Soldering, 10 sec).................. 300
C
LTC1734ES6-4.1
LTC1734ES6-4.2
T
JMAX
= 125
C,
JA
= 230
C/W
ABSOLUTE
M
AXI
M
U
M
RATINGS
W
W
W
U
PACKAGE/ORDER I
N
FOR
M
ATIO
N
W
U
U
S6 PART MARKING
LTHD
LTRG
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C. V
CC
= 5V, GND = 0V and V
BAT
is equal to the float voltage unless
otherwise noted. All current into a pin is positive and current out of a pin is negative. All voltages are referenced to GND, unless
otherwise specified.
ELECTRICAL CHARACTERISTICS
I
SENSE
1
GND 2
V
CC
3
6 DRIVE
5 BAT
4 PROG
TOP VIEW
S6 PACKAGE
6-LEAD PLASTIC SOT-23
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
CC
Supply
V
CC
Operating Supply Range (Note 5)
q
4.55
8
V
I
CC
Quiescent V
CC
Pin Supply Current
V
BAT
= 5V, (Forces I
DRIVE
= I
BAT
= 0),
q
670
1150
A
I
PROG
= 200
A,(7500
from PROG to GND)
I
SHDN
V
CC
Pin Supply Current in Manual Shutdown
PROG Pin Open
q
450
900
A
I
BMS
Battery Drain Current in Manual Shutdown
PROG Pin Open
q
1
0
1
A
(Note 3)
I
BSL
Battery Drain Current in Sleep Mode (Note 4)
V
CC
= 0V
q
1
0
1
A
V
UVLOI
Undervoltage Lockout Exit Threshold
V
CC
Increasing
q
4.45
4.56
4.68
V
V
UVLOD
Undervoltage Lockout Entry Threshold
V
CC
Decreasing
q
4.30
4.41
4.53
V
V
UVHYS
Undervoltage Lockout Hysteresis
V
CC
Decreasing
150
mV
Charging Performance
V
BAT
Output Float Voltage in Constant Voltage Mode
4.1V Version, I
BAT
= 10mA, 4.55V
V
CC
8V
q
4.059
4.10
4.141
V
4.2V Version, I
BAT
= 10mA, 4.55V
V
CC
8V
q
4.158
4.20
4.242
V
I
BAT1
Output Full-Scale Current When Programmed
R
PROG
= 7500
, 4.55V
V
CC
8V,
q
155
200
240
mA
for 200mA in Constant Current Mode
Pass PNP Beta > 50
I
BAT2
Output Full-Scale Current When Programmed
R
PROG
= 2143
, 4.55V
V
CC
8V,
q
620
700
770
mA
for 700mA in Constant Current Mode
Pass PNP Beta > 50
V
CM1
Current Monitor Voltage on PROG Pin
I
BAT
= 10% of I
BAT1
, R
PROG
= 7500
,
0.045
0.15
0.28
V
4.55V
V
CC
8V, Pass PNP Beta > 50,
0
C
T
A
85
C
V
CM2
Current Monitor Voltage on PROG Pin
I
BAT
= 10% of I
BAT2
, R
PROG
= 2143
,
0.10
0.15
0.20
V
4.55V
V
CC
8V, Pass PNP Beta > 50,
0
C
T
A
85
C
I
DSINK
Drive Output Current
V
DRIVE
= 3.5V
q
30
mA
Consult LTC Marketing for parts specified with wider operating temperature ranges.
3
LTC1734
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C. V
CC
= 5V, GND = 0V and V
BAT
is equal to the float voltage unless
otherwise noted. All current into a pin is positive and current out of a pin is negative. All voltages are referenced to GND, unless
otherwise specified.
ELECTRICAL CHARACTERISTICS
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Charger Manual Control
V
MSDT
Manual Shutdown Threshold
V
PROG
Increasing
q
2.05
2.15
2.25
V
V
MSHYS
Manual Shutdown Hysteresis
V
PROG
Decreasing from V
MSDT
90
mV
I
PROGPU
Programming Pin Pull-Up Current
V
PROG
= 2.5V
6
3
1.5
A
Protection
I
DSHRT
Drive Output Short-Circuit Current Limit
V
DRIVE
= V
CC
q
35
65
130
mA
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: The LTC1734E is guaranteed to meet performance specifications
from 0
C to 70
C ambient temperature range and 0
C to 100
C junction
temperature range. Specifications over the 40
C to 85
C operating
ambient temperature range are assured by design, characterization and
correlation with statistical process controls.
Note 3: Assumes that the external PNP pass transistor has negligible B-C
reverse-leakage current when the collector is biased at 4.2V (V
BAT
) and the
base is biased at 5V (V
CC
).
Note 4: Assumes that the external PNP pass transistor has negligible B-E
reverse-leakage current when the emitter is biased at 0V (V
CC
) and the
base is biased at 4.2V (V
BAT
).
Note 5: The 4.68V maximum undervoltage lockout (UVLO) exit threshold
must first be exceeded before the minimum V
CC
specification applies.
Short duration drops below the minimum V
CC
specification of several
microseconds or less are ignored by the UVLO. If manual shutdown is
entered, then V
CC
must be higher than the 4.68V maximum UVLO
threshold before manual shutdown can be exited. When operating near the
minimum V
CC
, a suitable PNP transistor with a low saturation voltage
must be used.
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
Float Voltage vs Temperature
and Supply Voltage
TEMPERATURE (
C)
50
4.19
FLOAT VOLTAGE (V)
4.20
4.21
0
50
75
1734 G01
25
25
100
125
I
BAT
= 10mA
PNP = FCX589
4.2V OPTION
V
CC
= 8V
V
CC
= 4.55V
I
BAT
(mA)
0
4.199
FLOAT VOLTAGE (V)
4.200
4.201
200
400
500
1734 G02
100
300
600
700
V
CC
= 5V
T
A
= 25
C
PNP = FCX589
4.2V OPTION
R
PROG
= 2150
TEMPERATURE (
C)
50
190
I
BAT1
(mA) 200
210
0
50
75
1734 G03
25
25
100
125
R
PROG
= 7.5k
PNP = FCX589
V
CC
= 4.55V AND 8V
Float Voltage vs I
BAT
I
BAT1
vs Temperature
and Supply Voltage
4
LTC1734
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
I
BAT2
vs Temperature
and Supply Voltage
I
BAT1
vs V
BAT
TEMPERATURE (
C)
50
660
I
BAT2
(mA) 700
740
0
50
75
1734 G04
25
25
100
125
R
PROG
= 2.15k
PNP = FCX589
V
CC
= 4.55V AND 8V
V
BAT
(V)
0
I
BAT1
(mA)
210
4
1734 G05
200
190
1
2
3
5
V
CC
= 5V
T
A
= 25
C
R
PROG
= 7.5k
PNP = FCX589
BAT PIN MUST BE DISCONNECTED
AND GROUNDED TO FORCE
CC MODE IN THIS REGION
I
BAT2
vs V
BAT
V
BAT
(V)
0
I
BAT2
(mA)
750
4
1734 G06
700
650
1
2
3
5
V
CC
= 5V
T
A
= 25
C
R
PROG
= 2.15k
PNP = FCX589
BAT PIN MUST BE DISCONNECTED
AND GROUNDED TO FORCE
CC MODE IN THIS REGION
Program Pin Pull-Up Current vs
Temperature and Supply Voltage
Program Pin Pull-Up Current
vs V
PROG
Program Pin Voltage
vs Charge Current (200mA)
TEMPERATURE (
C)
50
I
PROGPU
(
A)
3.4
3.5
3.6
25
75
1734 G07
3.3
3.2
25
0
50
100
125
3.1
3.0
V
PROG
= 2.5V
V
CC
= 8V
V
CC
= 4.55V
V
PROG
(V)
2
2.6
I
PROGPU
(
A)
2.8
3.0
3.2
3.4
3.6
3
4
5
6
1635 G08
7
8
V
CC
= 8V
T
A
= 25
C
I
BAT1
(mA)
0
V
PROG
(V)
0.8
1.0
1.2
200
1734 F09
0.6
0.4
0
50
100
150
0.2
1.6
1.4
V
CC
= 5V
T
A
= 25
C
R
PROG
= 7.5k
PNP = FCX589
LIMITS AT 25mV DUE TO
PROGRAMMING PIN PULL-UP
CURRENT (I
PROGPU
)
Program Pin Voltage
vs Charge Current (700mA)
Program Pin Voltage for I
BAT1
/10
vs Temperature and Supply Voltage
I
BAT2
(mA)
0
V
PROG
(V)
1.4
300
1734 G10
0.8
0.4
100
200
400
0.2
0
1.6
1.2
1.0
0.6
500
600
700
V
CC
= 5V
T
A
= 25
C
R
PROG
= 2.15k
PNP = FCX589
LIMITS AT 6mV DUE TO
PROGRAMMING PIN PULL-UP
CURRENT (I
PROGPU
)
TEMPERATURE (
C)
50
140
V
PROG
(mV)
150
160
0
50
75
1734 G11
25
25
100
125
R
PROG
= 7.5k
PNP = FCX589
V
CC
= 8V
V
CC
= 4.55V
Program Pin Voltage for I
BAT2
/10
vs Temperature and Supply Voltage
TEMPERATURE (
C)
50
140
V
PROG
(mV)
150
160
0
50
75
1734 G12
25
25
100
125
R
PROG
= 2.15k
PNP = FCX589
V
CC
= 8V
V
CC
= 4.55V
5
LTC1734
BLOCK DIAGRA
W
+
+
+
2
5
UVLO
VOLTAGE
REFERENCE
2.5V
SHUTDOWN
SHUTDOWN
SHUTDOWN
SHUTDOWN
GND
1734 BD
4
PROG
R
PROG
2.5V
10
F
A1
A2
A3
1.5V
+
C1
2.15V
3
A
BAT
I
BAT
SINGLE
Li-Ion
CELL
6
DRIVE
1
I
SENSE
V
CC
1
F
V
IN
I
BAT
/1000
TEMPERATURE AND
CURRENT LIMITING
60
0.06
3
REF
OUTPUT
DRIVER
I
BAT
PI
N
FU
N
CTIO
N
S
U
U
U
I
SENSE
(Pin 1): Sense Node for Charge Current. Current
from V
CC
passes through the internal current sense resis-
tor and reappears at I
SENSE
to supply current to the
external PNP emitter. The PNP collector provides charge
current to the battery.
GND (Pin 2): Ground. Provides a reference for the internal
voltage regulator and a return for all internal circuits.
When in the constant voltage mode, the LTC1734 will
precisely regulate the voltage between the BAT and GND
pins. The battery ground should connect close to the GND
pin to avoid voltage drop errors.
V
CC
(Pin 3): Positive Input Supply Voltage. This pin
supplies power to the internal control circuitry and exter-
nal PNP transistor through the internal current sense
resistor. This pin should be bypassed to ground with a
capacitor in the range of 1
F to 10
F.
PROG (Pin 4): Charge Current Programming, Charge
Current Monitor and Manual Shutdown Pin. Provides a
virtual reference voltage of 1.5V for an external resistor
(R
PROG
) tied between this pin and ground that programs
the battery charge current when the charger is in the
constant current mode. The typical charge current will be
1000 times greater than the current through this resistor
(I
BAT
= 1500/R
PROG
). This pin also allows for the charge
current to be monitored. The voltage on this pin is propor-
tional to the charge current where 1.5V corresponds to the
full programmed currrent. Floating this pin allows an
internal current source to pull the pin voltage above the
shutdown threshold voltage. Because this pin is in a signal
path, excessive capacitive loading can cause AC instabil-
ity. See the Applications Information section for more
details.
BAT (Pin 5): Battery Voltage Sense Input. A precision
internal resistor divider sets the final float voltage on this
pin. This divider is disconnected in the manual shutdown
or sleep mode. When charging, approximately 34
A
flows into the BAT pin. To minimize float voltage errors,
avoid excessive resistance between the battery and the
BAT pin. For dynamically stable operation, this pin usually
requires a minimum bypass capacitance to ground of 5
F
to frequency compensate for the high frequency inductive
effects of the battery and wiring.
DRIVE (Pin 6): Base Drive Output for the External PNP
Pass Transistor. Provides a controlled sink current that
drives the base of the PNP. This pin has current limiting
protection for the LTC1734.
6
LTC1734
4.2V (2.5V at amplifier A1's input) the amplifier will divert
current away from the output driver thus limiting charge
current to that which will maintain 4.2V on the battery. This
is the constant voltage mode.
When in the constant voltage mode, the 1000:1 current
ratio is still valid and the voltage on the PROG pin will
indicate the charge current as a proportion of the maxi-
mum current set by the current programming resistor.
The battery charge current is 1000 (V
PROG
/R
PROG
) amps.
This feature allows a microcontroller with an ADC to easily
monitor charge current and if desired, manually shut down
the charger at the appropriate time.
When V
CC
is applied, the charger can be manually shut
down by floating the otherwise grounded end of R
PROG
.
An internal 3
A current source pulls the PROG pin above
the 2.15V threshold of voltage comparator C1 initiating
shutdown.
For charging NiMH or NiCd batteries, the LTC1734 can
function as a constant current source by grounding the
BAT pin. This will prevent amplifier A1 from trying to limit
charging current and only A2 will control the current.
Fault conditions such as overheating of the die or exces-
sive DRIVE pin current are monitored and limited.
When input power is removed or manual shutdown is
entered, the charger will drain only tiny leakage currents
from the battery, thus maximizing battery standby time.
With V
CC
removed the external PNP's base is connected to
the battery by the charger. In manual shutdown the base
is connected to V
CC
by the charger.
OPERATIO
U
The LTC1734 is a linear battery charger controller. Opera-
tion can best be understood by referring to the Block
Diagram. Charging begins when V
CC
rises above the
UVLO (Undervoltage Lockout) threshold V
UVLOI
and an
external current programming resistor is connected be-
tween the PROG pin and ground. When charging, the
collector of the external PNP provides the charge current.
The PNP's emitter current flows through the I
SENSE
pin
and through the internal 0.06
current sense resistor.
This current is close in magnitude, but slightly more than
the collector current since it includes the base current.
Amplifier A3, along with the P-channel FET, will force the
same voltage that appears across the 0.06
resistor to
appear across the internal 60
resistor. The scale factor
of 1000:1 in resistor values will cause the FET's drain
current to be 1/1000 of the charge current and it is this
current that flows through the PROG pin. In the constant
current mode, amplifier A2 is used to limit the charge
current to the maximum that is programmed by R
PROG
.
The PROG pin current, which is 1/1000 of the charge
current, develops a voltage across the program resistor.
When this voltage reaches 1.5V, amplifier A2 begins
diverting current away from the output driver, thus limit-
ing the charge current. This is the constant current mode.
The constant charge current is 1000 (1.5V/R
PROG
).
As the battery accepts charge, its voltage rises. When it
reaches the preset float voltage of 4.2V (LTC1734-4.2
version), a precisely divided down version of this voltage
(2.5V) is compared to the 2.5V internal reference voltage
by amplifier A1. If the battery voltage attempts to exceed
APPLICATIO S I FOR ATIO
W
U
U
U
Charging Operation
Charging begins when an input voltage is present that
exceeds the undervoltage lockout threshold (V
UVLOI
), a
Li-Ion battery is connected to the charger output and a
program resistor is connected from the PROG pin to
ground. During the first portion of the charge cycle, when
the battery voltage is below the preset float voltage, the
charger is in the constant current mode. As the battery
voltage rises and reaches the preset float voltage, the
charge current begins to decrease and the constant
voltage portion of the charge cycle begins. The charge
current will continue to decrease exponentially as the
battery approaches a fully charged condition.
Should the battery be removed during charging, a fast
built-in protection circuit will prevent the BAT pin from ris-
ing above 5V, allowing the precision constant voltage
circuit time to respond.
7
LTC1734
APPLICATIO
N
S I
N
FOR
M
ATIO
N
W
U
U
U
Manual Shutdown
Floating the program resistor allows an internal 3
A
current source (I
PROGPU
) to pull the PROG pin above the
2.15V shutdown threshold (V
MSDT
), thus shutting down
the charger. In this mode, the LTC1734 continues to draw
some current from the supply (I
SHDN
), but only a negli-
gible leakage current is delivered to the battery (I
BMS
).
Shutdown can also be accomplished by pulling the other-
wise grounded end of the program resistor to a voltage
greater than 2.25V (V
MSDT
Max). Charging will cease above
1.5V, but the internal battery voltage resistor divider will
draw about 34
A from the battery until shutdown is
entered. Figure 1 illustrates a microcontroller configura-
tion that can either float the resistor or force it to a voltage.
The voltage should be no more than 8V when high and
have an impedance to ground of less than 10% of the
program resistor value when low to prevent excessive
charge current errors. To reduce errors the program
resistor value may be adjusted to account for the imped-
ance to ground. The programming resistor will prevent
potentially damaging currents if the PROG pin is forced
above V
CC
. Under this condition V
CC
may float, be loaded
down by other circuitry or be shorted to ground. If V
CC
is
not shorted to ground the current through the resistor will
pull V
CC
up somewhat.
Another method is to directly switch the PROG pin to a
voltage source when shutdown is desired (Caution: pull-
ing the PROG below 1.5V with V
CC
applied will cause
excessive and uncontrolled charge currents). The volt-
age source must be capable of sourcing the resulting
current through the program resistor. This has the ad-
vantage of not adding any error to the program resistor
during normal operation. The voltage on the PROG pin
must be greater than 2.25V (V
MSDT(MAX)
) to ensure
entering shutdown, but no more than 0.3V above V
CC
to
prevent damaging the LTC1734 from excessive PROG
pin current. An exception is if V
CC
is allowed to float with
no other circuitry loading V
CC
down. Then, because the
current will be low, it is allowable to have the PROG pin
shutdown voltage applied. A three-state logic driver with
sufficient pull-up current can be used to perform this
function by enabling the high impedance state to charge
or enabling the pull-up device to enter shutdown.
An NPN transistor or a diode can also be utilized to
implement shutdown from a voltage source. These have
the advantage of blocking current when the voltage source
goes low, thus automatically disconnecting the voltage
source for normal charging operation. The use of an NPN
allows for use of a weak voltage source due to the current
gain of the transistor. For an NPN connect the collector to
V
CC,
the base to the voltage source and the emitter to the
PROG pin. For a diode, connect the anode to the voltage
source and cathode to the PROG pin. An input high level
ranging from 3.3V to V
CC
should be adequate to enter
shutdown while a low level of 0.5V or less should allow for
normal charging operation. Use of inexpensive small
signal devices such as the 2N3904 or 1N914 is recom-
mended to prevent excessive capacitive loading on the
PROG pin (see Stability section).
Sleep Mode
When the input supply is disconnected, the IC enters the
sleep mode. In this mode, the battery drain current (I
BSL
)
is a negligible leakage current, allowing the battery to re-
main connected to the charger for an extended period of
time without discharging the battery. The leakage current
is due to the reverse-biased B-E junction of the external
PNP transistor.
Undervoltage Lockout
Undervoltage lockout (UVLO) keeps the charger off until
the input voltage exceeds a predetermined threshold level
(V
UVLOI
) that is typically 4.56V. Approximately 150mV of
hysteresis is built in to prevent oscillation around the
threshold level. In undervoltage lockout, battery drain
current is very low (< 1
A).
Figure 1. Interfacing with a Microcontroller
LTC1734
PROG
C
R
PROG
ADC INPUT
1734 F01
OPEN DRAIN
OR TOTEM
POLE OUTPUT
8
LTC1734
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Programming Constant Current
When in the constant current mode, the full-scale charge
current (C) is programmed using a single external resistor
between the PROG pin and ground. This charge current
will be 1000 times greater than the current through the
program resistor. The program resistor value is selected
by dividing the voltage forced across the resistor (1.5V)
by the desired resistor current.
The LTC1734 is designed for a maximum current of
approximately 700mA. This translates to a maximum
PROG pin current of 700
A and a minimum program
resistor of approximately 2.1k. Because the PROG pin is in
a closed-loop signal path, the pole frequency must be kept
high enough to maintain adequate AC stability by avoiding
excessive capacitance on the pin. See the Stability section
for more details.
The minimum full-scale current that can be reliably pro-
grammed is approximately 50mA, which requires a pro-
gram resistor of 30k. Limiting capacitive loading on the
program pin becomes more important when high value
program resistors are used. In addition, the current
monitoring accuracy can degrade considerably at very
low current levels. If current monitoring is desired, a
minimum full-scale current of 200mA is recommended.
Different charge currents can be programmed by various
means such as by switching in different program resistors
as shown in Figures 2 and 3. A voltage DAC connected
through a resistor to the PROG pin or a current DAC
connected in parallel with a resistor to the PROG pin can
also be used to program current (the resistor is required
with the I
DAC
to maintain AC stability as discussed in the
Stability section). Another means is to use a PWM output
from a microcontroller to duty cycle the charger into and
out of shutdown to create an average current (see Manual
Shutdown section for interfacing examples). Because
chargers are generally slow to respond, it can take up to
approximately 300
s for the charger to fully settle after a
shutdown is deasserted. This delay must be accounted for
unless the minimum PWM low duration is about 3ms or
more. Shutdown occurs within a few microseconds of a
shutdown command. The use of PWM can extend the
average current to less than the normal 200mA minimum
constant current.
V
IN
5V
CHARGE
CURRENT
MONITOR
(UNFILTERED)
CHARGE
CURRENT
MONITOR
(FILTERED)
7.5k
Q2
2N7002
CONTROL 2
FZT549
I
BAT
1734 F02
SINGLE
Li-Ion
BATTERY
10
F
GND
PROG
DRIVE
BAT
2
4
6
5
LTC1734
V
CC
I
SENSE
3
1
1
F
0.1
F TO
0.5
F
1k
OPTIONAL FILTER
3k
PIN 4
Q1
2N7002
CONTROL 1
CHARGE CURRENT
0
200mA
500mA
700mA
CONTROL 1
LOW
LOW
HIGH
HIGH
CONTROL 2
LOW
HIGH
LOW
HIGH
V
IN
5V
7.5k
Q2
2N7002
CONTROL 2
FZT549*
I
LOAD
*OBSERVE MAXIMUM TEMPERATURE
1734 F03
GND
PROG
DRIVE
BAT
2
4
6
5
LTC1734
V
CC
I
SENSE
3
1
1
F
3k
Q1
2N7002
CONTROL 1
CURRENT
0
200mA
500mA
700mA
CONTROL 1
LOW
LOW
HIGH
HIGH
CONTROL 2
LOW
HIGH
LOW
HIGH
LOAD
Figure 2. Logic Control Programming of Output Current to 0mA, 200mA, 500mA or 700mA
Figure 3. Programmable Current Source with Output Current of 0mA, 200mA, 500mA or 700mA
9
LTC1734
Monitoring Charge Current
The voltage on the PROG pin indicates the charge current
as a proportion of the maximum current set by the
program resistor. The charge current is equal to 1000
(V
PROG
/R
PROG
) amps. This feature allows a microcontrol-
ler with an ADC to easily monitor charge current and if
desired, manually shut down the charger at the appropri-
ate time. See Figure 1 for an example. The minimum PROG
pin current is about 3
A (I
PROGPU
).
Errors in the charge current monitor voltage on the PROG
pin are inversely proportional to battery current and can be
statistically approximated as follows:
One Sigma Error(%)
1 + 0.3/I
BAT
(A)
Dynamic loads on the battery will cause transients to
appear on the PROG pin. Should they cause excessive
errors in charge current monitoring, a simple RC filter as
shown in Figure 2 can be used to filter the transients. The
filter will also quiet the PROG pin to help prevent inadvert-
ent momentary entry into the manual shutdown mode.
Because the PROG pin is in a closed-loop signal path the
pole frequency must be kept high enough to maintain
adequate AC stability. This means that the maximum
resistance and capacitance presented to the PROG pin
must be limited. See the Stability section for more details.
Constant Current Source
The LTC1734 can be used as a constant current source by
disabling the voltage control loop as shown in Figure 3.
This is done by pulling the BAT pin below the preset float
voltages of 4.1V or 4.2V by grounding the BAT pin. The
program resistor will determine the output current. The
output current range can be between approximately 50mA
and 700mA, depending on the maximum power rating of
the external PNP pass transistor.
External PNP Transistor
The external PNP pass transistor must have adequate
beta, low saturation voltage and sufficient power dissipa-
tion capability (including any heat sinking, if required).
To provide 700mA of charge current with the minimum
available base drive of approximately 30mA requires a
PNP beta greater than 23. If lower beta PNP transistors are
used, more base current is required from the LTC1734.
This can result in the output drive current limit being
reached, or thermal shutdown due to excessive power
dissipation. Excessive beta can affect AC stability (see
Stability section)
With low supply voltages, the PNP saturation voltage
(V
CESAT
) becomes important. The V
CESAT
must be less
than the minimum supply voltage minus the maximum
voltage drop across the internal sense resistor and bond
wires (0.1
) and battery float voltage. If the PNP transis-
tor can not achieve the low saturation voltage required,
base current will dramatically increase. This is to be
avoided for a number of reasons: output drive may reach
current limit resulting in the charger's characteristics to
go out of specifications, excessive power dissipation may
force the IC into thermal shutdown, or the battery could
become discharged because some of the current from the
DRIVE pin could be pulled from the battery through the
forward biased collector base junction.
For example, to program a charge current of 500mA with
a minimum supply voltage of 4.75V, the minimum operat-
ing V
CE
is:
V
CE(MIN)
(V) = 4.75 (0.5)(0.1) 4.2 = 0.5V
The actual battery charge current (I
BAT
) is slightly smaller
than the expected charge current because the charger
senses the emitter current and the battery charge current
will be reduced by the base current. In terms of
(I
C
/I
B
),
I
BAT
can be calculated as follows:
I
BAT
(A) = 1000 I
PROG
[
/(
+ 1)]
If
= 50, then I
BAT
is 2% low.
If desired, the 2% loss can
be compensated for by increasing I
PROG
by 2%.
Another important factor to consider when choosing the
PNP pass transistor is the power handling capability. The
transistor's data sheet will usually give the maximum rated
power dissipation at a given ambient temperature with a
power derating for elevated temperature operation. The
maximum power dissipation of the PNP when charging is:
P
D(MAX)
(W) = I
BAT
(V
DD(MAX)
V
BAT(MIN)
)
V
DD(MAX)
is the maximum supply voltage and V
BAT(MIN)
is
the minimum battery voltage when discharged.
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10
LTC1734
Once the maximum power dissipation and V
CE(MIN)
are
known, Table 1 can be used as a guide in selecting some
PNPs to consider. In the table, very low V
CESAT
is less than
0.25V, low V
CESAT
is 0.25V to 0.5V and the others are 0.5V
to 0.8V all depending on the current. See the manufacturer's
data sheet for details. All of the PNP transistors are rated
to carry at least 1A continuously as long as the power
dissipation is within limits. The Stability section addresses
caution in the use of high beta PNPs.
Should overheating of the PNP transistor be a concern,
protection can be achieved with a positive temperature
coefficient (PTC) thermistor, wired in series with the
current programming resistor and thermally coupled to
the transistor. The PTH9C chip series from Murata has a
steep resistance increase at temperature thresholds from
85
C to 145
C making it behave somewhat like a thermo-
stat switch. For example, the model PTH9C16TBA471Q
thermistor is 470
at 25
C, but abruptly increase its
resistance to 4.7k at 125
C. Below 125
C, the device
exhibits a small negative TC. The 470
thermistor can be
added in series with a 1.6k resistor to form the current
programming resistor for a 700mA charger. Should the
thermistor reach 125
C, the charge current will drop to
238mA and inhibit any further increase in temperature.
Stability
The LTC1734 contains two control loops: constant voltage
and constant current. To maintain good AC stability in the
constant voltage mode, a capacitor of at least 4.7
F is
usually required from BAT to ground. The battery and
interconnecting wires appear inductive at high frequen-
cies, and since these are in the feedback loop, this capaci-
tance may be necessary to compensate for the inductance.
This capacitor need not exceed 100
F and its ESR can
range from near zero to several ohms depending on the
inductance to be compensated. In general, compensation
is optimal with a capacitance of 4.7
F to 22
F and an ESR
of 0.5
to 1.5
.
Using high beta PNP transistors (>300) and very low ESR
output capacitors (especially ceramic) reduces the phase
margin, possibly resulting in oscillation. Also, using high
value capacitors with very low ESRs will reduce the phase
margin. Adding a resistor of 0.5
to 1.5
in series with
the capacitor will restore the phase margin.
In the constant current mode, the PROG pin is in the
feedback loop, not the battery. Because of this, capaci-
tance on this pin must be limited. Locating the program
resistor near the PROG pin and isolating the charge
current monitoring circuitry (if used) from the PROG pin
with a 1k to 10k resistor may be necessary if the capaci-
tance is greater than that given by the following equation:
C
k
R
MAX pF
PROG
( )
=
400
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Table 1. PNP Pass Transistor Selection Guide
Maximum P
D
(W)
Mounted on Board
at T
A
= 25
C
Package Style
ZETEX Part Number
ROHM Part Number
Comments
0.5
SOT-23
FMMT549
Low V
CESAT
0.625
SOT-23
FMMT720
Very Low V
CESAT,
High Beta
1
SOT-89
FCX589 or BCX69
1.1
SOT-23-6
ZXT10P12DE6
Very Low V
CESAT,
High Beta, Small
1 to 2
SOT-89
FCX717
Very Low V
CESAT,
High Beta
2
SOT-223
FZT589
Low V
CESAT
2
SOT-223
BCP69 or FZT549
0.75
FTR
2SB822
Low V
CESAT
1
ATV
2SB1443
Low V
CESAT
2
SOT-89
2SA1797
Low V
CESAT
10 (T
C
= 25
C)
TO-252
2SB1182
Low V
CESAT,
High Beta
11
LTC1734
Higher charge currents require lower program resistor
values which can tolerate more capacitive loading on the
PROG pin. Maximum capacitance can be as high as 50pF
for a charge current of 200mA (R
PROG
= 7.5k).
Figure 4 is a simple test circuit for checking stability in both
the constant current and constant voltage modes. With
input power applied and a near fully charged battery
connected to the charger, driving the PROG pin with a
pulse generator will cycle the charger in and out of the
manual shutdown mode. Referring to Figure 5, after a
short delay, the charger will enter the constant current
mode first, then if the battery voltage is near the pro-
grammed voltage of 4.1V or 4.2V, the constant voltage
mode will begin. The resulting waveform on the PROG pin
is an indication of stability.
The double exposure photo in Figure 5 shows the effects
of capacitance on the program pin. The middle waveform
is typical while the lower waveform indicates excessive
program pin capacitance resulting in constant current
mode instability. Although not common, ringing on the
constant voltage portion of the waveform is an indication
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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.
of instability due to any combination of extremely low ESR
values, high capacitance values of the output capacitor or
very high PNP transistor beta. To minimize the effect of the
scope probe capacitance, a 10k resistor is used to isolate
the probe from the program pin. Also, an adjustable load
resistor or current sink can be used to quickly alter the
charge current when a fully charged battery is used.
Reverse Input Voltage Protection
In some applications, protection from reverse voltage on
V
CC
is desired. If the supply voltage is high enough, a
series blocking diode can be used. In other cases, where
the voltage drop must be kept low, a P-channel FET as
shown in Figure 6 can be used.
Figure 5. Stability Waveforms
LTC1734
PROG
Li-Ion*
6
TO
20
*FULLY CHARGED CELL
10k
R
PROG
3k
TO SCOPE
1734 F04
BAT
2.5V
f = 1kHz
0V
+
Figure 4. Setup for AC Stability Testing
5V
0V
PROG PIN
(20pF ON PIN)
PROG PIN
(200pF ON PIN)
PULSE
GENERATOR
2V
1V
0V
2V
SHUT
DOWN
DELAY
CONSTANT
CURRENT
HORIZONTAL SCALE: 100
s/DIV
1V
0V
CONSTANT
VOLTAGE
V
CC
V
IN
*
1734 F06
LTC1734
*DRAIN-BULK DIODE OF FET
Figure 6. Low Loss Reverse Voltage Protection
V
CC
Bypass Capacitor
Many types of capacitors with values ranging from 1
F to
10
F located close to the LTC1734 will provide adequate
input bypassing. However, caution must be exercised
when using multilayer ceramic capacitors. Because of the
self resonant and high Q characteristics of some types of
ceramic capacitors, high voltage transients can be gener-
ated under some start-up conditions, such as connecting
the charger input to a hot power source. To prevent these
transients from exceeding the absolute maximum voltage
rating, several ohms of resistance can be added in series
with the ceramic input capacitor.
Internal Protection
Internal protection is provided to prevent excessive DRIVE
pin currents (I
DSHRT
) and excessive self-heating of the
LTC1734 during a fault condition. The faults can be
generated from a shorted DRIVE pin or from excessive
DRIVE pin current to the base of the external PNP
transistor when it's in deep saturation from too low a V
CE
.
This protection is not designed to prevent overheating of
the external pass transistor. Indirectly though, self-heating
of the PNP thermally conducting to the LTC1734 and
12
LTC1734
LINEAR TECHNOLOGY CORPORATION 2001
sn1734 1734fs LT/TP 0801 2K PRINTED IN THE USA
PART NUMBER
DESCRIPTION
COMMENTS
LT
1510-5
500kHz Constant-Current/Constant-Voltage Battery Charger
Up to 1A Charge Current for Li-Ion, NiCd, NiMH or Lead-Acid
Batteries
LT1571-1/LT1571-2 200kHz/500kHz Constant-Current/Constant-Voltage Battery
Up to 1.5A Charge Current for 1-, 2- or Multiple Cell Li-Ion Batteries,
LT1571-5
Charger Family
Preset and Adjustable Battery Voltages, C/10 Charge Detection
LTC1729
Li-Ion Battery Charger Termination Controller
Can be Used with LTC Battery Chargers to Provide Charge Termina-
tion, Preset Voltages, C/10 Charge Detection and Timer Functions
LTC1730
Li-Ion Battery Pulse Charger
Minimizes Heat Dissipation, No Blocking Diode Required,
Limits Maximum Current for Safety
LTC1731
Linear Constant-Current/Constant-Voltage Charger Controller Simple Charger Uses External FET. Features Preset Voltages,
C/10 Charge Detection and Programmable Timer
LTC1732
Linear Constant-Current/Constant-Voltage Charger Controller Simple Charger Uses External FET. Input Power Good Indication
Features Preset Voltages, C/10 Charge Detection and Program-
mable Timer
LT1769
200kHz Constant-Current/Constant-Voltage Battery Charger
Up to 2A Charge Current for Li-Ion, NiCd, NiMH or Lead-Acid
Batteries with Input Current Limit
RELATED PARTS
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
q
FAX: (408) 434-0507
q
www.linear.com
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PACKAGE DESCRIPTIO
resulting in the IC's junction temperature to rise above
150
C, thus cutting off the PNP's base current. This
action will limit the PNP's junction temperature to some
temperature well above 150
C. The temperature
depends on how well the IC and PNP are thermally
connected and on the transistor's
JA
. See the External
PNP Transistor section for information on protecting the
transistor from overheating.
S6 Package
6-Lead Plastic SOT-23
(LTC DWG # 05-08-1634)
(LTC DWG # 05-08-1636)
L
DATUM `A'
.09 .20
(.004 .008)
(NOTE 2)
A1
S6 SOT-23 0401
A A2
1.90
(.074)
REF
.20
(.008)
1.50 1.75
(.059 .069)
(NOTE 3)
2.60 3.00
(.102 .118)
.25 .50
(.010 .020)
(6PLCS, NOTE 2)
2.80 3.10
(.110 .118)
(NOTE 3)
.95
(.037)
REF
PIN ONE ID
MILLIMETERS
(INCHES)
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS
2. DIMENSIONS ARE IN
3. DRAWING NOT TO SCALE
4. DIMENSIONS ARE INCLUSIVE OF PLATING
5. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR
6. MOLD FLASH SHALL NOT EXCEED .254mm
7. PACKAGE EIAJ REFERENCE IS:
SC-74A (EIAJ) FOR ORIGINAL
JEDEL MO-193 FOR THIN
.90 1.45
(.035 .057)
.00 0.15
(.00 .006)
.90 1.30
(.035 .051)
.35 .55
(.014 .021)
1.00 MAX
(.039 MAX)
A
A1
A2
L
.01 .10
(.0004 .004)
.80 .90
(.031 .035)
.30 .50 REF
(.012 .019 REF)
SOT-23
(Original)
SOT-23
(ThinSOT)
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