1
LT1511
Constant-Current/
Constant-Voltage 3A Battery
Charger with Input Current Limiting
The LT
®
1511 current mode PWM battery charger is the
simplest, most efficient solution to fast charge modern
rechargeable batteries including lithium-ion (Li-Ion), nickel-
metal-hydride (NiMH) and nickel-cadmium (NiCd) that
require constant-current and/or constant-voltage charg-
ing. The internal switch is capable of delivering 3A* DC
current (4A peak current). Full-charging current can be
programmed by resistors or a DAC to within 5%. With 0.5%
reference voltage accuracy, the LT1511 meets the critical
constant-voltage charging requirement for Li-Ion cells.
A third control loop is provided to regulate the current
drawn from the AC adapter. This allows simultaneous
operation of the equipment and battery charging without
overloading the adapter. Charging current is reduced to
keep the adapter current within specified levels.
The LT1511 can charge batteries ranging from 1V to 20V.
Ground sensing of current is not required and the battery’s
negative terminal can be tied directly to ground. A saturat-
ing switch running at 200kHz gives high charging effi-
ciency and small inductor size. A blocking diode is not
required between the chip and the battery because the
chip goes into sleep mode and drains only 3µA when the
wall adapter is unplugged.
Figure 1. 3A Lithium-Ion Battery Charger
SW
BOOST
COMP1
CLN
UV
OVP SENSE BAT
C1
1µF
R
S4
†
ADAPTER
CURRENT SENSE
R7
†
500Ω
R5
†
UNDERVOLTAGE
LOCKOUT
R6
5k
V
IN
(ADAPTER INPUT)
11V TO 28V
V
BAT
10µF
C
PROG
1µF
C
IN
*
10µF
300ΩR
PROG
4.93k
1%
0.33µF
1k
C2
0.47µF
R
S3
200Ω
1%
R
S2
200Ω
1%
L1**
20µHD2
MBR0540T 200pF
R
S1
0.033Ω
BATTERY CURRENT
SENSE
R3
390k
0.25%
BATTERY
VOLTAGE SENSE
R4
162k
0.25%
50pF
C
OUT
22µF
TANT
4.2V
4.2V
+
+
LT1511
NOTE: COMPLETE LITHIUM-ION CHARGER,
NO TERMINATION REQUIRED. R
S4
, R7
AND C1 ARE OPTIONAL FOR I
IN
LIMITING
*TOKIN OR UNITED CHEMI-CON/MARCON
CERAMIC SURFACE MOUNT
**20µH COILTRONICS CTX20-4
†
SEE APPLICATIONS INFORMATION FOR
INPUT CURRENT LIMIT AND UNDERVOLTAGE LOCKOUT
V
CC
TO MAIN
SYSTEM POWER
SPIN
D1
MBRD340
GND CLP
2 Li-Ion
D3
MBRD340
1511 • F01
PROG
V
C
+ +
+
, LTC and LT are registered trademarks of Linear Technology Corporation.
*See LT1510 for 1.5A Charger
■
Chargers for NiCd, NiMH, Lead-Acid, Lithium
Rechargeable Batteries
■
Switching Regulators with Precision Current Limit
■
Simple Design to Charge NiCd, NiMH and Lithium
Rechargeable Batteries—Charging Current
Programmed by Resistors or DAC
■
Adapter Current Loop Allows Maximum Possible
Charging Current During Computer Use
■
Precision 0.5% Accuracy for Voltage Mode Charging
■
High Efficiency Current Mode PWM with 4A Internal
Switch
■
5% Charging Current Accuracy
■
Adjustable Undervoltage Lockout
■
Automatic Shutdown When AC Adapter is Removed
■
Low Reverse Battery Drain Current: 3µA
■
Current Sensing Can Be at Either Terminal of the Battery
■
Charging Current Soft-Start
■
Shutdown Control
FEATURES
DESCRIPTIO
U
APPLICA TIO S
U
TYPICAL APPLICA TIO
U
2
LT1511
ABSOLUTE MAXIMUM RA TINGS
W
WW
U
PACKAGE/ORDER INFORMATION
W
UU
ORDER PART
NUMBER
LT1511CSW
LT1511ISW
T
JMAX
= 125°C, θ
JA
= 30°C/W**
1
2
3
4
5
6
7
8
9
10
11
12
TOP VIEW
SW PACKAGE
24-LEAD PLASTIC SO WIDE
24
23
22
21
20
19
18
17
16
15
14
13
GND**
SW
BOOST
GND**
GND**
UV
GND**
OVP
CLP
CLN
COMP1
SENSE
GND**
GND**
VCC1*
VCC2*
VCC3*
PROG
VC
UVOUT
GND**
COMP2
BAT
SPIN
Consult factory for Military grade parts.
*ALL VCC PINS SHOULD
BE CONNECTED
TOGETHER CLOSE TO
THE PINS
**ALL GND PINS ARE
FUSED TO INTERNAL DIE
ATTACH PADDLE FOR
HEAT SINKING. CONNECT
THESE PINS TO
EXPANDED PC LANDS
FOR PROPER HEAT
SINKING. 30°C/W
THERMAL RESISTANCE
ASSUMES AN INTERNAL
GROUND PLANE
DOUBLING AS A HEAT
SPREADER
(Note 1)
Supply Voltage
(V
MAX
, CLP and CLN Pin Voltage) ...................... 30V
Switch Voltage with Respect to GND...................... –3V
Boost Pin Voltage with Respect to V
CC
................... 25V
Boost Pin Voltage with Respect to GND ................. 57V
Boost Pin Voltage with Respect to SW Pin .............. 30V
V
C
, PROG, OVP Pin Voltage...................................... 8V
I
BAT
(Average)........................................................... 3A
Switch Current (Peak) .............................................. 4A
Operating Junction Temperature Range
Commercial ...........................................0°C to 125°C
Industrial ......................................... –40°C to 125°C
Operating Ambient Temperature
Commercial ............................................ 0°C to 70°C
Industrial ........................................... –40°C to 85°C
Storage Temperature Range................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec)..................300°C
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 16V, VBAT = 8V, VMAX (maximum operating VCC) = 28V,
RS2 = RS3 = 200Ω (see Block Diagram), VCLN = VCC. No load on any outputs unless otherwise noted.
PARAMETER CONDITIONS MIN TYP MAX UNITS
Overall
Supply Current V
PROG
= 2.7V, V
CC
≤ 20V ●4.5 6.8 mA
V
PROG
= 2.7V, 20V < V
CC
≤ 25V ●4.6 7.0 mA
Sense Amplifier CA1 Gain and Input Offset Voltage 8V ≤ V
CC
≤ 25V , 0V ≤ V
BAT
≤ 20V
(With R
S2
= 200Ω, R
S3
= 200Ω)R
PROG
= 4.93k ●95 100 105 mV
(Measured across R
S1
)(Note 2) R
PROG
= 49.3k ●81012 mV
T
J
< 0°C 7 13 mV
V
CC
= 28V, V
BAT
= 20V
R
PROG
= 4.93k ●90 110 mV
R
PROG
= 49.3k ●713mV
T
J
< 0°C 6 14 mV
V
CC
Undervoltage Lockout (Switch OFF) Threshold Measured at UV Pin ●678 V
UV Pin Input Current 0.2V ≤ V
UV
≤ 8V ●0.1 5 µA
UV Output Voltage at UV
OUT
Pin In Undervoltage State, I
UVOUT
= 70µA●0.1 0.5 V
UV Output Leakage Current at UV
OUT
Pin 8V ≤ V
UV
, V
UVOUT
= 5V ●0.1 3 µA
Reverse Current from Battery (When V
CC
Is V
BAT
≤ 20V, V
UV
≤ 0.4V 3 15 µA
Not Connected, V
SW
Is Floating)
3
LT1511
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 16V, VBAT = 8V, VMAX (maximum operating VCC) = 28V,
RS2 = RS3 = 200Ω (see Block Diagram), VCLN = VCC. No load on any outputs unless otherwise noted.
PARAMETER CONDITIONS MIN TYP MAX UNITS
Overall
Boost Pin Current V
CC
= 20V, V
BOOST
= 0V 0.1 10 µA
V
CC
= 28V, V
BOOST
= 0V 0.25 20 µA
2V ≤ V
BOOST
– V
CC
< 8V (Switch ON) 6 9 mA
8V ≤ V
BOOST
– V
CC
≤ 25V (Switch ON) 8 12 mA
Switch
Switch ON Resistance 8V ≤ V
CC
≤ V
MAX
, I
SW
= 3A,
V
BOOST
– V
SW
≥ 2V ●0.15 0.25 Ω
∆I
BOOST
/∆I
SW
During Switch ON V
BOOST
= 24V, I
SW
≤ 3A 25 35 mA/A
Switch OFF Leakage Current V
SW
= 0V, V
CC
≤ 20V ●2 100 µA
20V < V
CC
≤ 28V ●4 200 µA
Minimum I
PROG
for Switch ON ●2420 µA
Minimum I
PROG
for Switch OFF at V
PROG
≤ 1V ●1 2.4 mA
Maximum V
BAT
for Switch ON ●V
CC
– 2 V
Current Sense Amplifier CA1 Inputs (Sense, BAT)
Input Bias Current ●–50 –125 µA
Input Common Mode Low ●–0.25 V
Input Common Mode High ●V
CC
– 2 V
SPIN Input Current –100 –200 µA
Reference
Reference Voltage (Note 3) R
PROG
= 4.93k, Measured at OVP with
VA Supplying I
PROG
and Switch OFF 2.453 2.465 2.477 V
Reference Voltage All Conditions of V
CC
,T
J
> 0°C●2.441 2.489 V
T
J
< 0°C (Note 4) ●2.43 2.489 V
Oscillator
Switching Frequency 180 200 220 kHz
Switching Frequency All Conditions of V
CC
,T
J
> 0°C●170 200 230 kHz
T
J
< 0°C●160 230 kHz
Maximum Duty Cycle ●85 %
T
A
= 25°C9093%
Current Amplifier CA2
Transconductance V
C
= 1V, I
VC
= ±1µA 150 250 550 µmho
Maximum V
C
for Switch OFF ●0.6 V
I
VC
Current (Out of Pin) V
C
≥ 0.6V 100 µA
V
C
< 0.45V 3 mA
4
LT1511
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 16V, VBAT = 8V, VMAX (maximum operating VCC) = 28V.
No load on any outputs unless otherwise noted.
PARAMETER CONDITIONS MIN TYP MAX UNITS
Voltage Amplifier VA
Transconductance (Note 3) Output Current from 50µA to 500µA 0.25 0.6 1.3 mho
Output Source Current V
OVP
= V
REF
+ 10mV, V
PROG
= V
REF
+ 10mV 1.1 mA
OVP Input Bias Current At 0.75mA VA Output Current ±3±10 nA
At 0.75mA VA Output Current, T
J
> 90°C –15 25 nA
Current Limit Amplifier CL1, 8V
≤
Input Common Mode
Turn-On Threshold 0.75mA Output Current 93 100 107 mV
Transconductance Output Current from 50µA to 500µA 0.5 1 2 mho
CLP Input Current 0.75mA Output Current, V
UV
≥ 0.4V 0.3 1 µA
CLN Input Current 0.75mA Output Current V
UV
≥ 0.4V 0.8 2 mA
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: Tested with Test Circuit 1.
Note 3: Tested with Test Circuit 2.
Note 4: A linear interpolation can be used for reference voltage
specification between 0°C and –40°C.
TYPICAL PERFORMANCE CHARACTERISTICS
UW
Thermally Limited Maximum
Charging Current Efficiency of Figure 1 Circuit
DUTY CYCLE (%)
010305070
I
CC
(mA)
80
1511 • TPC03
20 40 60
8
7
6
5
4
3
2
1
0
125°C
0°C
25°C
V
CC
= 16V
ICC vs Duty Cycle
INPUT VOLTAGE (V)
5
MAXIMUM CHARGING CURRENT (A)
3.0
2.8
2.6
2.4
2.2
2.0 25
1511 • TPC01
10 15 20 30
(θ
JA
=30°C/W)
T
AMAX
=60°C
T
JMAX
=125°C
4.2V BATTERY
V
IN
≥ 8V
8.4V BATTERY
V
IN
≥ 11V
12.6V BATTERY
16.8V BATTERY
NOTE: FOR 4.2V AND 8.4V BATTERIES MAXIMUM
CHARGING CURRENT IS 3A FOR V
IN
– V
BAT
≥ 3V
I
BAT
(A)
0.2
EFFICIENCY (%)
100
98
96
94
92
90
88
86
84
82
80 1.0 1.8 3.02.62.2
1511 • TPC02
0.6 1.4
V
IN
= 16.5
V
BAT
= 8.4V
CHARGER EFFICIENCY
INCLUDES LOSS
IN DIODE D3
5
LT1511
TYPICAL PERFORMANCE CHARACTERISTICS
UW
Switching Frequency vs
Temperature VREF Line Regulation
TEMPERATURE (°C)
–20
FREQUENCY (kHz)
200 40 80 12060 100 140
1511 • TPC04
210
205
200
195
190
185
180
VCC (V)
0
ICC (mA)
7.0
6.5
6.0
5.5
5.0
4.5 510 15 20
1511 • TPC05
25 30
125°C
25°C
0°C
MAXIMUM DUTY CYCLE
ICC vs VCC
V
CC
(V)
0
∆V
REF
(V)
0.003
0.002
0.001
0
–0.001
–0.002
–0.003 510 15 20
1511 • TPC06
25 30
ALL TEMPERATURES
VC Pin CharacteristicsMaximum Duty Cycle
IVA vs ∆VOVP (Voltage Amplifier)
I
VA
(mA)
0
∆V
OVP
(mV)
4
3
2
1
00.8
1511• TPC07
0.20.1 0.3 0.5 0.7 0.9
0.4 0.6 1.0
125°C
25°C
TEMPERATURE (°C)
0
DUTY CYCLE (%)
120
1511 • TPC08
40 80
98
97
96
95
94
93
92
91
90 20 60 100 140
V
C
(V)
0 0.2 0.6 1.0 1.4 1.8
I
VC
(mA)
–1.20
–1.08
–0.96
–0.84
–0.72
–0.60
–0.48
–0.36
–0.24
–0.12
0
0.12 1.6
1511 • TPC09
0.4 0.8 1.2 2.0
Switch Current vs Boost Current
vs Boost Voltage
SWITCH CURRENT (A)
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.8 2.01.6
BOOST CURRENT (mA)
50
45
40
35
30
25
20
15
10
5
0
1511 • TPC11
V
CC
= 16V
V
BOOST
= 38V
28V
18V
TEMPERATURE
0
REFERENCE VOLTAGE (V)
2.470
2.468
2.466
2.464
2.462
2.460
2.458 25 50 75 100
LT1511 • TPC12
125 150
Reference Voltage
vs Temperature
PROG Pin Characteristics
V
PROG
(V)
0123 54
I
PROG
(mA)
6
0
–6
1511 • TPC10
125°C
25°C
6
LT1511
GND (Pins 1, 4, 5, 7, 16, 23, 24): Ground Pin.
SW (Pin 2): Switch Output. The Schottky catch diode must
be placed with very short lead length in close proximity to
SW pin and GND.
BOOST (Pin 3): This pin is used to bootstrap and drive the
switch power NPN transistor to a low on-voltage for low
power dissipation. In normal operation, V
BOOST
= V
CC
+
V
BAT
when switch is on. Maximum allowable V
BOOST
is
55V.
UV (Pin 6): Undervoltage Lockout Input. The rising thresh-
old is at 6.7V with a hysteresis of 0.5V. Switching stops in
undervoltage lockout. When the supply (normally the wall
adapter output) to the chip is removed, the UV pin has to
be pulled down to below 0.7V (a 5k resistor from adapter
output to GND is required) otherwise the reverse battery
current drained by the chip will be approximately 200µA
instead of 3µA. Do not leave UV pin floating. If it is
connected to V
IN
with no resistor divider, the built-in 6.7V
undervoltage lockout will be effective.
OVP (Pin 8): This is the input to the amplifier VA with a
threshold of 2.465V. Typical input current is about 3nA out
of pin. For charging lithium-ion batteries, VA monitors the
battery voltage and reduces charging when battery voltage
reaches the preset value. If it is not used, the OVP pin
should be grounded.
CLP (Pin 9): This is the positive input to the supply current
limit amplifier CL1. The threshold is set at 100mV. When
used to limit supply current, a filter is needed to filter out
the 200kHz switching noise.
CLN (Pin 10): This is the negative input to the amplifier
CL1.
COMP1 (Pin 11): This is the compensation node for the
amplifier CL1. A 200pF capacitor is required from this pin
to GND if input current amplifier CL1 is used. At input
adapter current limit, this node rises to 1V. By forcing
COMP1 low with an external transistor, amplifier CL1 will
be defeated (no adapter current limit). COMP1 can source
200µA.
PIN FUNCTIONS
UUU
SENSE (Pin 12): Current Amplifier CA1 Input. Sensing can
be at either terminal of the battery.
SPIN (Pin 13): This pin is for the internal amplifier CA1
bias. It has to be connected to R
S1
as shown in the 3A
Lithium Battery Charger (Figure 1).
BAT (Pin 14): Current Amplifier CA1 Input.
COMP2 (Pin 15): This is also a compensation node for the
amplifier CL1. It gets up to 2.8V at input adapter current
limit and/or at constant-voltage charging.
UV
OUT
(Pin 17): This is an open collector output for
undervoltage lockout status. It stays low in undervoltage
state. With an external pull-up resistor , it goes high at valid
V
CC
. Note that the base drive of the open collector NPN
comes from CLN pin. UV
OUT
stays low only when CLN is
higher than 2V. Pull-up current should be kept under
100µA.
V
C
(Pin 18): This is the control signal of the inner loop of
the current mode PWM. Switching starts at 0.7V. Higher
V
C
corresponds to higher charging current in normal
operation. A capacitor of at least 0.33µF to GND filters out
noise and controls the rate of soft-start. To shut down
switching, pull this pin low. Typical output current is 30µA.
PROG (Pin 19): This pin is for programming the charging
current and for system loop compensation. During normal
operation, V
PROG
stays close to 2.465V. If it is shorted to
GND the switching will stop. When a microprocessor
controlled DAC is used to program charging current, it
must be capable of sinking current at a compliance up to
2.465V.
V
CC
(Pins 20, 21, 22): This is the supply of the chip. For
good bypass, a low ESR capacitor of 20µF or higher is
required, with the lead length kept to a minimum. V
CC
should be between 8V and 28V and at least 3V higher than
V
BAT
. Undervoltage lockout starts and switching stops
when V
CC
goes below 7V. Note that there is a parasitic
diode inside from SW pin to V
CC
pin. Do not force V
CC
below SW by more than 0.7V with battery present. All three
V
CC
pins should be shorted together close to the pins.
7
LT1511
BLOCK DIAGRAM
W
–
+
–
+
–
+
–
+
–
+
VSW
0.7V
1.5V
VBAT
VREF
VC
GND
UV
SLOPE COMPENSATION
R2
R3
C1
PWM B1
CA2
–
+
–
+
CA1
VA
+
+
–
+
6.7V
+
VREF
2.465V
SHUTDOWN
200kHz
OSCILLATOR
S
R
R
R
R1
1k
RPROG
VCC
UVOUT
VCC
BOOST
SW
SENSE
SPIN
BAT
IPROG
RS3
RS2 RS1
IBAT
0VP
BAT
1511 BD
PROG
IPROG
IBAT = (IPROG)(RS2)
RS1
CPROG
75k
QSW
VCC
gm = 0.64
Ω
–
+
CL1
CLP
100mV
CLN
COMP1
COMP2
+
=
(RS3 = RS2)
2.465V
RPROG RS2
RS1
(())
8
LT1511
TEST CIRCUITS
Test Circuit 1
–
+
V
REF
≈ 0.65V
V
BAT
V
C
CA2
–
+
–
+
CA1
+
300Ω
20k
1k
1k
R
S1
10Ω
BAT
SENSE
SPIN
1511 • TC01
PROG
R
PROG
0.047µF
LT1511
1µF
60k
LT1006
+
R
S2
200Ω
R
S3
200Ω
V
REF
2.465V
–
+
–
+
VA
+
10k
10k
OVP
1511 • TC02
I
PROG
R
PROG
LT1511
PROG
LT1013
0.47µF
OPERATION
U
The LT1511 is a current mode PWM step-down (buck)
switcher. The battery DC charging current is programmed
by a resistor R
PROG
(or a DAC output current) at the PROG
pin (see Block Diagram). Amplifier CA1 converts the
charging current through R
S1
to a much lower current
I
PROG
fed into the PROG pin. Amplifier CA2 compares the
output of CA1 with the programmed current and drives the
PWM loop to force them to be equal. High DC accuracy is
achieved with averaging capacitor C
PROG
. Note that I
PROG
has both AC and DC components. I
PROG
goes through R1
and generates a ramp signal that is fed to the PWM control
comparator C1 through buffer B1 and level shift resistors
R2 and R3, forming the current mode inner loop. The
Boost pin drives the switch NPN Q
SW
into saturation and
reduces power loss. For batteries like lithium-ion that
require both constant-current and constant-voltage charg-
ing, the 0.5%, 2.465V reference and the amplifier VA
reduce the charging current when battery voltage reaches
the preset level. For NiMH and NiCd, VA can be used for
overvoltage protection. When input voltage is not present,
the charger goes into low current (3µA typically) sleep
mode as input drops down to 0.7V below battery voltage.
To shut down the charger, simply pull the V
C
pin low with
a transistor.
Test Circuit 2
9
LT1511
APPLICATIONS INFORMATION
WUU U
Input and Output Capacitors
In the 3A Lithium Battery Charger (Figure 1), the input
capacitor (C
IN
) is assumed to absorb all input switching
ripple current in the converter, so it must have adequate
ripple current rating. Worst-case RMS ripple current will
be equal to one half of output charging current. Actual
capacitance value is not critical. Solid tantalum capacitors
such as the AVX TPS and Sprague 593D series have high
ripple current rating in a relatively small surface mount
package, but
caution must be used when tantalum capaci-
tors are used for input bypass
. High input surge currents
can be created when the adapter is hot-plugged to the
charger and solid tantalum capacitors have a known
failure mechanism when subjected to very high turn-on
surge currents. Highest possible voltage rating on the
capacitor will minimize problems. Consult with the manu-
facturer before use. Alternatives include new high capacity
ceramic (5µF to 20µF) from Tokin or United Chemi-Con/
Marcon, et al., and the old standby, aluminum electrolytic,
which will require more microfarads to achieve adequate
ripple rating. Sanyo OS-CON can also be used.
The output capacitor (C
OUT
) is also assumed to absorb
output switching current ripple. The general formula for
capacitor current is:
IRMS =
(L1)(f)
VBAT
VCC
()
0.29 (VBAT) 1 –
For example, V
CC
= 16V, V
BAT
= 8.4V, L1 = 20µH,
and f = 200kHz, I
RMS
= 0.3A.
EMI considerations usually make it desirable to minimize
ripple current in the battery leads, and beads or inductors
may be added to increase battery impedance at the 200kHz
switching frequency. Switching ripple current splits be-
tween the battery and the output capacitor depending on
the ESR of the output capacitor and the battery imped-
ance. If the ESR of C
OUT
is 0.2Ω and the battery impedance
is rased to 4Ω with a bead or inductor, only 5% of the
current ripple will flow in the battery.
Soft-Start
The LT1511 is soft started by the 0.33µF capacitor on the
V
C
pin. On start-up, V
C
pin voltage will rise quickly to 0.5V,
then ramp at a rate set by the internal 45µA pull-up current
and the external capacitor. Battery charging current starts
ramping up when V
C
voltage reaches 0.7V and full current
is achieved with V
C
at 1.1V. With a 0.33µF capacitor, time
to reach full charge current is about 10ms and it is
assumed that input voltage to the charger will reach full
value in less than 10ms. The capacitor can be increased up
to 1µF if longer input start-up times are needed.
In any switching regulator, conventional timer-based soft
starting can be defeated if the input voltage rises much
slower than the time out period. This happens because the
switching regulators in the battery charger and the com-
puter power supply are typically supplying a fixed amount
of power to the load. If input voltage comes up slowly
compared to the soft start time, the regulators will try to
deliver full power to the load when the input voltage is still
well below its final value. If the adapter is current limited,
it cannot deliver full power at reduced output voltages and
the possibility exists for a quasi “latch” state where the
adapter output stays in a current limited state at reduced
output voltage. For instance, if maximum charger plus
computer load power is 30W, a 15V adapter might be
current limited at 2.5A. If adapter voltage is less than
(30W/2.5A = 12V) when full power is drawn, the adapter
voltage will be sucked down by the constant 30W load until
it reaches a lower stable state where the switching regu-
lators can no longer supply full load. This situation can be
prevented by utilizing
undervoltage lockout
, set higher
than the minimum adapter voltage where full power can be
achieved.
A fixed undervoltage lockout of 7V is built into the V
CC
pin,
but an additional adjustable lockout is also available on the
UV pin. Internal lockout is performed by clamping the V
C
pin low. The V
C
pin is released from its clamped state when
the UV pin rises above 6.7V and is pulled low when the UV
pin drops below 6.2V (0.5V hysteresis). At the same time
UV
OUT
goes high with an external pull-up resistor. This
signal can be used to alert the system that charging is
about to start. The charger will start delivering current
about 4ms after V
C
is released, as set by the 0.33µF
10
LT1511
APPLICATIONS INFORMATION
WUU U
capacitor. A resistor divider is used to set the desired V
CC
lockout voltage as shown in Figure 2. A typical value for R6
is 5k and R5 is found from:
R5= R6(V – V )
V
UV
UV
IN
V
UV
= Rising lockout threshold on the UV pin
V
IN
= Charger input voltage that will sustain full load power
Example: With R6 = 5k, V
UV
= 6.7V and setting V
IN
at 12V;
R5 = 5k (12V – 6.7V)/6.7V = 4k
The resistor divider should be connected directly to the
adapter output as shown, not to the V
CC
pin to prevent
battery drain with no adapter voltage. If the UV pin is not
used, connect it to the adapter output (not V
CC
) and
connect a resistor no greater than 5k to ground. Floating
the pin will cause reverse battery current to increase from
3µA to 200µA.
If connecting the unused UV pin to the adapter output is
not possible for some reason, it can be grounded. Al-
though it would seem that grounding the pin creates a
permanent lockout state, the UV circuitry is arranged for
phase reversal with low voltages on the UV pin to allow the
grounding technique to work.
being charged without complex load management algo-
rithms. Additionally, batteries will automatically be charged at
the maximum possible rate of which the adapter is capable.
This feature is created by sensing total adapter output
current and adjusting charging current downward if a
preset adapter current limit is exceeded. True analog
control is used, with closed loop feedback ensuring that
adapter load current remains within limits. Amplifier CL1
in Figure 2 senses the voltage across R
S4
, connected
between the CLP and CLN pins. When this voltage exceeds
100mV, the amplifier will override programmed charging
current to limit adapter current to 100mV/R
S4
. A lowpass
filter formed by 500Ω and 1µF is required to eliminate
switching noise. If the current limit is not used, both CLP
and CLN pins should be connected to V
CC
.
Charging Current Programming
The basic formula for charging current is (see Block
Diagram):
IBAT = IPROG = 2.465V
RPROG RS2
RS1
()()
R
S2
RS1
()
where R
PROG
is the total resistance from PROG pin to ground.
For the sense amplifier CA1 biasing purpose, R
S3
should
have the same value as R
S2
and SPIN should be connected
directly to the sense resistor (R
S1
) as shown in the Block
Diagram.
For example, 3A charging current is needed. To have low
power dissipation on R
S1
and enough signal to drive the
amplifier CA1, let R
S1
= 100mV/3A = 0.033Ω. This limits
R
S1
power to 0.3W. Let R
PROG
= 5k, then:
R
S2
= R
S3
=
= = 200Ω
(I
BAT
)(R
PROG
)(R
S1
)
2.465V
(3A)(5k)(0.033)
2.465V
Charging current can also be programmed by pulse width
modulating I
PROG
with a switch Q1 to R
PROG
at a frequency
higher than a few kHz (Figure 3). Charging current will be
proportional to the duty cycle of the switch with full current
at 100% duty cycle.
Figure 2. Adapter Current Limiting
Adapter Limiting
An important feature of the LT1511 is the ability to
automatically adjust charging current to a level which
avoids overloading the wall adapter. This allows the
product to operate at the same time that batteries are
100mV
–
+
500Ω
CLP
CLN
V
CC
UV
1511 • F02
R5
LT1511
R6
1µF
+
R
S4
*V
IN
CL1
AC ADAPTER
OUTPUT
*R
S4
= 100mV
ADAPTER CURRENT LIMIT
+
11
LT1511
Lithium-Ion Charging
The 3A Lithium Battery Charger (Figure 1) charges lithium-
ion batteries at a constant 3A until battery voltage reaches
a limit set by R3 and R4. The charger will then automati-
cally go into a constant-voltage mode with current de-
creasing to zero over time as the battery reaches full
charge. This is the normal regimen for lithium-ion charg-
ing, with the charger holding the battery at “float” voltage
indefinitely. In this case no external sensing of full charge
is needed.
Battery Voltage Sense Resistors Selection
To minimize battery drain when the charger is off, current
through the R3/R4 divider is set at 15µA. The input current
to the OVP pin is 3nA and the error can be neglected.
With divider current set at 15µA, R4 = 2.465/15µA = 162k
and,
R3 R4 V 2.465
2.465 162k 8.4 2.465
2.465
390k
BAT
=
()
−
()
=
−
()
=
Li-Ion batteries typically require float voltage accuracy of
1% to 2%. Accuracy of the LT1511 OVP voltage is ±0.5%
at 25°C and ±1% over full temperature. This leads to the
possibility that very accurate (0.1%) resistors might be
needed for R3 and R4. Actually, the temperature of the
LT1511 will rarely exceed 50°C in float mode because
charging currents have tapered off to a low level, so 0.25%
resistors will normally provide the required level of overall
accuracy.
APPLICATIONS INFORMATION
WUU U
When power is on, there is about 200µA of current flowing
out of the BAT and Sense pins. If the battery is removed
during charging, and total load including R3 and R4 is less
than the 200µA, V
BAT
could float up to V
CC
even though the
loop has turned switching off. To keep V
BAT
regulated to
the battery voltage in this condition, R3 and R4 can be
chosen to draw 0.5mA and Q3 can be added to disconnect
them when power is off (Figure 4). R5 isolates the OVP pin
from any high frequency noise on V
IN
. An alternative way is
to use a Zener diode with a breakdown voltage two or three
volts higher than battery voltage to clamp the V
BAT
voltage.
Figure 3. PWM Current Programming
Some battery manufacturers recommend termination of
constant-voltage float mode after charging current has
dropped below a specified level (typically around 10% of
the full current)
and
a further time out period of 30 minutes
to 90 minutes has elapsed. This may extend the life of the
battery, so check with manufacturers for details. The
circuit in Figure 5 will detect when charging current has
dropped below 400mA. This logic signal is used to initiate
a timeout period, after which the LT1511 can be shut down
by pulling the V
C
pin low with an open collector or drain.
Some external means must be used to detect the need for
additional charging or the charger may be turned on
periodically to complete a short float-voltage cycle.
Current trip level is determined by the battery voltage, R1
through R3 and the sense resistor (R
S1
). D2 generates
hysteresis in the trip level to avoid multiple comparator
transitions.
PWM
R
PROG
4.7k
300Ω
PROG
C
PROG
1µF
Q1
VN2222
5V
0V
LT1511
1511 • F03
I
BAT
= (DC)(3A)
R3
12k
0.25%
R4
4.99k
0.25%
OVP
V
IN
+
–
+
–
4.2V
4.2V
V
BAT
Q3
VN2222
LT1511
LT1511 • F04
R5
220k
Figure 4. Disconnecting Voltage Divider
12
LT1511
For 2A full current, the current sense resistor (R
S1
) should
be increased to 0.05Ω so that enough signal (10mV) will
be across R
S1
at 0.2A trickle charge to keep charging
current accurate.
For a 2-level charger, R1 and R2 are found from;
R1 2.465 4000
I R2 2.465 4000
II
LOW HI LOW
=
()()
=
()()
−
All battery chargers with fast charge rates require some
means to detect full charge state in the battery to terminate
the high charging current. NiCd batteries are typically
charged at high current until temperature rise or battery
voltage decrease is detected as an indication of near full
charge. The charging current is then reduced to a much
lower value and maintained as a constant trickle charge.
An intermediate “top off” current may be used for a fixed
time period to reduce 100% charge time.
NiMH batteries are similar in chemistry to NiCd but have
two differences related to charging. First, the inflection
characteristic in battery voltage as full charge is ap-
proached is not nearly as pronounced. This makes it more
difficult to use dV/dt as an indicator of full charge, and
change of temperature is more often used with a tempera-
ture sensor in the battery pack. Secondly, constant trickle
charge may not be recommended. Instead, a moderate
level of current is used on a pulse basis (≈ 1% to 5% duty
cycle) with the time-averaged value substituting for a
constant low trickle. Please contact the Linear Technology
Applications Department about charge termination cir-
cuits.
If overvoltage protection is needed, R3 and R4 should be
calculated according to the procedure described in Lithium-
Ion Charging section. The OVP pin should be grounded if
not used.
When a microprocessor DAC output is used to control
charging current, it must be capable of sinking current at a
compliance up to 2.5V if connected directly to the PROG pin.
Thermal Calculations
If the LT1511 is used for charging currents above 1.5A, a
thermal calculation should be done to ensure that junction
temperature will not exceed 125°C. Power dissipation in
the IC is caused by bias and driver current, switch resis-
tance and switch transition losses. The SO wide package,
with a thermal resistance of 30°C/W, can provide a full 3A
charging current in many situations. A graph is shown in
the Typical Performance Characteristics section.
NEGATIVE EDGE
TO TIMER
1511 • F04
3.3V OR 5V
ADAPTER
OUTPUT
38
7
14
2
D1
1N4148
C1
0.1µF
BAT
SENSE
R1*
1.6k
RS1
0.033Ω
R4
470k
R3
430k
R2
560k
LT1011
D2
1N4148
* TRIP CURRENT =
=≈ 400mA
R1(VBAT)
(R2 + R3)(RS1)
(1.6k)(8.4V)
(560k + 430k)(0.033Ω)
+
–
VBAT
BAT
RS3
200Ω
RS2
200ΩLT1511
IBAT
R2
5.49k
R1
49.3k
1k
PROG
0.33µFQ1
LT1511
1511 • F05
Nickel-Cadmium and Nickel-Metal-Hydride Charging
The circuit in the 3A Lithium Battery Charger (Figure 1) can
be modified to charge NiCd or NiMH batteries. For ex-
ample, 2-level charging is needed; 2A when Q1 is on and
200mA when Q1 is off.
Figure 6. 2-Level Charging
APPLICATIONS INFORMATION
WUU U
Figure 5. Current Comparator for Initiating Float Time Out
13
LT1511
APPLICATIONS INFORMATION
WUU U
Figure 7. Lower VBOOST
PAVV V
VW
DRIVER
=
()()()
+
()
=
384331
33
30
55 15 011
.. .
.
The average I
VX
required is:
PVW
VmA
DRIVER
X==
011
33 34
..
Fused-lead packages conduct most of their heat out the
leads. This makes it very important to provide as much PC
board copper around the leads as is practical. Total
thermal resistance of the package-board combination is
dominated by the characteristics of the board in the
immediate area of the package. This means both lateral
thermal resistance across the board and vertical thermal
resistance through the board to other copper layers. Each
layer acts as a thermal heat spreader that increases the
heat sinking effectiveness of extended areas of the board.
Total board area becomes an important factor when the
area of the board drops below about 20 square inches. The
graph in Figure 8 shows thermal resistance vs board area
for 2-layer and 4-layer boards with continuous copper
planes. Note that 4-layer boards have significantly lower
thermal resistance, but both types show a rapid increase
for reduced board areas. Figure 9 shows actual measured
lead temperatures for chargers operating at full current.
Battery voltage and input voltage will affect device power
dissipation, so the data sheet power calculations must be
used to extrapolate these readings to other situations.
Vias should be used to connect board layers together.
Planes under the charger area can be cut away from the
rest of the board and connected with vias to form both a
For example, V
X
= 3.3V then:
P 3.5mA V 1.5mA V
VV7.5mA 0.012 I
PIV
55 V
PIRV
VtVI f
BIAS IN BAT
BAT 2
IN BAT
DRIVER BAT BAT 2
IN
SW BAT 2SW BAT
IN OL IN BAT
=
()()
+
()
+
()
+
()()
[]
=
()( )
+
()
=
()()( )
+
()()( )()
130
V
BAT
R
SW
= Switch ON resistance ≈ 0.16Ω
t
OL
= Effective switch overlap time ≈ 10ns
f = 200kHz
Example: V
IN
= 15V, V
BAT
= 8.4V, I
BAT
= 3A;
P 3.5mA 15 1.5mA 8.4
8.4
15 7.5mA 0.012 3 0.27W
P3 8.4
5515 0.33W
P3 0.16 8.4
15 10 15 3 200kHz
0.81 0.09 0.9W
BIAS 2
DRIVER
2
SW
29
=
()()
+
()
+
()
+
()()
[]
=
=
()( )
+
()
=
=
()( )( )
+
()()( )
=+=
−
1
84
30
.
Total Power in the IC is: 0.27 + 0.33 + 0.9 = 1.5W
Temperature rise will be (1.5W)(30°C/W) = 45°C. This
assumes that the LT1511 is properly heat sunk by con-
necting the seven fused ground pins to expanded traces
and that the PC board has a backside or internal plane for
heat spreading.
The P
DRIVER
term can be reduced by connecting the boost
diode D2 (see Figure 1) to a lower system voltage (lower
than V
BAT
) instead of V
BAT
.
Then PDRIVER =
()( )()
+
()
IVV V
V
BAT BAT X X
IN
130
55
SW
BOOST
SPIN
1511 • F07
LT1511
V
X
I
VX
C2
D2
10µF
L1
+
14
LT1511
APPLICATIONS INFORMATION
WUU U
low thermal resistance system and to act as a ground
plane for reduced EMI.
Glue-on, chip-mounted heat sinks are effective only in
moderate power applications where the PC board copper
cannot be used, or where the board size is small. They
offer very little improvement in a properly laid out multi-
layer board of reasonable size.
Higher Duty Cycle for the LT1511 Battery Charger
Maximum duty cycle for the LT1511 is typically 90%, but
this may be too low for some applications. For example, if
an 18V ±3% adapter is used to charge ten NiMH cells, the
charger must put out 15V maximum. A total of 1.6V is lost
in the input diode, switch resistance, inductor resistance
and parasitics, so the required duty cycle is 15/16.4 =
91.4%. As it turn out, duty cycle can be extended to 93%
by restricting boost voltage to 5V instead of using V
BAT
as
is normally done. This lower boost voltage also reduces
power dissipation in the LT1511, so it is a win-win deci-
sion. Connect an external source of 3V to 6V at V
X
node in
Figure 10 with a 10µF C
X
bypass capacitor.
Even Lower Dropout
For even lower dropout and/or reducing heat on the board,
the input diode D3 should be replaced with a FET (see
Figure 11). It is pretty straightforward to connect a
P-channel FET across the input diode and connect its gate
to the battery so that the FET commutates off when the
input goes low. The problem is that the gate must be
pumped low so that the FET is fully turned on even when
the input is only a volt or two above the battery voltage.
Also there is a turn-off speed issue. The FET should turn
Figure 9. LT1511 Lead Temperature
BOARD AREA (IN2)
0
45
40
35
30
25
20
15
10 15 25
LT1511 • F08
510 20 30 35
THERMAL RESISTANCE (°C/W)
MEASURED FROM AIR AMBIENT
TO DIE USING COPPER LANDS
AS SHOWN ON DATA SHEET
2-LAYER BOARD
4-LAYER BOARD
Figure 8. LT1511 Thermal Resistance
BOARD AREA (IN2)
0
110
100
90
80
70
60
50
40 15 25
LT1511 • F09
510 20 30 35
LEAD TEMPERATURE (°C)
VIN = 16V
VBAT = 8.4V
ICHRG = 3A
TA = 25°C
NOTE: PEAK DIE TEMPERATURE WILL BE
ABOUT 10°C HIGHER THAN LEAD TEMPER-
ATURE AT 3A CHARGING CURRENT
2-LAYER BOARD
4-LAYER BOARD
4-LAYER BOARD
WITH VBOOST = 3.3V
SW
BOOST
SPIN
SENSE BAT
V
BAT
C3
0.47µF
D2 LT1511
SW
BOOST
SPIN
SENSE BAT
V
X
3V TO 6V C
X
10µF
V
BAT
1511 F10
C3
0.47µF
D2 LT1511
STANDARD CONNECTION HIGH DUTY CYCLE CONNECTION
+ +
V
IN
SW
BOOST
SPIN
SENSE BAT
V
CC
V
X
3V TO 6V C
X
10µF
V
BAT
1511 F11
C2
0.47µF
D2
D1
R
X
50k
Q2
Q1
LT1511
HIGH DUTY CYCLE CONNECTION
Q1 = Si4435DY
Q2 = TP0610L
+
+
Figure 11. Replacing the Input Diode
Figure 10. High Duty Cycle
15
LT1511
APPLICATIONS INFORMATION
WUU U
off instantly when the input is dead shorted to avoid large
current surges from the battery back through the charger
into the FET. Gate capacitance slows turn-off, so a small
P-channel (Q2) is to discharge the gate capacitance quickly
in the event of an input short. The body diode of Q2 creates
the necessary pumping action to keep the gate of Q1 low
during normal operation. Note that Q1 and Q2 have a V
GS
spec limit of 20V. This restricts V
IN
to a maximum of 20V.
For low dropout operation with V
IN
> 20V consult factory.
Optional Connection of Input Diode and
Current Sense Resistor
The typical application shown in Figure 1 on the first page
of this data sheet shows a single diode to isolate the V
CC
pin from the adapter input. This simple connection may be
unacceptable in situations where the main system power
must be disconnected from both the battery
and
the
adapter under some conditons. In particular, if the adapter
is disconnected or turned off and it is desired to also
Figure 13. High Speed Switching Path
LT1511 • F13
V
BAT
L1
V
IN
HIGH
FREQUENCY
CIRCULATING
PATH BAT
SWITCH NODE
C
IN
C
OUT
D1
SW
L1
CLP
CLN
ADAPTER
IN
TO
SYSTEM
POWER
R
S1
C
IN
R
S4
R7
500Ω
C1
1µF
D3
LT1511
PARASITIC
INTERNAL
DIODE
V
CC
1511 F12a
+
+
Figure 12a. Standard Connection
1511 F12b
SW
L1
CLP
CLN
ADAPTER
IN
TO
SYSTEM
POWER
R
S1
C
IN
R
S4
R7
500Ω
C1
1µF
D4
D3
LT1511
PARASITIC
INTERNAL
DIODE
V
CC
+
+
Figure 12b. Modified Input Diode Connection
disconnect the system load from the battery, the system
will remain powered through the parasitic diode from the
SW pin to the V
CC
pin.
The circuit in Figure 12b allows system power to go to 0V
without drawing battery current by adding an additional
diode, D4. To ensure proper operation, the LT1511 current
sense amplifier inputs (CLP and CLN) were designed to
work above V
CC
and not to draw current from V
CC
when the
inputs are pulled to ground by a powered-down adapter.
Layout Considerations
Switch rise and fall times are under 10ns for maximum
efficiency. To prevent radiation, the catch diode, SW pin
and input bypass capacitor leads should be kept as short
as possible. A ground plane should be used under the
switching circuitry to prevent interplane coupling and to
act as a thermal spreading path. All ground pins should be
connected to expanded traces for low thermal resistance.
The fast-switching high current ground path, including the
switch, catch diode and input capacitor, should be kept
very short. Catch diode and input capacitor should be
close to the chip and terminated to the same point. This
path contains nanosecond rise and fall times with several
amps of current. The other paths contain only DC and/or
200kHz tri-wave and are less critical. Figure 13 indicates
the high speed, high current switching path. Figure 14
shows critical path layout. Contact Linear Technology for
an actual LT1511 circuit PCB layout or Gerber file.
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.
16
LT1511
1511fb LT/TP 0399 REV B 2K • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 1995
APPLICATIONS INFORMATION
WUU U
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507
●
www.linear-tech.com
PACKAGE DESCRIPTION
U
PART NUMBER DESCRIPTION COMMENTS
LTC®1325 Microprocessor-Controlled Battery Management System Can Charge, Discharge and Gas Gauge NiCd and Lead-Acid
Batteries with Software Charging Profiles
LT1372/LT1377 500kHz/1MHz Step-Up Switching Regulators High Frequency, Small Inductor, High Efficiency Switchers, 1.5A Switch
LT1376 500kHz Step-Down Switching Regulator High Frequency, Small Inductor, High Efficiency Switcher, 1.5A Switch
LT1505 High Current, High Efficiency Battery Charger 94% Efficiency, Synchronous Current Mode PWM
LT1510 Constant-Voltage/Constant-Current Battery Charger Up to 1.5A Charge Current for Lithium-Ion, NiCd and NiMH Batteries
LT1512 SEPIC Battery Charger V
IN
Can Be Higher or Lower Than Battery Voltage
LT1769 Constant-Voltage/Constant-Current Battery Charger Up to 2A Charge Current for Lithium-Ion, NiCd and NiMH Batteries
RELATED PARTS
S24 (WIDE) 0996
NOTE 1
0.598 – 0.614*
(15.190 – 15.600)
22 21 20 19 18 17 16 15
12345678
0.394 – 0.419
(10.007 – 10.643)
910
1314
11 12
2324
0.037 – 0.045
(0.940 – 1.143)
0.004 – 0.012
(0.102 – 0.305)
0.093 – 0.104
(2.362 – 2.642)
0.050
(1.270)
TYP 0.014 – 0.019
(0.356 – 0.482)
TYP
0° – 8° TYP
NOTE 1
0.009 – 0.013
(0.229 – 0.330) 0.016 – 0.050
(0.406 – 1.270)
0.291 – 0.299**
(7.391 – 7.595)
× 45°
0.010 – 0.029
(0.254 – 0.737)
NOTE:
1. PIN 1 IDENT, NOTCH ON TOP AND CAVITIES ON THE BOTTOM OF PACKAGES ARE THE MANUFACTURING OPTIONS.
THE PART MAY BE SUPPLIED WITH OR WITHOUT ANY OF THE OPTIONS
DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
*
**
Figure 14. Critical Electrical and Thermal Path Layout
Dimensions in inches (millimeters) unless otherwise noted.
SW Package
24-Lead Plastic Small Outline (Wide 0.300)
(LTC DWG # 05-08-1620)
C
IN
C
IN
C
OUT
R
S1
D1
L1
GND
GND
LT1511 • F14
TO
GND TO
GND
GND
SW
GND
GND
GND
GND
GND
V
CC1
GND
NOTE: CONNECT ALL GND PINS TO EXPANDED PC LANDS FOR PROPER HEAT SINKING
