LT1769

Constant-Current/

Constant-Voltage 2A Battery

Charger with Input Current Limiting

Simple Solution to Charge NiCd, NiMH and Lithium
Rechargeable Batteries--Charging Current
Programmed by Resistors or DAC

Adapter Current Limit Allows Maximum Possible
Charging Current During System Use*

Precision 0.5% Accuracy for Voltage Mode Charging

High Efficiency Current Mode PWM with 3A Internal
Switch

5% Charge Current Accuracy

Adjustable Undervoltage Lockout

Automatic Shutdown When AC Adapter is Removed

Low Reverse Battery Drain Current: 3

Current Sensing Can Be at Either Terminal of the Battery

Charging Current Soft Start

Shutdown Control

Available in 28-Lead Narrow SSOP Package

The LT

1769 current mode PWM battery charger is a

simple, efficient solution to fast charge modern recharge-
able batteries including lithium-ion (Li-Ion), nickel-metal-
hydride (NiMH) and nickel-cadmium (NiCd) that require
constant-current and/or constant-voltage charging. The
internal switch is capable of delivering 2A** DC current
(3A peak current). Charge current can be programmed by
resistors or a DAC to within 5%. With 0.5% reference voltage
accuracy, the LT1769 meets the critical constant-voltage
charging requirement for Li-Ion cells.

A third control loop is provided to regulate the current
drawn from the input AC adapter. This allows simulta-
neous operation of the equipment and battery charging
without overloading the adapter. Charge current is reduced
to keep the adapter current below specified levels.

The LT1769 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 3

A when the

wall adapter is unplugged.

Figure 1. 2A Lithium-Ion Battery Charger

Chargers for NiCd, NiMH, Lead-Acid, Lithium
Rechargeable Batteries

Switching Regulators with Precision Current Limit

, LTC and LT are registered trademarks of Linear Technology Corporation.

*US patent number 5,723,970
**See LT1510 for 1.5A charger; see LT1511 for 3A charger

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

BOOST

COMP1

CLN

OVP SENSE

BAT

C1
1

ADAPTER
CURRENT SENSE

500

UNDERVOLTAGE
LOCKOUT

R6
5k

(ADAPTER INPUT)

11V TO 28V

BAT

PROG

300

PROG

4.93k
1%

0.33

C2
0.47

200

L1**
22

1N4148

2nF

10k

0.05

BATTERY CURRENT

SENSE

R3
390k
0.25%
BATTERY
VOLTAGE SENSE

R4
162k
0.25%

OUT

TANT

8.4V
Li-Ion

LT1769

NOTE: COMPLETE LITHIUM-ION CHARGER,
NO TERMINATION REQUIRED. R

, R7

AND C1 ARE OPTIONAL FOR I

LIMITING

*TOKIN OR UNITED CHEMI-CON/MARCON
CERAMIC SURFACE MOUNT
**22

H SUMIDA CDRH125

SEE APPLICATIONS INFORMATION FOR

INPUT CURRENT LIMIT AND UNDERVOLTAGE LOCKOUT

GENERAL SEMICONDUCTOR. FOR T

LESS THEN 100

MBRS130LT3 CAN BE USED

TO MAIN
SYSTEM LOAD

SPIN

SS24

GND

CLP

SS24

1511 ˇ F01

PROG

LT1769

ABSOLUTE

AXI

RATINGS

PACKAGE/ORDER I

FOR

ATIO

ORDER PART

NUMBER

Consult factory for Military grade parts.

*ALL V

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. 35

C/W

THERMAL RESISTANCE
ASSUMES AN INTERNAL
GROUND PLANE
DOUBLING AS A HEAT
SPREADER

LT1769CGN
LT1769IGN

JMAX

= 125

= 35

C/ W**

TOP VIEW

GN PACKAGE

28-LEAD PLASTIC SSOP

GND**

BOOST

GND**

OVP

CLP

CLN

COMP1

SENSE

GND**

CC1

CC2

CC3

GND**

PROG

OUT

COMP2

BAT

SPIN

GND**

ELECTRICAL CHARACTERISTICS

(Note 1)

Supply Voltage

, CLP and CLN Pin Voltage) ......................... 30V

BOOST Pin Voltage with Respect to V

................. 25V

BAT

(Average) ........................................................... 2A

Operating Junction Temperature Range

Commercial ........................................... 0

C to 125

Industrial ......................................... 40

C to 125

Operating Ambient Temperature

Commercial ............................................ 0

C to 70

Industrial ........................................... 40

C to 85

Storage Temperature Range ................. 65

C to 150

Lead Temperature (Soldering, 10 sec).................. 300

The

denotes specifications which apply over the full operating

temperature range, otherwise specifications are at T

= 25

C. V

= 16V, V

BAT

= 8V, R

= R

= 200

(see Block Diagram),

CLN

= V

. No load on any outputs unless otherwise noted.

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

Overall

Supply Current

PROG

= 2.7V, V

20V

4.5

6.8

PROG

= 2.7V, 20V < V

25V

4.6

7.0

Sense Amplifier CA1 Gain and Input Offset Voltage

25V , 0V

BAT

20V

(With R

= 200

, R

= 200

)

PROG

= 4.93k

100

107

(Measured across R

)(Note 2)

PROG

= 49.3k

< 0

= 28V, V

BAT

= 20V

PROG

= 4.93k

110

PROG

= 49.3k

< 0

Undervoltage Lockout (Switch OFF) Threshold

Measured at UV Pin

UV Pin Input Current

0.2V

0.1

UV Output Voltage at UV

OUT

In Undervoltage State, I

UVOUT

= 70

0.1

0.5

UV Output Leakage Current at UV

OUT

, V

UVOUT

= 5V

0.1

Reverse Current from Battery (When V

BAT

20V, V

0.4V

Not Connected, V

Is Floating)

LT1769

ELECTRICAL CHARACTERISTICS

The

denotes specifications which apply over the full operating

temperature range, otherwise specifications are at T

= 25

C. V

= 16V, V

BAT

= 8V, R

= R

= 200

(see Block Diagram),

CLN

= V

. No load on any outputs unless otherwise noted.

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

Overall

Boost Pin Current

= 20V, V

BOOST

= 0V

0.1

= 28V, V

BOOST

= 0V

0.25

BOOST

< 8V (Switch ON)

BOOST

25V (Switch ON)

Switch

Switch ON Resistance

MAX

, I

= 2A,

BOOST

0.15

0.25

BOOST

During Switch ON

BOOST

= 24V, I

mA/A

Switch OFF Leakage Current

= 0V, V

20V

100

20V < V

28V

200

Minimum I

PROG

for Switch ON

Minimum I

PROG

for Switch OFF

2.4

Maximum V

BAT

for Switch ON

Current Sense Amplifier CA1 Inputs (Sense, BAT)

Input Bias Current

125

Input Common Mode Low

0.25

Input Common Mode High

SPIN Input Current

100

200

Reference

Reference Voltage (Note 3)

PROG

= 4.93k, Measured at OVP with

VA Supplying I

PROG

and Switch OFF

2.448

2.465

2.477

Reference Voltage

All Conditions of V

, T

2.441

2.489

< 0

C (Note 4)

2.43

2.489

Oscillator

Switching Frequency

180

200

220

kHz

Switching Frequency

All Conditions of V

, T

170

200

230

kHz

< 0

160

230

kHz

Maximum Duty Cycle

Current Amplifier CA2

Transconductance

= 1V, I

150

250

550

mho

Maximum V

for Switch OFF

0.6

Current (Out of Pin)

0.6V

100

< 0.45V

LT1769

ELECTRICAL CHARACTERISTICS

TYPICAL PERFOR

CE CHARACTERISTICS

Efficiency of Figure 1 Circuit

BAT

(A)

0.2

EFFICIENCY (%)

100

1.0

1.8

2.2

1769 G01

0.6

1.4

= 16.5

BAT

= 8.4V

CHARGER EFFICIENCY

INCLUDES LOSS

IN DIODE D3

(V)

(mA)

7.0

6.5

6.0

5.5

5.0

4.5

1769 G03

= 125

= 25

= 0

MAXIMUM DUTY CYCLE

vs Duty Cycle

DUTY CYCLE (%)

(mA)

1769 G02

= 125

= 0

= 25

= 16V

vs V

The

denotes specifications which apply over the full operating

temperature range, otherwise specifications are at T

= 25

C. V

= 16V, V

BAT

= 8V. No load on any outputs unless otherwise noted.

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

Voltage Amplifier VA

Transconductance (Note 3)

Output Current from 50

A to 500

0.25

0.6

1.3

mho

Output Source Current

OVP

= V

REF

+ 10mV, V

PROG

= V

REF

+ 10mV

1.1

OVP Input Bias Current

VA Output Current at 0.5mA

VA Output Current at 0.5mA, T

> 90

Current Limit Amplifier CL1, 8V

Input Common Mode

Turn-On Threshold

0.5mA Output Current

100

107

Transconductance

Output Current from 50

A to 500

0.5

mho

CLP Input Current

0.5mA Output Current, V

0.4V

0.3

CLN Input Current

0.5mA Output Current V

0.4V

0.8

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

LT1769

TYPICAL PERFOR

CE CHARACTERISTICS

(V)

REF

(V)

0.003

0.002

0.001

0.002

0.003

1769 G04

ALL TEMPERATURES

REF

Line Regulation

OVP

(Voltage Amplifier)

(mA)

OVP

(mV)

0.8

1769 G05

0.2

0.1

0.3

0.5

0.7

0.9

0.4

0.6

1.0

= 125

= 25

Pin Characteristics

(V)

0.2

0.6

1.0

1.4

1.8

(mA)

1.20

1.08

0.96

0.84

0.72

0.60

0.48

0.36

0.24

0.12

1.6

1769 G07

0.4

0.8

1.2

2.0

JUNCTION TEMPERATURE (

DUTY CYCLE (%)

120

1769 G06

100

140

Maximum Duty Cycle

Reference Voltage
vs Temperature

JUNCTION TEMPERATURE

REFERENCE VOLTAGE (V)

2.470

2.468

2.466

2.464

2.462

2.460

2.458

100

1769 G09

125

150

PROG

(V)

PROG

(mA)

1769 G08

= 125

= 25

PROG Pin Characteristics

LT1769

GND (Pins 1 to 3, 7, 8, 14, 15, 22, 26 to 28): Ground Pins.
Must be connected to expanded PC lands for proper heat
sinking. See Applications Information section for details.

SW (Pin 4): Switch Output. The Schottky catch diode must
be placed with very short lead length in close proximity to
SW pin and GND.

BOOST (Pin 5): This pin is used to bootstrap and drive the
switch power NPN transistor to a low on-voltage for low
power dissipation. In Figure 1, V

BOOST

= V

+ V

BAT

when

switch is on. For lowest IC power dissipation, connect
boost diode D1 to a 3V to 6V at 30mA voltage source (see
Figure 10).

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 input supply (normally
the wall adapter output) to the IC is removed, the UV pin
must 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 IC will be approximately
200

A instead of 3

A. Do not leave the UV pin floating.

When connected to V

with no resistor divider, the built-

in 6.7V undervoltage lockout will be effective.

OVP (Pin 9): This is the input to amplifier VA with a
threshold of 2.465V. Typical bias current is about 3nA out
of this 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 10): This is the positive input to the input 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 11): This is the negative input to the input current
limit amplifier CL1.

COMP1 (Pin 12): This is the compensation node for the
input current limit amplifier CL1. 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 200

A. If this

function is not used, the resistor and capacitor on COMP1
pin, shown on the Figure 1 circuit, are not needed.

SENSE (Pin 13): Current Amplifier CA1 Input. Sensing can
be at either terminal of the battery.

SPIN (Pin 16): This pin is for the current amplifier CA1
bias. It must be connected to R

as shown in the 2A

Lithium Battery Charger (Figure 1).

BAT (Pin 17): Current Amplifier CA1 Input.

COMP2 (Pin 18): This is also a compensation node for
amplifier CL1. Voltage on this pin rises to 2.8V at input
adapter current limit and/or at constant-voltage charging.

OUT

(Pin 19): 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

. 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
100

(Pin 20): This is the inner loop control signal for the

current mode PWM. Switching starts at 0.7V. In normal
operation, a higher V

corresponds to higher charge

current. A capacitor of at least 0.33

F to GND filters out

noise and controls the rate of soft start. To stop switching,
pull this pin low. Typical output current is 30

PROG (Pin 21): This pin is for programming the charge
current and for system loop compensation. During normal
operation, V

PROG

stays close to 2.465V. If it is shorted to

GND switching will stop. When a microprocessor con-
trolled DAC is used to program charge current, it must be
capable of sinking current at a compliance up to 2.465V.

CC1

CC2

CC3

(Pins 23 to 25): Input Supply. For good

bypass, a low ESR capacitor of 15

F or higher is required,

with the lead length kept to a minimum. V

should be

between 8V and 28V and at least 3V higher than V

BAT

Undervoltage lockout starts and switching stops when
V

goes below 7V typical. Note that there is an internal

parasitic diode from SW pin to V

pin. Do not force V

below SW by more than 0.7V with battery present. All three
V

pins should be shorted together close to the pins.

CTIO

LT1769

TEST CIRCUITS

Test Circuit 1

REF

0.65V

BAT

CA2

CA1

300

20k

100

BAT

SENSE

SPIN

1769 TC01

PROG

0.047

LT1769

60k

LT1006

200

Test Circuit 2

REF

2.465V

10k

OVP

1769 TC02

PROG

LT1769

PROG

LT1013

0.47

OPERATIO

The LT1769 is a current mode PWM step-down (buck)
switcher. The battery DC charge current is programmed
by a resistor R

PROG

(or a DAC output current) at the PROG

pin (see Block Diagram). Amplifier CA1 converts the
charge current through R

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
control loop to force them to be equal. High DC accuracy
is achieved with averaging capacitor C

PROG

. Note that

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

into

saturation and reduces power loss. For batteries like
lithium-ion that require both constant-current and con-
stant-voltage charging, the 0.5%, 2.465V reference and
the amplifier VA reduce the charge current when battery
voltage reaches the preset level. For NiMH and NiCd, VA
can be used for overvoltage protection. When the input
voltage is removed, the V

pin drops to 0.7V below the

battery voltage, forcing the charger into a low battery drain
(3

A typical) sleep mode. To shut down the charger,

simply pull the V

pin low with a transistor.

LT1769

APPLICATIO

S I

FOR

ATIO

Input and Output Capacitors

In the 2A Lithium-Ion Battery Charger (Figure 1), the input
capacitor (C

) 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 the output charge 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
are possible 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. Selecting a high voltage rating on the
capacitor will minimize problems. Consult with the manu-
facturer before use. Alternatives include new high capacity
ceramic (5

F to 20

F) from Tokin or United Chemi-Con/

Marcon, et al. Sanyo OS-CON can also be used.

The output capacitor (C

OUT

) is also assumed to absorb

output switching ripple current. The general formula for
capacitor ripple current is:

RMS

(L1)(f)

BAT

( )

0.29 (V

BAT

) 1

For example, V

= 16V, V

BAT

= 8.4V, L1 = 20

and f = 200kHz, I

RMS

= 0.3A.

EMI considerations usually make it desirable to minimize
ripple current in the battery leads. Beads or inductors can
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 0.2

and the battery impedance

is raised to 4

with a bead or inductor, only 5% of the

ripple current will flow into the battery.

Soft-Start and Undervoltage Lockout

The LT1769 is soft-started by the 0.33

F capacitor on the

pin. On start-up, the V

pin voltage will quickly rise to

0.5V, then ramp at a rate set by the internal 45

A pull-up

current and the external capacitor. Charge current starts
ramping up when V

pin voltage reaches 0.7V and full

current is achieved with V

at 1.1V. With a 0.33

F capaci-

tor, the 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 1

F if longer input start-up times are

needed.

In any switching regulator, conventional time-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 the 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 25W, a 15V adapter might be
current limited at 2A. If adapter voltage is less than
(25W/2A = 12.5V) when full power is drawn, the adapter
voltage will be pulled down by the constant 25W 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 LT1769.
This 7V threshold can be increased by adding a resistive
divider to the UV pin as shown in Figure 2. Internal lockout
is performed by clamping the V

pin low. The V

pin is

released from its clamped state when the UV pin rises
above 7V and is pulled low when the UV pin drops below
6.5V (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

released, as set by the 0.33

F capacitor. A resistor divider

is used to set the desired V

lockout voltage as shown in

Figure 2. A typical value for R6 is 5k and R5 is found from:

LT1769

APPLICATIO

S I

FOR

ATIO

R5 =

R6(V

)

= Rising lockout threshold on the UV pin

= Charger input voltage that will sustain full load power

Example: With R6 = 5k, V

= 6.7V and setting V

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

pin, to prevent

battery drain with no adapter voltage. If the UV pin is not
used, connect it to the adapter output (not V

) and

connect a resistor no greater than 5k to ground. Floating
this pin will cause reverse battery current to increase from
3

A to 200

If connecting the unused UV pin to the adapter output is
not possible, it can be grounded. Although 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.

ally, batteries will automatically be charged at the maximum
possible rate of which the adapter is capable.

This is accomplished by sensing total adapter output
current and adjusting the charge 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 below the limit. Amplifier
CL1 in Figure 2 senses the voltage across R

, connected

between the CLP and CLN pins. When this voltage exceeds
100mV, the amplifier will override the programmed charge
current to limit adapter current to 100mV/R

. A lowpass

filter formed by 500

and 1

F is required to eliminate

switching noise. If the input current limit is not used, both
CLP and CLN pins should be connected to V

Charge Current Programming

The basic formula for charge current is (see Block
Diagram):

BAT

= I

PROG

2.465V

PROG

( )( )

( )

where R

PROG

is the total resistance from PROG pin to ground.

For the sense amplifier CA1 biasing purpose, R

should

have the same value as R

and SPIN should be connected

directly to the sense resistor (R

) as shown in the Block

Diagram.

For example, 2A charge current is needed. For low power
dissipation on R

and enough signal to drive the amplifier

CA1, let R

= 100mV/2A = 0.05

. This limits R

power

to 0.2W. Let R

PROG

= 5k, then:

= R

= 200

BAT

)(R

PROG

)(R

)

2.465V

(2A)(5k)(0.05)

2.465V

Charge 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). Charge current will be
proportional to the duty cycle of the switch with full current
at 100% duty cycle.

100mV

500

CLP

CLN

1769 F02

LT1769

AC ADAPTER

OUTPUT

100mV

ADAPTER CURRENT LIMIT

CL1

Figure 2. Adapter Input Current Limiting

Adapter Current Limiting

An important feature of the LT1769 is the ability to
automatically adjust charge current to a level which avoids
overloading the wall adapter. This allows the product to
operate at the same time the batteries are being charged
without complex load management algorithms. Addition-

LT1769

APPLICATIO

S I

FOR

ATIO

When power is on, there is about 200

A 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 200

A, V

BAT

could float up to V

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

. An alternative method

is to use a Zener diode with a breakdown voltage two or three
volts higher than battery voltage to clamp the V

BAT

voltage.

Lithium-Ion Charging

The 2A Lithium-Ion Battery Charger (Figure 1) charges at
a constant 2A until battery voltage reaches a limit set by R3
and R4. The charger will then automatically go into a
constant-voltage mode with current decreasing to near
zero over time as the battery reaches full charge. This is the
normal regimen for lithium-ion charging, 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 15

A. The input current

to the OVP pin is 3nA and the error can be neglected.

With divider current set at 15

A, V

BAT

= 8.4V, R4 =

2.465/15

A = 162k and,

R4 V

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 LT1769 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
LT1769 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.

Figure 4. Disconnecting Voltage Divider

PWM

PROG

4.7k

300

PROG

Q1
VN2222

5V
0V

LT1769

1769 F03

BAT

= (DC)(2A)

Figure 3. PWM Current Programming

R3
12k
0.25%

R4
4.99k
0.25%

OVP

8.4V

BAT

VN2222

LT1769

1769 F04

220k

Some battery manufacturers recommend terminating the
constant-voltage float mode after charge current has
dropped below a specified level (typically around 10% of
the full current)

and a further time out period of 30 to 90

minutes has elapsed. This may extend battery life, so
check with the manufacturer for details. The circuit in
Figure 5 will detect when charge current has dropped
below 270mA. This logic signal is used to initiate a timeout
period, after which the LT1769 can be shut down by
pulling the V

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

). D2 generates

hysteresis in the trip level to avoid multiple comparator
transitions.

LT1769

APPLICATIO

S I

FOR

ATIO

Figure 5. Current Comparator for Initiating Float Time Out

NEGATIVE EDGE
TO TIMER

1769 F04

3.3V OR 5V

ADAPTER

OUTPUT

D1
1N4148

0.1

BAT

SENSE

R1*
1.6k

0.05

R4
470k

R3
430k

R2
560k

LT1011

1N4148

* TRIP CURRENT =

270mA

R1(V

BAT

)

(R2 + R3)(R

)

(1.6k)(8.4V)

(560k + 430k)(0.05

)

BAT

200

LT1769

BAT

2.465 2000

LOW

( )( )

All battery chargers with fast charge rates require some
means to detect full charge in the battery and terminate the
high charge current. NiCd batteries are typically charged at
high current until a 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 also be used for a fixed
time period to reduce total 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 approached
is not nearly as pronounced. This makes it more difficult
to use dV/dt as an indicator of full charge, and an
increase in battery temperature is more often used with a
temperature sensor in the battery pack. Secondly, con-
stant trickle charge may not be recommended. Instead, a
moderate level of current is used on a pulse basis (

to 5% duty cycle) with the time-averaged value substitut-
ing for a constant low trickle. Please contact the Linear
Technology Applications department about charge termi-
nation circuits.

If overvoltage protection is needed, R3 and R4 can be cal-
culated 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
charge 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 LT1769 is used for charging currents above 1A, 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 GN package, with
a thermal resistance of 35

C/W, can provide a full 2A

charging current in many situations. A graph is shown in
the Typical Performance Characteristics section.

For 1A full current, the current sense resistor (R

) should

be increased to 0.1

so that enough signal (10mV) will be

across R

at 0.1A trickle charge to keep charging current

accurate.

For a 2-level charger, R1 and R2 are found from:

Nickel-Cadmium and Nickel-Metal-Hydride Charging

The 2A Lithium-Ion Battery Charger shown in Figure 1 can
be modified to charge NiCd or NiMH batteries. For ex-
ample, if a 2-level charge is needed; 1A when Q1 is on and
100mA when Q1 is off.

Figure 6. 2-Level Charging

R2
5.49k

R1
49.3k

300

PROG

LT1769

1769 F05

LT1769

APPLICATIO

S I

FOR

ATIO

Figure 7. Lower V

BOOST

For example, V

= 3.3V then:

Then P

DRIVER

( )(

)( )

( )

BAT

Total Power in the IC is: 0.35 + 0.43 + 0.5 = 1.3W

Temperature rise will be (1.3W)(35

C/W) = 46

C. This

assumes that the LT1769 is properly heat sunk by con-
necting the eleven 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 7) to a lower system voltage (lower
than V

BAT

) instead of V

BAT

BOOST

SPIN

1769 F07

LT1769

DRIVER

( )( )( )

( )

12 6

3 3

55 19

0 09

The average I

required is:

DRIVER

0 09

3 3

The previous example shows the dramatic drop in driver
power dissipation when the boost diode (D2) is connected
to an external 3.3V source instead of the 12.6V battery.
P

DRIVER

drops from 0.43W to 0.09W resulting in an

approximately 12

C drop in junction temperature.

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.

3.5mA V

1.5mA V

7.5mA

0.012 I

55 V

BIAS

BAT

DRIVER

BAT

(

)( )

(

)

(

)

(

)( )

[

]

( )(

)

( )

( ) ( )(

)

( )( )( )( )

BAT

3.5mA 19

1.5mA 12.6

12.6

7.5mA

0.012 2000mA

0.35W

2 12.6

55 19

0.43W

0.16 12.6

19 2 200kHz

0.42 0.08

0.5W

BIAS

DRIVER

(

)( )

( )

( )(

)

[

]

( )( )

( )

( ) ( )( )

( )( )(

)

12 6

= Switch ON resistance

0.16

= Effective switch overlap time

10ns

f = 200kHz

Example: V

= 19V, V

BAT

= 12.6V, I

BAT

= 2A:

LT1769

APPLICATIO

S I

FOR

ATIO

Figure 11. Replacing the Input Diode

CHARGE CURRENT (A)

LEAD TEMPERATURE ON PINS 1, 2, 3 (

1769 F09

0.5

1.5

NOTE: PEAK DIE TEMPERATURE
WILL BE ABOUT 15

C HIGHER AT

2A CHARGE CURRENT

= 19V

BAT

= 12.3V

BOOST

= 5V

2-LAYER BOARD
ROOM TEMP = 24

5 IN

BOARD

25 IN

BOARD

BOARD AREA (IN

)

1769 F08

THERMAL RESISTANCE (

MEASURED FROM AIR AMBIENT
TO DIE USING COPPER LANDS
AS SHOWN ON DATA SHEET

2-LAYER BOARD

4-LAYER BOARD

Figure 8. LT1769 Thermal Resistance

Figure 10. High Duty Cycle

BOOST

SPIN

SENSE

BAT

3V TO 6V

BAT

1769 F11

0.47

50k

LT1769

HIGH DUTY CYCLE CONNECTION

Q1 = Si4435DY
Q2 = TP0610L

BOOST

SPIN

SENSE

BAT

0.47

LT1769

BOOST

SPIN

SENSE

BAT

3V TO 6V

BAT

1769 F10

0.47

LT1769

STANDARD CONNECTION

HIGH DUTY CYCLE CONNECTION

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
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 LT1769 Battery Charger

Maximum duty cycle for the LT1769 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 approximaly 15V. A total of 1.6V is
lost in the input diode, switch resistance, inductor resis-
tance and parasitics, so the required duty cycle is
15/16.4 = 91.4%. The duty cycle can be extended to 93%
by restricting boost voltage to 5V instead of using V

BAT

is normally done. This lower boost voltage also reduces
power dissipation in the LT1769, so it is a win-win deci-
sion. Connect an external source of 3V to 6V at V

node in

Figure 10 with a 10

F C

bypass capacitor.

Figure 9. LT1769 Lead Temperature

LT1769

APPLICATIO

S I

FOR

ATIO

Lower Dropout Voltage

For even lower dropout and/or reducing heat on the board,
the input diode D3 can be replaced with a FET (see Figure
11). Connect a P-channel FET in place of the input diode
with its gate connected to the battery causing the FET to
turn off when the input voltage 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 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 added to
discharge the gate capacitance quickly in the event of an
input short. The Q2 body diode creates the necessary
pumping action to keep the gate of Q1 low during normal
operation. Note that Q1 and Q2 have a V

spec limit of

20V. This restricts V

to a maximum of 20V. For low

dropout operation with V

> 20V consult factory.

Optional Diode Connections

The typical application in Figure 1 shows a single diode
(D3) to isolate the V

pin from the adaptor input and to

block reverse input voltage (both steady state and tran-
sient). This simple connection may be unacceptable in
situations where the system load must be powered from
the battery when the adapter input power is removed. As
shown in Figure 12, a parasitic diode exists from the SW

pin to the V

pin in the LT1769. When the input power is

removed, this diode will become forward biased and will
provide a current path from the battery to the system load.
Because of diode power limitations, it is not recom-
mended to power the system load through the internal
parasitic diode. To safely power the system load from the
battery, an additional Schottky diode (D4) is needed. For
minimum losses, D4 could be replaced by a low R

DS(ON)

MOSFET which is turned on when the adapter power is
removed.

Layout Considerations

Switch rise and fall times are under 10ns for maximum
efficiency. To minimize 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
the LT1769 circuit PCB layout or Gerber file.

CLP

CLN

ADAPTER
IN

TO
SYSTEM
LOAD

500

C1
1

LT1769

INTERNAL

PARASITIC

DIODE

1769 F12a

Figure 12. Modified Diode Connection

Figure 13. High Speed Switching Path

1769 F13

BAT

HIGH

FREQUENCY

CIRCULATING

PATH

BAT

SWITCH NODE

OUT

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.

LT1769

1769f LT/TP 0999 4K ˇ PRINTED IN USA

LINEAR TECHNOLOGY CORPORATION 1999

APPLICATIO

S I

FOR

ATIO

OUT

GND

1769 F14

GND

NOTE: CONNECT ALL GND PINS TO EXPANDED PC LANDS FOR PROPER HEAT SINKING

GND
GND
GND
SW
BOOST
UV
GND
GND
OVP
CLP
CLN
COMP1
SENSE
GND

GND
GND
GND

CC1

CC2

CC3

GND

PROG

OUT

COMP2

BAT

SPIN

GND

Figure 14. Critical Electrical and Thermal Path Layout

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900

FAX: (408) 434-0507

www.linear-tech.com

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

LT1511

Constant-Voltage/Constant-Current Battery Charger

Up to 3A Charge Current for Lithium-Ion, NiCd and NiMH Batteries

LT1512/LT1513

SEPIC Battery Chargers

Can Be Higher or Lower Than Battery Voltage

LTC1729

Li-Ion Battery Charger Termination Controller

Preconditioning If Cell < 2.7V, 3hr Time-Out, C/10 Detection, Temp Sensor
Pin, Charger and Battery Detection

LTC1759

SMBus Smart Battery Charger

94% Efficiency with Input Current Limiting, Up to 8A I

CHG

RELATED PARTS

Dimensions in inches (millimeters) unless otherwise noted.

PACKAGE DESCRIPTIO

GN Package

28-Lead Plastic SSOP (Narrow 0.150)

(LTC DWG # 05-08-1641)

GN28 (SSOP) 1098

* 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

0.016 0.050

(0.406 1.270)

0.015

0.004

(0.38

0.10)

TYP

0.0075 0.0098

(0.191 0.249)

0.053 0.069

(1.351 1.748)

0.008 0.012

(0.203 0.305)

0.004 0.009

(0.102 0.249)

0.0250

(0.635)

BSC

0.386 0.393*

(9.804 9.982)

9 10 11 12

0.229 0.244

(5.817 6.198)

0.150 0.157**

(3.810 3.988)

19 18 17

13 14

1615

0.033

(0.838)

REF

Part Number LT1769