LT1769IGN#TRPBF Datasheet LT1769IGN#PBF, LT1769CGN#PBF, LT1769CFE#PBF | P10-P12

LT1769

1769fa

APPLICATIONS INFORMATION

WUU

= 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

3µA to 200µA.

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 volt-

ages on the UV pin to allow the grounding technique to work.

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 500Ω and 1µF 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 = 0.05Ω. This limits R

power

to 0.2W. Let R

PROG

= 5k, then:

= R

= = 200Ω

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.

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

100mV

500Ω

CLP

CLN

1769 F02

LT1769

1µF

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-

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

LT1769

1769fa

APPLICATIONS INFORMATION

WUU

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, V

BAT

= 8.4V, R4 =

2.465/15µA = 162k and,

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%. 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.

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 200µA, 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

Figure 4. Disconnecting Voltage Divider

PWM

PROG

4.7k

300Ω

PROG

1µF

VN2222

LT1769

1769 F03

BAT

= (DC)(2A)

Figure 3. PWM Current Programming

12k

0.25%

4.99k

0.25%

OVP

8.4V

BAT

VN2222

LT1769

1769 F04

220k

is to use a Zener diode with a breakdown voltage two or three

volts higher than battery voltage to clamp the V

BAT

voltage.

Battery manufacturers recommend terminating the con-

stant-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

Figure 5. Current Comparator for Initiating Float Time Out

NEGATIVE EDGE

TO TIMER

1769 F04

3.3V OR 5V

ADAPTER

OUTPUT

1N4148

0.1µF

BAT

SENSE

R1*

1.6k

0.05Ω

470k

430k

560k

LT1011

1N4148

* TRIP CURRENT =

= ≈ 270mA

R1(V

BAT

)

(R2 + R3)(R

)

(1.6k)(8.4V)

(560k + 430k)(0.05Ω)

–

BAT

200Ω

LT1769

BAT

LT1769

1769fa

APPLICATIONS INFORMATION

WUU

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 (≈ 1%

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.

P 3.5mA V 1.5mA V

7.5mA 0.012 I

55 V

IRV

tVI f

BIAS IN BAT

BAT

DRIVER

BAT BAT

BAT

SW BAT

OL IN BAT

()()

()

()()

[]

()( )













()

()()( )

()()( )()

BAT

= Switch ON resistance ≈ 0.16Ω

= Effective switch overlap time ≈ 10ns

f = 200kHz

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.

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

5.49k

49.3k

300Ω

PROG

1µF

LT1769

1769 F05

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

) should

be increased to 0.1Ω 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:

2.465 2000

LOW HI 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

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LT1769IGN#TRPBF

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Description:

Battery Management Const-C/Const-V 2A Bat Chr w/ In C Limin

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