12

LT1769

1769fa

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

V

V

7.5mA

0.012 I

P

IV

55 V

P

IR

V

V

tV

I

f

BIAS

IN

BAT

BAT

2

IN

BAT

DRIVER

BAT

BAT

2

IN

SW

BAT

2

SW

BAT

IN

OL

IN

BAT

=

()

+

()( )

[]

=

()(









()

)(

)

+

()( )( )( )

1

30

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

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

R2

5.49k

R1

49.3k

300

Ω

PROG

1

µF

Q1

LT1769

1769 F05

be increased to 0.1

Ω so that enough signal (10mV) will be

accurate.

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

R1

2.465 2000

I

R2

2.465 2000

II

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