Datasheet

12
LT1769
1769fa
APPLICATIONS INFORMATION
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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
IRV
V
tVI f
BIAS IN BAT
BAT
2
IN
BAT
DRIVER
BAT BAT
2
IN
SW
BAT
2
SW BAT
IN
OL IN BAT
=
()()
+
()
+
()
+
()()
[]
=
()( )
+






()
=
()( )( )
+
()()( )()
1
30
V
BAT
R
SW
= Switch ON resistance ≈ 0.16Ω
t
OL
= 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
S1
). 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
R2
5.49k
R1
49.3k
300Ω
PROG
1µF
Q1
LT1769
1769 F05
For 1A full current, the current sense resistor (R
S1
) should
be increased to 0.1Ω so that enough signal (10mV) will be
across R
S1
at 0.1A trickle charge to keep charging current
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