Datasheet

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
3
8
7
1
4
2
D1
1N4148
C1
0.1µF
BAT
SENSE
R1*
1.6k
R
S1
0.033Ω
R4
470k
R3
430k
R2
560k
LT1011
D2
1N4148
* TRIP CURRENT =
= ≈ 400mA
R1(V
BAT
)
(R2 + R3)(R
S1
)
(1.6k)(8.4V)
(560k + 430k)(0.033Ω)
+
–
V
BAT
BAT
R
S3
200Ω
R
S2
200Ω
LT1511
I
BAT
R2
5.49k
R1
49.3k
1k
PROG
0.33µF
Q1
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
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Figure 5. Current Comparator for Initiating Float Time Out