LT1511 Linear Technology, LT1511 Datasheet - Page 12

• Manufacturer Part Number: LT1511
• Description: Constant-Current/ Constant-Voltage 3A Battery Charger with Input Current Limiting
• Manufacturer: Linear Technology

APPLICATIONS
LT1511
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.
For 2A full current, the current sense resistor (R
be increased to 0.05 so that enough signal (10mV) will
be across R
current accurate.
For a 2-level charger, R1 and R2 are found from;
12
V
BAT
Figure 5. Current Comparator for Initiating Float Time Out
BAT
R1*
1.6k
R2
560k
R3
430k
S1
0.033
0.1 F
C1
1N4148
R
at 0.2A trickle charge to keep charging
Figure 6. 2-Level Charging
S1
D2
I
BAT
U
1k
0.33 F
3
2
200
200
R
R
–
+
INFORMATION
S3
S2
LT1011
U
ADAPTER
LT1511
OUTPUT
1
R1
49.3k
8
PROG
D1
1N4148
4
* TRIP CURRENT =
SENSE
BAT
LT1511
=
Q1
3.3V OR 5V
7
(560k + 430k)(0.033 )
W
R2
5.49k
(1.6k)(8.4V)
R4
470k
1511 • F05
NEGATIVE EDGE
TO TIMER
(R2 + R3)(R
S1
U
R1(V
) should
BAT
400mA
1511 • F04
)
S1
)
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.
R1
2.465 4000
I
LOW
R2
2.465 4000
I
HI
I
LOW