LT1511 Linear Technology, LT1511 Datasheet - Page 10

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

LT1511
APPLICATIONS
capacitor. A resistor divider is used to set the desired V
lockout voltage as shown in Figure 2. A typical value for R6
is 5k and R5 is found from:
V
V
Example: With R6 = 5k, V
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
battery drain with no adapter voltage. If the UV pin is not
used, connect it to the adapter output (not V
connect a resistor no greater than 5k to ground. Floating
the pin will cause reverse battery current to increase from
3 A to 200 A.
If connecting the unused UV pin to the adapter output is
not possible for some reason, it can be grounded. Al-
though it would seem that grounding the pin creates a
permanent lockout state, the UV circuitry is arranged for
phase reversal with low voltages on the UV pin to allow the
grounding technique to work.
10
Adapter Limiting
An important feature of the LT1511 is the ability to
automatically adjust charging current to a level which
avoids overloading the wall adapter. This allows the
product to operate at the same time that batteries are
UV
IN
R5 =
= Charger input voltage that will sustain full load power
= Rising lockout threshold on the UV pin
CL1
R6(V – V )
LT1511
+
–
V
Figure 2. Adapter Current Limiting
IN
100mV
UV
+
U
UV
CLP
CLN
V
UV
*R
CC
UV
INFORMATION
S4
U
=
+
= 6.7V and setting V
ADAPTER CURRENT LIMIT
1 F
100mV
W
R
S4
*
500
CC
1511 • F02
pin to prevent
U
IN
AC ADAPTER
CC
OUTPUT
R5
R6
at 12V;
) and
CC
V
IN
being charged without complex load management algo-
rithms. Additionally, batteries will automatically be charged at
the maximum possible rate of which the adapter is capable.
This feature is created by sensing total adapter output
current and adjusting charging current downward if a
preset adapter current limit is exceeded. True analog
control is used, with closed loop feedback ensuring that
adapter load current remains within limits. Amplifier CL1
in Figure 2 senses the voltage across R
between the CLP and CLN pins. When this voltage exceeds
100mV, the amplifier will override programmed charging
current to limit adapter current to 100mV/R
filter formed by 500
switching noise. If the current limit is not used, both CLP
and CLN pins should be connected to V
Charging Current Programming
The basic formula for charging current is (see Block
Diagram):
where R
For the sense amplifier CA1 biasing purpose, R
have the same value as R
directly to the sense resistor (R
Diagram.
For example, 3A charging current is needed. To have low
power dissipation on R
amplifier CA1, let R
R
Charging current can also be programmed by pulse width
modulating I
higher than a few kHz (Figure 3). Charging current will be
proportional to the duty cycle of the switch with full current
at 100% duty cycle.
S1
I
R
=
BAT
power to 0.3W. Let R
S2
(3A)(5k)(0.033)
= R
PROG
= I
2.465V
PROG
S3
PROG
is the total resistance from PROG pin to ground.
=
(I
R
R
with a switch Q1 to R
BAT
S2
S1
S1
)(R
= 100mV/3A = 0.033 . This limits
= 200
=
2.465V
S1
and 1 F is required to eliminate
S2
PROG
PROG
and enough signal to drive the
2.465V
R
and SPIN should be connected
PROG
)(R
S1
= 5k, then:
) as shown in the Block
S1
)
R
R
PROG
S2
S1
CC
.
S4
at a frequency
S4
, connected
. A lowpass
S3
should