Datasheet

8
LT1511
TEST CIRCUITS
Test Circuit 1
–
+
V
REF
≈ 0.65V
V
BAT
V
C
CA2
–
+
–
+
CA1
+
300Ω
20k
1k
1k
R
S1
10Ω
BAT
SENSE
SPIN
1511 • TC01
PROG
R
PROG
0.047µF
LT1511
1µF
60k
LT1006
+
R
S2
200Ω
R
S3
200Ω
V
REF
2.465V
–
+
–
+
VA
+
10k
10k
OVP
1511 • TC02
I
PROG
R
PROG
LT1511
PROG
LT1013
0.47µF
OPERATION
U
The LT1511 is a current mode PWM step-down (buck)
switcher. The battery DC charging current is programmed
by a resistor R
PROG
(or a DAC output current) at the PROG
pin (see Block Diagram). Amplifier CA1 converts the
charging current through R
S1
to a much lower current
I
PROG
fed into the PROG pin. Amplifier CA2 compares the
output of CA1 with the programmed current and drives the
PWM loop to force them to be equal. High DC accuracy is
achieved with averaging capacitor C
PROG
. Note that I
PROG
has both AC and DC components. I
PROG
goes through R1
and generates a ramp signal that is fed to the PWM control
comparator C1 through buffer B1 and level shift resistors
R2 and R3, forming the current mode inner loop. The
Boost pin drives the switch NPN Q
SW
into saturation and
reduces power loss. For batteries like lithium-ion that
require both constant-current and constant-voltage charg-
ing, the 0.5%, 2.465V reference and the amplifier VA
reduce the charging current when battery voltage reaches
the preset level. For NiMH and NiCd, VA can be used for
overvoltage protection. When input voltage is not present,
the charger goes into low current (3µA typically) sleep
mode as input drops down to 0.7V below battery voltage.
To shut down the charger, simply pull the V
C
pin low with
a transistor.
Test Circuit 2