Datasheet

12
LT1505
1505fc
APPLICATIONS INFORMATION
WUU
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This is accomplished by sensing total adapter output
current and adjusting charge 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
S4
, connected between the
CLP and CLN pins. When this voltage exceeds 92mV, the
amplifier will override programmed charge current to limit
adapter current to 92mV/R
S4
. A lowpass filter formed by
500Ω and 1µF is required to eliminate switching noise. If
the current limit is not used, then the R7 /C1 filter and the
COMP1 (R1/C7) compensation networks are not needed,
and both CLP and CLN pins should be connected to V
CC
.
Charge Current Programming
The basic formula for charge current is (see Block
Diagram):
I
BAT
= I
PROG
=
2.465V
R
PROG
R
S2
R
S1
()()
R
S2
R
S1
()
where R
PROG
is the total resistance from PROG pin to ground.
For the sense amplifier CA1 biasing purpose, R
S3
should
have the same value as R
S2
and SPIN should be connected
directly to the sense resistor (R
S1
) as shown in the Block
Diagram.
For example, 4A charging current is needed. For low power
dissipation on R
S1
and enough signal to drive the amplifier
CA1, let R
S1
= 100mV/4A = 0.025Ω. This limits R
S1
power
to 0.4W. Let R
PROG
= 5k, then:
R
S2
= R
S3
=
= = 200Ω
(I
BAT
)(R
PROG
)(R
S1
)
2.465V
(4A)(5k)(0.025)
2.465V
Charge current can also be programmed by pulse width
modulating I
PROG
with a switch Q1 to R
PROG
at a frequency
higher than a few kHz (Figure 3). Charge current will be
proportional to the duty cycle of the switch with full current
at 100% duty cycle.
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.
Note that for charge current accuracy and noise immu-
nity, 100mV full scale level across the sense resistor RS1
is required. Consequently, both RS2 and RS3 should be
200Ω.
It is critical to have a good Kelvin connection on the
current sense resistor RS1 to minimize stray resistive
and inductive pickup. RS1 should have low parasitic
inductance (typical 3nH or less, as exhibited by Dale or
IRC sense resistors). The layout path from RS2 and RS3
to RS1 should be kept away from the fast switching SW
node. Under low charge current conditions, a low quality
sense resistor with high ESL (4nH or higher) coupled
with a very noisy current sense path might false trip
comparator A12 and turn on BGATE at the wrong time,
potentially damaging the bottom power FET. In this case,
an RC filter of 10Ω and 10nF should be used across RS1
to filter out the noise (see Figure 4).
PWM
R
PROG
4.7k
PROG
C
PROG
1µF
Q1
VN2222
5V
0V
LT1505
1505 F03
I
BAT
= (DC)(4A)
Figure 3. PWM Current Programming
Figure 4. Reducing Current Sensing Noise
1505 F04
LT1505
SPIN
SENSE
BAT
BAT2
+
–
L1
RS1
RS2
RS3
10Ω
10nF
+ V
RS1
–
BATTERY
Lithium-Ion Charging
The 4A Lithium Battery Charger (Figure 1) charges lithium-
ion batteries at a constant 4A until battery voltage reaches
the preset value. The charger will then automatically go
into a constant-voltage mode with current decreasing to
near zero over time as the battery reaches full charge.