LT1505CG Linear Technology, LT1505CG Datasheet - Page 12

• Manufacturer Part Number: LT1505CG
• Description: IC BATT CHARGER CONST I/V 28SSOP
• Manufacturer: Linear Technology

Specifications of LT1505CG

• Function: Charge Management
• Battery Type: Li-Ion, NiCd, NiMH
• Voltage - Supply: 11 V ~ 24 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP (0.200", 5.30mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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
CLP and CLN pins. When this voltage exceeds 92mV, the
amplifier will override programmed charge current to limit
adapter current to 92mV/R
500 and 1 F 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
Charge Current Programming
The basic formula for charge 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, 4A charging current is needed. For low power
dissipation on R
CA1, let R
to 0.4W. Let R
LT1505
APPLICATIONS
12
I
R
=
BAT
S2
(4A)(5k)(0.025)
= R
PROG
= I
S1
2.465V
PROG
S3
= 100mV/4A = 0.025 . This limits R
Figure 3. PWM Current Programming
is the total resistance from PROG pin to ground.
I
5V
0V
BAT
=
PROG
S1
= (DC)(4A)
(I
PWM
R
R
and enough signal to drive the amplifier
BAT
S2
S1
U
= 5k, then:
)(R
= 200
2.465V
=
S2
INFORMATION
PROG
LT1505
U
2.465V
S4
R
and SPIN should be connected
PROG
R
4.7k
Q1
VN2222
. A lowpass filter formed by
S4
PROG
PROG
)(R
S1
, connected between the
) as shown in the Block
S1
W
)
R
R
S2
S1
C
1 F
1505 F03
PROG
U
S3
S1
should
power
CC
.
Charge current can also be programmed by pulse width
modulating I
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
200 .
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 10 and 10nF should be used across RS1
to filter out the noise (see Figure 4).
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.
LT1505
Figure 4. Reducing Current Sensing Noise
SENSE
PROG
BAT2
SPIN
BAT
with a switch Q1 to R
RS2
RS3
L1
10
+ V
10nF
RS1
RS1
–
1505 F04
PROG
+
–
at a frequency
BATTERY
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