LT1505 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1505' PDF : 16 Pages View PDF
LT1505
APPLICATIONS INFORMATION
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 RS4, 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/RS4. 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 VCC.
Charge Current Programming
The basic formula for charge current is (see Block
Diagram):
( ) ( ) ( ) IBAT = IPROG
RS2
RS1
=
2.465V
RPROG
RS2
RS1
where RPROG is the total resistance from PROG pin to ground.
For the sense amplifier CA1 biasing purpose, RS3 should
have the same value as RS2 and SPIN should be connected
directly to the sense resistor (RS1) as shown in the Block
Diagram.
For example, 4A charging current is needed. For low power
dissipation on RS1 and enough signal to drive the amplifier
CA1, let RS1 = 100mV/4A = 0.025Ω. This limits RS1 power
to 0.4W. Let RPROG = 5k, then:
LT1505
PROG
5V
0V
PWM
IBAT = (DC)(4A)
RPROG
4.7k
Q1
VN2222
CPROG
1µF
1505 F03
Figure 3. PWM Current Programming
RS2
=
RS3
=
(IBAT)(RPROG)(RS1)
2.465V
=
(4A)(5k)(0.025) =
2.465V
200Ω
Charge current can also be programmed by pulse width
modulating IPROG with a switch Q1 to RPROG 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.
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.
Preset Battery Voltage Settings
The LT1505 provides four preset battery voltages: 12.3V,
12.6V, 16.4V and 16.8V. See the Pin Functions section for
pin setting voltage selection. An internal switch connects
the resistor dividers to the battery sense pin, BAT2. When
shutting down the LT1505 by removing adaptor power or
by pulling the SHDN pin low, the resistor dividers will be
disconnected and will not drain the battery. The BAT2 pin
should be connected to the battery when any of the preset
battery voltages are used.
External Battery Voltage Setting Resistors
When an external divider is used for other battery voltages,
BAT2 should be grounded. Pins 4.1V, 4.2V and 3CELL
should be left floating. To minimize battery drain when the
charger is off, current through the R3/R4 divider (Figure 4)
is set at 15µA . The input current to the VFB pin is 3nA and
the error can be neglected.
12
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