Datasheet

13
LT1505
1505fc
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 volt-
ages, 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 5) is set at 15µA . The input current to the V
FB
pin
is 3nA and the error can be neglected.
With divider current set at 15µA, R4 = 2.465/15µA = 162k
and,
R3
R4 V 2.465
2.465
162k 8.4 2.465
2.465
390k
BAT
=
()
−
()
=
−
()
=
APPLICATIONS INFORMATION
WUU
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Lithium-Ion Charging Completion
Some battery manufacturers recommend termination of
constant-voltage float mode after charge current has
dropped below a specified level (typically around 20% of
the full current) and a further time-out period of 30
minutes to 90 minutes has elapsed. Check with manufac-
turers for details. The LT1505 provides a signal at the
FLAG pin when charging is in voltage mode and current is
reduced to 20% of full current, assuming full charge
current is programmed to have 100mV across the current
sense resistor (V
RS1
). The comparator E6 in the Block
Diagram compares the charge current sample I
PROG
to the
output current I
VA
voltage amplifier VA. When the charge
current drops to 20% of full current, I
PROG
will be equal to
0.25 I
VA
and the open-collector output V
FLAG
will go low
and can be used to start an external timer. When this
feature is used, a capacitor of at least 0.1µF is required at
the CAP pin to filter out the switching noise and a pull-up
resistor is also needed at the FLAG pin. If this feature is not
used, C6 is not needed.
Very Low Dropout Operation
The LT1505 can charge the battery even when V
CC
goes
as low as 0.5V above the combined voltages of the
battery and the drops on the sense resistor as well as
parasitic wiring. This low V
CC
sometimes requires a duty
factor greater then 99% and TGATE stays on for many
switching cycles. While TGATE stays on, the voltage
V
BOOST
across the capacitor C2 drops down because
TGATE control circuits require 2mA DC current. C2 needs
to be recharged before V
BOOST
drops too low to keep the
topside switch on. A unique design allows the LT1505 to
operate under these conditions; the comparator A2 moni-
tors V
BOOST
and when it drops from 9.1V to 6.9V, TGATE
will be turned off for about 0.2µs to recharge C2. Note that
the LT1505 gets started the same way when power turns
on and there is no initial V
BOOST
.
It is important to use 0.56µF or greater value for C2 to hold
V
BOOST
up for a sufficient amount of time.
When minimum operating V
CC
is less than 2.5V above the
battery voltage, D3 and C4 (see Figure 1) are also needed
to bootstrap V
BOOSTC
higher than V
CC
to bias the current
V
BAT
1505 F04
8.4V
R3
390k
0.25%
R4
162k
0.25%
+
V
FB
LT1505
Figure 5. External Resistor Divider
Li-Ion batteries typically require float voltage accuracy of
1% to 2%. Accuracy of the LT1505 V
FB
voltage is ±0.5%
at 25°C and ±1% over the full temperature range. This
leads to the possibility that very accurate (0.1%) resistors
might be needed for R3 and R4. Actually, the temperature
of the LT1505 will rarely exceed 50°C in float mode
because charging currents have tapered off to a low level,
so 0.25% resistors will normally provide the required level
of overall accuracy.