LTC4040_15 Datasheet, PDF (18/26 Pages) Linear Technology – 2.5A Battery Backup Power Manager

LTC4040
Applications Information
Choosing a Charge Voltage for the Battery:
The LTC4040 offers 4 different charge voltage options for
each of the two battery chemistries (Li-Ion and LiFePO₄)
and these levels are selected by the digital inputs F0, F1
and F2. Choosing a higher charge voltage increases the
battery capacity to provide a longer product run-time but
reduces the battery lifetime, usually measured by the
number of charge/ discharge cycles. Battery manufacturers
usually consider the end of life for a battery to be when
the battery capacity drops to 80% of the rated capacity.
The curves in Figure 1 show the relationship between
cell capacity and cycle life for a typical Li-Ion battery cell.
Using 4.2V as the charge voltage, a typical Li-Ion battery
is considered at 100% initial capacity but delivers about
500 charge/ discharge cycles before the capacity drops
to 80%. However, if the same battery uses 4.1V as the
charge voltage, it is at 85% initial capacity but the number
of charge/discharge cycles can be almost doubled to 1000
before the capacity drops to 80%. Lowering the charge
voltage even further to 4.0V can increase the battery lifetime
more than three times to 1800 charge/ discharge cycles.
Since LTC4040 is a backup product, the battery is likely
to spend the majority of its lifetime fully charged. This
makes it even more critical to charge at a lower charge
voltage to maximize battery lifetime since battery capacity
degrades even faster when batteries remain fully charged.
Because of the different Li-Ion battery chemistries and
other conditions that can affect battery lifetime, the curves
shown here are only estimates of the number of charge
cycles and battery-capacity levels.
Programming the Input Voltage Threshold
for the Power-Fail Comparator
The input voltage threshold below which the power-fail
status pin PFO indicates a power-fail condition and the
LTC4040 activates the backup boost operation can be
programmed by using a resistor divider from the supply
to GND via the PFI pin such that:
V_SUPP(PFO)
=
V_PFI
•


1+
R_PF1
R_{PF 2}


=
1.19V
•


1+
R_PF1
R_{PF 2}


V_PFI is approximately 1.19V. See Block Diagram. The PFI
threshold voltage should be set to a level between 200mV
to 300mV below the nominal input supply voltage so that
the supply transients do not trip the comparator. On the
other hand, it should be set high enough so that the V_SYS
voltage does not drop too much to trip the reset compara-
tor during the transition to backup mode.
2000
130
1750
120
1500
1250
110
CAPACITY
100
1000
90
750
80
500
70
250 ^{NUMBER OF CYCLES}
60
0
50
4
4.1
4.2
4.3
4.4
4.5
CHARGE VOLTAGE (V)
4040 F01
Figure 1. Battery Cycle Life and Capacity
as a Function of Charge Voltage
Programming the Battery Charge Current
Battery charge current is programmed using a single resis-
tor from the PROG pin to ground. To set a charge current
of I_CHG, the PROG pin resistor value can be determined
using the following equation:
R_PROG
=
2500
•
0.8V
I_CHG
=
2000V
I_CHG
For example, to set the charge current to 1A, the value
of the PROG pin resistor should be 2k. The minimum
recommended charge current is 500mA, below which the
accuracy of the charge current suffers. This corresponds
to a maximum R_PROG resistor of 4k.
4040fa
18
For more information www.linear.com/LTC4040