Hw Config: Input Current Sense And Limit - Analog Devices LINEAR LT8491 Manual

LT8491
APPLICATIONS INFORMATION
Setting the Stage 2 Battery Voltage (V
network shown in Figure 14 is used to set the Stage 2
battery voltage and to measure V
(see TELE_VBAT register). Battery manufacturers typi-
cally call for a higher Stage 2 voltage limit than the nomi-
nal battery voltage. For example, a 12V lead-acid battery
used in automotive applications commonly has a Stage 2
charging voltage limit of 14.2V. If temperature compen-
sated charging is used (see the Configure Temperature
Compensation section) then use the 25˚C value of V
the equations below.
R is often selected to be between 4.99kΩ and
FBOUT2
49.9kΩ. Choosing higher values for R
the amount of current draw from the battery through the
feedback network.
⎡
R = R • V
⎢
FBOUT1 FBOUT2
⎣
R =
DACO2
R •R
FBOUT1 FBOUT2
1.241
⎛
R • V •
⎝ ⎜
FBOUT2 S2
1.211
R = 0.2 •R
DACO1 DACO2
1
=
C
DACO
500 •R
DACO1
For greater charging voltage accuracy, it is recommended
that 0.1% tolerance resistors are used for the output feed-
back resistor network.
Due to the granularity of standard resistor values, simply
rounding the calculated results to their nearest standard
values may result in unwanted errors. Consider using
multiple resistors in series to match the calculated results.
Otherwise, use standard resistor values and check the
final results with the following equations:
⎛
R
FBOUT1
V =
⎝ ⎜
X3
R +R
DACO1 DACO2
where
64
): The resistor
S2
via the FBOR pin
BAT
reduces
FBOUT2
1.241
⎛ ⎞
• – 0.128 – 1
⎝ ⎜ ⎠ ⎟
S2
1.211
• 0.833
Ω
⎞
–R –R
⎠ ⎟
FBOUT2 FBOUT1
Ω
F
⎞
•(X – 1.89)
⎠ ⎟
For more information
X = 1.211•
⎡
⎛
R
DACO1
1+
⎢
⎝ ⎜
R
⎣
FBOUT2
V indicates the actual 25°C V
X3
selected resistors.
X – 1.89
N1=
X – 3.3
in
S2
N1 should be as close as possible to 1.22.
1.89
N2 = 1–
X
N2 should be as close as possible to 0.806. Iterations may
be required to determine best standard resistor values.
⎤
⎥ Ω
Table 23 shows good sets of standard value components
⎦
for charging nominal battery voltages of 12V, 24V, 36V,
48V and 60V. Iterative calculations were performed to
select these values that achieve the best overall results.
Table 23. Standard Value Output Feedback Network vs Output
Regulation Voltage
BATTERY TARGET
VOLTAGE V (V)
S2
12 14.2
24 28.4
36 42.6
48 56.8
60 71.0

HW Config: Input Current Sense and Limit

This section discusses the LT8491's input current sens-
ing and limiting circuit and how to properly configure it.
Solar Panel Supply Considerations: Solar panels are
inherently current limited and may not be able to provide
maximum power at the lowest input voltages. The LT8491
uses its MPPT algorithm to scan the panel voltage as low
as 6V to find the maximum power point. Make sure that
the charger's input current limit is set higher than the
maximum panel current capability, plus at least 20% to
30% margin, in order to achieve the maximum charging
capability of the system.
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⎞ ⎛
+R R +R
DACO2 DACO1
+
⎠ ⎟ ⎝ ⎜
R
FBOUT1
voltage using the
S2
R R R
FBOUT1 FBOUT2 DACO1
(kΩ) (kΩ) (kΩ)
274 23.2 26.1
487 20 28
787 21 22.6
1000 20 22.6
866 13.7 13.3
⎤
⎞
DACO2
⎥
⎠ ⎟
⎦
R C
DACO2 DACO
(kΩ) (nF)
124 82
107 68
121 100
115 100
80.6 150
Rev. 0