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

APPLICATIONS INFORMATION
In addition, note that the LT8491 uses the same circuit
(shown in Figure 15) to measure the input current as to
limit it. The input current is measured by an A/D conver-
sion of the IIR pin which is connected to IMON_IN and is
proportional to input current. The digitized input current
measurement is used to locate the maximum power point
of the solar panel. Setting a higher input current limit
reduces the resolution of the digitized reading of the input
current. Avoid setting the input current limit significantly
higher than necessary, as this may affect the accuracy of
the maximum power point calculations.
DC Power Supply Considerations: When charging a battery
at the maximum charging current limit, and thus power, a
low voltage input supply must provide more current than
a high voltage input supply. This can be seen by equating
output power to input power, less some efficiency loss.
V • I • η = V • I
IN IN BAT BAT
or
V •I
BAT BAT(MAX)
I =
IN(MAX)
• η
V
IN(MIN)
Where the efficiency factor η is typically between 0.95
and 0.99.
When powered by a DC voltage supply, appropriate input
current limiting is recommended for supplies that might
(1) become overloaded as the supply ramps up or down
through 6V or (2) provide more input current than the
charger components can tolerate.
Configuring the Input Current Sense and Limit: The input
current is sensed through R
The current through R
SENSE1
on the IMON_IN pin according to the following equation:
⎡ I •R
⎛
IN
SENSE1
V =
⎝ ⎜
⎢
IMON_IN
1000
⎣
The LT8491 performs an A/D conversion of the IIR pin
voltage to report input current telemetry in the TELE_IIN
register. IMON_IN voltages exceeding 1.208V (typical) are
detected by EA2 and cause the V
limiting the input current. R
as shown in Figure 15.
SENSE1
is converted to a voltage
⎤
⎞
+ 7µA •R ⎥ V
⎠ ⎟
IMON_IN
⎦
voltage to reduce, thus
C
should be 21kΩ ±1%
IMON_IN
For more information
or better. Using this information, the appropriate value for
R can be calculated using the following equation:
SENSE1
1.208V
⎛
1000 •
⎝ ⎜
21kΩ
R =
SENSE1
I
IN(MAX)
where I is the desired maximum input current
IN(MAX)
limit in Amps. R
SENSE1
not recommended.
R
FROM SOLAR
SENSE1
PANEL OR
DC POWER SUPPLY
OUTPUT
CURRENT
CSPIN
LT8491
–
+
+
g
m
–
1.61V
FAULT
CONTROL
+
IIR
21k
R
IMON_IN
Figure 15. Input Current Regulation Loop
C reduces IMON_IN ripple and stabilizes the input
IMON_IN
current limit control loop. Reducing C
stability and minimizes inductor current overshoot.
However, this is at the expense of increased IMON_IN
ripple that can introduce more noise into the ADC mea-
surements. The higher frequency pole created at IMON_IN
must be adequately separated from the lower frequency
pole at the V pin for proper stability. A C
C
of 4.7nF to 22nF is suitable for most applications.

HW Config: Output Current Sense and Limit

The maximum battery charging current, commonly
referred to as C, is configured with the output current
sensing and limiting circuit. The output current is sensed
through R and converted to a proportional current
SENSE2
flowing out of the IMON_OUT pin (see Figure 16). An A/D
measurement of the IOR pin voltage is used to determine
www.analog.com
LT8491
⎞
– 7nA
⎠ ⎟
0.0505
= Ω
I
IN(MAX)
values greater than 25mΩ are
TO REMAINDER
OF SYSTEM
CSNIN
7mV
–
= 1m
A7
+
1.208V
EA2
–
IMON_IN
C
IMON_IN
improves
IMON_IN
capacitor
IMON_IN
65
V
C
8491 F15
Rev. 0