Optional Features; Optional: Low Power Mode; Optional: Output Feedback Resistor Disconnect - Analog Devices LINEAR LT8491 Manual

LT8491
APPLICATIONS INFORMATION

OPTIONAL FEATURES

Optional: Low Power Mode

When current from the solar panel is not high enough to
reliably measure the maximum power point, the LT8491
will automatically begin operating in low power mode.
Low power mode is automatically disabled when operat-
ing from a DC supply in power supply mode. The low
power mode feature is enabled by default register set-
ting CFG_CHRG_MISC→LP_MODE_EN=1 and allows the
LT8491 to charge a battery under very low light conditions
that would otherwise cause the LT8491 to stop charging.
Low power mode can be disabled by setting CFG_CHRG_
MISC→LP_MODE_EN=0. See Configure Misc. section for
more information.
In low power mode, the LT8491 momentarily stops charg-
ing, allowing the panel voltage to rise. When the panel has
sufficiently charged the input capacitor, the LT8491 trans-
fers energy from the input capacitor to the battery while
drawing down the panel voltage. This behavior repeats
rapidly, delivering charge to the battery as shown in the
Panel Voltage in Low Power Mode plot in the Typical
Performance Characteristics section.
Minimum Input Capacitance for Low Power Mode: A
minimum amount of energy must be transferred from
the input capacitor to the battery during each charge
transfer cycle. Otherwise the battery may be drained
instead of being charged. Figure 28 shows the minimum
input capacitance required when the charger is operating
near the 10V minimum input voltage. As the panel volt-
age rises, due to increased illumination, more energy is
stored in the input capacitor and a corresponding increase
of energy is delivered to the battery. Carefully check the
solar panel voltage for good stability and minimal ripple
when operating with low input capacitance.
Minimum Input Voltage: With low power mode enabled,
the panel voltage must initially exceed 10V (typical – as
measured through the VINR pin) before the charger will
attempt to charge the battery. If the panel voltage is below
10V, the charger may temporarily wait for more voltage
before transferring the input charge to the battery.
72
Exiting Low Power Mode: The charger will automatically
exit low power mode and resume normal charging after
adequate input power is detected. The charger typically
requires the input current to exceed 2.5% to 3% of the
maximum input current limit to make a valid power point
reading and exit low power mode. The panel voltage may
be adjusted as low as 6V when searching for the maxi-
mum power point.

Optional: Output Feedback Resistor Disconnect

To measure and regulate the battery voltage, the LT8491
uses the resistor feedback network discussed in the HW
Config: V
BAT
tors are disconnected from the battery, they will draw
current from the battery even when it is not charging as
shown in Figure 29. This may be undesirable when using
small capacity batteries.
Figure 29. Battery Discharge when Not Charging
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250
200
150
100
50
0
0 10 20 30 40 50 60
BATTERY VOLTAGE (V)
Figure 28. Minimum Input Capacitance
Required for Low Power Mode
in Stage 2 (V ) section. Unless these resis-
S2
I
LT8491
DRAIN
FBOUT
FBOR
R R
DACO1 DACO2
FBOW
C
DACO
SWEN
SWENO
200k
GND
70 80
8491 F28
+
R
FBOUT1
V
BAT
R
FBOUT2
–
8491 F29
Rev. 0