Optional: Remote Battery Voltage Sensing - Analog Devices LINEAR LT8491 Manual

LT8491
APPLICATIONS INFORMATION
low, as discussed in the HW Config: MODE Pin – Current
Conduction Mode section, eliminates most of the current
draw from EXTV when the charging current becomes
CC
low. However, there is a 305kΩ (typical) path from EXTV
to ground through the LT8491 at all times. If desired, this
current path can be disconnected through the optional
circuit shown in Figure 32.
TO CHARGER OUT
AT R
SENSE2
LT8491
EXTV
CC
1µF
ECON
GND
M6: ZVP3310F
Q4: MMBT5550L
Z2: BZT52C13
Figure 32. Optional EXTV
The LT8491, via the ECON signal, disconnects EXTV
from the battery when charging current becomes low.
Charging current is monitored by measuring the IMON_
OUT pin voltage with the IOR pin's A/D input. When
IMON_OUT falls below 122mV (typical) the ECON signal
goes low and EXTV is disconnected from the LT8491.
CC
When IMON_OUT rises above 195mV (typical) the ECON
signal goes high and EXTV
LT8491.
Follow the same recommendations and equations from
the previous section for choosing components for the
optional EXTV disconnect circuit.
CC

Optional: Remote Battery Voltage Sensing

The LT8491 senses the battery voltage continually during
charging. The apparent battery voltage is sensed from
ground of the LT8491 to the top of R
ing, resistance in the battery cables (R
Figure 33) causes the apparent voltage to be higher than
the actual battery voltage by 2 • V
74
R
VGS2
Z2
100k
(OPT.)
M6
+
OPTIONAL
10
EXTV
CC
DISCONNECT
CIRCUIT
–
Q4
R
LIM4
200k
26.1k
8491 F32
Disconnect Circuit
CC
is reconnected to the
CC
. During charg-
FBOUT1
+
/R
CABLE CABLE
.
IR
For more information
TO CHARGER OUT
LT8491
FBOUT
CC
FBOR
R
FBOW
GND
Figure 33. IR Drop Present in Battery Connection
The effects of this cable drop are most significant when
charging low voltage batteries at high currents. As an
example, a 4-foot battery cable using 14 AWG wire can
V
BAT
have a voltage drop exceeding 0.5V at 15A of current. Note
however that the voltage drop, along with the charging
current, reduces automatically as the battery approaches
full charge.
The most significant effects from the V
are as follows:
1. When approaching full charge in Stage 2, the V
causes the charger to reduce the charging current ear-
lier than otherwise necessary. This increases the total
CC
charging time.
2. Terminating at C/10 in Stage 2 will occur at a reduced
battery voltage equal to C/10 • (R
which is 10% of the voltage drop at full charging
current.
3. The STATUS pin will indicate a transition from Stage 1
to Stage 2 earlier than would otherwise occur without
the cable drop.
4. The V telemetry measurement will have some error
BAT
since the measurement is taken at the FBOR pin of the
LT8491.
Again, these effects become less significant at higher bat-
tery voltages because the charging current is typically
lower, and the cable drop becomes a smaller percentage
of the total battery voltage. Using thicker and/or shorter
battery cables is the simplest method for reducing these
–
in
effects. Otherwise the remote battery sensing circuit in
Figure 34 can correct for these effects.
www.analog.com
I
CHARGE
+
AT R
SENSE2
R
CABLE
R
FBOUT1
R
DACO1 DACO2
R
CABLE
R
C
FBOUT2
DACO
–
IR
CABLE
–
V
IR
+
+
V
BAT
–
–
+
V
IR
8491 F33
voltage drops
error
IR
+ –
+ R )
CABLE
Rev. 0