Hw Config: Current Sense Filtering - Analog Devices LINEAR LT8491 Manual

LT8491
APPLICATIONS INFORMATION
R
FROM
SENSE2
CONTROLLER
V
OUT1
OUTPUT
CURRENT
CSPOUT
LT8491
+
g = 1m
m
A6
–
1.61V
FAULT
CONTROL
+
IOW
R
IOW
R
IMON_OUT
Figure 16. Output Current Regulation Loop
the output current telemetry reported in the TELE_IOUT
register. The IOR voltage measurement is also used to
determine C/10 for charging state changes and termina-
tion as illustrated in Figure 8.
IMON_OUT voltages above 1.208V (typical) cause V
reduce due to EA1, and thus limit the output current. The
IOW pin is either driven to ground or floated depend-
ing on charging conditions. This allows the current limit
for Stage 0 (I ) to be set independently of the
OUT(MAXS0)
remaining stages (I
OUT(MAX)
R and R . Use the following equations to
IOW IMON_OUT
configure the current limits:
0.0497
R = Ω
SENSE2
I
OUT(MAX)
R =
IMON_OUT
I
OUT(MAXS0)
24.3k •R
IMON_OUT
=
R
IOW
R – 24.3k
IMON_OUT
= 3.01k
R
IOR
C = read below
IMON_OUT
where I is the maximum charging current (C)
OUT(MAX)
in Amps, I is the maximum trickle charging
OUT(MAXS0)
current in Stage 0 and I
66
TO BATTERY
CSNOUT
–
+
1.208V
EA1
–
IMON_OUT IOR
R
IOR
3.01k
C
IMON_OUT
) with proper selection of
1208
Ω
•R
SENSE2
Ω
is no greater than
OUT(MAXS0)
For more information
I . For cases where I
OUT(MAX)
OK to exclude R
I must be at least 20% of I
OUT(MAXS0)
C reduces IMON_OUT ripple and stabilizes the
IMON_OUT
constant charging current control loop. Reducing C
improves stability and minimizes inductor current
OUT
overshoot that can occur if a discharged battery is quickly
disconnected then reconnected to the charger. However,
this is at the expense of increased IMON_OUT ripple that
can introduce more noise into the ADC measurements.
V
C
The higher frequency pole created at IMON_OUT must be
8491 F16
adequately separated from the lower frequency pole at the
V pin for proper stability. A C
C
range of 4.7nF to 22nF is suitable for most applications.

HW Config: Current Sense Filtering

The C and R
SX SX
in Figure 17 can improve the accuracy of the input and
output current measurements at low average current lev-
els. Amplifiers A6 and A7 (Figure 15 and Figure 16) can
only amplify positive R
to
C
age R voltage is always positive, the voltage ripple
SENSE
at low average current levels may contain negative com-
ponents that are averaged out by the filter. Recommended
values for R , R
S1
C
C1
Figure 17. Recommended Current Sense Filter
C and C may be required, depending on board lay-
C1 C2
out, to reduce common mode noise that may reach the
LT8491 pins. 100nF ceramic capacitors, with the appro-
priate voltage ratings, work well in most cases. Be sure
to place all the filter components (C
the LT8491 for best performance.
Finally, note that a small voltage drop (typically ~0.25mV
per 10Ω) will occur across R
www.analog.com
= I
OUT(MAX)
from the PCB and float the IOW pin.
IOW
OUT(MAX)
IMON_OUT
current sense filter components shown
voltages. Although the aver-
SENSE
and C , C are 22Ω and 470nF.
S2 S1 S2
R R
SENSE1 SENSE2
R
S1
C C
S1
C
C2
CSPIN CSNIN CSPOUT CSNOUT
LT8491 LT8491
, R
SX SX
and R due to the input
S1 S2
it is
OUT(MAXS0)
.
IMON_
capacitor in the
R
S2
S2
8491 F17
, C ) close to
CX
Rev. 0