LTC4110
35
4110fb
APPLICATIONS INFORMATION
the highest current fl ow between charge and calibration
modes, whichever is greater.
R
mV
I
SNS BAT
MAX
(
)
=
100
See Table 10 for example values.
R
SNS(BAT)
accuracy is intentionally made very high to permit
development of an accurate host software based capacity
measurements of standard batteries. Use resistors with
1% accuracy or better or use a 4-terminal Kelvin sensing
resistor. See the PCB Layout section for a reasonable no
cost Kelvin sensing layout that permits the use of less
expensive standard two terminal sense resistors. For more
electrical information relating to R
SNS(BAT)
itself, see the
Component Selection section.
As designed, any signifi cant AC ripple voltage seen by
CSP and CSN pins can lead to current sensing errors for
both current programming and capacity measurements.
To prevent the Flyback’s AC ripple voltage from interfering
with DC accuracy, R
SNS(BAT)
must have a RC fi lter network
installed between the R
SNS(BAT)
and CSP and CSN pins.
The CSP and CSN pins have an input bias current of ±10nA
typically. A very large R
CSP1
+ R
CSP2
value will cause a
large current mismatch error. The current fl owing into
the CSP and CSN pins equals V
SNS
/(R
CSP1
+ R
CSP2
) =
100mV/(R
CSP1
+ R
CSP2
), a very small R
CSP1
+ R
CSP2
value
will result in a large current. Typically a value between 3k
and 30k gives the best performance.
Figure 12. CSP, CSN RC Filter
C
CSP
330n
C
CSN
330n
R
SNS(BAT)
100mV
R
CSP2
2k
R
CSN2
2k
R
CSP1
1k
RCSN1
1k
4110 F12
–
+
LTC4110
CA
Recommended starting values for the fi lter is:
R
CSP1
= R
CSN1
between 1K and 2K
R
CSP1
+ R
CSP2
= R
CSN1
+ R
CSN2
= about 3K
C
CSP
= C
CSN
= about 3 • C
ITH
.
Figure 12 shows typical values for C
ITH
= 0.1μF
R
SNS(FET)
The LTC4110’s Flyback converter operates in current mode
with R
SNS(FET)
monitoring cycle-by-cycle transformer cur-
rent in both Charge and Calibration modes. The LTC4110’s
I
SENSE
pin serves two functions. First is to regulate the
primary current as required by the feedback loop. Second
is to monitor the secondary current and check for short
circuits. The traditional Flyback primary and secondary
currents look like the following:
Figure 13. Flyback Primary and Secondary Current
ΔI
I
PRI
SECONDARY CURRENT
PRIMARY CURRENT
I
PRI
N
0
0
4110 F13
The waveforms in Figure 13 each assume a view of posi-
tive current fl ow into the load. The value N represents the
ratio of the secondary to the primary with the primary
set to a value of 1. Unlike a traditional Flyback topology,
the LTC4110 Flyback is bi-directional, so the meaning of
“primary” is a function of the operating mode. In order
