LTC4110_15 Datasheet, PDF (35/52 Pages) Linear Technology – Battery Backup System Manager

LTC4110
APPLICATIONS INFORMATION
the highest current flow between charge and calibration
modes, whichever is greater.
R_SNS(BAT)
=
100mV
I_MAX
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 significant 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 filter 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 flowing 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.
Recommended starting values for the filter 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:
ΔI
I_PRI
PRIMARY CURRENT
0
0
I_PRI
N
SECONDARY CURRENT
4110 F13
Figure 13. Flyback Primary and Secondary Current
The waveforms in Figure 13 each assume a view of posi-
tive current flow 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
LTC4110
CA
R_CSP2
2k
R_CSN2
2k
C_CSP
330n
R_CSP1
1k
RCSN1
1k
C_CSN
330n
R_SNS(BAT)
100mV
Figure 12. CSP, CSN RC Filter
4110 F12
4110fb
35