LTC4000
29
4000fb
For more information
applicaTions inForMaTion
This same procedure is then repeated for the other four
loops: the input current regulation, the output voltage
regulation, the battery float voltage regulation and finally
the charge current regulation when V
OFB
> V
OUT(INST_ON)
.
Note that the resulting optimum values for each of the loops
may differ slightly. The final values of C
C
and R
C
are then
selected by combining the results and ensuring the most
conservative response for all the loops. This usually entails
picking the largest value of C
C
and the smallest value of
R
C
based on the results obtained for all the loops. In this
particular example, the value of C
C
is finally set to 47nF
and R
C
= 14.7kΩ.
BOARD LAYOUT CONSIDERATIONS
In the majority of applications, the most important param-
eter of the system is the battery float voltage. Therefore,
the user needs to be extra careful when placing and routing
the feedback resistor R
BFB1
and R
BFB2
. In particular, the
battery sense line connected to R
BFB1
and the ground return
line for the LTC4000 must be Kelvined back to where the
battery output and the battery ground are located respec-
tively. Figure 19 shows this Kelvin sense configuration.
For accurate current sensing, the sense lines from R
IS
and R
CS
(Figure 19) must be Kelvined back all the way
to the sense resistors terminals. The two sense lines of
each resistor must also be routed close together and away
from noise sources to minimize error. Furthermore, cur-
rent filtering capacitors should be placed strategically to
ensure that very little AC current is flowing through these
sense resistors as mentioned in the applications section.
The decoupling capacitors C
IN
and C
BIAS
must be placed
as close to the LTC4000 as possible. This allows as short
a route as possible from C
IN
to the IN and GND pins, as
well as from C
BIAS
to the BIAS and GND pins.
In a typical application, the LTC4000 is paired with an
external DC/DC converter. The operation of this converter
often involves high dV/dt switching voltage as well as high
currents. Isolate these switching voltages and currents
from the LTC4000 section of the board as much as pos-
sible by using good board layout practices. These include
separating noisy power and signal grounds, having a good
low impedance ground plane, shielding whenever neces-
sary, and routing sensitive signals as short as possible
and away from noisy sections of the board.
Figure 19. Kelvin Sense Lines Configuration for LTC4000
4000 F19
V
IN
CSN
CLN
IN
CSP
BAT
GND
LTC4000
R
C
ITH
GND
SWITCHING
CONVERTER
BGATE
ITH
IID
CC
C
C
IGATE
R
BFB1
R
IS
R
CS
R
BFB2
BFB
FBG
SYSTEM LOAD
