LTC4000
18
4000f
Charge Current Limit Setting and Monitoring
The regulated full charge current is set according to the
following formula:
RCS =
VCL
20 ICLIM
where VCL is the voltage on the CL pin. The CL pin is
internally pulled up with an accurate current source of
50A. Therefore, an equivalent formula to obtain the input
current limit is:
RCL =
ICLIM RCS
2.5A
ICLIM =
RCL
RCS
2.5A
The charge current through the sense resistor is available
for monitoring through the IBMON pin. The voltage on
the IBMON pin varies with the current through the sense
resistor as follows:
VIBMON = 20 IRCS RCS = 20 VCSP – VCSN
(
)
Similar to the IIMON pin, the regulation voltage level at
the IBMON pin is clamped at 1V with an accurate internal
reference. At 1V on the IBMON pin, the charge current
limit is regulated to the following value:
ICLIM(MAX)(A) =
0.050V
RCS(Ω)
When this maximum charge current limit is desired, leave
the CL pin open or set it to a voltage >1.05V such that
amplifierA5canregulatetheIBMONpinvoltageaccurately
to the internal reference of 1V.
WhentheoutputcurrentwaveformoftheDC/DCconverter
or the system load current is noisy, it is recommended that
a capacitor is connected to the CSP pin (CCSP). This is to
reduce the AC content of the current through the sense
resistor (RCS). Where the highest accuracy is important,
pick the value of CCSP such that the AC content is less
than or equal to 50% of the average voltage across the
sense resistor. Similar to the IIMON pin, the voltage on the
IBMON pin is filtered further by putting a capacitor on the
pin (CIBMON). This filter capacitor should not be arbitrarily
large as it will slow down the overall compensated charge
applicaTions inForMaTion
current regulation loop. For details on the loop compensa-
tion, refer to the Compensation section.
Battery Float Voltage Programming
WhenthevalueofRBFB1ismuchlargerthan100Ω,thefinal
float voltage is determined using the following formula:
RBFB1 =
VFLOAT
1.136V
– 1
RBFB2
When higher accuracy is important, a slightly more ac-
curate final float voltage can be determined using the
following formula:
VFLOAT =
RBFB1 +RBFB2
RBFB2
1.136V
–
RBFB1
RBFB2
VFBG
where VFBG is the voltage at the FBG pin during float
voltage regulation, which accounts for all the current
from all resistor dividers that are connected to this pin
(RFBG = 100Ω typical).
Low Battery Trickle Charge Programming and Bad
Battery Detection
When charging into an over-discharged or dead battery
(VBFB<VLOBAT),thepull-upcurrentattheCLpinisreduced
to 10% of the normal pull-up current. Therefore, the trickle
charge current is set using the following formula:
RCL =
ICLIM(TRKL) RCS
0.25A
ICLIM(TRKL) = 0.25A
RCL
RCS
Therefore, when 50ARCL is less than 1V, the following
relation is true:
ICLIM(TRKL) =
ICLIM
10
Oncethebatteryvoltagerisesabovethelowbatteryvoltage
threshold, the charge current level rises from the trickle
charge current level to the full charge current level.
The LTC4000 also features bad battery detection. This
detection is disabled if the TMR pin is grounded or tied
to BIAS. However, when a capacitor is connected to the