LTC4000_15 Datasheet, PDF (17/40 Pages) Linear Technology – High Voltage High Current Controller for Battery Charging and Power Management

LTC4000
Applications Information
The voltage on the IIMON pin can be filtered further by putting
a capacitor on the pin (C_IIMON). The voltage on the IIMON
pin is also the feedback input to the input current regulation
error amplifier. Any capacitor connected to this pin places
a pole in the input current regulation loop. Therefore, this
filter capacitor should NOT be arbitrarily large as it will slow
down the overall compensated loop. For details on loop
compensation please refer to the Compensation section.
Charge Current Limit Setting and Monitoring
The regulated full charge current is set according to the
following formula:
R_CS
=
20
V_CL
• I_CLIM
where V_CL is the voltage on the CL pin. The CL pin is
internally pulled up with an accurate current source of
50µA. Therefore, an equivalent formula to obtain the input
current limit is:
R_CL
=
I_CLIM • R_CS
2.5µA
⇒ I_CLIM
=
R_CL
R_CS
• 2.5µA
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:
V_IBMON = 20 • I_RCS • R_CS = 20 • ₍V_CSP – V_{CSN )}
reduce the AC content of the current through the sense
resistor (R_CS). Where the highest accuracy is important,
pick the value of C_CSP 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 (C_IBMON). This filter capacitor should not be arbitrarily
large as it will slow down the overall compensated charge
current regulation loop. For details on the loop compensa-
tion, refer to the Compensation section.
Battery Float Voltage Programming
When the value of R_BFB1 is much larger than 100Ω, the final
float voltage is determined using the following formula:
R_BFB1
=


V_FLOAT
1.136V
–
1^{}
R_BFB2
When higher accuracy is important, a slightly more ac-
curate final float voltage can be determined using the
following formula:
V_FLOAT
=



R_BFB1 + R_BFB2
R_BFB2
•
1.136V



–
 _RRBBFFBB21
•

V_FBG


where V_FBG 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
(R_FBG = 100Ω typical).
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:
I_CLIM(MAX
)^(A)
=
0.050V
R_CS(Ω)
When this maximum charge current limit is desired, leave
the CL pin open or set it to a voltage >1.05V such that
amplifier A5 can regulate the IBMON pin voltage accurately
to the internal reference of 1V.
When the output current waveform of the DC/DC converter
or the system load current is noisy, it is recommended that
a capacitor is connected to the CSP pin (C_CSP). This is to
Low Battery Trickle Charge Programming and Bad
Battery Detection
When charging into an over-discharged or dead battery
(V_BFB < V_LOBAT), the pull-up current at the CL pin is reduced
to 10% of the normal pull-up current. Therefore, the trickle
charge current is set using the following formula:
R_CL
=
I_CLIM(TRKL) • R_CS
0.25µA
⇒ I_CLIM(TRKL)
= 0.25µA
•
R_CL
R_CS
Therefore, when 50µA•R_CL is less than 1V, the following
relation is true:
I_{CLIM( TRKL )}
=
I_CLIM
10
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