LTC4000
18
4000fb
For more information
applicaTions inForMaTion
Once the battery voltage rises above the low battery voltage
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
TMR pin, a bad battery detection timer is started as soon
as trickle charging starts. If at the end of the bad battery
detection time the battery voltage is still lower than the
low battery threshold, charging is terminated and the part
indicates a bad battery condition by pulling the
FLT
pin low
and leaving the
CHRG
pin high impedance.
The bad battery detection time can be programmed ac-
cording to the following formula:
C
TMR
(nF)
=
t
BADBAT
(h) • 138.5
Note that once a bad battery condition is detected, the
condition is latched. In order to re-enable charging, re-
move the battery and connect a new battery whose voltage
causes BFB to rise above the recharge battery threshold
(V
RECHRG(RISE)
). Alternatively toggle the ENC pin or remove
and reapply power to IN.
C/X Detection, Charge Termination and Automatic
Recharge
Once the constant voltage charging is reached, there are
two ways in which charging can terminate. If the TMR pin
is tied to BIAS, the battery charger terminates as soon as
the charge current drops to the level programmed by the
CX pin. The C/X current termination level is programmed
according to the following formula:
R
CX
=
I
C/X
•R
CS
(
)
+
0.5mV
0.25µA
⇒
I
C/X
=
0.25µA •R
CX
(
)
−
0.5mV
R
CS
where R
CS
is the charge current sense resistor connected
between the CSP and the CSN pins.
When the voltage at BFB is higher than the recharge
threshold (97.6% of float), the C/X comparator is enabled.
In order to ensure proper C/X termination coming out of
a paused charging condition, connect a capacitor on the
CX pin according to the following formula:
C
CX
= 100C
BGATE
where C
BGATE
is the total capacitance connected to the
BGATE pin.
For example, a typical capacitance of 1nF requires a capaci-
tor greater than 100nF connected to the CX pin to ensure
proper C/X termination behavior.
If a capacitor is connected to the TMR pin, as soon as the
constant voltage charging is achieved, a charge termina-
tion timer is started. When the charge termination timer
expires, the charge cycle terminates. The total charge
termination time can be programmed according to the
following formula:
C
TMR
(nF)
=
t
TERMINATE
(h) • 34.6
If the TMR pin is grounded, charging never terminates and
the battery voltage is held at the float voltage. Note that
regardless of which termination behavior is selected, the
CHRG
and
FLT
pins will both assume a high impedance
state as soon as the charge current falls below the pro-
grammed C/X level.
After the charger terminates, the LTC4000 automatically
restarts another charge cycle if the battery feedback voltage
drops below 97.1% of the programmed final float voltage
(V
RECHRG(FALL)
). When charging restarts, the
CHRG
pin
pulls low and the
FLT
pin remains high impedance.
Output Voltage Regulation Programming
The output voltage regulation level is determined using
the following formula:
R
OFB1
=
V
OUT
1.193
−
1
• R
OFB2
As in the battery float voltage calculation, when higher
accuracy is important, a slightly more accurate output is
determined using the following formula:
V
OUT
=
R
OFB1
+
R
OFB2
R
OFB2
• 1.193V
–
R
OFB1
R
OFB2
• V
FBG
where V
FBG
is the voltage at the FBG pin during output
voltage regulation, which accounts for all the current from
all resistor dividers that are connected to this pin.
