Datasheet
LTC4000-1
18
40001fa
For more information www.linear.com/LTC4001-1
applicaTions inForMaTion
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 F LT 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 LTC4000-1 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 F LT 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.
Battery Instant-On and Ideal Diode External PMOS
Consideration
The instant-on voltage level is determined using the fol-
lowing formula:
V
OUT(INST _ON)
=
R
OFB1
+R
OFB2
R
OFB2
• 0.974V
Note that R
OFB1
and R
OFB2
are the same resistors that
program the output voltage regulation level. Therefore,
the output voltage regulation level is always 122.5% of
the instant-on voltage level.
During instant-on operation, it is critical to consider the
charging PMOS power dissipation. When the battery volt-
age is below the low battery threshold (V
LOBAT
), the power
dissipation in the PMOS can be calculated as follows:
P
TRKL
= 0.86 • V
FLOAT
– V
BAT
[ ]
• I
CLIM(TRKL)
where I
CLIM(TRKL)
is the trickle charge current limit.
On the other hand, when the battery voltage is above the
low battery threshold but still below the instant-on thresh-
old, the power dissipation can be calculated as follows:
P
INST _ ON
= 0.86 • V
FLOAT
– V
BAT
[ ]
• I
CLIM
where I
CLIM
is the full scale charge current limit.
For example, when charging a 3-cell Lithium Ion battery
with a programmed full charged current of 1A, the float
voltage is 12.6V, the bad battery voltage level is 8.55V and
the instant-on voltage level is 10.8V. During instant-on
operation and in the trickle charge mode, the worst case