LTC4000
26
4000fb
For more information
applicaTions inForMaTion
DESIGN EXAMPLE
In this design example, the LTC4000 is paired with the
LT3845A buck converter to create a 10A, 3-cell LiFePO
4
battery charger. The circuit is shown on the front page
and is repeated here in Figure 14.
• The input voltage monitor falling threshold is set at
14.3V according to the following formula:
R
VM1
=
14.3V
1.193V
−
1
• 100k
Ω ≈
1.10M
Ω
• The IL pin is left open such that the voltage on this pin
is >1.05V. The regulation voltage on the IIMON pin is
clamped at 1.0V with an accurate internal reference.
Therefore, the input current limit is set at 10A according
to the following formula:
R
IS
=
0.050V
10A
=
5m
Ω
• R
CL
is set at 24.9kΩ such that the voltage at the CL pin
is 1.25V. Similar to the IIMON pin, the regulation voltage
on the IBMON pin is clamped at 1V with an accurate
internal reference. Therefore, the charge current limit
is set at 10A according to the following formula:
I
CLIM(MAX)
=
0.050V
R
CS
=
0.050V
5m
Ω
=
10A
• The trickle charge current level is consequently set at
1.25A, according to the following formula:
I
CLIM(TRKL)
=
0.25µA • 24.9k
Ω
5m
Ω
=
1.25A
• The battery float voltage is set at 10.8V according to
the following formula:
R
BFB1
=
10.8
1.136
−
1
• 133k
Ω ≈
1.13M
Ω
• The bad battery detection time is set at 43 minutes
according to the following formula:
C
TMR
(nF)
=
t
BADBAT
(h) • 138.5
=
43
60
• 138.5
=
100nF
Figure 14. 48V to 10.8V at 10A Buck Converter Charger for Three LiFePO
4
Cells
1.13M
14.7k
127k
10k
10k
3-CELL Li-Ion
BATTERY PACK
10.8V FLOAT
10A MAX CHARGE
CURRENT
NTHS0603
N02N1002J
1.15M
47nF
5mΩ
12V, 15A
15V TO 60V
133k
CSN
CSP
BGATE
IGATE
BAT
OFB
FBG
BFB
NTC
CX
LTC4000
ITH
CC
IID
10M
5mΩ
LT3845A
100µF
OUT
V
C
SHDN
IN
RST
CLN
IN
ENC
CHRG
FLT
VM
IIMON
IBMON
22.1k
CL
IL
TMR
GND BIAS
22.1k
1µF
1.10M
100k
10nF
10nF
1µF
0.1µF
4000 F14
Si7135DP
Si7135DP
3.0V
