Datasheet

LTC4000-1
23
40001fa
For more information www.linear.com/LTC4001-1
applicaTions inForMaTion
On the other hand, the LTC4000-1 is also easily configu-
rable to handle lead-acid based battery charging. One of
the common methods used in lead-acid battery charging
is called 3-step charging (Bulk, Absorption and Float).
Figure14 and Table 3 summarize the normal steps involved
in a typical 3-step charging of a lead-acid battery.
Figure 14. Lead-Acid 3-Step Charging Cycle
Figure 15. 3-Step Lead-Acid Circuit Configuration
CHARGE TIME
40001 F14
FLOAT
(STORAGE)
BULK
CHARGE
CHARGE
CURRENT
BATTERY VOLTAGE
ABSORPTION
R
BFB2
R
BFB3
BFB
CSP
CX
CHRG
FBG
LTC4000-1
CSN
BAT
R
CS
R
CX
CL
R
CL
R
BFB1
40001 F15
LEAD-ACID
BATTERY
FROM DC/DC
OUTPUT
When a charging cycle is initiated, the CHRG pin is pulled
low. The charger first enters the bulk charge step, charg-
ing the battery with a constant current programmed at
the CL pin:
I
CLIM
=
MIN 50mV, 2.5µA R
CL
( )
R
CS
When the battery voltage rises to the Absorption voltage
level:
V
ABSRP
=
R
BFB1
R
BFB2
+R
BFB3
( )
R
BFB2
R
BFB3
+1
1.136V
the charger enters the Absorption step, charging the bat-
tery at a constant voltage at this absorption voltage level.
As the charge current drops to the C/X level:
I
CLIM
=
0.25µA R
CX
( )
0.5mV
R
CS
the CHRG pin turns high impedance and now the charger
enters the Float (Storage) step, charging the battery volt-
age at the constant float voltage level:
V
FLOAT
=
R
BFB1
R
BFB2
+1
1.136V
Table 3. Lead-Acid Battery Charging Steps
STEP CHARGE METHOD DURATION
Bulk Charge Constant Current Until Battery Voltage Reaches
Absorption Voltage
No Time Limit
Absorption Constant Voltage
at the Absorption
Voltage Level
Terminate When Charge
Current Falls to the
Programmed Level at the CX
Pin
Float (Storage) Constant Voltage
at the Lower Float
Voltage Level (Float
Voltage Is Lower
than the Absorption
Voltage)
Indefinite
Recharge Initiate Bulk Charge
Again When Battery
Voltage Drops Below
Recharge Threshold
Figure 15 shows the configuration needed to implement
this 3-step lead-acid battery charging with the LTC4000-1.