Datasheet
LTM8062/LTM8062A
14
8062fd
For more information www.linear.com/LTM8062
applicaTions inForMaTion
Figure 5. Lead-Acid 6-Cell Float Charge Voltage vs Temperature
with a –19.8mV/°C Temperature Coefficient Using LM234 with
the Feedback Network
network shown in Figure 5, with the desired temperature
coefficient (TC) and 25°C float voltage (V
FLOAT
(25°C))
specified, and using a convenient value of 2.4k for R
SET
,
necessary resistor values follow the relations:
R
FB1
= –R
SET
• (TC • 4405)
= –2.4k • (–0.0198 • 4405) ⇒ 210kΩ
R
FB2
=
R
FB1
V
FLOAT
(25°C)+R
FB1
•(0.0674 /R
SET
)
V
FB
-
1
=
210k
13.5+210k •(0.0674 / 2.4k)
3.3
-1
⇒ 43kΩ
R
FB3
= 250k – R
FB1
||R
FB2
= 250k – 210k||43k ⇒ 215kΩ
(see the Battery Float Voltage Programming section)
While the circuit in Figure 5 creates a linear tempera-
ture characteristic that follows a typical –3.3mV/°C per
cell lead-acid specification, the theoretical float charge
voltage characteristic is slightly nonlinear. This nonlinear
characteristic follows the relation:
V
FLOAT
= 4 • 10
–5
(T
2
) – 6 • 10
–3
(T) + 2.375
(with a 2.18V minimum)
where T = temperature in °C. A thermistor-based network
can be used to approximate the nonlinear ideal tempera
-
ture characteristic across a reasonable operating range,
as shown in Figure 6.
Figure 6. Thermistor-Based Temperature Compensation Network
Programs V
FLOAT
to Closely Match Ideal Lead-Acid Float Charge
Voltage for 6-Cell Charger
TEMPERATURE (°C)
–10
V
FLOAT
(V)
10
5040
60
0 20 30
8062 F05b
12.6
12.8
13.0
13.2
13.4
13.6
13.8
14.0
14.2
14.3
–19.8mV/°C
TEMPERATURE (°C)
–10
V
FLOAT
(V)
10
5040
60
0 20 30
8062 F06b
12.8
13.0
13.2
13.4
13.6
13.8
14.0
14.6
14.4
14.2
14.8
THEORETICAL V
FLOAT
PROGRAMMED V
BAT(FLOAT)
8062 F06a
LTM8062
196k
69k 22k
B = 3380
69k
BAT
ADJ
198k
6-CELL
LEAD-ACID
BATTERY
+
8062 F05a
LTM8062
LINEAR
TECHNOLOGY
LM234
R
FB1
210k
R
FB2
43k
V
+
V
–
R
R
SET
2.4k
BAT
ADJ
R
FB3
215k
6-CELL
LEAD-ACID
BATTERY
+
Status Pins
The LTM8062/LTM8062A report charger status through
two open-collector outputs, the CHRG and FAULT pins.
These pins can be pulled up as high as V
IN
, and can sink
up to 10mA. The CHRG pin indicates that the charger is
delivering current at greater than a C/10 rate, or one-tenth