Data Sheet

LT3652
18
3652fe
For more information www.linear.com/LT3652
APPLICATIONS INFORMATION
or –19.8mV/°C. Using the feedback network shown in
Figure 9, 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) = 210k
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)
= 43k
R
FB3
= 250k - R
FB1
||R
FB2
= 250k – 210k||43k = 215k (see the Battery Float
Voltage Programming section)
While the circuit in Figure 9 creates a linear temperature
characteristic that follows a typical –3.3mV/°C per cell
lead-acid specification, the theoretical float charge voltage
characteristic is slightly nonlinear. This nonlinear charac
-
teristic follows the relation V
FLOAT(1-CELL)
= 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 temperature
characteristic across a reasonable operating range, as
shown in Figure 10.
LT3652
R
FB3
215k
R
FB2
43k
R
SET
2.4k
R
FB1
210k
6-CELL
LEAD-ACID
BATTERY
LM234
3652 F09a
V
+
V
–
R
BAT
V
FB
+
Figure 9. Lead-Acid 6-Cell Float Charge Voltage vs
Temperature Has –19.8mV/°C Characteristic Using LM234 with
Feedback Network
TEMPERATURE (°C)
–10
V
FLOAT
(V)
10
5040
60
0 20 30
3652 F09b
12.6
12.8
13.0
13.2
13.4
13.6
13.8
14.0
14.2
14.3
–19.8mV/°C
LT3652
196k
198k
6-CELL
LEAD-ACID
BATTERY
22k
B = 3380
3652 F10a
BAT
69k
69k
V
FB
+
TEMPERATURE (°C)
–10
V
FLOAT
(V)
10
5040
60
0 20 30
3652 F10b
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)
Figure 10. Thermistor-Based Temperature Compensation
Network Programs V
FLOAT
to Closely Match Ideal Lead-Acid
Float Charge Voltage for 6-Cell Charger