Datasheet

LT3652HV
16
3652hvfa
power tracking (MPPT) application. As the temperature
characteristic for a typical solar panel V
MP
voltage is highly
linear, a simple solution for tracking that characteristic can
be implemented using an LM234 3-terminal temperature
sensor. This creates an easily programmable, linear tem-
perature dependent characteristic.
In the circuit shown in figure 8,
APPLICATIONS INFORMATION
Battery Voltage Temperature Compensation
Some battery chemistries have charge voltage require-
ments that vary with temperature. Lead-acid batteries in
particular experience a significant change in charge volt-
age requirements as temperature changes. For example,
manufacturers of large lead-acid batteries recommend
a float charge of 2.25V/cell at 25°C. This battery float
voltage, however, has a temperature coefficient which is
typically specified at –3.3mV/°C per cell.
In a manner similar to the MPPT temperature correction
outlined previously, implementation of linear battery
charge voltage temperature compensation can be ac-
complished by incorporating an LM234 into the output
feedback network.
For example, a 6-cell lead acid battery has a float charge
voltage that is commonly specified at 2.25V/cell at 25°C,
or 13.5V, and a –3.3mV/°C per cell temperature coefficient,
or –19.8mV/°C. Using the feedback network shown in
Figure 9, with the desired temperature coefficient (TC)
Figure 8. MPPT Temperature Compensation Network
V
IN_REG
LT3652HV
V
IN
V
IN
LM234
3658 F08
V
+
V
–
R
R
SET
R
IN1
R
IN2
R
IN1
= –R
SET
• (TC • 4405), and
R
IN2
= R
IN1
/({[V
MP(25°C)
+ R
IN1
• (0.0674/R
SET
)]/V
IN_REG
} – 1)
Where: TC = temperature coefficient (in V/°C), and
V
MP(25°C)
= maximum power voltage at 25°C
For example, given a common 36-cell solar panel that has
the following specified characteristics:
Open Circuit Voltage (V
OC
) = 21.7V
Maximum Power Voltage (V
MP
) = 17.6V
Open-Circuit Voltage Temperature Coefficient (V
OC
) =
–78mV/°C
As the temperature coefficient for V
MP
is similar to that
of V
OC
, the specified temperature coefficient for V
OC
(TC) of –78mV/°C and the specified peak power voltage
(V
MP(25°C)
) of 17.6V can be inserted into the equations to
calculate the appropriate resistor values for the tempera-
ture compensation network in Figure 8. With R
SET
equal
to 1000Ω, then:
R
SET
= 1k
R
IN1
= –1k • (–0.078 • 4405 ) = 344k
R
IN2
= 344k/({[17.6 + 344k • (0.0674/1k)]/2.7} – 1)
= 24.4k
LT3652HV
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