Datasheet

LTM8062/LTM8062A
13
8062fd
For more information www.linear.com/LTM8062
applicaTions inForMaTion
efficient implementation of various temperature compensa-
tion schemes for a maximum peak 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 a
Linear Technology LM234 3-terminal temperature sensor.
This creates an easily programmable, linear temperature
dependent characteristic.
In the circuit shown in Figure 4,
100V
MP
25°C
(
)
V
INREG
–100
1–
1 0000 0 •0.0677
V
INREG
•R
SET







kΩ
R
SET
=100
1
TC•4405
+
0.0677
V
INREG
–



R
IN
=







V
MP
25°C
(
)
TC•4405• V
INREG



kΩ
where TC = temperature coefficient (in V/°C), and
V
MP
(25°C) = maximum power voltage at 25°C.
Figure 4. MPPT Temperature Compensation Network
V
INREG
8062 F04
LTM8062
LINEAR
TECHNOLOGY
LM234
R
IN
V
IN
V
IN
V
+
V
–
R
R
SET
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 temperature
compensation network in Figure 4. Initially, determine the
R
SET
value using the following equation:
R
SET
=100
1
–78mV/°C• 4405


+
0.0677
2.7
–
17.6
–78mV / °C• 4405•2.7




kΩ ⇒ 4.12kΩ
Then, R
IN
can be determined using the calculated R
SET
value:
R
IN
=
100•17.6V
2.7
–100
1–
100000 •0.0677
2.7 • 4120




kΩ ⇒1400kΩ






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 accomplished by
incorporating a Linear Technology 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 tem
-
perature coefficient, or –19.8mV/°C. Using the feedback
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