Data Sheet

LT3652
16
3652fe
For more information www.linear.com/LT3652
cleaning measures should be implemented to prevent
board contamination, and low-leakage solder flux is
recommended.
Input Supply Voltage Regulation
The LT3652 contains a voltage monitor pin that enables
programming a minimum operational voltage. Connect
-
ing a resistor divider from V
IN
to the V
IN_REG
pin enables
programming of minimum input supply voltage, typically
used to program the peak power voltage for a solar panel.
Maximum charge current is reduced when the V
IN_REG
pin
is below the regulation threshold of 2.7V.
If an input supply cannot provide enough power to satisfy
the requirements of an LT3652 charger, the supply voltage
will collapse. A minimum operating supply voltage can
thus be programmed by monitoring the supply through
a resistor divider, such that the desired minimum volt
-
age corresponds to 2.7V at the V
IN_REG
pin. The LT3652
servos the maximum output charge current to maintain
the voltage on V
IN_REG
at or above 2.7V.
Programming of the desired minimum voltage is ac
-
complished by connecting a resistor divider as shown in
Figure 6. The ratio of R
IN1
/R
IN2
for a desired minimum
voltage (V
IN(MIN)
) is:
R
IN1
/R
IN2
= (V
IN(MIN)
/2.7) – 1
If the voltage regulation feature is not used, connect the
V
IN_REG
pin to V
IN
.
Figure 6. Resistor Divider Sets Minimum V
IN
V
IN
V
IN_REG
R
IN2
R
IN1
LT3652
INPUT
SUPPLY
3652 F06
APPLICATIONS INFORMATION
Figure 7. Temperature Characteristics for Solar Panel
Output Voltage
TEMPERATURE (°C)
5
PANEL VOLTAGE (V)
25
45
55
15 35
3652 F07
V
OC(25°C)
V
MP(25°C)
V
MP
V
OC
V
OC
TEMP CO.
V
OC
– V
MP
MPPT Temperature Compensation
A typical solar panel is comprised of a number of series-con
-
nected cells, each cell being a forward-biased p-n junction.
As such, the open-circuit voltage (V
OC
) of a solar cell has
a temperature coefficient that is similar to a common p-n
diode, or about –2mV/°C. The peak power point voltage
(V
MP
) for a crystalline solar panel can be approximated as
a fixed voltage below V
OC
, so the temperature coefficient
for the peak power point is similar to that of V
OC
.
Panel manufacturers typically specify the 25°C values for
V
OC
, V
MP
, and the temperature coefficient for V
OC
, making
determination of the temperature coefficient for V
MP
of a
typical panel straight forward.
The LT3652 employs a feedback network to program the
V
IN
input regulation voltage. Manipulation of the network
makes for efficient implementation of various temperature
compensation schemes for a maximum peak power track
-
ing (MPPT) application. As the temperature characteristic
for a typical solar panel V
MP
voltage is highly linear, a