Datasheet

LT3652HV
15
3652hvfa
To satisfy the 250k equivalent resistance to the V
FB
pin:
R
FB3
= 250k − 27.5k
R
FB3
= 223k.
Because the V
FB
pin is a relatively high impedance node,
stray capacitances at this pin must be minimized. Special
attention should be given to any stray capacitances that
can couple external signals onto the pin, which can pro-
duce undesirable output transients or ripple. Effects of
parasitic capacitance can typically be reduced by adding
a small-value (20pF to 50pF) feedforward capacitor from
the BAT pin to the V
FB
pin.
Extra care should be taken during board assembly. Small
amounts of board contamination can lead to significant
shifts in output voltage. Appropriate post-assembly board
cleaning measures should be implemented to prevent
board contamination, and low-leakage solder flux is
recommended.
Input Supply Voltage Regulation
The LT3652HV 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 LT3652HV charger, the supply volt-
age will collapse. A minimum operating supply voltage can
thus be programmed by monitoring the supply through
a resistor divider, such that the desired minimum voltage
corresponds to 2.7V at the V
IN_REG
pin. The LT3652HV
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
APPLICATIONS INFORMATION
If the voltage regulation feature is not used, connect the
V
IN_REG
pin to V
IN
.
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 LT3652HV 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
Figure 6. Resistor Divider Sets Minimum V
IN
V
IN
V
IN_REG
R
IN2
R
IN1
LT3652HV
INPUT
SUPPLY
3652 F06
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