LTC4000-1 Datasheet, PDF (26/40 Pages) Linear Technology – High Voltage High Current Controller for Battery Charging with Maximum Power Point Control

LTC4000-1
Applications Information
R_IS
IN
C_IN
C_CLN
(OPTIONAL)
R_VM3
IN
CLN
RST
DC/DC INPUT
TO DC/DC EN PIN
VM
–
CP1
C_VM
1.193V +
R_VM4
IFB
–
–
R_IFB2
1V +
A4
+
LTC4000-1
CC
C_C
R_C
ITH
40001 F16
TO DC/DC
COMPENSATION
PIN
Figure 16. Input Voltage Monitoring and Input Voltage
Regulation Resistor Divider Combined
In this configuration use the following formula to determine
the values of the three resistors:
R_VM3
=

 ^1–
1.193V
V_{VM _ RST}




V_{IN _ REG}
1V


R_IFB2
R_VM4
=



1.193_
V_{IN _ REG}
V_{VM _ RST}


–

1_

R_IFB2
Note that for the R_VM4 value to be positive, the ratio of
V_{IN_REG} to V_{VM_RST} has to be greater than 0.838.
When the RST pin of the LTC4000-1 is connected to the
SHDN or RUN pin of the converter, it is recommended that
the value of V_{IN_REG} is set higher than the V_{VM_RST} pin by a
significant margin. This is to ensure that any voltage noise
or ripple on the input supply pin does not cause the RST
pin to shut down the converter prematurely, preventing
the input regulation loop from functioning as expected.
As discussed in the input current monitoring section,
noise issues on the input node can be reduced by placing
a large filter capacitor on the CLN node (C_CLN). To further
reduce the effect of any noise on the monitoring function,
another filter capacitor placed on the VM pin (C_VM) is
recommended.
MPPT Temperature Compensation – Solar Panel
Example
The input regulation loop of the LTC4000-1 allows a user to
program a minimum input supply voltage regulation level
allowing for high impedance source to provide maximum
available power. With typical high impedance source such
as a solar panel, this maximum power point varies with
temperature.
A typical solar panel is comprised of a number of series-
connected 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 junction diode, about –2mV/°C. The peak
power point voltage (V_MP) for a crystalline solar panel
can be approximated as a fixed percentage of 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.
V_OC(25°C)
V_MP(25°C)
V_OC TEMP CO.
V_OC
V_MP
V_OC – V_MP
5
15
25
35
45
55
TEMPERATURE (°C)
40001 F17
Figure 17. Temperature Characteristic of a Solar
Panel Open Circuit and Peak Power Point Voltages
In a manner similar to the battery float voltage temperature
compensation, implementation of the MPPT temperature
compensation can be accomplished by incorporating an
LM234 into the input voltage feedback network. Using the
40001f
26