LTC4000-1 Datasheet, PDF (27/40 Pages) Linear Technology – High Voltage High Current Controller for Battery Charging with Maximum Power Point Control

LTC4000-1
Applications Information
feedback network in Figure 18, a similar set of equations
can be used to determine the resistor values:
R_IFB1 = –R_SET • (TC • 4405)
and
R_IFB2
=


V_MP(25°C)
R_IFB1 • 1V
+
R_IFB1
•


0.0677
R_SET




–
1V
Where: TC = temperature coefficient in V/°C, and
V_MP(25°C) = maximum power point voltage at 25°C in V.
V_IN
LM234
V⁺
V^–
R
R_SET
1k
R_IFB1
348k
R_IFB2
8.66k
IN
LTC4000-1
IFB
40001 F18
Figure 18. Maximum Power Point Voltage Temperature
Compensation Feedback Network
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 temperature compensa-
tion network in Figure 18. With R_SET equal to 1kΩ, then:
R_SET = 1kΩ, R_IFB1 = 348kΩ, R_IFB2 = 8.66kΩ.
Compensation
In order for the LTC4000-1 to control the external DC/DC
converter, it has to be able to overcome the sourcing bias
current of the ITH or VC pin of the DC/DC converter. The
typical sinking capability of the LTC4000-1 at the ITH pin
is 1mA at 0.4V with a maximum voltage range of 0V to 6V.
It is imperative that the local feedback of the DC/DC con-
verter be set up such that during regulation of any of the
LTC4000-1 loops this local loop is out of regulation and
sources as much current as possible from its ITH/VC pin.
For example for a DC/DC converter regulating its output
voltage, it is recommended that the converter feedback
divider is programmed to be greater than 110% of the
output voltage regulation level programmed at the OFB pin.
There are four feedback loops to consider when setting
up the compensation for the LTC4000-1. As mentioned
before these loops are: the input voltage loop, the charge
current loop, the float voltage loop and the output volt-
age loop. All of these loops have an error amp (A4-A7)
followed by another amplifier (A10) with the intermediate
node driving the CC pin and the output of A10 driving the
ITH pin as shown in Figure 19. The most common com-
pensation network of a series capacitor (C_C) and resistor
(R_C) between the CC pin and the ITH pin is shown here.
Each of the loops has slightly different dynamics due to
differences in the feedback signal path. The analytic de-
scription of the input voltage regulation loop is included
in the Appendix section. Please refer to the LTC4000 data
sheet for the analytic description of the other three loops.
In most situations, an alternative empirical approach to
compensation, as described here, is more practical.
LTC4000-1
CC
A4-A7
g_m4-7 = 0.2m
+
–
R_O4-7
A10
g_m10 = 0.1m
–
ITH
+
C_C
R_C
R_O10
40001 F11
Figure 19. Error Amplifier Followed by Output Amplifier Driving
CC and ITH Pins
Empirical Loop Compensation
Based on the analytical expressions and the transfer
function from the ITH pin to the input and output current
of the external DC/DC converter, the user can analytically
40001f
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