This is a
design circuit for Low
Dropout Voltage (LDO) control is a variation of the previously posted 12V LDO
control. It is optimized for charging a 6V lead-acid battery with a 9V solar
panel. Minimum voltage drop is less than 1V. It uses a simple differential
amplifier and series P channel MOSFET linear regulator. This is the figure of
the circuit;
In
this circuit,
the voltage output is adjustable. It may also be applied in two or four cell
lead-acid applications (4V & 8V). It is not recommended for 12V
applications. Max solar panel rating: 50W (8A, 6V nominal) (open circuit
voltage: 9 to 10V). This circuit has output voltage range: 4.7 to 9.8V. While
designed for 8A, 50W, it will function just as well at much lower current
/power. The input voltage exceeds the input voltage by 0.9V when charging at
the maximum rate—the lower, the better. Low Dropout Voltage (LDO) is the catch
phrase for anything under approximately 2V.
U1 is an LM317LZ
TO-92 voltage regulator that is set to put out 3.1V. Low voltage zeners (below
6.2V) are too sloppy to use as voltage references, so the LM317 is used. Q1
& Q2 make up the classic differential amplifier that amplifies the
difference between the reference voltage and the feedback voltage from the arm
of potentiometer R6. The output is taken from the collector of Q2 and drives
the gate of P Channel MOSFET Q3. Differential voltage gain is probably in the
order of 100 to 200. For best performance, I selected Q1 & Q2 for matched
hFE (approx 300). As the feedback voltage increases at the arm of R6, Q2 turns
on harder and steals some of the emitter current away from Q1. The collector
current of Q1 follows the emitter current and drops less voltage across R1 thus
reducing Vgs of Q3 and turning it off. C2 provides frequency compensation to
prevent the amplifier from oscillating.
