LM7805 Minimun Input Voltage Require

That depends. A standard 7805 will work above 7v (unfortunately) and there's high efficiency version that could work as low as 6.

Instead of that 7805 regulator, look for DC-DC converter, they can handle voltage more efficiently than the regulator.

If you can't find that, you could try this quick and dirty emitter-follower transistor design. Input of minimum of 5.7v to collector, a resistor from collector to base (to turn it on), a 5.7v Zener diode in reverse bias from base to ground, and the output should be a 5v. Not efficient but cheap.

That also varies depending on the load and the ripples.

That design takes about a four volt drop across the NPN pass transistor. You can lower the drop if your bias voltage (the resistor to the collector) isn't connected to the collector, but connected to a source that is ~4 volts higher than your output.

That technique is used in linear supplies to make them more efficient since the bias supply is just saturating the transistor - not supplying the output current.

Some standard 780x regulators require 3V headroom. Low drop out types can work at lower supply voltages..

Here is one example but I'm not saying it's the best for your application...

The summary says this one works down to 5V + 0.47 = 5.47V so you should be ok at 5.8V min. but you don't say how much current you need?

Sometimes LDO types can be unstable (prone to oscillate). You normally have to use capacitors with the correct ESR (refer to the data sheet/Apps note) to ensure stability.

"stanley" wrot in messag

capacito

Some standard 780x regulators require 3V headroom. Low drop out type ca work at lower supply voltages.

Here is one example but I'm not saying it's the best for you

application..

The summary says this one works down to 5V + 0.47 = 5.47V so yo

should b ok at 5.8V min. but you don't say how much current you need

Sometimes LDO types can be unstable (prone to oscillate). You normall

hav to use capacitors with the correct ESR (refer to the data sheet/App note to ensure stability.[/quote:8398df63ed

Thanks All for the useful suggestion given

The maximun load current is 800mA