You have a number of options.
- find a low dropout regulator (say 100 mv or less), rated at the current you need. Make sure that the maximum voltage input is 7 to 8 volts or more (to all situations). I know of some where the maximum input voltage is 6, but that's for a 3.3 volt output. These are surface mount chips at 200 to 300 ma.
- arrange a current limiter by putting in an NPN transistor. Take the output from the emitter. From the base run a 5.2 volt (easy value to find) zener to ground. Take a 100 ohm resistor and run it to the 6 volt input, the collector also goes to the 6 volt input. This is an emitter follower with the base clamped to 5.1 volts (or more if you want). The emitter is 0.6 volts less (4.6 volts for a 5.2 volt zener). Not efficient, but it works
Definitely not elegant solutions, especially the last.
Do not use LM79 series regulators, they need at least a 3 volt greater supply, so a 5.0 volt output regulator would need 8 volts input.
There are some chinese made buck regulators, but you must be careful with the allowed voltage drop across them.
I don't know what the SPV1040's output voltage is, but there might be other substitutes.
a VERY crude result would be a high wattage zener diode at 5.2 volts and a series resistor.
For a better answer, you need to consider your maximum current drain, the maximum expected voltage, and your regulator's desired output (3.3 volts? 5.0 volts?)
