The MAX710 has something like that - a step-up switching regulator followed by a low-dropout linear regulator. Its intended purpose is to use with four 1.5 volt batteries and output 5 volts through the whole battery life. When the battery is putting out 6 volts, just the linear regulator is operating. When the battery voltage is low, the flyback regulator starts up. You have the option of bypassing the linear regulator (for efficiency) or using it and getting less switching noise on the output. In that mode it needs to run the switcher about .6 volts higher than the output, to allow the linear regulaor to do its job. A useful part when you are forced by marketing to use 4 cells on a 5 volt project ;-) Cheers, Bob On Thu, Mar 5, 2015, at 09:38 PM, Ryan O'Connor wrote: > Theoretically this would reduce ripple after a SMPS too, correct? >=20 > Ryan >=20 > On 6 March 2015 at 17:34, Bob Blick wrote: >=20 > > It wouldn't work the same. As the load on the circuit increases, the > > base current also increases, so the voltage drops both due to the > > overall voltage drop, and also due to the increases base drive. > > Hopefully keeping the transistor from dropping out (and letting ripple > > through). > > > > If you used a MOSFET, you could get gobs of ripple reduction (the > > resistor can be much larger), more than with a BJT. But as you increase > > the load, the output voltage only tracks the voltage drop, not the > > increased load current. > > > > A 78xx regulator would need the ground lead to be less than the output, > > unlike a BJT or MOSFET. So you'd need to use a voltage divider (off the > > input, to ground). It would track the input voltage, but due to the > > voltage divider, to a lesser extent than a BJT or MOSFET. > > > > Friendly regards, Bob > > > > On Thu, Mar 5, 2015, at 07:25 PM, Ryan O'Connor wrote: > > > Does it have to be a BJT? What about a mosfet or a 78xx regulator wit= h a > > > cap on its adjust pin? --=20 http://www.fastmail.com - Choose from over 50 domains or use your own --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .