LM2940 direct replacement for LM7805?

eagletalontim

I have a project I am tinkering with that needs to run the SX28 at 50Mhz. When everything is hooked up, the 7805 will stop putting out power on first boot. The power source is a 16v solar array I have assembled. There is one circuit that detects the power from the solar array and when over 13v, it turns on the SX28 circuitry which controls a small 6v DC motor using a mosfet and PWM. Once the SX28 turns on, it displays a sequence on an 7 segment display, then rotates the motor for 2 to 5 seconds. Everything works until a cloud appears. The voltage drops to 13.5 to 14.2 and the entire SX circuit dies. Could I simply pull the 7805 out and replace it with a 2940 with no other modifications? If there are mods, what would need to be changed?

Phil Pilgrim (PhiPi)

In general, the LM2940-5.0 is a much-improved replacement for the LM7805 and shares the same pinout. However, it has much stricter requirements for its input and output capacitors, which must be adhered to. See the LM2940 datasheet for details.

-Phil

pedward

Just to elaborate, you would probably be best served with a buck-boost regulator. Maxim makes a bunch of ICs to do this in various flavors. You won't have to worry about dropout until you get pretty far down there, and you can use a super capacitor or battery to handle low voltage situations.

eagletalontim

Ok, so I would also have to change both input and output caps as well since I am only using a 10uf axial cap and a .001uf disc cap on the 7805. According to what I have read on these forums, a 1000uf cap is recommended on the input side instead of the 47uf which is recommended. Is this the same when using an SX28?

RDL2004

As Phil mentioned, this regulator has very specific capacitor requirements. You must be certain to use a low ESR capacitor on the output.

Ron Czapala

You might find this interesting Measuring Capacitor ESR

eagletalontim

Are both caps non polarized? I have been using one non polarize cap that is on the output pin of the 7805. The 10uf is on the input to the 7805. Based on the schematic above, it looks like both are non polarized and the caps will be backwards from how I have it now.

Phil Pilgrim (PhiPi)

The polarization or non-polarization is not important in and of itself. As RDL2004 mentioned, the important factor is the ESR (in addition to the capacitance and voltage rating). Figure 1 on page 13 of the LM2940 datasheet shows what the acceptable limits are for stable operation. Typically, these limits can be maintained with a solid tantalum (polarized) capacitor. Aluminum electrolytics will almost always have too high an ESR; ceramics, usually too low.

You also have to pay attention to the capacitance tolerances. If the minimum required capacitance is 22uF, a capacitor rated at 22uF +/- 10% may not do the job; one rated at +20%/-0% will be okay.

-Phil

RDL2004

You know, the 7805 should work fine down to 7.5 volts. Perhaps your problem isn't voltage related, maybe the SX (or the regulator) just isn't getting enough power or maybe there's something else going on entirely.