Load regulation L7808?

[max0412]

improve load regulation

Does anyone have any suggestion on how to improve load regulation of ST’s L7808 three terminal regulator (1.5A, 8V, TO220)? The over and undershoots overshoots particularly are high. The regulators supplying 1A pulses for 2.3uS per 26uS. During no load the regulator is supplying about 50mA to 60mA.

The schematic of my current setup is below, also a scope capture of the 38 kHz ripple, (800mVpp!). Is this typical of these regulators?

Ripple

Magnified view of overshoot

 

[rowokii]

l7808 circuit

I'm assuming this only happens with your load on, but it actually looks like your LC filter that is ringing, not the regulator. You can try putting smaller value, lower ESR caps in parallel with you 330uF caps. You might also have to experiment with different values for the LC filters (both L & C). ESR on the caps can make big difference, so you might try different type of caps.

Good luck.

 

[IanP]

I would start with moving L1 beyond C4/R1 (see picture below) and then experiment with different types of caps ..

Regards,
IanP

 

[max0412]

Thanks for the reply.

Thats a ferrite bead I put there to suppress a 25MHz oscillation,now its about 6MHz but the number of oscillation's have reduced. The aluminum caps are all quality panasonic low impedance (ESR) types. I've tried several different values of caps,single and in parallel but still the large overshoot. It's short duration but it's wreaking havoc with the other side of the board that I want to be quiet. It's screwing my reference up on the other half of my board.The noise is coupling into the return.

The other half of the board gets supplied from a 78l05 which is supplied from the 12VDC supply. There is about 100mil isolation between the two ccts.I'm using an RC filter and a diode for decoupling from the 12vdc for the 78L05. Each regulator has it's own return to the twelve volt input.

For know I'm just trying to get the 7808 as quiet as possible,I don't want to have to do another board.

Thanks Ian I'll try that.

The load for 7808 is an IRLED driver.Low side mosfet 2xIRLED + current limiting resistor + mosfet driver.The FET is switching at 50nS rise and fall.Maybe slow it down increase the noload current to 100mA?