Voltage Regulation

[Henk Visser]

I am using a LM3488 voltage regulator from National Semiconductor.
The goal is to produce 12 VDC from an input of ~9V-18V. When I attach
my multimeter it shows a value of about 12.5 volts. Not bad! But
when I hook up my oscilloscope I see huge voltage spikes!

My circuit is almost exactly that found in the datasheet at the link
below. Page 20 is where you can find the ciruit.The only differences
are some of the resistances to configure the output voltage.

http://www.national.com/ds/LM/LM3488.pdf

Any ideas why I am getting these huge spikes and how I can correct
them would be greatly appreciated!

-Henk Visser
henk@networkcar.com

 

[Brian]

Henk Visser wrote:

I am using a LM3488 voltage regulator from National Semiconductor.
The goal is to produce 12 VDC from an input of ~9V-18V. When I attach
my multimeter it shows a value of about 12.5 volts. Not bad! But
when I hook up my oscilloscope I see huge voltage spikes!

My circuit is almost exactly that found in the datasheet at the link
below. Page 20 is where you can find the ciruit.The only differences
are some of the resistances to configure the output voltage.

http://www.national.com/ds/LM/LM3488.pdf

Any ideas why I am getting these huge spikes and how I can correct
them would be greatly appreciated!

-Henk Visser
henk@networkcar.com

Two things are very important in switching regulators, grounding paths
and the type of filter capacitor. The filter capacitors need to have low
ESR and ESL values.

This is information taken from a different switching regulator, but it
applies to your regulator too.

Taken from application note AN920/D:
High frequency circuit layout techniques are imperative with switching
regulators. To minimize EMI, all high current loops should be kept as
short as possible using heavy copper runs. The low current signal and high
current switch and output grounds should return on separate paths back to
the input filter capacitor. The R1, R2 output voltage divider should be
located as close to the IC as possible, to eliminate any noise pick-up
into the feedback loop.
Hope this helps,
Brian



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