LDO Output Voltage High

to generate my 5V and 3V3 rails (the 5V feeds the 3V3 regulator). I've received the boards back and have been going through my checkout of the board and have noted that the outputs of my LDO's are high. My 5V rail is at 5.2V and my 3V3 rail is at 3.44V. I don't know why the outputs are high, and was wondering if anyone knew why.

from the wall wart is 5.36V. I've bypassed the input to the 5V regulator with a

10u cap, and the output with a 10u cap. I have similar bypass caps on the 3V3 regulator. All the caps are ceramic.

type of caps (the regulator datasheet recommends ceramic)?

Have you checked your meter? If it's reading ~4% high, that would explain all three readings. Maybe it needs a fresh battery.

ther to generate my 5V and 3V3 rails (the 5V feeds the 3V3 regulator). =A0I= 've received the boards back and have been going through my checkout of the= board and have noted that the outputs of my LDO's are high. =A0My 5V rail = is at 5.2V and my 3V3 rail is at 3.44V. =A0I don't know why the outputs are= high, and was wondering if anyone knew why.

Same here, different parts: ST L4931CZ50-AP (5V 250mA), Diode Inc. AP130-33WL-7 (3.3V 300mA). My outputs are slightly lower than spec, with 200mA load.

voltage from the wall wart is 5.36V. =A0I've bypassed the input to the 5V = regulator with a 10u cap, and the output with a 10u cap. =A0I have similar = bypass caps on the 3V3 regulator. =A0All the caps are ceramic.

=A0Wrong type of caps (the regulator datasheet recommends ceramic)?

What are your load currents?

On a sunny day (Thu, 14 Jul 2011 09:05:05 -0700 (PDT)) it happened PlayDough wrote in :

Multimeter battery empty?

to generate my 5V and 3V3 rails (the 5V feeds the 3V3 regulator). I've received the boards back and have been going through my checkout of the board and have noted that the outputs of my LDO's are high. My 5V rail is at 5.2V and my 3V3 rail is at 3.44V. I don't know why the outputs are high, and was wondering if anyone knew why.

from the wall wart is 5.36V. I've bypassed the input to the 5V regulator with a

10u cap, and the output with a 10u cap. I have similar bypass caps on the 3V3 regulator. All the caps are ceramic.

type of caps (the regulator datasheet recommends ceramic)?

Scope the regulator outputs. They may be oscillating.

John

voltage from the wall wart is 5.36V.

That's not going to work with any reasonable load. We are using 6.5V to 7V plugs.

voltage from the wall wart is 5.36V.

Umm.. that's a touch on the brave side, given the 6.5V absolute maximum input voltage of the MCP1824 regulator.

I'm using a scope. Peak-to-peak ripple is about 400mV.

However, I think I figured out the initial problem. My measurement was set= to 'max', which recorded 5.2V. When set to 'avg', I get 5.1V for the 5V r= ail and 3.36V for the 3V3 rail. Both of these values are within the +/- 2.=

5% swing in the spec. I was concerned because the 5.2V and 3.44V were outs= ide the range.

Looks like I'm ok. I just need to run my scope correctly...

voltage from the wall wart is 5.36V.

The MCP1824 has a max 6VDC input...

Charlie

That's a lot. What's the waveform? If it's regular, the regs are probably oscillating. A triangle wave is common.

John

Ok, I must admit my scope idiocy. I'm trying to zoom in on the waveform to= see what I get, and the more I zoom in, the smaller the ripple. Zoomed in= as far as I can, I see an 80mV peak-to-peak swing on the 5V and 160mV on t= he 3V3 rail. As for the waveform, it looks like noise, not oscillations.

Then, as I fiddled with the settings, I screwed things up and did an autose= t to cleanup. Now my 'avg' on the 5V reads 4.99V and on the 3V3 I get 3.26= V.

voltage from the wall wart is 5.36V.

On the 5V LDO? Then switch to the ST AP130. We are running it at

7V. Max is 20V, but i am not brave enough to try that.

ut voltage from the wall wart is 5.36V.

V

Sorry, should be:

ST L4931CZ50-AP (5V 250mA)

what I get, and the more I zoom in, the smaller the ripple. Zoomed in as far as I can, I see an 80mV peak-to-peak swing on the 5V and 160mV on the 3V3 rail. As for the waveform, it looks like noise, not oscillations.

cleanup. Now my 'avg' on the 5V reads 4.99V and on the 3V3 I get 3.26V.

Obviuosly your test equipment is the problem. Use a Simpsom 260 to verify your circuit & I guarantee everyting will be fine :)

H.

see what I get, and the more I zoom in, the smaller the ripple. Zoomed in as far as I can, I see an 80mV peak-to-peak swing on the 5V and 160mV on the 3V3 rail. As for the waveform, it looks like noise, not oscillations.

to cleanup. Now my 'avg' on the 5V reads 4.99V and on the 3V3 I get 3.26V.

Unless it oscillateth ... :-)

--
Regards, Joerg

http://www.analogconsultants.com/

put voltage from the wall wart is 5.36V.

This is purely Microcrap marketing. It's a perfectly good LDO for

0.8V, 1,2V, 1.8V, 2.0V, 2.5V and 3.3V. But, marketing came in and said they got to support 5.0V. Who in his right mind would pick a LDO with 1.5V margin? Oh yeah, you did.

Perhaps. I'm a digital guy, and this analog stuff is FBM to me. I've never done board design, so to learn I've been designing my own board to get a better grasp of it.

The scope I'm using is a Tek TDS2014B. As an embedded software developer, I've only ever been worried about logic levels, and never have had to worry about these types of issues.

Where does this 1.5V margin come from? Looking at the MCP1824 datasheet, t= he max drop out at 300mA is about 180mV. My max load is about 250mA, so I'= m not worried about coming anywhere near a 1.5V drop. The 5.36V I get from= my wall wart is well above the 180mV drop.

As for why I chose this par, there are several reasons: price, 250mA+ load,= package, distributor, and availability. Perhaps if this project were a co= mmercial product, I'd choose differently. But for a hobby project, it seem= ed appropriate.

6.5V - 5.0V =3D 1.5V

V. =A0My max load is about 250mA, so I'm not worried about coming anywhere = near a 1.5V drop. =A0The 5.36V I get from my wall wart is well above the 18=

0mV drop.

If you are seeing ripple in the output, your input to the LDO is too low.

d, package, distributor, and availability. =A0Perhaps if this project were = a commercial product, I'd choose differently. =A0But for a hobby project, i= t seemed appropriate.

Yes, Microcrap marketing.