The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> Regulator Problem
https://designers-guide.org/forum/YaBB.pl?num=1161740216

Message started by uncle_ezra on Oct 24^th, 2006, 6:36pm

Title: Regulator Problem
Post by uncle_ezra on Oct 24^th, 2006, 6:36pm

I have an on-chip amplifier followed by a Class-AB buffer driving an off-chip PNP in a feedback mode, providing a 3.3-to-1.8V regulated output. In my measurements the base of the PNP is railed to 3.3V on standy-mode and somehow have 180uA from emitter side (can PNP provide this current in cutoff region :o). The base voltage moves to 2.5V when there is a large loading current. At both instances the collector is at 1.9V and emitter at 3.3V. Any suggestions on what is going on? In simulation everything seems ok at all corners even if I put a large input offset in the on-chip amplifier. Ideally I want the base voltage to be at 2.6V and collector at 1.8V in standy mode

Title: Re: Regulator Problem
Post by ACWWong on Oct 25^th, 2006, 2:36am

if everything you say is true... then i would say your pnp is device is broken... try replacing it.
if vbe is 0 and vce is > 0.3V then i can't see how 180uA flows into the emitter.... try testing the pnp alone to confirm.

Title: Re: Regulator Problem
Post by uncle_ezra on Oct 25^th, 2006, 2:43am

Thanks for your reply.

I got a more sensible measurement today. Now my emitter sits at 3.3V, collector at 2.5V and base at 3.3V and no current flows! Seems like the feedback is not working properly. There might be something wrong with the connection on the PCB board ... still checking. One more thing, when I pull a large current from the collector the voltage pulls down to 1.9V and base move down to 2.6V.

Title: Re: Regulator Problem
Post by ccd on Oct 25^th, 2006, 6:56pm

I'm thinking you could be capacitively loading your circuit when taking measurements causing your feedback not working. How much capacitance is your probe?

Title: Re: Regulator Problem
Post by uncle_ezra on Oct 25^th, 2006, 11:42pm

Hmm ... but with or without the probe there is no current, the BJT is off. I have placed a 30pF and 1MOhms loading in the simulation so it shouldnt be a problem. Still puzzled why the feedback is railing the base to 3.3V in standby mode and move down to 2.5V when there is large load current. Basically the BJT is going from cutoff to active region so its bound to oscillate when I go from standy mode to normal mode. Ideally in simulation the BJT should be in active region and remain there regardless of the mode of operation. Also I have added 22uF capacitor at the collector to add a zero for stability.

Title: Re: Regulator Problem
Post by uncle_ezra on Oct 26^th, 2006, 12:58am

Would a multimeter load the collector? What kind of input impedance does a multimeter have?

Title: Re: Regulator Problem
Post by ACWWong on Oct 26^th, 2006, 2:12am

uncle_ezra wrote on Oct 26^th, 2006, 12:58am:

Would a multimeter load the collector? What kind of input impedance does a multimeter have?

it should say what it is on the back... but usually they're 10Mohm... and if you can tolerate 30pF, then make sure you are using short (<0.5m) probe leads, then there shouldn't be a capacitance problem. If you're unsure you should use an oscilloscope, which has defined input R and C (typically 1Mohm//10pF on x1).

Title: Re: Regulator Problem
Post by uncle_ezra on Oct 26^th, 2006, 2:13am

Yep I am using an oscilloscope.

Title: Re: Regulator Problem
Post by ccd on Oct 26^th, 2006, 6:43am

I'm curious what your phase margin is between no load and large load. I designed a linear voltage regulator before with mosfet driver and had to provide a dc path to ground as a minimum current load for loop stability reason. My phase margin decreases with lower current load.

Another question I have is since there seems to be some discrepancies between simulation and reality, so I'm wondering if you included the bondwire and such between the chip and the bjt in your simulation.

Title: Re: Regulator Problem
Post by uncle_ezra on Oct 26^th, 2006, 6:02pm

Yep I have included all the bondwire and parasitics in the simulation. The phase margin are all above 80 deg in simulation. I think the main problem now is that the DC points are all wrong, so phase margin and everything is wrong. Not sure if it has to do with the BJT model since the manufacture does not provide one. I tried changing some of the spice paremeters and the DC point changes.

Title: Re: Regulator Problem
Post by uncle_ezra on Oct 30^th, 2006, 9:06pm

Thanks for everyone's input. I found out that I needed a higher Q capacitor ;D

Does anyone know how to design regulator for higher ESR? What changes should one make in order to cope with lower Q capacitor? I found out that using a small signal analysis can be misleading in regulator design because even for high ESR >5Ohms the small signal tells me that the circuit is stable but if you run a transient it oscillates :o

Title: Re: Regulator Problem
Post by loose-electron on Nov 2^nd, 2006, 8:06am

The ESR of the load plays into the phase margin of the feedback system in a big way. What you are describing is a common problem in LDO regulators.

Jerry

Title: Re: Regulator Problem
Post by uncle_ezra on Nov 7^th, 2006, 5:46pm

Has anyone experienced probing an unstable regulator and then releasing to find out it is stable ????? :o Anyone can explain this phenomena? I am still talking abt the same regulator described above. Basically probing either the base or collector I see oscillation on the oscilloscope when I powerup but then somehow it disappears after probing it. Seems like after pertubing the regulator becomes stable, totally confused ???

Title: Re: Regulator Problem
Post by uncle_ezra on Jan 7^th, 2007, 3:50pm

Attached is a single tone output of the LO. The tone seems noisy and the source was found to be from the regulator. As the current is increased further two "sidebands" seems to enlarge. Is the regulator unstable?

Title: Re: Regulator Problem
Post by uncle_ezra on Jan 8^th, 2007, 5:03pm

I know the source is from the regulator. From the oscilloscope the output of the regulator is clean but maybe the harmonics are too small to be seen in an oscilloscope.

Title: Re: Regulator Problem
Post by uncle_ezra on Jan 9^th, 2007, 1:21am

After playing with the measurement I am now sure the regulator is stable. I think the regulator is too noisy so the noise floor increases at high current. But I still don't know why the regulator would be that noisy. Anyone had any experience with noisy regulator? I did not add any guard ring to my regulator and no low pass filter at the reference voltage from bandgap.