The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> Startup circuit
https://designers-guide.org/forum/YaBB.pl?num=1343893864

Message started by mixed_signal on Aug 2^nd, 2012, 12:51am

Title: Startup circuit
Post by mixed_signal on Aug 2^nd, 2012, 12:51am

Hi,
M1-M5 constitute startup circuit for the self bias refernce. VDD is 1.2 and V1 is usuall 700mV. M1 is sized for current=30nA.

After circuit startsup V2 is 1v and M1 goes to triode, M5 turns off. Is M1 going to triode after startup a problem?
Thanks!

Title: Re: Startup circuit
Post by AnalogDE on Aug 2^nd, 2012, 2:36pm

That should be fine -- as long as M5 is fully turned off you're good.

Title: Re: Startup circuit
Post by analogbeginer on Aug 12^th, 2012, 10:58pm

Wont M5 going to linear introduce offset between the branches?

Title: Re: Startup circuit
Post by AnalogDE on Aug 13^th, 2012, 2:37pm

Size M1 strong enough so the the gate of M5 is high enough so M5 doesn't inject current into the circuit that will mess up your desired bias point.

Title: Re: Startup circuit
Post by analog_wiz on Nov 19^th, 2012, 9:49pm

The transistors M2 and M3 should be sized weakly,so that as soon as the circuit is out of its degenerate state ..the startup should not be disturbing the bias of the main circuit. Hence..once current starts flowing in the main bias block...it will get mirrored into M1(Supposedly should be enough current so that the gate voltages of the diode connected transistors would be enough to turn off the current injecting transistor). Once this happens M1 will naturally go into triode..if M1 is not going to triode then you have a problem infact :)

Title: Re: Startup circuit
Post by Kevin Aylward on Jul 6^th, 2013, 8:14am

This is one of those typical, sure fail start-up circuits. If everything was ideal, as drawn, there would be two stable states for this design, zero current and the wanted one, therefore NEVER USER IT. It’s an accident waiting to happen. E.g. leakage of M1 could hold the circuit off. Never rely on leakages and non ideal behavior for a startup circuit. It’s probably why I have problem with switching on my Tom Tom. I have a tutorial on how to design a reliable startup circuit here http://www.kevinaylward.co.uk/ee/ptat/PTAT.xht. The ONLY topological way to RELIABLY design a startup circuit that removes the startup bias current is to use a depletion device. Example shown here http://www.kevinaylward.co.uk/ee/zeropowerstartup/zeropowerstartup.html

Title: Re: Startup circuit
Post by RobG on Jul 6^th, 2013, 12:34pm

Ooo, a bandgap discussion. I must participate or people will think I've died ;).

I mostly agree with Kevin - the startup device is one of 5 gate-source voltages in series. Even the tiniest of currents will shut off M5. This is a really bad idea if you are using bipolars to generate the bias voltage across the resistor since they might not get enough current to operate correctly (low level injection, yada yada...). With MOSFETs... unless you made that bottom NMOS ratio every small you would have to have really bad mismatch between the PMOS mirror before it wouldn't start up. However, it could take a very long time to start up.

A better circuit (arguably better than the good one on Kevin's site) is to
1) replace M2-M4 with a single long device or large resistor (pull-down device)
2) connect source of M5 to Vdd
3) size the M1 device so that when the circuit is operating properly it provides 5 and 10 times the current necessary to pull up the gate of M5 and shut it off over all corners.
4) M5 should be sized so that the current it provides during startup is about the same as the normal current in the bias circuit.

Items 3&4 are my own rules of thumb. If the devices are too strong you can get into a large signal oscillation during startup. I feel this is the best circuit because it directly senses the current in the bias loop and won't turn off until it is well on its way to fully starting up.

All that said, with an entirely MOS circuit it would be unusual for it to not eventually start up - basically the low-current gain around the loop would have to be less than one. Your mismatches and leakage would have to be horrible before that would happen. (again bipolar is a different story). Simply getting rid of M1-M4 and diode connecting M5 is becoming a popular circuit for this MOS bias circuit at low Vdd, and I'd say it is simpler and more robust than the one you show, but not as robust as the one I described above or on Kevin's great site.

Title: Re: Startup circuit
Post by Kevin Aylward on Jul 6^th, 2013, 12:55pm

Good points RobG. The overall point of my start-up tutorial, is that it has to be possible to do basic calculations that ensures the design will work. Leakages are ill-defined and vary all over the place over input voltage, process and temperature. Designs need to be able go I=V/R, I1=m.I2 etc. If this can’t be done, there isn’t a design at all. It’s just pissing about on a breadboard/simulator.