The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> smart voltage regulator
https://designers-guide.org/forum/YaBB.pl?num=1162404526

Message started by ccd on Nov 1^st, 2006, 10:08am

Title: smart voltage regulator
Post by ccd on Nov 1^st, 2006, 10:08am

Hello. I've been thinking about a way to build voltage regulator that runs on a battery, let's say 3.3V or something. With a fresh battery, this voltage regulator is on and regulates down to, say 0.8V, but as the battery voltage drops down to 0.8V, the regulator senses this low battery voltage and bypasses itself, so that the battery is connected directly to the regulator output. Basically, this regulator shuts itself off when regulation is not needed when it senses low supply voltage.

I appreciate any idea you may have.

Regards,
ccd

Title: Re: smart voltage regulator
Post by loose-electron on Nov 2^nd, 2006, 8:02am

What you are describing already exists. It is called an LDO (Low Drop Out ) Regulator.

Google it, lots of manufactureres make them, and lots of papaers on the subject.

Title: Re: smart voltage regulator
Post by ccd on Nov 2^nd, 2006, 9:16am

Thanks for your response, but what I'm looking for is NOT how to design a LDO regulator, but a way to sense low supply voltage and bypass the regulator because the regulator is no longer functional at this low supply voltage. What I could do is use a low voltage bandgap reference (0.6V perhaps) to detect the voltage drop to pull the gate of the pmos regulator driver low to short the load to the supply at that point.

Title: Re: smart voltage regulator
Post by Raul on Nov 3^rd, 2006, 4:17pm

What you describe will naturally happen with an LDO. When the battery is too low the LDO will go into dropout. If you want to turn off the bias current you will need to have a comparator comparing the voltage at the feedback input of the error amp of the LDO to some reference slightly lower to the voltage reference used by the LDO. Then feed this signal to the digital control to disable the LDO.

Title: Re: smart voltage regulator
Post by ccd on Nov 3^rd, 2006, 6:58pm

Thanks Raul.

I'm concerned about LDO not naturally going into dropout when the error amp and bandgap reference are also on the same battery as the LDO. When the battery voltage drops, the error amp becomes non-functional or partially functional, will it still correctly drive the gate of the LDO pmos device low? This is why I think a comparator and another voltage reference are needed to ensure the LDO is disabled.

I hope my thinking is wrong and that the comparator and voltage reference are not needed. Please give me your feedback.

Thanks again,
ccd

Title: Re: smart voltage regulator
Post by loose-electron on Nov 7^th, 2006, 8:06am

An LDO if propoerly designed drives the pass transistor into triode region (slammed on) as the power supply decreases. This pegs the output to the battery supply. Well designed LDO's use about 25-35 uA of current to make this all happen. No extra circuits required.