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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> opamp for ldo https://designers-guide.org/forum/YaBB.pl?num=1156252990 Message started by grosser on Aug 22nd, 2006, 6:23am |
Title: opamp for ldo Post by grosser on Aug 22nd, 2006, 6:23am hello Can you recommend an opamp for LDO regulator which is suitable to work under low voltage (Vin=1V-3.3V)? if you have, upload the schematics please regards |
Title: Re: opamp for ldo Post by loose-electron on Aug 22nd, 2006, 7:50am two questions - Input voltage range? What are you driving for a load? Jerry |
Title: Re: opamp for ldo Post by grosser on Aug 22nd, 2006, 9:04am loose-electron wrote on Aug 22nd, 2006, 7:50am:
input range 1V-3.3V Iload 1mA-100mA |
Title: Re: opamp for ldo Post by loose-electron on Aug 23rd, 2006, 8:25am That large an output current, you will need a source follower type buffer for an output driver. Also, your input voltage is not going down to ground so a NMOS differential pair input should work. Add an NMOS source follower to the attached and it should get the job done. The compensation here uses pole splitting, there are other options as well. |
Title: Re: opamp for ldo Post by Visjnoe on Aug 23rd, 2006, 10:55am Hi, I would just like to comment on the OTA topology given above and note that: 1. It has a certain systematic offset, which might be kept low enough if you have enough gain in the first stage. 2. It has a very bad PSRR. Insert a cascode to improve this. Kind Regards Peter |
Title: Re: opamp for ldo Post by loose-electron on Aug 23rd, 2006, 6:35pm cascode will help with the PSRR a bit yes, agreed. The systematic offset will probably be overshadowed by the differential pair mismatch. An LDO usually ends up getting trimmed (usually the bandgap reference) to get to the right operating point output, so that should fall out of the equation. |
Title: Re: opamp for ldo Post by grosser on Aug 24th, 2006, 12:36am hi i have tried this simple topology, but it is not enough because of the source follower. For the lowest Vin=1V it can drive the full load (100mA) but can't drive low load current. I'm looking for something with simple architecture and good slew-rate current. regards |
Title: Re: opamp for ldo Post by loose-electron on Aug 24th, 2006, 10:07am grosser - Not sure exactly what the problem is, if you could define the problem a bit more perhaps I can help. Jerry |
Title: Re: opamp for ldo Post by grosser on Aug 24th, 2006, 1:18pm loose-electron wrote on Aug 24th, 2006, 10:07am:
the problem is that for low input Voltage=1V when i use source follower with nmos input opamp can drive large load, but can't drive low load and can't shutt off when it's needed. When i use source follower with pmos input i can't drive large load. i know this topology is not suitable for low voltage and i want to check other possibilities. Regards |
Title: Re: opamp for ldo Post by richard88 on Aug 24th, 2006, 10:02pm grosser, I think you have to specify more info for an LDO : What is your VIN (or VDD), Vref, Vout ? is the feedback factor=1 (no resistor divider, direct feedback) ? Richard |
Title: Re: opamp for ldo Post by loose-electron on Aug 25th, 2006, 8:40am Quote:
Ah, if that is the case, you can use an OTA type output, with lots of gain. You do this in a Class-AB type structure with a PMOS and NMOS outputs being controlled. With lots of gain it give the capability for either the PMOS or NMOS output to be driven triode. That will get your control for the PMOS power transistor to go from ground to power (or pretty bloody close) in concept (sorry dont have a picture) 2 NMOS differential pairs 2 current mirror structures above the differential pairs opposite polarity on the high gain output of each differential pair one output drives the PMOS output of the OTA other output drives a PMOS current source, that mirrors into a NMOS (diode connected) which drives the NMOS output of the OTA. There are other ways of doing this as well, with one differential pair, and you may need additional gain before getting to the NMOS/PMOS outputs. The key thing is the A/B output stage. Jerry |
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