Hi, I asked about a LK112SM33TR LDO regulator recently (datasheet below). To achieve the output ESR rating for stability I have decided to use a 10 uF tantalum capacitor. Figure 21 of the datasheet says that the min. required ESR is 0.03 ohms and the 10 uF tantalum should meet that. I will be placing it very close to the output of the regulator. My question now has to do with by-pass caps. I've got a PIC, MCC card, graphic LCD, buzzer and a OEM GPS board all connected to this power supply and I designed the board with by-pass caps scattered around the board to prevent glitches on the supply lines. I was going to use either 0.1 uF or 1 uF ceramic caps. These have very low ESR and since everything is very compact on a small board I'm wondering if the close proximity of the by-pass caps to the regulator will mean that the ESR of the tantalum cap will be negated and I may run into stability problems. The closest by-pass cap is about 1.3 inches away from the regulator/ reg output cap and has a 0.024" trace width with 1 oz copper. My multimeter is not very sensitive on the 200 ohm scale and shows 0.01 ohm for the trace, if that. Should I leave the PIC by-pass out as well as some of the others and let the 10 uF tantalum do the job? Common wisdom says that the by-passes should be right by the power pins, which is where I have placed them. Thanks, Gordon Williams ----- Original Message ----- From: "Gordon Williams" To: "pic microcontroller discussion list" Sent: Monday, March 29, 2010 5:25 PM Subject: LDO regulator > I would like to put a 3.3V LDO into a circuit to power it normally. The > data sheet for LK112SM33TR is > > http://www.st.com/stonline/books/pdf/docs/8059.pdf > > I have two questions: > > One: > During ICSP the programmer will put 5V into the board. The battery will be > disconnected so no voltage will be applied to the reg. input. The input to > the reg. will have a cap on it and there will be a line to ground with more > than 5Kohms of resistance. (All the rest of the components can take the 5V > OK) > > I don't see anything in the Max. Ratings section that would cover "back > driving" like this. Would this typically be acceptable for this > configuration. > > Two: > The output cap ESR stability range is given in Fig. 21. The allowable range > is roughly 0.05 to 2 Ohms for a 2uF output cap. The caps that I'm going to > use are ceramic SMD 805 (no name, Chinese). Will the ceramic cap meet the > 0.05 Ohms ESR requirement typically? From what I've been able to see is > that ceramic caps are < 0.015 ESR therefore I would be below the > requirement. Suggestions? > > Thanks, > > Gordon Williams > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist