Hi Martin, sorry but I totally forgot. It has been a while to read a message from you here...
The two resistors form a voltage divider which reduces the output voltage according to the formula V = Vo * R2/(R1 + R2) where R2 is the resistor between the adjust pin and ground and R1 is the resistor between the output pin and the adjust pin. Vo is the output voltage. Internally the regulator compares this divided voltage with an internal reference voltage, which is 1.25 volts for the standard series (positive and negative regulators). In order to get an output voltage of 5 volts, you need to choose the resistors in a way that 1.2 = 5 * R2/(R1 + R2) If you now keep R1 constant for all your designs, this equation only has one variable and is easy to solve. R2 = R1 * 1.25/(5 - 1.25) or in general R2 = R1 * 1.25/(Vo - 1.25) You normally don't want to burn off too much power in this divider, hence you choose a bias current in the low mA range. The bias current is the output voltage divided by (R1 + R2). Given an R1 of 4.7k you would need an R2 of 1.567k to get 5 volts out. The bias current would then be I = 5 / (R1 + R2) = 0.8mA All these calculations neglect the input current into the adjustment pin (Iadj), which according to the datasheet is in the range of 70 microamps. The error you make if you follow the above equations is below 1 percent for a bias current of 1mA. If you want the accurate value then the equation would be Vo = 1.25 * (1 + R2/R1) + (Iadj * R2) And finally: all voltages above are absolute values, this means you should use +5 even for the nagative regulators, where you of course will get an output voltage of -5 volts. Uwe. >If one is looking at the LM337T TO220 package such that the >tab is against the table, the mounting hole is at 12 o'clock and the 3 >leads are at 6 o'clock, what function is each of the 3 leads? The adjust pin is the left one, the input pin is in the middle and the output is to the right. The tab is connected to the middle pin, as for the positive regulators or other components in TO220. > Cercuits using 3-terminal adjustable voltage regulators like > the LM317 or LM338K 3-terminal adjustable regulators have a > resistor from the Adjust pin to ground that one must set carefully to > determine the output voltage of the regulator. There is also a > fixed resistor that connects the Output pin of the regulator to the > Adjust pin which appears to be a feedback path. What exactly is the > formula for calculating the exact value for the feedback resistor if > any? > I read lots of stuff these days via speech synthesizer and > this is the kind of information that typically gets put in to graphs > or otherwise made difficult to read by putting it in to images rather > than text. I can certainly understand why, but that is why I am > asking here. > Martin McCormick WB5AGZ Stillwater, OK > OSU Information Technology Division Network Operations Group -- Author: Uwe Zimmermann INET: [EMAIL PROTECTED] Fat City Hosting, San Diego, California -- http://www.fatcity.com --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
