application note 1 of 2 www.xicor.com october, 2000 an 46 remotely adjustable voltage regulators using xdcps by applications staff, october 1994 this note shows how an x9312 can be used to imple- ment a remote, adjustable regulator. the lm317 is probably the most common adjustable three- terminal voltage regulator in use today. often it would be desir- able to adjust these regulators remotely, but extending the adjustment rheostat any distance introduces noise and instability. xicor s x9312 xdcp can provide a simple solution. by using this xdcp, the voltage output of the lm317 can be set using a simple 3-wire digital interface over practically any distance. figure 1 shows the lm317 s usual adjustment resistor replaced with the 10k ? version of the x9312. the push- button control circuitry would be located remotely (on a separate board). the adjustment range can be set remotely to ? an application by changing r7, just remember not to exceed the v h or v l maximum limit of +15 volts. the standard formula for output voltage naturally still applies: where vref = 1.25v rref = r7 rpot = 0 to 10k ? value for u1 in 99 steps. iadj = approx. 50? of current from the adjustment pin of the lm317 figure 1 shows the control circuitry connected via a 10- pin connector. this allows an lm317 voltage regulator to be set and then for the control circuitry to be removed. in this con?uration, it s a good idea to include a pull-up resistor on all of the control lines (r9, r10, and r11) to assure that inadvertant xdcp changes do not occur. v out v ref v ref r pot r ref ---------- i adj r ref () ++ =
2 of 2 an 46 application note www.xicor.com october, 2000 j1/p1 p1/j1 figure 1. 1.25v to 10v dc supply
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