Step 1: use a voltage divider with a ratio of say 1:13 from the output to ground. This can be a relatively high impedance divider, say 120K from +Vout to the midpoint and 10K from the midpoint to ground. The max voltage on the midpoint of this divider is now 1/13 * Max Vout = 1/13 * 55V = 4.2 volts Step 2: set up an op-amp as a voltage follower to reduce the impedance of the output of the voltage divider. Step 3: now use the output of the op-amp as the input to a 50K digital pot, with a small resistor to ground. Step 4: the output of the digital pot is your voltage feedback signal. Bob Ammerman RAm Systems ----- Original Message ----- From: "Ishaan Dalal" To: Sent: Friday, June 18, 2004 4:25 PM Subject: [EE:] Digital Pots for HV Power Supply > Hi folks, > > I'd like to use an LM2576 adjustable "Simple Switcher" (National) as a > buck regulator to give me voltages from 1.2V to 55V, with an input of > 60V. There is a design in the datasheet that does just this, using a 50K > POT in series with 1.2K fixed for the voltage feedback/control. The 50K > + 1.2K is from the output voltage to ground, and the feedback "tap" is > at the junction of the 50K & 1.2K. > > For soft control of the voltage with a PIC, I'd like to use a digital > POT, but the most tolerant ones I've found so far have a max Vdd of 30V > (from AD). > > Are there any tricks to use a limited voltage digipot in such a > situation (float Vdd?). Alternatively, what other ways could I > "simulate" a digipot, e.g. maybe PWM a MOSFET with an RC filter? Or > jerry rig some sort of op-amp in between, with the digipot controlling > the opamp? > > For reference, the design is on page 18 of > http://cache.national.com/ds/LM/LM2576.pdf. > > Thanks, > -Ishaan > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body