Electronic – Adjustable LDO Regulator


I'm using Microchip TC1071 in a circuit to provide an adjustable output voltage (appears to be pin compatible with LT1761-SD). My plan is to use a digital potentiometer (Microchip MCP4251) to control the output voltage within over some range. The schematics and layout are on solder pad here. The adjustable regulator circuits are in the upper left quadrant of both the schematics and layout.

I've got it all hooked up per their typical application circuit and it doesn't seem to work the way I'd expected it to. I have an 0805 100nF ceramic capacitor near the input voltage (pin 1) of the regulator. I put this 10uF electrolytic capacitor near the output voltage (pin 5). The high side of the voltage divider is connected to my digital potentiometer. The low side is connected to a 56 kOhm 0805 resistor. The \$\mathrm{\overline{SHDN}}\$ (pin 3) measures 5V as does the input voltage.

So I blew the digital potentiometer off the board and hooked up a fixed 47kOhm resistor. If I read the datasheet correctly, the voltage I should get out of the regulator in this configuration is {1.2 * [( 47 / 56 ) + 1]} = 2.21V. Instead, I measure about 1.6V. I'm staring at this datasheet and am somewhat baffled. Are my capacitors inappropriate to support this regulator (seems like there are some ESR requirements, but I can't tell what the ESR of my caps is)? They suggest using somewhat larger resistors for the voltage divider:

The ohmic values of these
resistors should be between 470K and 3M to minimize
bleeder current.

But could this actually impact the accuracy of the output voltage (that much)? Does anyone have any experience with these adjustable regulators that can offer me some practical advice on getting the thing to work, and what I might be doing wrong?


I should note that I only need to source 50mA max, though 100mA would give some more breathing room.


Once I resolved the netlist error with some trace cuts and softwires, I was able to control the voltage as I had intended per the design.

Best Answer

It looks to me like a netlist/layout error:

schematic screenshot

If you are placing your fixed resistor on the dig pot footprint, it would also be messed up. Looks like you accidentally wired the CO+ and NO2- together and the NO2+ and CO- together so the LDO(Vout-Vadj) is not across a resistance as you believe.