Today I have been powering up a new home project…for “dealing with” garden slugs. One component is a 5V LM4040 reference in 3-pin SOT23. When I was reviewing my BOM and what I needed, I found that I had an adequate supply of ADR550 in the same package. The ADR550 datasheet says “Pin compatible with LM4040/LM4050”… So I deleted the LM4040 from my distributor order and moved ahead.
Both the original National Semiconductor and newer TI LM4040 datasheet says (of the 3-pin SOT package) that pin 3 must be left floating or connected to pin 2. Pin 2 is grounded in my design, so for ESD protection and mechanical mounting I connected pin 3 to the same grounded zone.
At least one application schematic diagram on the ADR550 datasheet shows connecting pin 3 to the wiper of a 10K potentiometer. I have my doubts as to whether that recommendation is good as shown, but that is another discussion.
When I powered up my board on Wednesday I was getting 2.3 to 3V across this shunt reference. (The reading differed on two boards.) Well, it turns out that unlike the LM4040, the ADR550 pin 3 is for trimming the output voltage. The ADR550 datasheet says nothing about connecting pins 1 and 2 together, although presumably you should get that when the potentiometer wiper is set to one extreme. I think the two different devices might work similarly when pin 3 is left floating but I had not done that in my pcb layout.
Anyway, I lifted pin 3 off the board (and the ground zone connection) and slipped a piece of electrical tape in there for insulation. When I powered up again, I obtained 5.00 Volts across the reference as I wanted.
Lessons:
- “Pin compatible” does not necessarily mean “pin compatible.”
- As some co-workers used to say, “RTDSS” meaning “Read the Datasheet, Stupid.”
