About 6 months ago we had an Essential Post on Power Distribution:

 

element14 Essentials - Power IV: Smart Power Distribution

 

Tonight as I studied a junk circuit board in my bin I came across a good example of the De-Centralized Architecture.

 

This is a board from an upscale Audio Video Sound System Receiver. The board has multiple micro processors which usually don't have much interest for me but in the lower right corner of the board I saw a parts layout that was repeated 4 times. The IC at the center of each repetition was an MP2363DN which turns out to be a 3A 365 kHz Step Down Converter.

 

Here is the Data Sheet if you are interested:

 

https://datasheet.octopart.com/MP2363DN-LF-Monolithic-Power-Systems-datasheet-13696477.pdf

 

The section of the Data Sheet that first caught my eye was the typical application circuit:

 

As I traced out and checked the circuits on the board it appeared that the designer had used the Typical Application for the configuration. The data Sheet also had information on using a resistor voltage divider to set the voltage output. I decided that it would be a fun exercise to figure out what resistors were used in each section and apply the formulas in the data sheet to predict the output voltage  of each section. I numbered the sections 1 to 4 left to right. Here are the formulas for setting output voltage:

 

 

This was extra fun as the designer had used parallel resistors to trim his required value for R-1 in each section. I tried to measure the R1 combination in circuit with my meter but it was obvious that other components were messing with my readings. Since the readings on the resistors were legible I was able to read and calculate them as #1 - 1.525k, #2 - 10k, #3 - 26k, #4 - 46k. Inserting these into the formulas yielded the following output voltages respectively:  1 volt, 1.84 volts, 3.3 volts, and 5.1 volts.

 

Now I wanted to test my calculations. I had no way to power the board like it was in the receiver but I could easily power the rails that fed these four circuits. I set up a couple of 10 volt power wires with Non-Slip Probe Tips and placed them so that I had 10 volts plus on pin 2 of chip #1 and ground on pin 4 of the same chip. Since this fed the rails for all four circuits no further movement was needed. I had already tied my multimeter negative probe to ground and while holding the power probes with one hand I probed the output of each of the 4 DC to DC converters. The result was 1.2 volts, 1.9 volts, 3.4 volts, and 5.2 volts respectively which matched very well with my calculations.

 

 

This is the first time I have encountered such an obvious example of the Distributed Power Architecture that was mentioned in the Essential Power Blog. It was fun to experiment with the circuit and who knows somewhere down the line I may even have an application for the MP2363 Step Down Converter Chip.

 

John