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I recently came across this circuit for measuring the output of a TPS54620 switching regulator (12V->3.3V). I'm not a power engineer, so I'd like some help understanding the parts in red boxes (presumably used by the power engineer to measure the power supply?)

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1) Is this Hirose U.FL coax connection (J4) used for measuring the output ripple and/or noise on the regulated 3.3V output? I think the 49.9 ohm resistor is used to provide a 50-ohm source impedance to match a 50-ohm coax cable, which is presumably terminated via a 50-ohm BNC pass-through (or the 50-ohm setting on a scope). Is 49.9-ohm used instead of 50-ohm to ensure that the resistance is precise to within 0.1-ohm? (just curious why you'd use a 49.9 ohm...)

If the designer was just concerned with ripple/noise, why wouldn't they add a DC blocking capacitor in series before the 49.9 ohm resistor? Since there is no DC blocking, this presents a 100-ohm DC load (49.9-ohm + 50-ohm), which results in a static current draw. For 3.3V output, this is probably ok (3.3V * (3.3V / 100-ohm) = 0.1W), but if the output was 12V, you'd get 12V * (12V / 100-ohm) = 1.44W, which I think is too much power for the 50-ohm input on my scope. Do people normally put some kind of DC block inline with the coax cable? What is the best practice for putting an onboard coax connection like this? What tests is this connection typically used for?

2) I believe R134 (the 10-ohm resistor) is for loop stability measurements (see http://www.tij.co.jp/jp/lit/an/snva364a/snva364a.pdf) TI recommends adding a 20-ohm resistor into the feedback loop of the power supply for bode measurements, and I think the 10-ohm resistor is for the same purpose here. Is this correct? Any other reason for this resistor?

3) Why have another large resistance (R137) in parallel with the upper half of the feedback divider? The TI reference design uses 31.6k/10k resistors in the feedback loop, so 3.16k/1k seem like they should be the correct value without R137. I'm not sure why you'd want this resistor?

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Your scope should have a capacitor in series to block DC when making AC measurements. Putting a blocking capacitor in the test circuit would be redundant, and also would prevent you from monitoring the DC voltage (which might be useful for certain tests).

The reason for R133 being 49.9 Ohms instead of 50 Ohms is simply that 49.9 is the nearest value in the E96 1% resistor series.

You are probably right about the purpose of R134.

R137 compensates for the combined resistance of R135 + R134, which would otherwise make the output voltage about 1% too high (it would not be needed if R135 was 3115 Ohms, but that is not a standard value).

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  • \$\begingroup\$ Abbot Thanks! When you turn on the "AC" coupling mode at the same time as the "50-ohm" input mode on the scope, does the DC blocking cap come before or after the 50-ohm termination resistor inside the scope? (I've got a Tek scope) If the DC blocking cap comes after the 50-ohm termination resistor, then a DC current would flow into the scope through the termination resistor, right? In that case, I would still have to be careful not to exceed the input power rating for the scope's 50-ohm input, right? \$\endgroup\$
    – cdwilson
    Commented Sep 25, 2014 at 16:19
  • \$\begingroup\$ The DC blocking cap is switched in before the 50 Ohm terminator. \$\endgroup\$
    – Bruce Abbott
    Commented Sep 25, 2014 at 20:40

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