Finally – decent-looking gain/phase response for a differential current sensing LED driver

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I´m not sure if I´m allowed to link to commercial websites or not, so just to be safe I´ll simply state that I was reading an LT Journal entry about the LT3790, a part that can sense it´s output current with a diff amp, and at the end of the article a gain/phase injection point and method is shown for such a part when running in constant-current mode. It´s almost identical to a suggestion made by a member of the Power Supply Design Center forum when I wrote about this previously. The suggestion was to move the reference points of the probes A and B from system ground to the negative input of the diff amp. (Assuming that the signal is being injected across a resistor placed in series with the positive input of the diff amp.) Here´s a picture:

Diff amp injection

LT suggests injecting in series with the negative and referencing to the positive, but I figured it wouldn’t make much difference. After mixing up the polarity and overdriving the diff amp I settled on a 250 mV injection at 100 Hz, scaling down to 20 mV at 1 MHz, and the result seems very realistic: 1.82 kHz bandwidth with nearly 90º of phase margin. Here it is:

Diff Amp Gain and Phase 1

Next up: check this with LTspice, which will take time and patience!

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