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I'm designing a buck LED driver using the LT8376 and I'm following the typical application for a low EMI solution (page 26).

On the output, the schematic proposes a 100 nF capacitor in parallel with the LED string. I tried to reproduce the schematic and adapt it to my application on a LTspice simulation but my problem is that when I have this capacitor, I see peak of current at each positive transition.

enter image description here

I checked the data measured on the evaluation board, which uses exactly the same capacitor. The usefulness I found there is that it acts as a low-pass filter with the ferrite for high frequencies, but it also seems to adjust the transition response.

The only way I've found to avoid the peaks of current is to reduce the value of the capacitor as with a 1 nF, but in this case the transition is as fast as if there were none.

So my question is: how can I eliminate current spikes while smoothing the transition to reduce electromagnetic interference? Is it possible that these current peaks are a simulation problem?

Here is my design: enter image description here

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2 Answers 2

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Flipping through the datasheet, I don't see any mention of the capacitor in question -- your C1. It appears in the headline / example circuit (a slightly modified version of the EMI example on page 26), but nowhere else (including the EMI example itself). I don't find any discussion of it in the text, no mention of filtering the sense signal. Nor any mention of the eval board's C26 in its document.

I would simply remove the capacitor; it's doubtful that the error amp it's feeding, can respond quickly enough to be affected by a leading-edge blip (due to gate charge plus resuming the load). The capacitor seems intended to reduce these effects -- but clearly it wasn't important enough to document in the datasheet, and I don't see any obvious-at-a-glance reason why it should be important.

You can also, I think, add a gate resistor between PWMTG and the PMOS gate, to slow the transition and reduce such blips, as well as reduce EMI emissions if that is a concern. At a PWM rate of 100s Hz, a transition time of ~µs seems reasonable, or for a ~3nF equivalent gate, maybe 330 to 3.3kΩ. (Note the pin has a ~300mA nominal drive strength, so resistors less than about 30Ω will have little effect.)

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  • \$\begingroup\$ Thank you very much the addition of a gate resistor has completely removed the peak of current. \$\endgroup\$
    – msch
    Mar 21 at 15:40
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You are activating/controlling the LEDs with a P-channel MOSFET (allowed) but, you are current sensing as if it were an N-channel MOSFET (with the 2.2 μF capacitor across the current sense resistor): -

enter image description here

If you read the LT8376 data sheet, they only use this capacitor (C1, 2.2 μF) when using an N-channel MOSFET and, it will certainly slow down the ability to current sense at high-speed and possibly produce the spikes into the LEDs.

enter image description here

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    \$\begingroup\$ Wow, facepalm for LT -- that's a typo, but not in the more common way you would think (source down, N-ch intended?), it's simply upside down and therefore always-on. I was wondering if the SYNC pin was configuring that, but nah, doesn't say so. The note specifies SQ7415AEN, a P-ch MOSFET. \$\endgroup\$ Mar 20 at 18:50
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    \$\begingroup\$ @TimWilliams yes, they repeated that error throughout on the circuits using the N-channel MOSFET. I've amended my answer because I hadn't noticed that error in the DS. \$\endgroup\$
    – Andy aka
    Mar 20 at 19:25
  • \$\begingroup\$ @Andyaka thank you very much for noticing this error on the circuit, I removed the capacitor but the current peak is still present. \$\endgroup\$
    – msch
    Mar 21 at 15:38

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