LTC3824 switching not as expected - Page 1
Forum section: electronics / projects

author: Ashwin619
author_rank: Regular Contributor
timestamp: February 15, 2025, 01:45:31 pm
content: Hello everyone,I am currently debugging a Buck switcher that uses a LTC3824 from analog devices. The design was made quite some time ago using a different reference board. The OG design was supposed to work for peak currents upto 20A, however that might have been exaggerated considering the small dimension and inappropriately placed heatsink.Schematic->Here's the specs of the current regulator,Vin = 6-50VVout = 5-9V (set using a changeable voltage divider)Iout = 10AFsw = 200kHz,The IC is designed for PMOS based Buck with an internal regulator to drive the Vgs with -8V.I am currently using a shunt of 6.25mΩ (actual desired shunt values was 7mΩ) which as per the datasheet (100mV/Rshunt) should keep a current limit of 15.38A and thus should allow me to draw a max o/p current of ->(0.1V/6.25mΩ)-(ILp-p/2) = 13.12Awhere ILp-p = 5.74A.I have obtained ILp-p with worst case of Vin = 50V, Vout = 5V, L = 4.7uH, Fsw = 200kHz.I am aware of the fact that the inductor value is somewhat low and will lead to high ripple currently. Unfortunately the value had been selected for the 20A application before and thus I need to use it for now. Ill probably replace it if that's the source of the problems that I am facing.The issues that I am currently facing are as follows.1. The Inductor current always stays in DCM irrespective of load2. The switching frequency is much lower than the switcher is set for (27kHz vs 200kHz)The below waveform if for Vin = 24V, Vout = 5V, Iout = 5AThis one for Iout = 7AFor Iout = 9AVoltage across shunt when Iout = 5ALooking at the voltage across the shunt (reaching the threshold of 100mV) and the inductor current of 16A max tells me that the IC might hitting its OCP value and thus causing it to go in some kind of low frequency mode. Datasheet specifies that the Fsw is made 50kHz when the Vout is shorted to GND and Vfb falls below 0.5V. Although I am not fully convinced since the o/p voltage was stable.Here's the simulation result with similar component valuesI am also kind of skeptic that the IC that I am using might be fake due to the fact that the older reference board's ic had it part number etched in coloured letter (like most of the LT and AD ic's) unlike the one that I am using which doesn't have coloured part number.And also due to the fact that I have managed to destroy two of these ic's when i was soldering and de soldering the filter capacitor across the shunt. Most probable reason might be due to ESD since i didn't make any contact with the ic.I believe I have done some measurements with the original reference ic along with the current one with different shunt values. Will upload once I find them.I would really appreciate if someone could shed some more light into these issues that I am facing.Regards,Ashwin619.

author: moffy
author_rank: Super Contributor
timestamp: February 15, 2025, 03:13:59 pm
content: If you think they could be fake ICs, the question is, where did you purchase them from? I am assuming not Digikey, Mouser etc.

author: Ashwin619
author_rank: Regular Contributor
timestamp: February 15, 2025, 03:34:45 pm
content: Hi moffy,My apologies, I forgot to mention that the ic's were purchased from LCSC. We have manufactured several boards using components from LCSC and I am beleive they are pretty reputed as well. Although the part code marking makes me feel skeptic about its authenticity.Is anyone aware if Analog devices is still making these ics and whether they have changed the way of printing part numbers on them.

author: D2236
author_rank: Regular Contributor
timestamp: February 15, 2025, 05:27:27 pm
content: I would take alook at compensation, and measure gain/phase margin if you have the tools (FRA) available. The C5/C9 connection/value may be causing a problem with forcing low zero-crossover point. It should be possible to do a simulation in LTSpice and get some idea of stability and crossover frequencies at various load currents."To simplify the creation of Bode plots, LTspice includes a frequency response analysis (FRA) component and directive, which can be used to provide the test signal and analysis, respectively."

author: moffy
author_rank: Super Contributor
timestamp: February 15, 2025, 09:13:36 pm
content: Quote from: Ashwin619 on February 15, 2025, 03:34:45 pmFrom what I've seen though not having used them, LCSC do appear reputable.A quote from the datasheet:"VFB (Pin 5): Error Amplifier Inverting Input. A resistordivider to this pin sets the output voltage. When VFB isless than 0.5V, the switching frequency will fold back to50kHz to reduce the minimum on-cycle."P.S. According to the Analog website, they are still in production.

author: D2236
author_rank: Regular Contributor
timestamp: February 16, 2025, 01:22:04 am
content: Take a look at your compensation values. I simulated the feedback loop with C5/C9 removed and making R14 (39k), C8 (220p).It is important to model taking into account the expected ESR of the 270uF cap.With your C5/C9 in the circuit, I get bad simulation results with oscillations on step loads..I(R22) is just the current sense resistor for the load in my simulation.

author: Psi
author_rank: Super Contributor
timestamp: February 16, 2025, 01:28:14 am
content: Quote from: Ashwin619 on February 15, 2025, 03:34:45 pmI received some fake polyfuses from LCSC (no fuse action, the solder would melt without them blowing).So there can be fakes there, but it's pretty rare, mostly LCSC are ok.It's also possible to come across factory reject parts that are legit but out of spec and were supposed to end up in the trash but didnt.

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 16, 2025, 03:08:20 pm
content: I would first look into whether the loop compensation is the issue. Could you get scope captures showing both inductor current and Vc pin voltage at the same time?Another thing worth considering is filtering on the current shunt signal. This is especially important when the shunt is on the input of a buck, as it is in this case. Putting 100pF directly across the terminals of a 0.01ohm shunt makes no sense (a cutoff frequency of >100GHz, yeah right). You have should add some resistance in between the shunt and the filter capacitor (on the SENSE pin side, not the VCC pin side) so the filter actually does some filtering. Often what you should aim for is a RC time constant equal to the L/R time constant formed by the sense resistor's resistance and its ESL.For some reason, Linear Tech datasheets almost always gives poor or no guidance on current sense filtering. But if you look atthe schematic for this chip's demo board (DC960A), you'll see that there is a resistor (R6) exactly where I described. Its value is 0ohms on the schematic, but I guarantee it's there because the designers knew that some customers would need to increase its value to get proper operation (particularly when operating at higher currents like you are).Not sure what that is for your sense resistor, I'd bet L is at least 1nH so aim for a time constant of ~100ns to start. So you could try setting the filter resistor to 1K. One thing to watch out for is that there are different types of current shunt amplifiers that LT uses, and some are affected adversely by the filtering components. For example sometimes the inputs to the amplifier draw significant bias current (like tens of uA), meaning that adding series resistance causes a large input-referred offset voltage. In this case you would want to increase the filter cap and reduce the filter R. Other times the amplifier may become unstable with too much capacitance directly on its inputs. Hard to tell from such a barebones datasheet in this case.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: February 17, 2025, 05:49:03 pm
content: If you post the question on the Analog Devices forum, you will receive the answer directly from AD. They are really helpful, even helping a hobby user like myself with a design question about one of their ICs.SJ

author: Ashwin619
author_rank: Regular Contributor
timestamp: February 18, 2025, 03:56:05 am
content: Quote from: D2236 on February 16, 2025, 01:22:04 amHello D2236,Thanks for taking time to do the stability simulation. I need to go through the compensation value design since I might have used some generic values for the LPF capacitor across FB and the feed forward capacitor. The current values seems to b quite high which probably introduces a pole at much lower frequency giving rise to a low Fc as well.Ill try to perform the FRA in LTspice to get some more insights.

author: Ashwin619
author_rank: Regular Contributor
timestamp: February 18, 2025, 04:00:40 am
content: Quote from: moffy on February 15, 2025, 09:13:36 pmHello moffy,I have seen this statement about the reduction in Fsw when VFB is <0.5V, although I am a little doubtful since the o/p looked to be stable enough. Ill try to measure the VFB pin along Vgs to confirm this.

author: Ashwin619
author_rank: Regular Contributor
timestamp: February 18, 2025, 02:10:36 pm
content: Quote from: mtwieg on February 16, 2025, 03:08:20 pmHello mtwieg,I agree with you that placing a filter cap directly across the shunt with no series resistance is most probably a bad idea. All the measurements that i have shown here is without having the C11 cap. I have seen that placing C11 significantly reduces the amplitude of the Current sense voltage signals near the sense pins as compared to what I am seeing near the Shunt. Here's some waveform which shows this difference ->1. With filter Cap across ShuntVoltage across shunt->Voltage across Sense pin->2. With filter Cap removed across ShuntVoltage across shunt->Voltage across Sense pin->You'll notice that keeping the filter cap significantly reduces the switching noise but also dampens the amplitude of the sensed current via shunt. Ill try placing the series resistance once I get a new board for me to test and share the waveform. Thanks for your feedback !!!

author: Ashwin619
author_rank: Regular Contributor
timestamp: February 18, 2025, 02:23:45 pm
content: Quote from: Solder_Junkie on February 17, 2025, 05:49:03 pmHello Solder_Junkie,I definitely need to check with the AD guys regarding this, since I have some concerns regarding the authenticity of the chip as well. I remember that the last time I had posted one of my queries on their Analog engineer zone forum regarding their PSFB controller chips, they were pretty clumsy to reply back. The folks over at TI's e2e forum were much better than them when it came to replying back to queries.