LTC4368 over current protection? - Page 1
Forum section: electronics / projects

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: June 22, 2023, 07:47:01 am
content: The LTC4368 ic provides over/under/reverse Voltage protection, as well as over current protection.However, the intended use appears to be in the module/equipment supplied, as opposed to the power supply side. The problem that I cannot overcome is avoiding the surge current tripping the protection when equipment is turned on.There is an “inrush current” R/C on the gate of the series mosfets, but I think this relates to limiting the initial supply current to the board itself.The overcurrent shutdown occurs in as little as 8uS, if it were around 20mS it would cater for surges due to external equipment being switched on.The circuit in question is attached. The LTC4368 data sheet is at:https://www.analog.com/en/products/ltc4368.htmlMaybe not the right device for the power supply side, although the over Voltage side works fine.Your thoughts are appreciated.SJ

author: thm_w
author_rank: Super Contributor
timestamp: June 22, 2023, 10:15:17 pm
content: Did you read the recommendation in the datasheet?"Limiting Inrush Current During Turn OnCharging large capacitors on VOUT can lead to excessive inrush currents when LTC4368 turns on the external N-channel MOSFET. The maximum slew rate at the GATE pin can be reduced by adding a capacitor on the GATE pin:"https://www.analog.com/media/en/technical-documentation/data-sheets/ltc4368.pdfWhat is the capacitance on the device being driven?

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: June 22, 2023, 10:23:07 pm
content: Quote from: thm_w on June 22, 2023, 10:15:17 pmYes I did read the data sheet. The limiting of inrush current refers to the current when power is applied to the board, not necessarily the current resulting from a suddenly load change, such as turning on equipment down stream with large capacitors creating surge currents of tens of amps.SJ

author: thm_w
author_rank: Super Contributor
timestamp: June 22, 2023, 10:44:36 pm
content: If you create 10's of amps of surge, then its going to trip, not much can be done other than choose another IC or increase the current limit.You could also increase the capacitance on the output to reduce the surge current amount. Have you measured with an oscilloscope to see how long it lasts?

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: June 25, 2023, 08:40:42 am
content: No, I’ve not measured the duration, or current. Other similar equipment surged at 50 to 70 Amps for 1mS, when measured by the author of the original article. The LTC4368 “trips” in around 8uS.Linear Technology suggested adding a resistor in series with the sense line to the current sense resistor and a capacitor between the sense and Vout pins. This works on an LTSpice model where the switched load is a 1000uF cap with an ESR of 0.16 Ohm in parallel with a 10 Ohm resistor. Over the next few days I will try modifying a board to see if that works in practice.LTSpice model attachedSJ

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: June 26, 2023, 09:21:20 am
content: Adding 3.9K in series with the sense pin and a 220nF cap between the sense pin and Vout cures the problem. I've modified 2 boards so far and a complete success.SJ

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: June 29, 2023, 03:57:27 pm
content: I’ve modified 4 boards and the fix works well. Measuring the Voltage drop across the 1.5 milli Ohm current sense resistor shows the switching surge current was up to 45 Amps, peaking at 500 uS and decaying to around 20 Amps after a millisecond, then dropping to 2 Amps after the caps in the supplied equipment had all charged up and started running.The CR time constant for the additional R and C was 0.000858 using 3.9K and 220nF. Linear Technology suggested reducing the resistor value and increasing the capacitor, while keeping the same time constant. Perhaps 820 Ohms and 1uF?Incidentally, I powered the board (and its load) from battery power. The battery powered equipment was isolated from ground/mains earth. I was then able to directly connect an ordinary oscilloscope probe across the sense resistor to calculate the peak current. Equally, if I had a sensitive differential probe or a decent battery powered oscilloscope, I could have used a mains power supply to test the board.SJ

author: PeteH
author_rank: Unknown
timestamp: June 29, 2023, 04:39:53 pm
content: I assume your intent was overcurrent protection here.If you intend on short circuit protection, may want to check that still works as intended with the additional delays added by the sense network.I didn't see any inductance though, so it's unlikely that this would have been okay for SCP, but depends on the cabling and source.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: June 29, 2023, 04:52:05 pm
content: The main purpose is over Voltage protection using an external board. This allows it to be added to any power supply without needing to modify the PSU. In addition, it is easy to test the unit from time to time, whereas an in-built protection unit tends to be fitted and forgotten.I have tested the modified boards by placing a low value fuse across the output, which isn’t very scientific but shows the over current shutdown works without blowing the fuse.The equipment supplied through the board costs upwards of $5000 US, so relying on the power supply to not fail in a high Voltage output condition is a bit risky, as is expecting a PSU over Voltage circuit to really protect the load.SJ