LDO Failure - Page 1
Forum section: electronics / projects

author: rokspy
author_rank: Contributor
timestamp: October 11, 2022, 01:05:13 pm
content: I'm using MCP1792T-4102H/DB LDO, and I'm dropping from 48V to 4V with a load of approximately 10 to 15mA. When input voltage is increased gradually there are no issues and everything works okay, but when the 48V are plugged in, it instantly burns the LDO and releases the magic smoke. The 48V is coming from PSU, so maybe PSU transients. Any ideas what could be the cause and how to reliably fix it?? Guessing there is some transient stuff going on, but from the specs, it should survive it. No large output capacitance so inrush probably is not a big issue. My next move would be to place a fuse and TVS diode, but not fully sure.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 11, 2022, 01:13:25 pm
content: Add a RC filter in series with the input, which would give it a soft start as well as reduce the dissipation in the regulator.

author: wraper
author_rank: Unknown
timestamp: October 11, 2022, 01:41:24 pm
content: Add a series resistor between PSU and input cap/LDO input. Probably some inductive kick or PSU overcompensates on sharp voltage drop while charging input cap.

author: magic
author_rank: Super Contributor
timestamp: October 11, 2022, 01:48:31 pm
content: Either the PSU control loop overshoots as suggested above or you haven't included large enough input capacitor and voltage at your end overshoots due to cable inductance. Add 100µF elco and try again.

author: ConKbot
author_rank: Super Contributor
timestamp: October 11, 2022, 04:24:55 pm
content: Check out AN88 from analog devices. Given your input is so high, and your load is so low, I absolutely would put a 500 to 1kohm resistor in series, getting some dissipation off the device, and making sure the input doesn't overshoot. If you were more sensitive to input voltage drop, a higher ESR bulk electrolytic to damp the input could make sense. But you're not, so brute forcing it with a series input resistor works.

author: jkostb
author_rank: Regular Contributor
timestamp: October 11, 2022, 05:44:19 pm
content: Hi,Did you use ceramic input capacitors? If you use ceramic input capacitors and power it over cable from 48V power supply, then this can destroy your LDO. Please note that ceramic caps have very low ESR, this combined with inductance of cable gives a LC resonant circuit. The overshoot kills the LDO. Probably using an Elco at input solves the problem

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 12, 2022, 12:16:14 am
content: Try adding a small series resistor of 2.2-10R between the psu and the LDO, in the way the cap is at the ldo side, it will dampen any huge spikes that could be doing this.Are you connecting the banana terminals to the psu already set at 48v, or turning it on/off?What's the max. psu output? Some not-so-good psu can cause spikes at power on.Take the scope and monitor the psu output, set it to DC, single shot, 60V trigger level, about 1us/div, you might find some scary stuff...

author: rokspy
author_rank: Contributor
timestamp: October 12, 2022, 05:08:23 am
content: Ok, thanks all for the answers! Good insights and helpful! Will look into possibilities to add series resistor or/and electrolytic cap at the input.

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: October 13, 2022, 12:45:24 am
content: Sounds like an inductive kick from the combination of your input wiring length and the input capacitor. A sudden startup voltage has inrush into the cap, which charges the input wiring (inductor) and then when the input cap is full the inductor pushes more energy into it, overshooting the regulator's maximum voltage rating.A resistor will ruin the Q and calm it down.. and also make life a bit easier for the reg.

author: sicco
author_rank: Regular Contributor
timestamp: October 13, 2022, 08:05:07 am
content: Reverse current is a killer for LDO regulators. A Schottkey diode from Vout (anode) to Vin (cathode) prevents the trouble. I recently found out the hard way that Teensy4.1 modules can suffer from the same. Application notehttps://e2e.ti.com/blogs_/b/powerhouse/posts/ldo-basics-preventing-reverse-current-in-ldosexplains and lists mitigations.If your LDO Vin, your 48V rail, is hard wired to other circuitry that pulls it to GND, and you ‘plug-in’ manually the 48V power supply, then a (too large) Cout on Vout results in reverse current from a charged Cout back to Vin and from there to GND (through someting else, through ‘other circuits’ on Vin. Plugging in manually is never a single off to on transition, but a series of off-on events on a micro/millisecond scale, followed eventually by the aspired steady on.

author: magic
author_rank: Super Contributor
timestamp: October 13, 2022, 08:21:07 am
content: Right, another thing to worry about if there is a large output cap.In this case, it would be better to just add a diode in series with the input.Whenever you use a Schottky diode, remember that they have leakage, particularly at high temperature and particularly if it's a power diode, like you would probably use for surge protection. I recall seeing figures up to a few mA

author: rokspy
author_rank: Contributor
timestamp: November 04, 2022, 11:28:20 am
content: UPDATE: SOLVEDConsidering the low current for the application a simple resistor in series worked like a charm. Was not able to kill the LDO or the circuit when trying to intentionally kill it with what killed it before.Thanks!