PJ75AL33SA LDO - Crap or design fault? (Solved: Crap) - Page 1
Forum section: electronics / projects

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 17, 2023, 06:26:54 pm
content: PJ75AL33SA Datasheet.Looks pretty nice and it was dirty cheap. So I bought 50 to replace the more expensive MCP1703.Basic specs:- 36V max in.- 3.5uA quiescent current.- 25mV drop @ 1mA.- 100mA max.The board I'm using this in only powers a small MCU (2-3mA), a IR receiver (200uA), a Led (2mA) and a ULN2003, top current peak could be 40mA at most.Powered from USB + series 1N5819 Schottky diode as reverse polarity protection.The MCP1703 was fine with 4.7uf at input,100nF at outputwas 1uF, and 100nF at MCU VDD.(I think I hit my head with something that day LOL)The IR sensor uses minijack plug, so I put a series 120 Ohm resistor to protect VDD from possible shorts when inserting it, never had any issue with this.With this LDO, inserting the IR causes a MCU reset. Shorting the resistor to gnd (Measured 25mA) also causes this reset.Input is completely stable at all times at 4.7V, no surge of any kind, but I'm seeing a large negative spike at the output dropping to 2.7V for ~200uS (See attached picture).I thought, maybe this LDO needs larger cap? Soldered 4.7uF also at the output... same!Could be that 200nF capacitance is just too much for this small load, causing oscillation? Maybe too high Q? Doesn't seem to me?Can't really tell the capacitor specs, I simply stole 20x 0603 100nF caps from a random production board.I made 10 boards, all are board are doing this.Maybe just... Chinesium?

author: wraper
author_rank: Unknown
timestamp: November 17, 2023, 06:47:58 pm
content: QuoteSo what let you think 100n is enough when datasheet clearly shows otherwise on the first page? It's not a 78xx, and it works by lucky chance.QuoteAs of PJ75AL33SA did you even spare a single glance on its datasheet? Hint: it needs 10uF electrolytic or tantalum (not MLCC).

author: wraper
author_rank: Unknown
timestamp: November 17, 2023, 06:56:37 pm
content: If you want dirt cheap, here you gohttps://www.lcsc.com/product-detail/Linear-Voltage-Regulators-LDO_HXY-MOSFET-XC6206P332MR_C5148692.htmlRequires 1uF MLCC on the output. Maximum input voltage is 8V.

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 17, 2023, 07:01:26 pm
content: Yeah I supposed so, that's why I said I tried larger values. The input has zero drop, so not an input capacitance issue.I tried 4.7uF (MLCC X7R), then 10uF...22uF electrolytics (Also with/without the MLCC) at the output, basically the same thing, slighly reduced to 2.9V with 22uF.It's just switching a resistive load of 25mA, if this needs 100uF to be stable then it must be the worst LDO in Earth!Look at this, 1KHz PWM feeding a LED through 1K resistor, current about 1.5-2mA, causing 160mVPP ripple with 22uF!

author: wraper
author_rank: Unknown
timestamp: November 17, 2023, 07:10:39 pm
content: Again it should be 10uF and not MLCC due to too low ESR. If you say take 1117 LDO from the most reputable manufacturer, it would be no different in this regard. Like what is easier, looking into datasheet for a few minutes or doing tests with random capacitors?

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 17, 2023, 07:11:07 pm
content: Yep, I also tried removing the MLCC from the output, 10 and 22uF caps where electrolytics, no improvement.

author: wraper
author_rank: Unknown
timestamp: November 17, 2023, 07:16:04 pm
content: What is input voltage?EDIT: it's not really an LDO FWIW, it requires 2V Vin-Vout difference for 70mA current. Basically FET based 78L33, low dropout only for very small current.

author: wraper
author_rank: Unknown
timestamp: November 17, 2023, 07:18:43 pm
content: In any case, I would not use PJ75AL33SA or any other LDO that requires electrolytic/tantallum caps in new designs. They are outdated.

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 17, 2023, 07:39:37 pm
content: Quote from: wraper on November 17, 2023, 07:16:04 pmI was starting to suspect this, but the datasheet really sucks.The temperature coefficient triggered the alarm: "VIN= VOUT+2.0V, IOUT=10mA".Why Vin+2V for just 10mA? Oh wait. Fuck this chinese crap.I'll be makign a very little contribution to the E-waste soon.

author: DavidAlfa
author_rank: Super Contributor
timestamp: January 21, 2024, 01:40:59 pm
content: I had forgot about this... Today I was making more of these boards and got a flashback!I kept using the MCP1703T-3302, this was just a test of cheap LDO from lcsc when I ordered the PY32F002A.With 4.7uF X7Rs at input & output, switching a 100 Ohm load makes a 20us, ~150mV drop.Is it normal or my fault due keep using ceramic caps?The normal load is just 5-10mA, so will be good enough, no analog signals here, just a led and a ULN2003 switching external signals.The footprint is for 0402 and I'm not making new boards for this nuisance!Just for testing: Adding a 100uF electrolytic to the input made no difference, but did a terrific job at the output.