Replace LM2596s-ADJ with smaller? - Page 1
Forum section: electronics / beginners

author: Falcon69
author_rank: Super Contributor
timestamp: April 01, 2015, 09:24:21 pm
content: So, I was looking at redoing my power supply for my circuit.Now, I am not sure if switching frequency matters much, as I am only going to be operating Logic Gates (74HC) and LED's with this circuit.The LM2596s-adj is just too big. It works, and works fine, but I am trying to make the PCB smaller, therefore, I need to look for a smaller alternative.I need to produce ~5.3v to compensate for voltage drop I will have along the wiring going to the switches I have. In addition to that, I also need to produce a flat out 5v output from the PCB to run other things. The voltage supplied to the PCB will be from a WalWart that is 7.2v-24v output, depending on which is on hand at the time.I was thinking maybe a 1.0A-1.5A max load on the 5.3v, and a max load of 1.0A-1.5A on the 5v at any given time. Most likely, the load will be half that amperage.The circuit I have could have 42 LED's on at a given time, they don't all draw the 20mA though, but if I calculate at 20mA each, that is 840mA current draw for the circuit, not taking into account the draw for the sensors or the logic gates, which I don't think is much. The sensors draw 25mA each, but they will NEVER be on at the same time. So worse case scenario, the circuit will draw less than 1A at a given time.It would be nice to fine a small footprint that has 2 outputs that can achieve that.For the 5v output, I was thinking maybe this chip....http://www.mouser.com/ds/2/631/ACT4088_Datasheet-345917.pdfbut it doesn't show anything about the recommended land pattern. How do I figure that out? Which pin should I extend the copper out for heat dissipation and vias through to copper side?I am not sure what chip can use for the 5.3v.It looks like the ACT4088 can also do adj. Using the formula 49900/((5.3/.81)-1)=9002 So using a 49.9K resistor for RFB1 as the data sheet suggests, that would make RFB2 a 9k resistor. Or am I limited to voltages out BELOW 5volts with this chip?If this chip works, I think I can get the overall footprint fairly small for 2 of these chips, both operating at a different voltage output, as it looks like it only uses surface mount components, i.e. ceramic capacitors.If this would work, I could run two of these chips and be okay.Any suggestions? Maybe a different chip? Would this chip be sufficient enough to do both outputs?If you think this chip will work, what is the recommendation for footprint/placement of components for heat dissipation and optimal operation? The data sheet was vague on that.All the datasheet size isPC Board LayoutThe high current paths at G, IN and SW should beplaced very close to the device with short, directand wide traces. The input capacitor needs to be asclose as possible to the IN and G pins. The externalfeedback resistors should be placed next to the FBpin. Keep the switch node traces short and awayfrom the feedback network and use shieldedinductors.That ACT4088 seems like a nice chip. It is small, works off SMD ceramic caps, internal start, thermal shutdown, and enable pin.

author: Falcon69
author_rank: Super Contributor
timestamp: April 01, 2015, 09:28:38 pm
content: Oh, one other thing, which I don't understand. The temp of the chip is 85°c while it recommend X7R chips for the caps. Those are 125°c chips. *shrug*. Maybe they are going off the ESR Value of the X7R vs. the X5R? Although, it does suggest using the XR5 as well.I just find it funny that they recommend a higher temp chip, then what the chip itself can handle.

author: Falcon69
author_rank: Super Contributor
timestamp: April 02, 2015, 01:14:57 am
content: Noone has any input? Noone has used this particular cheap or can recommend one better?I also like the fact it is much less cost than the LM2596s-ADJ. I don't really need a 3A output on the voltage regulator, so using the LM2596s-ADJ is not idea for my circuit (although, it is currently working fine)

author: Falcon69
author_rank: Super Contributor
timestamp: April 02, 2015, 03:42:31 am
content: NevermindI think what I am going to do is use a 1A Schottky Diode with a voltage drop of 320mV. I think that should work. I will have 9 inputs going out at 5 volts, each of which should be limited by the OptoMOS I am using which is limited to 100mA. That should keep me right around 5volts on each of the 9 outputs.I just hope my thinking is correct on this.http://www.mouser.com/ds/2/302/PMEG2010ER-353417.pdf

author: mariush
author_rank: Super Contributor
timestamp: April 02, 2015, 04:05:53 am
content: X5R and X7R are recommended because compared to cheaper types ceramic capacitors, they're better at holding energy at various voltages. It has very little to do with temperature.See this article which explains how capacitance of a ceramic capacitor varies with the voltage and temperature :http://www.maximintegrated.com/en/app-notes/index.mvp/id/5527Basically, a 1uF 10v ceramic capacitor will not be 1uF when you use it as output capacitor for a 5.3v regulator, the actual capacitance would be much less. A smart choice would be to use a ceramic capacitor rated for at least 25v, and preferably x7r so that temperature will have less of an effect.-The datasheet for that chip explains what you should do with the traces, and there's pages explaining how to pick parts and what's important.-other observations... led are current devices, you need to control the current not the voltage... i don't know how you limit the current on each led so I can't give you more advice.

author: Falcon69
author_rank: Super Contributor
timestamp: April 02, 2015, 04:28:43 am
content: Ya, I know the LED's are current based, but you still need to know the voltage applied, forward voltage of the LED, and the current required for the LED to figure out the series resistor needed for that LED.But, the outputs that need to be 5volts, are not for LED's. The outputs would be for other things requiring a 5volt source.

author: Rick Law
author_rank: Super Contributor
timestamp: April 02, 2015, 05:14:48 am
content: Quote from: Falcon69 on April 01, 2015, 09:24:21 pmTake a look atXL1509-ADJfromXL-Semi. It may suit your needs.2A output, adjustable Vout, 4.5V to 40V input. It has a small foot print inSOIC-8package.http://www.xlsemi.com/datasheet/XL1509%20datasheet.pdf(I don't know how well this chip works, but the spec seem to fit. XL1509 came to my attention as I am repairing something with an XL1509 in it. I am rather impressed by how small the foot print of the entire XL1509-adj's circuit is.)Rick

author: mariush
author_rank: Super Contributor
timestamp: April 02, 2015, 06:12:21 am
content: Quote from: Falcon69 on April 02, 2015, 04:28:43 amAs a suggestion, have a look at these very basic led drivers, they're basically like diodes that limit the current to a particular fixed value.. so no matter the input voltage, you get same current to the led:10mA :http://uk.farnell.com/on-semiconductor/nsi50010yt1g/ic-led-driver-50v-0-01a-sod123/dp/179497920mA :http://uk.farnell.com/on-semiconductor/nsi45020at1g/ic-led-driver-45v-0-02a-sod123/dp/1794973RLMay also want to look into led drivers with lots of channels, for example a 16 channel led driver IC like TLC5928 (example link :http://uk.farnell.com/texas-instruments/tlc5928dbqr/ic-led-display-driver/dp/1697163) could replace 16 resistors and also allow you to power the leds with up to about 17v (so for example you power your project from a 12v dc adapter and feed the leds directly from 12v, while using a dc-dc converter to produce 5v for the logic gates and whatever.Also you can daisy chain 3 of these chips and use just 3 wires (serial data) to turn on or off all leds at once using a microcontroller or something... look at the datasheet.

author: JesusCB
author_rank: Contributor
timestamp: April 04, 2015, 11:10:53 pm
content: The LM2597 is available in 8PDIP and 8 SOIC, I don´t know if that is smaller for you

author: Evil Lurker
author_rank: Regular Contributor
timestamp: April 05, 2015, 05:51:24 pm
content: Actually I have see some new switching IC's on the Chinese seller's sites that use a small SOIC-8 integrated switching package switching at around a purported 1.5mhz. Thanks to the high switching frequency they supposedly can get away with using SMD ceramic caps instead of electrolytics in addition to using 3.3-10uH inductors while generating a whole lot less heat. By the looks of them I'm gonna guess that they are about half the size of a typical LM2596 module and cost roughly the same or less.

author: Falcon69
author_rank: Super Contributor
timestamp: April 08, 2015, 06:46:21 pm
content: I just discovered that Texas Instruments has a simulation thingy for their voltage regulators.Pretty Neat, if it's accurate.Anyway, If the information is correct (and I'm not sure yet, as I have not fully read the data sheet), then the TPS54332 Chip should work great for my application.It's a small footprint with all components needed.Some pics of BOM, Simulation Temps, Board Layout, and Schematic below with values.I just have a couple questions.....The TI program says this chip is 86.693% efficient @ 24v input at 2A load with voltage out at 5.3v.My circuit will probably have 12volt supplied and a MAX of 2A, but most likely averaging out at about 400mA.So, based on worse case scenario of my circuit at 12V input 2A load 5.3v out, I am guessing efficient at 90%?But, what exactly is efficiency? When do I need to worry about it, or what type of circuit requires better/less?Do I need to have a debounce circuit for the switch needed for the enable pin (to turn the regulator on/off manually without plugging/unplugging the voltage source), or just use a standard toggle/rocker/tactile switch hooked up to it directly to EN pin via the voltage input (i.e. 7.5-24v), like in the schematic below with it drawn in Orange (needed voltage divider to keep the input from going above 6v (EN max input voltage)) The regulator is turned off when the voltage goes below 1.25v If this is what is needed, what size resistors do I need for the voltage divider part? 1/2W for the 150k and 1/16W for the 4K7? I will probably have a voltage divider on this circuit as well, just before the 1M resistor, so the voltage can drop enough from the 7.5v-24v input coming from the Walwart. I don't want to damage the Inverter.Should I have an Inductor/Capacitor filter on the output of the voltage regulator, after the voltage regulator components, but before the out to the circuit? (drawn below with values). I have it drawn with a 10uF 16v 1206 size. Can I go smaller footprint?The simulation shows the following temperatures for 24v IN, 5.3v OUT, and 2A load...1oz. Copper 80°c/176°F (Diode at 82°c)2oz. Copper 73°c/163.4°F (Diode at 74°c)at those temps, it's warm/hot to touch, but is still well below the rated 150°c the chip is rated for, and the chip does have an internal thermal shutdown of 165°c. Are these temps okay? I prefer to use 1oz. copper, it is cheaper.I'm new to this chip, (i am familiar with the LM2596s-ADJ somewhat). I have a reverse polarity circuit just before my LM2596s-ADJ I am using. Will that reverse Polarity circuit still be okay for this chip? Or do I need to seek out another design for that. Currently, it works just FINE with the LM2596-ADJ.I am assuming the parts list on the BOM is just fine, or do I need to update anything? I will probably use 0603 resistors instead of the 0201 and 0402 the BOM calls out for.I'm sure I will have more questions later. Thank you all in advance for your help and suggestions. It is very much appreciated.Jason