Can I connect switching regulator's EN pin directly to the car's ACC? - Page 1
Forum section: electronics / beginners

author: sidlauskas
author_rank: Contributor
timestamp: October 09, 2018, 08:36:54 am
content: I am developing an electronic circuit that will be installed in a car and I'm wondering how I should properly turn the regulator on from the car's ignition (ACC) using ENABLE pin. I'm planning to use BD9778F switching regulator (link to datasheet). Can I connect it directly? If not, how this should be implemented then?By the way, someone suggested me to power my project directly from ACC, but I want to highlight that I can not do this because the ACC I get from the car's radio the max load is 150mA and my circuit could drain up to 500mA, so I need a proper schematic for enabling/disabling regulator using ACC.

author: Psi
author_rank: Super Contributor
timestamp: October 09, 2018, 08:53:24 am
content: ACC can provide much more than 150mA.ACC doesn't come from the car radio, it comes from the car ignition.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 09, 2018, 09:04:09 am
content: ACC comes from when you put car key in ON position, regardless if ignition is on or off... somesort of relay will just connect it to fuse and battery. so basically the current limit is the fuse limit. almost every car's appliances will use ACC line to turn themselves ON.

author: Ian.M
author_rank: Super Contributor
timestamp: October 09, 2018, 09:07:01 am
content: A typical automotive battery supply is really hostile to electronics with transients of either polarity that can peak at an order of magnitude higher than the supply voltage, and the possibility of significant overvoltage for long enough that the power rating of clamping networks can be a significant issue, and supply dropouts of significant duration.Quote from: sidlauskas on October 09, 2018, 08:36:54 amThat would be disastrous if the main Vbatt supply to Vin was ever interrupted, even momentarily, while ACC was on.QuoteAssuming a nominally 12V vehicle, I would suggest a potential divider, 1K lower arm, 1.8K upper arm, to feed about 5V to EN while drawing 5mA from ACC (enough to prevent stray voltages turning it on), with 10nF to Gnd and Schottky diode clamping to Vin and Gnd at the EN pin.Don't forget you also need to adequately protect the regulator Vin, and any other signals to your circuit from the rest of the car's electrical and electronic systems . . .Quote from: Psi on October 09, 2018, 08:53:24 amOTOH if ACC is tapped from the car radio rather than the supply to it, it may well be limited by the current capability of tracks on the radio PCB, or may even be a switched output from the radio after its power control circuit (which is fairly common to support electrically operated antennae, CD changers etc.) Ignoring any warnings about a 150mA limit would be foolish . . .

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 09, 2018, 10:16:00 am
content: i've opened and repaired a few car electronics equipments such as player and power amp i dont see any over complicated overvoltage protection other than reverse polarity diode protection and fuse similar to what usually available in other non-automotive products. i rechecked my log, for example there is TL494 connected directly to the Vbatt (car battery), granted its a 40V max chip but in other is PT2313, 10V max chip, so it must be fed from some jellybean regulator or simply emitter follower bjt by a voltage divider directly from battery, since i dont see any fancy regulator in there. if there is high transient reaching ACC, i will be more worry on the car's battery health life.

author: sidlauskas
author_rank: Contributor
timestamp: October 11, 2018, 07:46:27 pm
content: Quote from: Ian.M on October 09, 2018, 09:07:01 amYou're correct, the ACC which comes from the radio is basically for enabling/disabling the CD changer and it has a 150mA limit. Not sure if I correctly understood the second part of your answer where you say "with 10nF to Gnd and Schottky diode clamping to Vin and Gnd at the EN pin". Could you please clarify if capacitor should be at Vin or En and if Schottky diode should be inserted between Vin and En or between En and Gnd? Simple schematic would be very helpful as I'm total beginner. Thank you!

author: Ian.M
author_rank: Super Contributor
timestamp: October 11, 2018, 11:23:46 pm
content: I don't normally draw schematics for such simple stuff, but as you are a noob, here it is.N.B. You *MUST* protect the regulator Vin against transients and overvoltage. How much protection it needs is a matter of opinion - I prefer 'bullet-proof' for my own projects, as I'm protecting a lot of time, effort and materials invested in them, but for a cheap commercial product, it would often only be the minimum protection needed for most of them to survive long enough to go out of warranty. How to protect the regulator input in an automotive application is heavily dependent on the level of protection required, the load current, and whether the regulator must be capable of riding through the worst transients at full load current or if the application can shed loads or even shutdown, so I am *NOT* going to discuss it here at this time. There have been many threads about such protection on the forum, so do some research, come up with a proposal, and ask for criticism.

author: sidlauskas
author_rank: Contributor
timestamp: October 13, 2018, 08:07:59 pm
content: Thank you so much for your help and schematic, I really do appreciate it! Here's the final schematic I came up with. It would be great if you could give me some feedback on this.

author: sidlauskas
author_rank: Contributor
timestamp: October 16, 2018, 10:30:53 am
content: Could anyone give me feedback on the schematic in my previous post? Thanks!

author: Ian.M
author_rank: Super Contributor
timestamp: October 19, 2018, 09:03:45 pm
content: I don't see any input over-voltage protection, or any transient protection, other than that provided by the low pass filter formed by the input capacitors with the supply wiring, which wont be very effective. Its essential that spikes be kept under 35V (regulator abs max input voltage is 36V), so it would be advisable to fit a beefy unidirectional TVS diode with a clamping voltage under 35V across C1, cathode positive, and move the fuse to before C1. An 18V TVS diode would probably be suitable. It should have a sufficient surge rating to be capable of blowing the fuse without failing, to protect your circuit if one of the battery post connectors comes loose under load. As a rough place to start, look at TVS diodes with a peak current rating of at least an order of magnitude greater than the fusing current at the shortest time shown on the fuse's I2t characteristic graph.For the rest of it, if you CAREFULLY follow the recommended PCB layout round the regulator from its datasheet, it should work as expected, as long as the inductor has a high enough saturation current and a low enough internal resistance.