Can I bypass my AP2112 regulator ? - Page 1
Forum section: electronics / projects

author: voidptr
author_rank: Contributor
timestamp: November 30, 2016, 08:43:39 am
content: I have a ucontroller powered through an AP2112 3.3v from 5v, but I already have a stable3.3v so here my question,can I just feed 3.3v on the output pin 5 of the ap2112?if I understand the ic when turn off have inside discharge resistors is this really, 60Ohm or is it going into some High resistor?if I do that will I blow the AP2112 ?I try to measure the input current ... I should see 55mA but no im more like 20mA who is the uctrl running...I could not really understand the spec sheet on that...

author: trophosphere
author_rank: Frequent Contributor
timestamp: November 30, 2016, 09:16:11 am
content: The AP2112 will shunt any current on its output through an internal 60 ohm discharge resistor when it is disabled. If you have 3.3v on the output then the amount of current through that resistor will be 55mA. The power being dissipated will be ~182mW and the corresponding temperature rise will be dependent upon the thermal resistances inherent to whatever package the chip is in as well as what kind of heat-sinking/interface is available.If the thermal calculations show an acceptable temperature rise that is within the operating temperature of the device (can technically use storage temperature but will assume lower tolerance for longer life and in case the device needs to go into operation) then the device should be okay. That is assuming you are okay with burning off that much current as well.

author: voidptr
author_rank: Contributor
timestamp: November 30, 2016, 09:49:22 am
content: but if I try to measure the input current going on the output pin ... I should see more than 55mA but no im more like 20mA who is the uctrl running current sortof ...that is what bug me ? ...so if the 60ohm just a minimal value and can varie from ic to other ic or the fact that not powering the ap2112 at all change the shunt resistor in some way...:-)

author: trophosphere
author_rank: Frequent Contributor
timestamp: November 30, 2016, 03:17:17 pm
content: The 60 Ohm figure is listed as typical so it can certainly vary and I very much doubt that the manufacturer would go any lengths to trim it. Having the regulator unpowered means that the shutdown logic is not powered. If you look at figures 3 and 4 in the datasheet, it shows a N-channel MOSFET controlling the discharge pathway so a positive gate voltage is needed to turn it on.Now, whether or not current is allowed to flow backwards into the shutdown logic for cases when Vin is floating the datasheet does not seem to say as all test conditions require Vin to have a voltage present. Your question of if the chip will blow in this instance cannot be answered with confidence as there is no information provided in the instance of a floating Vin with a voltage present on Vout. The current of 20mA (I am assuming that is just the current going into Vout) could be going through the discharge path or it could be going through something else that happens to provide an intrinsic path in this scenario.Is there anything preventing you from just desoldering the chip? Another possibility is to feed 3.3v into Vin and set the enable pin to on. The regulator would not be able to regulate as Vin=Vout but that is insignificant as the source of your 3.3v can keep regulation. The current consumed by the device would then be around its quiescent current.

author: voidptr
author_rank: Contributor
timestamp: November 30, 2016, 05:47:38 pm
content: hey that seems to be good !!!I thought vin should be something like vout + 1vBut if I feed 3.3v to Vin I got 3.3v on vout ( like on 1 of the datasheet diagram)vdrop for me should be really low, the max current I will used might be 100mA but more typically 20mAso this might be how I will used it finally...ps. btw R shunt is around 80ohms when I measure with vin and enable off ... so a bit more than 60ohms