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Question about LDO regulators — Parallax Forums

Question about LDO regulators

KyeKye Posts: 2,200
edited 2011-04-25 09:38 in Propeller 1
I'm working on a new design using the propeller chip and I plan to use an LDO 5V regular to power some componets in the design.

My question is this: Lets say I have 5V applied to the VOUT pin of the LDO regulator and leave the VIN pin unconnected. Will I have a problem? The VIN pin is also connected to a 33uF to capacitor to ground but nothing else.

Thanks for your help,

Comments

  • LeonLeon Posts: 7,620
    edited 2011-04-23 17:59
    It's probably OK. When the input supply is removed, the capacitance on the output will maintain the voltage on the output for a short while, which is similar.
  • KyeKye Posts: 2,200
    edited 2011-04-23 18:54
    Think I would have problems if the volatge on the VIN pin is negative instead of positive?

    EDIT: I mean VOUT instead of VIN.
  • Cluso99Cluso99 Posts: 18,069
    edited 2011-04-23 18:59
    Kye:
    Your actual question is not specific enough to understand precisely what you mean. Do you mean when unplugging the power source?

    You have to be careful with some regulators as if the voltage remains on the output (such as feeding from another supply, or in the case of extra large output capacitance vs input capacitance) the regulators may self destruct by backfeeding the power into the vout pin. If the regulator is not protected then a diode feeding back from Vo to Vin should protect this case.
  • KyeKye Posts: 2,200
    edited 2011-04-23 19:08
    Err, sorry I meeant VOUT above. The situation is, VOUT = negative voltage, VIN = floating (unconnected), GND is connected to GND.
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2011-04-23 20:34
    Kye,

    Most regulators (All that I have seen) have a reverse biased diode from their Vreg to their Vsupply, so an input voltage at the VOUT shouldn't harm the regulator. However if the voltage is negative at VOUT, then the regulator will be active in it's forward conducting mode, still this may be ok with the regulator, but I would be more concerned with the device(s) that are powered from the regulator in this scenario.
  • KyeKye Posts: 2,200
    edited 2011-04-23 21:16
    @Beau - I have a diode leading to another regulator to prevent this problem you mention above. I'm want to make sure its just not easily possible (with small voltages)to damge the regulator in question.
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2011-04-23 21:25
    Kye,

    Can you offer a circuit diagram? (partial would be ok) ... also how negative is the voltage and what is the source of it?
  • Cluso99Cluso99 Posts: 18,069
    edited 2011-04-24 05:58
    Beau: There are some regulators without the diodes. I have seen the data sheets with the advisories, just cannot recall which ones.
  • LeonLeon Posts: 7,620
    edited 2011-04-24 06:20
    The ST LExx parts have a PNP on the output. I think that is fairly common with LDOs.
  • KyeKye Posts: 2,200
    edited 2011-04-24 08:06
    Here's the image.
    001.jpg
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    001.jpg 152.8K
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2011-04-24 09:30
    Polarised capacitors do not tolerate being reverse-biased (if that's what you mean by "negative voltage".)
  • KyeKye Posts: 2,200
    edited 2011-04-24 09:51
    I can just use non polarized capacitors - if I can find 10uF ones. I care more about the regulator.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-04-24 10:07
    Why not move your diode in such a way as to prevent the negative voltage in the first place? Here are two different circuit topologies that will work. The simplest is series protection. However, if the diode's forward voltage drop is not tolerable, you can use the shunt protection method.

    attachment.php?attachmentid=80573&d=1303664818

    -Phil
    reverse_voltage_prot.gif
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  • Cluso99Cluso99 Posts: 18,069
    edited 2011-04-24 18:39
    Kye, I am not sure why you say you may have a negative voltage on the connector.

    Presuming you mean that the input can be reversed. To prevent this, the diode needs to be in from your input connector. If it is isolated ground, you could place it in series with the ground lead.
    Another method is to use a 3pin connector, with the outside pins connected together. Then it does not matter which way it is connected. I am not sure if there is a common way, perhaps center +ve and gnd outside.

    Another simple method my be to use a 3pin connector where a shunt is used on 2 pins to pass the 5V regulated output to the 3v3 Vin. Remove the shunt and use the common pin and the other pin on the 3pin header to feed the ne input voltage to the 3v3 regulator Vin.
  • KyeKye Posts: 2,200
    edited 2011-04-24 21:06
    I think the solution is just to punish the user when they plug the power in backwards. I will put proper silk masks on the board to tell the user not to plug the power in backwards.

    I'm working on a board designed to be sold to hobbyists... basic stamp/aurdino lovers etc. Keeping the cost low is the most important factor.

    The solutions presented above will not work in my sitautation since I need to have 5V come in and out of the connector I highlighted above.

    Thanks for all the help.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-04-24 21:17
    Kye wrote:
    The solutions presented above will not work in my sitautation since I need to have 5V come in and out of the connector I highlighted above.
    Um, the "shunt protection" circuit I presented above will satisfy those conditions and provide reverse voltage protection.

    -Phil
  • JonnyMacJonnyMac Posts: 9,208
    edited 2011-04-24 21:20
    Um, the "shunt protection" circuit I presented above will satisfy those conditions and provide reverse voltage protection.

    I'm with Phil -- we do that (PTC fuse) in all new EFX-TEK products.
  • KyeKye Posts: 2,200
    edited 2011-04-25 07:07
    You're right Phil. Any recs for poly fuses?
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-04-25 07:40
    Kye,

    Go to digikey.com, search "PTC", then click on "PTC Resettable Fuses". There's a huge variety, which you can narrow down parametrically. Your choice will mainly depend on the needed trip point, working current, and response time. Also, make sure the diode you choose is able to withstand the momentary current spike before the fuse "opens".

    -Phil
  • KyeKye Posts: 2,200
    edited 2011-04-25 08:23
    Alright, thanks!
  • MacTuxLinMacTuxLin Posts: 821
    edited 2011-04-25 09:38
    Kye wrote: »
    I think the solution is just to punish the user when they plug the power in backwards. I will put proper silk masks on the board to tell the user not to plug the power in backwards.

    Erm...how about using bridge rectifier?
    BridgeRectifier.jpg
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