Jump to content
Electronics-Lab.com Community


  • Posts

  • Joined

  • Last visited


brutus's Achievements


Newbie (1/14)



  1. Hi, I want to change a PWM into a smooth analog signal. I have the circuit, but I was wondering if it mattered what capacitor I would be using. I want to use a ceramic cap... does it matter? The PWM is a 100 KHz square wave.
  2. Hi, I'm reading the following pdf: http://www.national.com/an/AN/AN-1482.pdf Can someone explain what a pole is? "The capacitor connected to the ouput formsa pole with the load resistor whose frequency is given by" and so on... What is a Load Pole. I'm a layman, so please "dim it down a shade." Thanks.
  3. Thanks! I will do that. I was a bit unclear... I meant the chip maker of the Qt301 that needs a ceramic cap for high frequency supression, not the regulator. Anyways... this has been most educational. I appreciate your help.
  4. Yes. You are right. The guy from the chip company said: "ceramic capacitors are needed for high frequency supression" He said this regarding the chip I need to 'voltage regulate' Will adding a 1 Ohm resistor in series to the ceramic cap solve this problem, and would it affect the ceramic caps' ability for "high frequency supression"???
  5. Hi, I just spoke to the guys that make the chip (that I need to 'voltage regulate') as well as a tech guy from digikey who knew about this issue. They both said that this issue was true in the past (1980's 1990's) but is no longer an issue at all because the LDOs have internal components to deal with exactly that. So as far as I can tell, YES you can use ceramic caps on the LDO output. Just thought I would post my findings...
  6. > Even additional ceramic caps on the output cause trouble. Ok... Does this mean that any cap that is between the supply voltage (in this case the output of the regulator) and Ground can cause problems? _BUT_ Capacitors that are in other parts of the circuit (are not directly connected to the supply voltage and GND) will not cause problems, and can be film or ceramic???
  7. Hi, I read an article about LDO and mistakes people make that result in an unstable output. http://www.national.com/nationaledge/jul02/article2.html In that article (which is way over my head) they mention that tantalum and ceramic are not compatible (or something). I will post the two paragraphs from my pdf again and ask a question after: "If the power supply is shared with another electronic system, make sure the supply is free of spikes, sags, and surges. The supply is best locally regulated using a conventional 78L05 type regulator, or almost any 3-terminal LDO device from 3V to 5V. For proper operation, a 0.1uF or greater bypass capacitor must be used between Vdd and Vss; the bypass cap should be placed very close to the device pins." In the link article (www. national.com) it talks about interference (oscillation) caused by ceramic caps. If each of my chips will have it's own regulator, maybe a bypass cap between Vdd and Vss is not necessary? Maybe the 10uF cap I will use for the regulator output will be enough? Thanks.
  8. >I assume you are trying to use as little board space as possible? I design with "surface mount" tantalums instead of "through hole" Electrolytics in such a case. Solves many other problems as well. You assume correctly, except it's not only board space but I also need a very low profile, like 3-4 mm. They have 2.2uF axial electrolytic caps that I might use, but your idea of using surface mount tantalum caps sounds like a good idea. I will look into it... >I would not recommend metallic or ceramic caps for power supply bypass. Just curious... why? Does it have to do with the speed that it takes to charge and discharge? Thanks MP, B.
  9. >It is a nice regulator, but: >Can you buy a 2.2uF capacitor that is guaranteed to be at least 2.2uF? >Did you notice that every example circuit in the datasheet has a 10uF output >capacitor? Ok... So I'll use a 10uF outpur cap... Solves the overshoot problem maybe? I still want to use as small a cap as possible. Can I get away with a metallized film cap or maybe a X7R ceramic cap? _OR_ is electrolytic the only way to go for the output cap? ???
  10. Hmmm.... 100uF seems really high... Attached is a JPEG of the "test circuit" where they use 2.2 uF instead of 100uF. The whole pdf document can be found at: http://www.st.com/stonline/books/pdf/docs/2573.pdf Am I missing something? B
  11. Thanks for the reply. I did find a circuit. It uses a 0.1uF for the input film capacitor (poly...ester?), and a 2.2uF electrolytic capacitor for the output. Final question: Do I really need to use an electrolytic cap? They are too big. I would prefer to use something smaller, maybe another film or ceramic... Having a stable voltage is hugely important, so I don't want to use something unsuitable. Thanks again, B.
  12. Where do I say that I want to use a 78L05??? I say I dont. I want to use something like 78L033 or 78L04, except in a TO-92 package. Anyway... read my post if you're interested in replying...
  13. Hi, In the data sheet for a chip (qt301 from qprox.com) I am using it says the following: "If the power supply is shared with another electronic system, make sure the supply is free of spikes, sags, and surges. The supply is best locally regulated using a conventional 78L05 type regulator, or almost any 3-terminal LDO device from 3V to 5V". Ok... I can't use 78L05 because the supply is at 5V. So I will need to use a regulator that gives 3.3V or 4V ouput. As always in electronics, there are HUNDREDS of different parts to choose from, and many don't have basic circuits that show you how to make a basic "fixed" voltage regulator (i think it's called fixed). Also, there a very few regulators that are called "fixed", so I'm guessing the "fixed" regulators have the capacitors in the IC already??? I believe if they are not designated as "fixed" I need to use two capacitors... If anyone can point to a part and a circuit that would be very nice... B.
  • Create New...