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| Why are specific types of capacitors called for in terms of buffering an LDO? |
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| jnz:
I have an LDO I'd like to use. It calls for 10uF on the input and 22uF on the output. I have the output covered, but on the input side I can't fit the electrolytic it calls for. I have a height limitation of 3.8mm, there are no electrolytic in that 10uF and 40V+ range. IIRC electrolytic suck at high frequencies, but I don't remember if a ceramic, a tantalum, MLCC, could effectively replace an electrolytic or not for input buffering (assume you met the voltage, ESR, and capacitance requirements)? Does my datasheet say to us a 10uF aluminum electrolytic because it usually makes the most sense to do so? How much freedom do I have for low frequency input buffering? |
| spec:
+ jnz It would not be possible to answer your OP without knowing what LDO voltage regulator chip you intend to use. LDOs are normally sensitive to the input and output capacitor characteristics. Some LDO voltage regulators even go unstable with too lower ESR! |
| jnz:
--- Quote from: spec on October 19, 2018, 05:43:18 pm ---+ jnz It would not be possible to answer your OP without knowing what LDO voltage regulator chip you intend to use. LDOs are normally sensitive to the input and output capacitor characteristics. Some LDO voltage regulators even go unstable with too lower ESR! --- End quote --- Ignoring ESR for a moment. I can make that work. Lets say all the options have the same 5mOhm ESR. Is there a reason an MLCC or Tant or Ceramic all of equal values buffer the input to an LDO like an electrolytic would? edit: I'm interested in the question right there because I'm finding it extremely difficult to find a 40V+ rated 10uF that fits in my height requirements. If I can looked at other styles it would help. Unless someone has a 3mm tall 10uF electrolytic that's cheap! |
| T3sl4co1l:
Does it have to be a picky LDO? Are you really getting 39.5V out from a 40.0V input? Use a HDO like the old fashioned LM317 or 7805 or what have you (it'll have to be an -HV version most likely, but those exist, too) and you're fine. ;) Tim |
| jnz:
--- Quote from: T3sl4co1l on October 19, 2018, 07:18:56 pm ---Does it have to be a picky LDO? Are you really getting 39.5V out from a 40.0V input? Use a HDO like the old fashioned LM317 or 7805 or what have you (it'll have to be an -HV version most likely, but those exist, too) and you're fine. ;) Tim --- End quote --- Not really. It's 40V LDO, 12V automotive, 30-35V claming TVS, so I can get away with a 35V electrolytic. Nichicon makes a 3.95mm which is a hair too tall (literally, about a human hair). I MIGHT be able to lower the clamping on the TVS and use a lower rated cap, but this just risks blowig the TVS. That said, I'm not planning on this thing being plugged in when jumping, or even driving. It's a test tool for another device. So I may be planning on voltage spikes it'll never see. I could look into a HDO, but it's nice if this thing keeps working down to 3V like it's supposed to now. Goes with another question I posted here about having enough time to write to EEPROM if someone unplugged it when they shouldn't. So for the space issues I have, it would really help out if I could replace the electrolytic with a ceramic just to buffer the LDO on either/both sides. But can't seem to get that answer. |
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