LM–XX. Input. Cin*. µF. CO**. Output. 1. MOTOROLA ANALOG IC DEVICE DATA. Three-Terminal Positive. Fixed Voltage Regulators. This family of fixed. the LM is available in the SOT package, as well .. †The specifications for the LMH/ devices are not contained in this datasheet. Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet. 2All trademarks are the property of their respective.
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It is assumed from here on that lifetime has been properly designed for and is not an issue. Using a tantalum capacitor as the input capacitor invites capacitor death at any time if the input power rail can have voltage spikes on it from any datxsheet. A spike more than a small fraction above a tantalum capacitor’s rated value risks it’s total destruction in a high energy circuit, such as this one is. The input capacitor is a typical reservoir capacitor, its value is relatively non critical.
Tantalum serves no technical purpose here. If ultra low impedance is desired then use of a smaller parallel ceramic is indicated. The output capacitor is NOT a filter capacitor in any traditional sense. Its principal role is to provide loop stability for the regulator. An eg 10 ohm resistor could be placed in series with the capacitor without impeding its functionality.
No normal filter cap would tolerate this without impaired functionality. The characteristics of aluminum wet electrolytic capacitors of the correct capacitance datasheeh voltage rating are well suited to the output capacitor’s role.
There is no reason to not use them there. The LM datasheet provides clear guidance on the essential and desirable characteristics of the input and output capacitors. Any capacitor which meets these specifications is suitable. Tantalum is an OK choice but is not the best datassheet. There are various factors and cost is datasheeh. Tantalum offers OK cost per capability at capacitances from about 10 uF up.
The output capacitor is “safe” against spikes in most cases. The input capacitor is at risk l340 “bad behavior” from other parts of the system. Spikes above dafasheet value will produce a literally flaming melt down. Smoke, flame, noise, bad smell and explosion all optional – I have seen one tantalum cap do all of these in turn: The input capacitor is not overly critical when the regulator is fed from an already well decoupled system bus.
Under the diagram on the ,m340 page they note “Required if the regulator is located far from the power supply filter” – to which you could add “or another well lm3400 portion of the supply”. Dtasheet output capacitor is more crucial.
Many modern low drop out high performance regulators are unconditionally unstable as supplied. To provide loop stability they require an output capacitor which has both capacitance and ESR in selected ranges. Meeting these conditions is essential for stability under all load conditions.
Output capacitance required for stability: Higher capacitances are also stable. This requirement could be met by an aluminum wet electrolytic cap or l3m40 ceramic cap.
National Semiconductor – datasheet pdf
ESR is a “Goldilocks requirement”: Required ESR is stated as. This is an extremely wide dtaasheet unusual requirement. Even quite modest ripple currents in this capacitor would induce far larger than acceptable voltage variations.
It’s clear that they do not expect high ripple currents and that the capacitor’s role is primarily related to loop stability than to noise control per se. If needed, use 0. A tantalum capacitor is not required to meet this specification. A wet aluminum capacitor will meet this spec with ease. Here are some typical new maximum ESRs for new aluminum wet electrolytic capacitors. The first group are capacitors that might be used in practice in this application at the low end of the capacitance range.
The 10 uF, 10V is about half the allowed ESr – perhaps a bit close for comfort across lifetime. The second group are what would datashee used with Cadj bypassed and could be used anyway – ESRs are far away from limits in both directions. They say in the LM datasheet. Lmm340 output capacitor is critical in maintaining regulator stability, and must meet the required conditions for eatasheet minimum amount of capacitance and ESR Equivalent Series Resistance.
Any increase of the output capacitance will merely improve the loop stability and transient response.
The ESR of the output capacitor should range between 0. I’ve read this so many times – daatasheet regulator stability”. What would be an example of an unstable regulator?
Would the output oscillate with high ripple or be undefined or what exactly would happen? Regulator instability, in my experience, and as you’d expect results in the regulator oscillating, with large level and often high frequency signal at the output and a DC voltage measured with a non-RMS meter that appears to be stable DC at an incorrect value. The following is comment on what you may see in typical circumstances – actual results vary widely but this is a guide.
Look at the output with an oscilloscope and you may see an eg kHz semi sine wave of ‘s of mV to some Volts of amplitude on a nominal 5VDC output.
Depending on feedback parameters you might get low frequency oscillation, slow enough to see as variations on a “DC” meter and you might get more like MHz signals. BUT anything can happen.
Also, how exactly does the ESR come into play here? A naive passerby like myself would expect lower series resistance to be better. The intuitive and the logical do not always match.
A regulator is essentially a feedback controlled power amplifier. If the feedback is negative overall the system is stable and the output is DC. If the net loop feedback is positive you get oscillation. The overall feedback is described by a transfer function involving the components involved.
You can look at stability from the point of view of eg Nyquist stability criteria or related no poles on right half plane and all poles inside unit circle or It’s adequate to say that the feedback from output to input does not reinforce oscillation and that a resistance that is too large or too small may lead to an overall reinforcement when considered as part of the overall system.
Only slightly more complex – good Sueful – stack exchange. Lots of related pictures. And one final note, did you refer to the ripple voltage on the cap being large even for small currents as an inherent issue due to the small size? The active regulator can work to reduce this ripple, but it is nice to not have your filter capacitor adding to the problem you want it to fix.
Tantalums were specified because Electrolytic can have their ESR increase up to 30x on very low temps. It is possible to connect a small tantalum in parallel to a large electrolytic if cost is an issue. The ESR of the output capacitor will cause loop instability if it is too high or too low. The acceptable range of ESR plotted versus load current is shown in Figure It is essential that the output capacitor meet these requirements, or oscillations can result.
However, the designer must ensure that the ESR will stay inside the limits shown over the entire operating temperature range for the design. This type of capacitor is not well-suited for low temperature operation. Solid tantalum capacitors have a more stable ESR over temperature, but are more expensive than aluminum electrolytics.
LM340 Datasheet PDF
If two capacitors are paralleled, the effective ESR is the parallel of the two individual values. Electolytics have poor high frequency performance compared to tantalums. With the price of tantalum these days I would recommend using one of your electrolytics with a small ceramic capacitor in parallel – say nF.
It depends on what you are powering but it’s not usually all that critical unless ripple and transient response are especially critical for you.
Home Questions Tags Users Unanswered. Why does the LM data sheet specifically specify tantalum capacitors? Why are they so specific about using tantalum capacitors? Peter Mortensen 1, 3 14 Jon Bright 2 3 9. Could it be that tantalums typically have a higher ESR than electrolytic? Matt, having read a little more about it, ESR certainly seems likely to be the reason.
ESR of ceramics is apparently very low, whereas for electrolytics, it increases over time.
If this is indeed the reason, electrolytics should be fine for now – I’m trying something out, not making 10, of them. Still, I’d like to know for sure. Tantalums have a low ESR! MattYoung Actually yes, it is. Because if someone like you finds something wrong you are able to dtasheet it.
It’s called “peer review”, and it’s basically what all the scientific papers go through. In this case the peers are scattered all over the internet. MattYoung And how long do you think that edit would last?