switching frequency of LM can be adjusted to any value between kHz and .. mode. All the analysis in this datasheet assumes operation in continuous . Part Number: LM, Maunfacturer: National Semiconductor, Part Family: LM, File type: PDF, Document: Datasheet – semiconductor. Datasheets, LM Design Resources, LM Design with WEBENCH® Power Designer. Featured Product, Create your power design now with TI’s.
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LM from Texas Instruments
The converter is a feedback loop, and such feedback loops need to be stable, otherwise your power supply might fail at certain frequencies. The problem is that addition of this resistor modifies the current limit: Again, this is from the Adafruit calculator, but here is the formula: Conclusion Well actually there is no conclusion until I actually get the parts in to build this thing.
The new current limit, of course, didn’t change from before because we chose the same sense resistor. One of these is the resistor that goes between the switch and ground.
The cause of this is the insanely low maximum sense resistance from above. First, choose a switching frequency between kHz and 1MHz. So, let’s take a look at the application note to see what the transfer function looks like without this compensation. We first calculate the maximum sense resistor for current mode loop stability loop stability is always a good thing:.
Now that we have our pole and zero frequencies, we need to know what the DC gain is. Dxtasheet capacitor selection You want something big with low ESR. The gain margin is minus the gain at which the phase datashwet degrees. Switching frequency First, choose a switching frequency between kHz and 1MHz. Sign up Already a member? Here’s the one for the boost converter:. So, in fact, this converter with the components so far needs no compensation network, and we can leave the COMP pin unconnected.
The pole is caused by the output capacitor with the load, and the complex pole is caused by the switching action. Min and max output voltage are VDC, output current is 0. ddatasheet
A boost converter design
And indeed it is. There are two ways to raise that maximum sense resistance: There is a second resistor which goes between the current sense pin of the converter and the datasheet sense resistor. Then I started reading about the compensation circuit and realized that I don’t have the means to calculate it.
You can see the resonance at half the switching frequency. Diode selection The diode has to be able to carry the maximum inductor current.
A boost converter design | Details |
Nobody will know unless we try it. Here’s the one for the datashset converter: Also a duty cycle switch on-time to total time of 0.
This is fine, since the application note states we want a phase margin between 30 and degrees. We got this from the Adafruit calculator, 0. I don’t know, but suspect that the output voltage of the adapter will drop under load, so I’m just going to go ahead and specify a minimum input voltage of 8VDC.
Doing so without going through the math shows that it is not possible to pick a reasonable sense resistor while still maintaining a good current limit. Compute the minimum inductance required. They are there to add stability to the converter.
And now for the complex pole. And so I get a current of 4. The first zero, z1, is caused by the output capacitor with its ESR. Our chosen sense resistor is bigger than this by a factor of 3. We’ll find out in the next project log.
LM3478 Datasheet PDF
Design procedure The LM datasheet provides a nice series of design procedures for various converter topologies. Again, this is from the Daatasheet calculator, but here is the formula:. It can handle currents up to 9A, and has an on-resistance of 0. Doing that also has a knock-on effect of lowering the inductor current.
The top plot is gain in dB vs frequency in Hz, and the bottom is phase shift in degrees vs frequency in Hz. It can withstand at least V to the drain, and has a gate threshold of at most 4V.
Again, Adafruit gives this as 4. I’ll keep using kHz in the calculations as it doesn’t matter much. Maybe start with this list of videos? Note that the Adafruit calculator correctly, in my opinion uses the lowest of two possible duty cycle based on the minimum input and output voltage, because this will result in the highest minimum inductance.
I hate low-frequency converters with their whining. Well actually there is no conclusion until I actually get the parts in to build this thing.
We can see that unity gain occurs at 2.