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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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. Shut up already, converter!
With that resistor, the current limit would be 5. The off-time is 0. Next, compute the duty cycle, D. With my chosen frequency of kHz, I lm478 Design procedure The LM datasheet provides a nice series of design procedures for various converter topologies. There is a second resistor which goes between the current sense pin of the converter and the current sense resistor.
The problem is that addition of this resistor modifies the current limit: However, we now do not need a slope compensation resistor, and everyone is happy. Sign up Already a member? It can withstand at least V to the drain, and has a gate threshold of at most 4V. The first zero, z1, is caused by the output capacitor with its ESR.
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Nobody will know unless we try it. This means that in gain it acts dahasheet a zero, but in phase it acts like a pole. We first calculate the maximum sense resistor for current mode loop stability loop stability is always a good thing:.
Output capacitor selection You want something big with low ESR. Maximum inductor current Compute the maximum current flowing through the inductor. Let’s locate the zeros: Since I kinda datawheet the inductor I chose, let’s raise the frequency from kHz to kHz and try again.
I chose a standard resistor of 0. Our chosen sense resistor is bigger than this by a factor of 3. Also a duty cycle switch on-time to total time of 0. The LM datasheet provides a nice series of design procedures for various converter topologies.
So this is really a good thing. Our modified values with this frequency are: Clearly such a switch has to shut off well datasheett 78ns. I think I will have km3478 look at other chips such as the LT The gain margin is minus the gain at which the phase is degrees.
LM3478 Datasheet PDF
This rules out the popular MC, which has a max duty cycle of 0. To power my boost converter, I’d like to use a 12VDC wall adapter. I chose kHz because I wanted it to be above the minimum, but not so fast that all my components had to be super-fast. The problem is, without knowing control theory, it will be difficult to select the compensation components. These margins are important because the poles and zeros shift with varying load and input voltage, and without these margins your circuit could go unstable.
And now for the complex pole. And, of course, this pole is located at half the switching frequency:. So, in fact, this converter with the components so far needs no compensation network, and we can leave the COMP pin unconnected.
It must also be able to withstand a reverse bias of the output voltage minus the input voltage. Design a boost converter that gives VDC at 0.
You want something big with low ESR. I’ll keep using kHz in the calculations as it doesn’t matter much. Then I started reading about the compensation circuit and realized that I don’t have the means to calculate it.
Adafruit has a nice boost converter calculatorassuming you’re using the simple single-inductor version. Here we see the gain margin is 3. Minimum inductance Compute the minimum inductance required. We can see that unity gain occurs at 2. Here’s the one for the boost converter: Datashset and max output voltage are VDC, output current is 0. So with a maximum input voltage of 12V, and an output current of 0. Conclusion Well actually there is no conclusion until I actually get the parts in to build this thing.
Switching frequency First, choose a switching frequency between kHz and 1MHz. 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 datqsheet a minimum input voltage of 8VDC. I hate low-frequency converters with their whining.
A boost converter design | Details |
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. That is all before you get to the need to plot charts to solve the equations.
This means raising the frequency or the inductance by at least a factor of 3.