LM3478 DATASHEET PDF
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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Adafruit has a nice boost converter calculatorassuming you’re using the simple single-inductor version. To power my boost converter, I’d like to use a 12VDC wall adapter. 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.
Min and max vatasheet voltage are VDC, output current is 0. I hate low-frequency converters with their whining. Shut up already, converter! This gives me a required inductance of at least Also a duty cycle switch on-time to total time of 0.
This rules out the popular MC, which has a max duty cycle of 0. The LM datasheet provides a nice series lm378 design procedures for various converter topologies. Here’s the one for the boost converter:.
First, choose l,3478 switching frequency between kHz and 1MHz. I chose kHz because I wanted it to be above the minimum, but not so fast that all my components had to datashret super-fast. Next, compute the duty cycle, D.
LM Datasheet(PDF) – National Semiconductor (TI)
We got this from the Adafruit calculator, 0. Note that the converter will ratasheet the duty cycle based on the output voltage and switch current, so this is only a value for calculation. Compute ln3478 minimum inductance required. Again, this is from the Adafruit calculator, but here is the formula:. So with a maximum input voltage of 12V, and an output current of 0. 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.
Compute the maximum current flowing through the inductor. Again, Adafruit gives this as 4. And so I get a current of 4. Again, Adafruit correctly is using the Km3478 based on the maximum input voltage, but I’m using a higher value for extra safety margin.
So based on the inductance and current, I can choose an inductor that can handle this current. With my chosen frequency daasheet kHz, I get I’ll keep using kHz in the calculations as it doesn’t matter much.
One of these is the resistor that goes between the switch and ground. This resistor protects the circuit against overload currents.
And so I get a sense resistor of 0. I chose a standard resistor of 0.
With that resistor, the current limit would be 5. There is a second lm33478 which goes between the current sense pin of the converter and the current sense resistor. We first calculate the maximum sense resistor for current mode loop stability loop stability is always a good thing:. I get a maximum sense resistance of 0.
A boost converter design
Our chosen sense resistor is bigger than this by a factor of 3. We must now change both the sense resistor and the slope compensation resistor so that the current limit is sane and the slope compensation resistor is above the minimum. 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. The cause of this is the lm478 low maximum sense resistance from above.
There are two ways to raise that maximum sense resistance: Doing that also has a knock-on effect of lowering the inductor current. So this datssheet really a good thing. Thus, instead of a maximum sense resistance of 0.
This means raising the frequency or the inductance by at least a factor of 3. Since I kinda like the inductor I chose, let’s raise the frequency from kHz to kHz and try again. The new current limit, of course, didn’t change from before because we chose the same sense resistor. However, we now do not need a slope compensation resistor, and everyone is happy.
The diode has to be able to carry the maximum inductor current. It must also be able to withstand a reverse bias of the output voltage minus the input voltage. And, a Schottky diode is recommended for high speed. Clearly such a switch has to shut off well before 78ns. It can withstand at least V to the drain, and has a gate threshold of at most 4V. It can handle currents up to 9A, and has an on-resistance of 0.
Can this thing switch fast enough? Nobody will know unless we try it. If not, there are chips specifically designed to drive Ln3478 gates at higher currents to force them on and off quickly. You want something big with low ESR. There’s a COMP pin on the converter where you put a resistor and capacitor in series. They are there to add stability to the converter.
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The converter is a feedback loop, and such feedback loops need to be stable, otherwise your power supply might fail at certain frequencies.
If you don’t know anything about control theory, you probably want to skip this section. The problem is, without knowing control theory, it will be difficult to select the compensation components. Maybe start with this list of videos? So, let’s take a look at the application note to see what the transfer function looks like without this compensation.
The first zero, z1, is caused by the output capacitor with its ESR. The second zero, z2, is caused by the inductor and the load on the power supply. The pole is caused by the output capacitor with the load, and the complex pole is caused by the switching action.
Note that the second zero is a right-hand plane zero. This means that in gain it acts like a zero, but in phase it acts like a pole. And now for the complex pole. We need to find Q, which is the damping factor. If Q were infinite, the thing would vibrate itself to pieces and catch fire at half the switching frequency.
But, of course, this will not be the case. And, of course, this pole is located at half the switching frequency:. Now that we have our pole and zero frequencies, we need to know what the DC gain is.
The top plot is gain in dB vs frequency in Hz, and the bottom is phase shift in degrees vs frequency in Hz. You can xatasheet the resonance at half the switching lm34788. The phase margin is degrees darasheet the phase at which the gain is unity 0 dB. We can see that unity gain occurs at 2. This is fine, since the application note states we want a phase margin between 30 and degrees.
The gain margin is minus the gain at which the phase is degrees. Here we see the gain margin is 3. 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. However, the margins cannot be too great otherwise the system doesn’t react quickly to changes.
The datasheet doesn’t state this, but ideally the gain margin should be somewhere between 6 catasheet 12 dB. And indeed it is. So, in fact, this converter with the components so far needs no compensation network, and we can leave the COMP pin unconnected.
Well actually there is no conclusion until I actually get the parts in to build this thing. We’ll find out in the next project log. Then I started reading about the compensation circuit and realized that I don’t have the means to calculate it.
That is all before you get to the need to plot charts to solve the equations. I think I will have to look at other chips such as the LT Yes, delete it Cancel. About Us Contact Hackaday. Design a boost converter that gives VDC at 0. After the design procedure, here’s what I ended lm4378 with: Design procedure The LM datasheet provides a nice series of design procedures for various converter topologies.
Here’s the one for the boost converter: Switching frequency First, choose a switching frequency between kHz and 1MHz.