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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So, let’s take a look at the application note to see what the transfer function looks like without this compensation. With that resistor, the current limit would be 5.

We need to find Q, which is the damping factor. This rules out the popular MC, which has a max duty cycle of 0. Duty cycle Next, compute the duty vatasheet, D.

We see that there are two zeros, one pole, and one complex pole: It can handle currents up to 9A, and has an on-resistance of 0. Minimum inductance Compute the minimum inductance required.

Again, Adafruit correctly is using the D based on the maximum input voltage, but I’m using a higher value for extra safety margin. The second zero, datashest, is caused by the inductor and the load on the power supply. 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 problem is that addition of this resistor modifies the current limit: Clearly such a switch has to shut off well before 78ns.


Shut up already, converter! There is a second resistor which goes between the current sense pin of the converter and the current sense resistor. They datasheeh there to add stability to the converter.

Design a boost converter that gives VDC at dataseet. Since I kinda like the inductor I chose, let’s raise l3478 frequency from kHz to kHz and try again.

Anyway, this is a good worked example, and very relevant!

LM3478 Boost Controller overview

This gives me a required inductance of at least That is all before you get to the need to plot charts to solve the equations. Maximum inductor current Compute the maximum current flowing through the inductor. Then I started reading about the compensation circuit and realized that I don’t have the means to calculate it. We dataxheet 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.

LM overview, product comparison and replacement search

But, of course, this will not be the case. This means that in gain it acts like a zero, but in phase it acts like a pole. And indeed it is. And so I get a sense resistor of 0.

A boost converter design | Details |

Our chosen sense resistor is bigger than this by a factor of 3. The pole is caused by the output capacitor with the load, and the complex pole is caused by the switching dayasheet. The datasheet xatasheet state this, but ideally the gain margin should be somewhere between 6 and 12 dB. And, of course, this dataheet is located at half the switching frequency: So based on the inductance and current, I can choose an inductor that can handle this current.


The gain margin is minus the gain at which the phase is degrees. With my chosen frequency of kHz, I get After the design procedure, here’s what I ended up with: Let’s locate the zeros: Nobody will know unless we try it.

And, of course, this pole is located at half the switching frequency:.

LM3478 Datasheet

There are two ways to raise that maximum sense resistance: We’ll find out in the next project log. And now for the complex pole. This resistor protects the circuit against overload currents. We first calculate the maximum sense resistor for current mode loop stability loop stability is always a good thing:.

If not, there are chips specifically l3m478 to drive MOSFET gates at higher currents to force them on and off quickly. You can see the resonance at half the switching frequency. We first calculate the maximum sense resistor for current mode loop stability loop stability is always a good thing: