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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If Q were infinite, the thing would vibrate itself to pieces and catch fire at half the switching frequency. If not, there are chips specifically designed to drive MOSFET gates at higher currents to force them on and off quickly.
Diode selection The diode has to be able to carry ddatasheet maximum inductor current. And indeed it is.
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. With my chosen frequency of kHz, I get 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.
This means that in gain it acts like a zero, but in phase it acts like a pole. The off-time is 0. Compute the minimum inductance required. 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.
LM3478 Boost Controller overview
Our chosen sense resistor is bigger than this by a factor of 3. I hate low-frequency converters with their whining.
Adafruit has a nice boost converter calculatorassuming you’re using the simple single-inductor dztasheet. Then I started reading about the compensation circuit and realized that I don’t have the means to calculate it. This means raising the frequency or the inductance by at least a factor of 3.
Shut up already, converter! If you don’t know datasheeg about control theory, you probably want to skip this section. Next, compute the duty cycle, D. You want something big with low ESR. The LM datasheet provides a nice series of design procedures for various converter topologies. The problem is, without knowing control theory, it will be difficult to select the compensation components.
Our modified values with this frequency are: The diode has to be able to carry the maximum inductor current. This is fine, since the application note states we want a phase margin between 30 and degrees. Maybe start with this list of videos? Again, this is from the Adafruit calculator, but here is the formula:. The first zero, z1, is caused by the output capacitor lk3478 its ESR.
Here’s the one for the boost converter: The pole is caused by the output capacitor with the load, and the complex pole is caused by the switching action. So this is really a good datasheef.
A boost converter design | Details |
I chose a standard resistor of 0. Minimum inductance Compute the minimum inductance required. Note that the second zero is a right-hand plane zero. Also a duty cycle switch on-time to total time of 0. We got this from the Adafruit calculator, 0. Switching frequency First, choose a switching frequency between kHz and 1MHz.
Again, this is from the Adafruit calculator, but here is the formula: With that resistor, the current limit would be 5.
Anyway, this is a good worked example, and very relevant! There is a second resistor which goes between the current sense pin of the converter and the current sense resistor. This gives me a required inductance of at least And, of course, this pole is located at half the switching frequency: The top plot is gain in dB vs frequency in Hz, and the bottom is phase shift in degrees vs frequency in Hz.
Clearly such a switch has to shut off well before 78ns. This rules out the popular MC, which has a max duty cycle of 0. About Us Contact Hackaday. 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.
Here we see the gain margin is 3. We can see that unity gain occurs at 2.