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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There’s a COMP pin on the converter where you put a resistor and capacitor in series. However, we now do not need a slope compensation resistor, and everyone is happy. Sign up Already a member?
Our chosen sense resistor is bigger than this by a factor of 3. It can withstand at least V to the drain, and has a gate threshold of at most 4V.
Note that the converter will adjust the duty cycle based on the output voltage and switch current, so this is only a value for calculation. And, a Schottky diode is recommended for high speed. With my chosen frequency of kHz, I get I hate low-frequency converters with their whining. This gives me a required inductance of at least The gain margin is minus the gain at which the phase is degrees.
Again, Adafruit correctly is using the D based on the maximum input voltage, but I’m using a higher value for extra safety margin. We first calculate the maximum sense resistor for current mode loop stability loop stability is always a good thing:. 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.
You want something big with low ESR. Now that we have our pole and zero frequencies, we need to know what the DC gain is. Yes, delete it Cancel.
Doing that also has a knock-on effect of lowering dztasheet inductor current. I chose a standard resistor of 0. The converter is a feedback loop, and such feedback loops need to be stable, otherwise your power supply might fail at certain frequencies.
So this is really a good thing. Our modified values with this frequency are: Here we see the gain margin is 3. Again, this is from the Adafruit calculator, but here is the formula: We first calculate the maximum sense resistor for current mode loop stability loop stability is always a good thing: This rules out the popular MC, which has a max l3m478 cycle of 0.
To power my boost converter, I’d like to use a 12VDC wall adapter. The cause of this is the insanely low maximum sense resistance from above. So with a maximum input voltage of 12V, and an output current of 0.
If not, there are chips specifically designed to drive MOSFET gates at higher currents to force them on and off quickly. 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. Duty cycle Next, compute the duty cycle, D. The off-time is satasheet. Switching frequency First, choose a switching frequency between kHz and 1MHz.
LM Datasheet(PDF) – National Semiconductor (TI)
But, of dztasheet, this will not be the case. That is all before you get to the need to plot charts to solve the equations. So, in fact, this converter with the components so far needs no compensation network, and we can leave the COMP pin unconnected. Since I kinda like the inductor I chose, let’s raise the frequency from kHz to kHz and try again. Again, Adafruit gives this as 4. If Q were infinite, the thing would vibrate itself to pieces and catch fire at half the switching frequency.
I’ll keep using kHz in the calculations as it doesn’t matter much. The datasheet doesn’t state this, but ideally the gain margin should be somewhere between 6 and 12 dB. This means raising the frequency or the inductance by at least a factor of 3.