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LM311 PWM to MOSFET

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xpassionzxc

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Hello,

I have a configuration drawing for the LM311 that will generate a PWM signal to the MOSFET. This MOSFET is to drive the boost converter.

May I ask what is the use of the R1 ( 1k ohms) to the input of the gate of the MOSFET?

I realized when the dc control signal ( 0 - 1V ) is around 0.8 to 1V , my waveform ( square waveform) before the R1 resistor is very unstable. However, after placing the R1 resistor, the waveform becomes stable.

Anyone know why placing the 1K ohms before the input of the gate MOSFET will make the square waveform stable ?

MOSFET is N channel IRF740.

LM311.JPG
 
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The gate of a MOSFET has significant capacitance; often several nanofarads.
The series resistor limits the current through the 311 output when that switches on.
 
The gate of a MOSFET has significant capacitance; often several nanofarads.
The series resistor limits the current through the 311 output when that switches on.

Hello, thank you for your reply.

How does the current affect the stability of the waveform of the output of LM311?
Lower current = Better stability of the output ?

If that is the case, how would a higher duty cycle (my dc control signal is the duty cycle) affect the output waveform and the output current of the LM311 in a boost converter?
 
I'm guessing the 311 has a current limiter in the output stage, or excess current messes up the internal operation.

In general op amps and similar do not work well directly connected to capacitive loads; it's normal to use a series resistor to the load and take any feedback after the resistor.

If you are just switching a MOSFET slowly, you can use a resistor to limit the peak current and drive it from a normal IC pin.
For applications that need fast response, you have to use a high-current gate driver with a low value series resistor (eg. 10R) ; this is a typical low-side driver (when the FET source is connected to negative supply):

That can provide 1.5A to very quickly charge or discharge the FET gate.
 
The LM311 has an "open collector" output. That is, the internal amplifiers are connected to the base of a bipolar transistor and the emitter and collector of that transistor are concted to external pins (col out, emit out in image below) that can be connected by he circuit designer. That means, current is not automatically sourced or sunk like an op amp. This is pretty common a comparitor. You must add the "pull up" resistor to the collector and connect the emitter to ground.

the schematic below from the Texas Instruments lm311 shows the internal workings and the labels are the pin names (the transistor sown is part of the comparitor, not an external component.

EA150B61-A428-4304-AADC-8F7D4CEC3161.jpeg
 
Typically a small series resistor (a hundred ohms or so) is often put in series with the MOSFET gate to minimize ringing due to the tank circuit cause by the lead inductance and the gate capacitance.

I has nothing to do with the output characteristics, since it's just an open collector which is insensitive to capacitance.
 
I has nothing to do with the output characteristics, since it's just an open collector which is insensitive to capacitance.

The 311 is not a simple "open collector" device, it has a fairly complex current sense and limit circuit controlling the output transistor.
Loads which cause excess current as the transistor turns on will not switch cleanly.

See the internal circuit on page 10 of the datasheet:
 
The 311 is not a simple "open collector" device, it has a fairly complex current sense and limit circuit controlling the output transistor.
Loads which cause excess current as the transistor turns on will not switch cleanly.

See the internal circuit on page 10 of the datasheet:

Abit confused about whether who is right and wrong.

Anyway, I tried to put a capacitor across the output of the LM311 to ground but it doesn't work. Only a resistor in series with the output of LM311 provides a stable waveform whenever the duty cycle is above 0.7.
 
Abit confused about whether who is right and wrong.

Anyway, I tried to put a capacitor across the output of the LM311 to ground but it doesn't work. Only a resistor in series with the output of LM311 provides a stable waveform whenever the duty cycle is above 0.7.

you can believe everyone - just account for the details referenced by rjenkinsgb mentions.
The input common mode voltage concern of rjenkinsgb essentially means, don't set your threshold values within 0.5 volts of your lower supply (I.e. ground) or within about 2v of your upper supply. That means, for a 5v single supply, Dont expect accurate switching if both inputs are below 0.5v or if both are above 3. As I interpreted the datasheet.

see 5th row of the table on page 6 - Vicr and the details in 9.2.1 to 9.2.2.1 of Ti.com datasheet
 
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Hello,

I have a configuration drawing for the LM311 that will generate a PWM signal to the MOSFET. This MOSFET is to drive the boost converter.

May I ask what is the use of the R1 ( 1k ohms) to the input of the gate of the MOSFET?

I realized when the dc control signal ( 0 - 1V ) is around 0.8 to 1V , my waveform ( square waveform) before the R1 resistor is very unstable. However, after placing the R1 resistor, the waveform becomes stable.

Anyone know why placing the 1K ohms before the input of the gate MOSFET will make the square waveform stable ?

MOSFET is N channel IRF740.

View attachment 127425

What do you mean by "unstable"?

Can you post a snapshot of both the unstable and stable waveform?
 
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