TL3842 PWM with buck converter problem

ronv

Joined Nov 12, 2008
3,770
To turn on the NFET the gate needs to be 5 volts or so higher than the source. I don't think that can happen with your circuit. Use a logic level PFET.
 

Thread Starter

mah

Joined Mar 15, 2010
393
To turn on the NFET the gate needs to be 5 volts or so higher than the source. I don't think that can happen with your circuit. Use a logic level PFET.
why do you think isn't able to generate 5 volts . the output of this circuit is 15 volt at VCC=15.5
 

alfacliff

Joined Dec 13, 2013
2,458
he isnt talking about the output voltage of the ciecuit, he is talking about the supply for the gate driver
also, what is the supply for the tl3842? it muxt be over 16 volts or it shuts down.
 

Thread Starter

mah

Joined Mar 15, 2010
393
he isnt talking about the output voltage of the ciecuit, he is talking about the supply for the gate driver
also, what is the supply for the tl3842? it muxt be over 16 volts or it shuts down.
the supply for tl3842 is 15.5 volt and the output of the circuit is also 15 volt it can't be more than the supply . so what is the problem now ?
 

Thread Starter

mah

Joined Mar 15, 2010
393
why do you give a short brief please anyone explain in detail and what does logic level differ from normal fet ?
 

crutschow

Joined Mar 14, 2008
28,155
If your supply to the TL3842 is 15.5V (including 15.5V to Vc pin 11 and pin 8 grounded) and the voltage to the MOSFET drain is 5V then you should be fine.

A logic-level type MOSFET will fully turn ON with a gate-source voltage of 3V to 5V depending upon the specific type selected. A normal MOSFET typically takes at least 10V Vgs to fully turn on.
 

Thread Starter

mah

Joined Mar 15, 2010
393
If your supply to the TL3842 is 15.5V (including 15.5V to Vc pin 11 and pin 8 grounded) and the voltage to the MOSFET drain is 5V then you should be fine.

A logic-level type MOSFET will fully turn ON with a gate-source voltage of 3V to 5V depending upon the specific type selected. A normal MOSFET typically takes at least 10V Vgs to fully turn on.
why should i connect pin 8 to ground however it isn't in datasheet ? and what is the need here for logic level ? it was mentioned in datasheet of tl3842 that its output can drive mosfet directly .
 

crutschow

Joined Mar 14, 2008
28,155
why should i connect pin 8 to ground however it isn't in datasheet ? and what is the need here for logic level ? it was mentioned in datasheet of tl3842 that its output can drive mosfet directly .
Pin 8, labeled as power ground, certainly is in the data sheet. Just because it wasn't on the connection diagram you have doesn't mean you can arbitrarily connect it.

It can drive a MOSFET directly but it has to be the right kind and polarity of MOSFET for the particular requirements of your circuit. You have to use some thought as to what the circuit is doing.

You need to learn to look at the data sheet and understand how the circuit works (particularly the Functional Block Diagram on page 3), not just blindly go ahead with somewhat arbitrary connections to the device. You need to know what every pin on the circuit does. If you can't understand how the circuit works and how MOSFETs work as switches then you will have real difficulty at getting the circuit to work. :rolleyes:
 

Thread Starter

mah

Joined Mar 15, 2010
393
Pin 8, labeled as power ground, certainly is in the data sheet. Just because it wasn't on the connection diagram you have doesn't mean you can arbitrarily connect it.
pin 8 is power ground in 14 pin package but the attached circuit is for 8 pin package so pin 8 as shown in figure 6 page 10 isn't ground
 

crutschow

Joined Mar 14, 2008
28,155
I didn't know what package you were using so I assumed the common 14-pin one. You should state which package you are using.

Please post a circuit of your exact connections to all the parts and power supplies.
 

Thread Starter

mah

Joined Mar 15, 2010
393
I didn't know what package you were using so I assumed the common 14-pin one. You should state which package you are using.

Please post a circuit of your exact connections to all the parts and power supplies.
it is clear in the attached connection , there is no power ground nor Vc so it is the other one :). ok it's fine
 
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