Doubs about best way to drive a logic level mosfet with 3.3V

[Sink0]

Hi, i need to drive a logic level mofet from a LPC1758 microcontroller. For the mosfet i will be using 3.3V is too low so i will need 5V at the gate.

As a initial idea i will be using a npn transisor to drive the mosfet. I will get an inverted logic, but that is not a big problem.

My doubt is about the initialization of the system. During the MCU boot, the pins will be inputs with a internal pull-up. I could not find on the manual how weak or strong is that pull-up, so i dont know if i will get enough current to turn on the NPN (MMBT3904 for instance). If that does not happen the FET will be ON and the load active, for a few microseconds or even miliseconds, what is not good.

In order to solve that problem i was thinking on using an external pull-up resistor to ensure the that the transistor will be conducting, so the FET will be OFF.

The circui would be the following (the diodes at the FET and the load are missing here, and VCC is 3.3V)



Is that a good idea? Any other better solution?

Thank you!

 

[Tahmid]

I would increase the value of R18 to about 10k. This would decrease current draw from the output of the microcontroller and thus output voltage won't drop anywhere as much. Then, it would be fine.

Hope this helps.
Tahmid.

 

[dick_freebird]

Consider using something like a quad NOR (parallel as many
as you like for drive - 74ACT02 - with the other input being
a RC network with C to 5V, R to GND forming a power-on
reset / holdoff. The natural state of the dormant uC + pullup
(1) will not energize the FET. There may be some elevated
Idd on the 5V-operated logic IC but ought to be tolerable I
expect if you use the "TTL" (T) version.

Of course a purpose-designed FET driver won't cost much
more and often has both senses of input, so you could use
the active low one from the uC and the active high for your
holdoff.

 

[Sink0]

Do you think even using the external pull-up i will have problems? So the NOR gate would be placed between the MMBT3904Colector and the FET GATE, with a RC circuit o the the other NOR gate leg? Thats it?

Do you hava any very common and cheap fet driver partnumber? My switching requirements are almost null as the MOSFET will be used as a on/off switch.

Thank you!

 

[Tahmid]

You can use the circuit without problems. Just increase R18 as I stated before. You can use a TLP250. Or you could just use an opto-coupler. The opto-coupler wouldn't turn on if the input is left floating. So, there shouldn't be a problem there.

Hope this helps.
Tahmid.