# how does N-channel level shifter simplifies driving from logic

Discussion in 'General Electronics Chat' started by sharmilla, Feb 2, 2009.

1. ### sharmilla Thread Starter Member

Nov 3, 2008
17
0
hie all ,

How does using an N-channel level shifter simplifies driving from logic. why can't just drive the p-channel mosfet straight away. please advise

Many thanks

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2. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,699
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1) Can your logic take the voltage?
2) Can your logic provide/sink the current needed to turn the gate on quickly?

John

3. ### sharmilla Thread Starter Member

Nov 3, 2008
17
0
so u mean by using n-channel mosfet in this way, it simplifies the gate drive of p-channel for a high voltage...and also to provide enough current to flow to the gate of p-channel.

4. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,699
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I am not sure what you mean by simplify. Running the gate directly form the logic chip is simple, but simple may not work.

What I listed are some considerations for you as the designer. To get more detailed than that, we would need more information on your circuit. Particularly, what is the voltage provided to the P-mosfet source? I assume it is considerably higher than the logic supply.

The main issue, however, is probably the current capability of the logic circuit. Gates of big mosfets can have instantaneous turn-on/off currents of amperes.

John

5. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
In addition to potential problems with incompatible voltages, the o/p will be inverted if we run the P-MOSFET directly from the logic. P-channel devices conduct when the gate goes low.

6. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,699
909
That depends on what you want do do, doesn't it?

I am running a small P-mosfet (STS5PF20V) directly from a PIC MCU (200 ohm gate resistor for current limiting and a pull-up resistor for startup). It has a logic-level gate and stays off because its gate is held high by the PIC. Of course, I couldn't do that so simply, if the mosfet source was at 20 volts. John

John

Last edited: Feb 3, 2009
7. ### Skeebopstop Active Member

Jan 9, 2009
358
3
I feel you could do it directly with the N-FET, so long as the N-FET were below the load.

8. ### vansterritory New Member

Aug 5, 2011
1
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Hello,

at last i found a forum discussing about level shifter for driving high side MOSFET..
I wanna use this scheme for my buck converter. but im quite confused with the value of each component. how can i calculate each R and zener?

thanks!

9. ### SgtWookie Expert

Jul 17, 2007
22,183
1,729
Have a look in here:
http://www.ti.com/lit/ml/slup169/slup169.pdf
P-channel MOSFET drivers are discussed starting on page 18, but the whole document is recommended reading.

You would really be better off using an N-ch MOSFET and a high-side driver. The selection of P-ch MOSFETs is much more limited than for N-ch, and the main reason for doing so is that for all other parameters being equivalent, P-channel MOSFETs have 2.5 times the total gate charge requirement than N-ch MOSFETs do.

[eta]
You might find the attached interesting; I'm using a common 555 timer IC as a high-side driver for an N-ch MOSFET and current regulator for a couple of LEDs. It's about 85% efficient in the configuration shown, which isn't bad at all.

• ###### 555 LED Current Reg High Side Switching.PNG
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