🚀 go-pugleaf

"Phil in Melbourne" <reply@tonewsgroup> wrote in message 
news:481eee7b$0$17511$afc38c87@news.optusnet.com.au...
>
>
> So I would have thought that this would be a simple problem but the more 
> information I get the more I get confused......
>
>
> I want to PWM a whole lot of LEDs. The current consumption of the bank of 
> LEDs is in the order of 15 Amps, at 12V.
>
> For reasons I won't go into, my only option is to use a high side mosfet.
>
> I have chosen an N-channel MOSFET (IRL3803) mainly because of its huge 
> current capability and very low on resistance of 7 milli ohms.
>
> I am hoping to use PWM at a reasonably high frequency, say 20khz or 
> higher, and I want to use an efficient MOSFET driver IC that is designed 
> for the job.
>
> I understand that to switch an N channel mosfet on the high side of the 
> supply line, I have to drive the gate with 10V **more** than the supply 
> voltage. This is correct yes?
>
> This is where the question comes in. I am looking at the option of using 
> the IRS2001 to switch the high side fet. But looking at the datasheet, I 
> am confused. How can the  IRS2001 generate a voltage greater than VCC 
> without actually being connected to the high side voltage????? There is no 
> talk of a charge pump in the description and in any case it does not know 
> what potential the fet is connected to..
>
> Maybe it is not suitable for my application as the IRS2001 seems to be 
> used for switch mode power supplies? I can't make sense of the first 
> diagram on the datasheet.
>
> here is a link to the datasheet:
>
> http://www.irf.com/product-info/datasheets/data/irs2001spbf.pdf
>
>
>
> OK so if the above IC is not suitable, I have been looking at the MAX621.
>
> http://datasheets.maxim-ic.com/en/ds/MAX620-MAX621.pdf
>
> This one looks to be more straight forward. But I'm not sure how this IC 
> will go with 20Khz switching. There are many **low side** mosfet drivers 
> that are in the 2 to 7 amp range to ensure that the mosfets are hard on or 
> hard off within a few nS. This one doesn't look like it will do anywhere 
> near that switching current.
>
>
> Any other ideas for a high side N channel driver IC that packs a punch?
>


For a gate capacitance that large ou might need more than the 20mA or so of 
peak gate charge current avaialble from the IRS part.

The IR2110 is a very commonly used part that is similar to the irs2001 in 
functionality.  The high side gate bias voltage is essentially free due to 
the flying bootstrap capacitor supply.

The way it works is that when the low side fet is turned on the vs node is 
at ground potential (or 1 diode drop lower for a buck) and the VB node 
(bootstrap) is held to 1 diode drop below the 12V rail. This changes up the 
capacitor between Vb and Vs.  When the high side is turned on, the high side 
circuit uses the capacitor as a floating power supply to supply the gate 
drive current.

Pretty easy to use and works great.  You will need to generate your own PWM 
signals to feed this guy as it is only a driver.

Read up on the ap notes in the IRF website for more operational use and 
proper layout considerations for that chip.  wirewrap is NOT recomended.