I've done simple brightness as low as a couple of hundred Hz with no  
visual issues, but in this case the device is scanning pixels at a  
rate of about 10kHz, and the 1MHz PWM frequency is to control the  
brightness of each pixel.

I've settled for the idea of using 2-transistor arrays and have that  
portion of the layout mostly figured out - what are your thoughts on  
either of the following?  (they are pin compatible, so all that would  
change in the layout is the orientation of the base/gate resistor):
FFB3904 BJT (http://www.fairchildsemi.com/ds/FF/FFB3904.pdf)
FDG6301 MOSFET (http://www.fairchildsemi.com/ds/FD/FDG6301N.pdf)

Seems like maybe it's a good idea to go with the MOSFET?

Thanks,
-JNS.


On Mar 23, 2008, at 1:47 AM, Bob Axtell wrote:

> James Nick Sears wrote:
>> Hi all,
>>
>> I'm having a surprising amount of difficulty locating a part that
>> meets the following needs:
>>
>> Drive multiple LEDs with PWM at a maximum current of 20mA per  
>> channel.
>> PWM frequency: 1MHz (this rules out i.e. the TD620xx darlington  
>> arrays
>> with turnon/off times at 10 and 20% of the period)
>> Small size: Preferably available in a TSSOP package, SOIC at the
>> absolute largest (willing to deal with QFN, etc. if the parts are
>> available)
>> As many as possible per package: I'm driving several hundred LEDs and
>> want to minimize handling for efficient assembly.
>> Available now: This project needs to be finished in a few weeks time.
>>
>> I'm just about to give up and use discrete transistors, but this will
>> be a nightmare both in terms of layout and assembly.  Please help!
>>
>> Thanks,
>> -JNS.
>>
> At 1Mh? It rules out most everything except for MOSFET high-current
> switches.
>
> Why so fast? For simple brightness applications, 50Khz would be fast  
> enough.
>
> --Bob A
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