MOSFET selection for boost converter problem

On 2016-11-28 17:04, P E Schoen wrote:
> "Joerg"  wrote in message news:ea3cn9Fe1ikU1@mid.individual.net...
>
>> As suggested before a CMOS version of the 555 will probably go higher
>> (check first though) but would have to be ordered.
>
>> For a quick fix hang a bus driver between 555 and FET gate. Maybe with
>> several sections paralleled. That is usually something that can be
>> found in the parts bins.
>
> The real problem appears to be the low voltage from the USB power lines.
> The drain voltage on the MOSFET when it is OFF is only about 3.5 volts.
> I think the USB switch chosen for this circuit limits current to 300 mA
> by becoming a current source when overloaded.
>
> A CMOS 555 might provide a voltage close to the rail, but the output
> current drive is much less than the ~200 mA of the standard part. The
> ICM7555 CMOS part has a typical output high of 4.3 volts with 5 volt
> supply, at 0.8 mA. It is actually a CMOS inverter with a rated load of 2
> TTL gates. For an 8 mA load, the voltage drops to 2 volts below supply
> rail. This is equivalent to a 250 ohm source impedance. The IRL3915
> MOSFET has an input capacitance of almost 2000 pF, and this corresponds
> to a TC of about 500 nSec. With a 100 kHz PWM and about 10% duty cycle,
> the ON pulse is 1000 nSec, so the TC effectively cuts that in half. This
> is also seen in the scope trace.
>

That's why I suggested a bus driver. They provide low tens of ohms and 
you can usually parallel sections inside the same IC.

Also, you don't need a FET with 2000pF gate capacitance for this job.

-- 
Regards, Joerg

http://www.analogconsultants.com/