555 EFI again...

[xxalexx at ix.netcom.com]

> Date:          Fri, 10 Jul 1998 09:18:21 -0400
> From:          Wen Yen Chan <chanwe at ecf.utoronto.ca>
> To:            diy_efi at efi332.eng.ohio-state.edu
> Subject:       Re: 555 EFI again...
> Reply-to:      diy_efi at efi332.eng.ohio-state.edu

You can send this to Electronic Design News Magazine for publication
and maybe win a prize. http://www.ednmag.com
alex
> A while back when I was designing a linear PWM for a resonant mode
> switching supply (the Prof. would not let us use a PWM chip) my partner
> and I used a current source to charge the timing cap. We used a lin. 
> voltage reg. (78L05...dirt cheap $0.07) rigged as a current source. We
> tied the output of the device to one end of a resistor and the 'ground'
> pin to the other end of the resistor. The current was taken off of the
> 'ground' pin. Since the regulator maintains a constant voltage between its
> output and 'ground' pin a constant current can be pulled off the 'ground'
> pin. The output current is set by the (reg. voltage)/(reference
> resistance) + about 25uA. Assuming the current your planning on sourcing
> is much greater than 25uA the regulator's supply current can be ignored. 
> 
> We found that this setup was much stiffer than any current mirror we could
> design for the same price. Temperature stability was a small nuisance but
> it was still better than what we would have achieved with discrete parts. 
> 
> l8tr
> 
> Wen
>