555 EFI again...

[Chris Morriss]

In message <199807121420.JAA16116 at dfw-ix2.ix.netcom.com>,
xxalexx at ix.netcom.com writes
>> 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
>> 
Even better than a 7805, use a LM317.  This has an internal reference of
1.2V so the current source has better compliance.  For a fixed current
source the technique works well.  Current is 1.2V/R.  The config is
shown on the data sheet from Nat Semi for the LM317.

(difficult to make a voltage controlled current source this way though)
-- 
Chris Morriss