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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Another design
Date: Fri, 10 May 2024 17:20:01 -0000 (UTC)
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Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
> piglet <erichpwagner@hotmail.com> wrote:
>> On 10/05/2024 4:11 am, Edward Rawde wrote:
>>> "Edward Rawde" <invalid@invalid.invalid> wrote in message
>>> news:v1ho21$4ps$1@nnrp.usenet.blueworldhosting.com...
>>>> Now with 9V to 15V input range, direct gate drive and 500mA LED current.
>>>> 
>>>> Startup behaviour is not ideal but maybe that can be fixed if it needs
>>>> fixing.
>>>> 
>>>> What's the cheapest op amp I can use for U2 instead of OP07?
>>>> 
>>>> Is there a cheaper comparator instead of LT1719 which will work?
>>>> Also the fet is bigger than it needs to ne.
>>>> 
>>>> Is there a better device for D4? Particularly if R4 can be reduced.
>>>> 
>>>> Will it work at all? Except in simulation.
>>>> 
>>> 
>>> I think I've answered my own question for the current controller but I don't
>>> think it's feasible to attempt a discrete comparator on cost grounds.
>>> 
>>> Version 4
> (Sniiip)
>> 
>> 
>> The venerable LM311 is still a good value comparator, if you want to 
>> penny pinch omit the gate driver buffers and lower the pullup resistor 
>> but efficiency will drop a lot from 85% to ca 72%.
>> 
>> 
>> 
>> 
>> 
> I’m not following through all the spicery, but fwiw:
> The positive drive of an LM311 can be improved by 100x or more, by adding a
> 2-cent NPN and a 2-cent diode.  The NPN is an emitter follower, and the
> diode goes from base to emitter, reverse biases when the NPN is on. 
> 
> It pulls down to a diode drop above ground, but that can be a Schottky, and
> only has to handle the base bias, so it can be below 0.2 V, but you don’t
> care about 0.7 V in a gate driver anyway. 
> 
> When the 311 goes low, the diode doesn’t start conducting till the
> transistor is off, and there’s no stored charge to worry about, since the
> NPN never saturates. 
> 

Oh, and take the output from the emitter. 

Cheers 

Phil Hobbs 

-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics