From: Tony Williams
Subject: Re: Constant current source with low voltage drop
Date: Wed, 16 Oct 2002 09:26:33 +0100
References: <3D9A5E9A.firstname.lastname@example.org> <3D9AF62C.email@example.com> <firstname.lastname@example.org> <email@example.com>
NNTP-Posting-Date: Wed, 16 Oct 2002 08:25:41 +0000 (UTC)
User-Agent: Pluto/1.14i (RISC-OS/3.60)
In article ,
Winfield Hill wrote:
> > +---------+---+-----+-------5v
> > | | | |
> > \ | \ [LOAD]
> > /4.7k | /47k |
> > \ | \ |
> > | 5 | | |<---Iout and Vout wrt 0v.
> > +-----|+ \|8 | |-+
> > | |393>---+--||N-type
> > | +-|- / 7 | |-+
> > | |6 | |
> > | +---------|-----+
> > | | |
> > \ | \
> > /150 0.47=== /1
> > \ | \
> > | | |
> > +-------------++++--+-----0v
> > ||||
> > Pins 1234
> ............... However, I'd like to send
> you back to the drawing board to modify your design to
> drive 150mA into ground - that apparently was what the
> OP needed; at least that's what everyone was giving him.
A LED backlight just has two leads coming out of it,
one red, one black. So the choice of which polarity
to use should be open. However......
Our old friend the TL061 has a CMV capability up
to the positive supply rail. The real question is
the negative output swing when running off a 5v rail.
A quick test showed that Vout would go to about 0.65v
above 0v. A 1k pulldown only took it to about 0.6v,
showing that there is an internal pullup mechanism
I don't have any logic level P-type MOSFETs in stock,
so had to strap a pair of IRF9520 in parallel. They
need a Vgs of about -4v at 150mA, so things would be
a little tight. Near enough to try though.
Similar circuit as before, but totally inverted.
TL061 Vout lifted off when [V(1ohm) + Vds] was 270mV,
and it was fully regulating by 280mV, at 153mA.
So yes, it is possible. Logic level P-types are
thinner on the ground though... Zetex ZXM62P03E6
looks nice, RdsON= 0.15 Ohms, VgsThr = -1V.