I use a lot of LDO's in set top box PSU's that I design.It is true that
most of them are a bit of a problem as regards caps, specially ceramic
caps across the output, but most designers do use ceramics across the
power all over the place. The latest generation are much better. What to
look for is the pass element feeding the output from the cillector or
drain of a pnp or p channel device, though some like this are stable in
all conditions. The high open loop o/p impedance causes an additional
phase shift and the reg relies on the ESR of the output cap to stabilise
the response.

George

----- Original Message -----
From: "Spehro Pefhany" <speff@INTERLOG.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Sunday, March 14, 2004 5:49 PM
Subject: Re: [EE:] Soft power switch circuit


> At 02:49 PM 3/14/2004 +0000, you wrote:
>
>
> >         I've largely abandoned 7805 voltage regulators for my hobby
> >stuff, especially for battery operation.  They waste too much
> >current and require several volts of input headroom.  I now use
> >the LP2951 low-dropout regulator from National Semiconductor; it
> >draws only about a hundred microamps for itself (vs. several
> >milliamps for a 7805), will regulate well with an input only a
> >few hundred mV above the output, and it also has a handy ERR
> >output which you can use as a PIC reset input.  You can set it to
> >5 volts just by strapping a couple of pins together, or program
> >it to a different voltage with a resistor voltage divider.  It
> >comes in an 8-pin DIP.  IMO, it's the ideal hobby voltage
> >regulator.
>
> The LM2931 is kinda cool too, "load dump" (automotive transient)
> and reverse polarity (eg. battery) protection built in (also
mirror-image
> insertion protection). Cheap like the LP2951. Iq is around
> 400uA typically, which < 1/10 that of a LM7805.
>
> All these bipolar LDO regulators are fussy about output capacitors
> compare the to the 78xxx regulators. They can't be too lousy *or*
> too good ESR, and have to be in the right general range of
> capacitance or you can get oscillation. They also tend to draw more
> current as the output current goes up (roughly 10% of output current
> gets wasted) because of the lousy characteristics of the lateral PNP
> on-chip pass transistors.
>
> Then you have the great new CMOS parts like this one:
>
> http://cache.national.com/ds/LP/LP8345.pdf
> Unfortunately, no reverse battery protection, and 10V maximum input.
8-(
>
> I doubt we'll see another real jellybean part like the 78xx regulators
> anytime soon, all the upsides seem to have downsides.
>
> Best regards,
>
> Spehro Pefhany --"it's the network..."            "The Journey is the
reward"
> speff@interlog.com             Info for manufacturers:
http://www.trexon.com
> Embedded software/hardware/analog  Info for designers:
http://www.speff.com
>
> --
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>

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