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Subject: Re: What is LDO ( regulator ) ?
Date: Sat, 11 Jan 2003 19:59:12 +0700
Organization: C.S. Communications Co. Ltd.
References: <3E1FE33B.4FF09DBF@fanwap.com> <3E1FEE91.D547646F@autometer.de>
NNTP-Posting-Date: Sat, 11 Jan 2003 12:59:13 +0000 (UTC)
X-Mailer: Mozilla 4.8 [en] (Windows NT 5.0; U)
Thank you very much. Your answer is very clear.
You can be a teacher.
Tilmann Reh wrote:
> Tong schrieb:
> > Sorry for a basic question.
> > I guess it means = minimum voltage different between
> > input and output at full load ?
> > At high load current these IC require only low head room voltage, right ?
> > ex:
> > 7809 require head room 2V
> > LM317/350 require head room 5V
> > LM1084 require head room 1.2V
> > Am I correct ?
> Basically yes. LDO means "Low Drop Out", with drop-out being
> the minimum voltage between input and output for that regulation
> can be maintained (regardless of the load).
> Minimum drop out for the LM317 is significantly lower than 5V, BTW.
> 78XX needs about 2V, modern LDOs about 0.2V.
> There is one important difference between "standard" regulators
> and LDOs. Standard regulators have NPN output transistors, so the
> base current of the power stage also flows into the load. This
> leads to a rather constant supply current of the regulator itself.
> LDOs normally have PNP output stages to achieve the low drop-out.
> So the base current of the output stage flows into groud, which
> leads to significantly increasing quescient current for higher
> loads. There are only a few LDOs with P-MOS output stages where
> supply current stays small even at high loads.
> However, it all depends on the application. If you have a low-power
> circuit running at 5V (say, 3 mA supply current) and look for a
> good regulator, you'll find that the 7805 tykes about 4-5 mA for
> its own supply, more than doubling total power consumption. Using
> an LP2950 instead (which is an LDO with PNP output) reduces regulator
> current to about 200 uA (since even the LDO needs very littly current
> at this small load). For load currents of 100 mA, things however look
> Take care.
> Dipl.-Ing. Tilmann Reh
> Autometer GmbH Siegen - Elektronik nach Maß.
> In a world without walls and fences, who needs Windows and Gates ?
> (Sun Microsystems)
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