Russell,

Good catch on Figure 20.  I missed that completely.

I'll look into what kind of trace length and width will be required.  I was
trying to lay things out with short and wide supply traces - I guess that
will change now.

Thanks for the tip on measuring ESR.  Rather neat.

Regards,

Gordon Williams

----- Original Message ----- 
From: "Russell McMahon" <apptechnz@gmail.com>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Monday, March 29, 2010 6:01 PM
Subject: Re: LDO regulator


> c> I don't see anything in the Max. Ratings section that would cover "back
> > driving" like this.  Would this typically be acceptable for this
> > configuration.
>
> Fig 20 seems to cover your situation explicitly.
> I read that as meaning that with the regulator set to a nominal 3.8V
> output and with Vin = 0 then applying a voltage to the output produces
> a linear resistive response of about 80 k.
> E&OE YMMV.
>
> Vin = 0 MAY assume hard grounding of Vin, as happens when there is a
> solid resistive load sharing the input voltage at Vin and power is
> removed. It seems likely that having Vin floating is less damaging
> again but again YMMV.
>
> Classic ye older regulator problems happened when Vin was crow barred
> and Vout cap discharged through regulator. Death happened. Solution
> was a back diode across regulator. It seems unlikely to be needed,
> given Fig 20, but adding a reverse Schottky diode across LDO may add
> safety. And, if load drops to near zero power may add problems at very
> high temperatures as Schottky leakage exponentially rises - BUT also
> not usually a problem.
>
> So. Common sense and data sheet seem to suggest it's OK. I don't
> recall a fig 20 type diagram in other data sheets - well done ST -
> assuming I'm interpreting it correctly.
>
> > The output cap ESR stability range is given in Fig. 21.  The allowable
range
> > is roughly 0.05 to 2 Ohms for a 2uF output cap.  The caps that I'm going
to
> > use are ceramic SMD 805 (no name, Chinese).  Will the ceramic cap meet
the
> > 0.05 Ohms ESR requirement typically?  From what I've been able to see is
> > that ceramic caps are < 0.015 ESR therefore I would be below the
> > requirement.  Suggestions?
>
> Re cap ESR - I'd agree with you that it seems marginal.
> However, 0.1 ohm track etc resistance probably not too hard to dind
> with suboptimal placement - shame when components get "too good" for a
> circuit :-).
>
> Cap ESr will vary with brand and more.
>
> If this is one off or small quantity you can measure actual ESR with a
> VERY simple tester.
> Apply a square wave via a small resistor to CAP. At square wave
> transition the voltage will step jump in magnitude and then ramp
> exponentially as capacitor charges. The step jump is    Vin x ESR/(ESR
> + series_R) or rearranging
>
> ESR = Vstep x Rseries / (Vin - Vstep)
> Oscilloscope allows actual Vin at series resistor and Vstep to be
measured.
>
> Too small Rseries loads square wave muchly.
> Too large Reseries reduces Vstep size muchly.
>
>
>             Russell McMahon
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