At 16:53 9/11/99 -0500, you wrote:
>At 08:56 AM 11/9/99 +0000, Michael wrote:
>>Note that a 2200uF capacitor on the output is really way too much.  This
>>amount of capacitance does not help the regulator work better, it slugs
>>the transient response.  A single 100nF will suffice in most cases with
>>maybe a 10uF cap if you are expecting big load spikes.
>
>Although I'm not an EE, it's my understanding that the criteria for
>post-regulator capacitors are as follows:
>
>1. The 78xx (and presumably LM317) design specs require a minimum 0.1 uF
>bypass capacitor for compensation of the internal feedback loop.
>
>2. Too high a capacitor will result in overloading of the regulator at
>lower frequencies (i.e., too low an impedance load), resulting in too long
>a rise time for voltage to reach spec.
>
>Since C V = i t, and the PIC requires power up within 10 ms, the maximum
>capacitor on the output of, say, the 7805 regulator would be
>
>         C = (1 A) (0.01 s)/(5 V) = 2000 uF
>
>With a low-current device, this could be reduced even further to 200 uF.
>

Been a long time, I would guess that Paul or Sean will correct me, but I
think that the formulee is wrong.
Q = IT
=> Q = 1(Assume max current) * 0.01 = 0.01 Joules

EK = 1/2C(V)2 => 0.01 = 1/2C*25 = 800uF

Now this assumes that everything is hunky dory.



>So the output capacitor should be no smaller than 0.1 uF and no larger than
>200 uF. Usually the breakpoint is in the cost of the capacitor. Since you
>can buy 0.47-1.0 uF tantalum caps for the same price as 0.1 uF, using a
>0.47 uF or 1.0 uF tantalum is the best choice.

You have made suumptions here in that the maximum current from the REG is
1A, also if the circuit peaks at 1A for a period of 10mS, the coapacitor
would be effectily discharged (2000uF), and the regulator would make up the
other bit to hold the votage up, and rough rule of thumb the output will be
down by 63% och! This is due to the CAP taking the available current from
the reg to hold const voltage on its terminals (OK so not quite correct but
should give the purpose) This does not take into account other influences
like ESR etc.


>
>Larger capacitors provide no advantage at low frequency, since the output
>impedance of the regulator is very low (e.g., 1 mOhm). At high frequencies,
>distributed bypassing works better in any even.

Only if the CAPS are suitable for high frequency stuff. There is no need to
use a large cap (Say 1uF) that has a high impedance at RF. In this case a
small cap say 100pF ceramic would be better.

>
>There is apparently never a reason to use higher than 100 uF or lower than
>0.1 uF on the regulator output. And with lower values, you don't need a
>discharge resistor or diode to protect the regulator.

You have never had to meet Tempest ratings have you.


Dennis


>
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>Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ral@lcfltd.com
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