I have done several LPC systems and have found no power sequencing
issues. I do tend to drive the V1.8 from either the V3.3 regulator or
from a 5V pre-regulator. I took some fast scope traced and found that
V1.8 comes up ahead or with V3.3.
Prior postings on this site indicated that sequencing is not a big
issue, but ideally V1.8 comes up first.
Is it possible that you are exceeding the current capacity of your
regulator? The LPC comes up with ports in input mode. If these are
driving external loads - especially externally switched loads - it might
be possible that these loads are all turning on and exceeding power
supply.
Another issue I have seen is not enough bulk bypass in the power supply
input/output stages. IF you have low ripple input, a .1uF is usually
all the input that most linear regulators require. But if it is
rectified AC, like a wall wart supply, you may have extreme ripple that
you need to filter out.
In either case, you need some bulk bypass caps on the output of the
regulator. Otherwise, it will form a great oscillator when you put it
under load - with the frequency changing with the load. I found this in
a design the hard way. I had checked the V1.8 with a meter - looked
good. I moved on. Later, having problems with JTAG, I went back to
basics. The board worked most of the time, but would crash. I found
very high frequency dips in the V1.8. A 10uF cap on the supply fixed
all that.
Hope this helps
Dan
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From: lpc2000@yahoogroups.com [mailto:lpc2000@yahoogroups.com] On Behalf
Of rohan patil
Sent: Monday, June 27, 2005 6:16 AM
To: lpc2000@yahoogroups.com
Subject: [lpc2000] 3.3 v power supply problem
hi everybody
i have developed lpc2106 board for my project
but after making some on off operation 3.3v pin gets
shorted
is there any special power supply requirement?
or
is it power sequencing problem????????
please help....................
########## ROHAN ###########################
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