Re: direct replacement to a LM1785T

legg wrote:

On Sat, 06 Sep 2008 18:24:14 -0400, Jamie
<jamie_ka1lpa_not_valid_after_ka1lpa_@xxxxxxxxxxx> wrote:

Hmm, well since I was the one the originally designed the circuit, I really
don't know by whom I would be presenting this to?


If a forensic study is inconclusive, presenting the failed part to the
mfr (Nat Semi - after determining valid origin and provenance), along
with all relevent paper from the study, is the lowest-cost method of
determining what internal damage is present in the sample.


What has been done to attempt to induce failure in your standard test
units? Will the units run continually into a load short/open/grounded?
What are the assembly's demonstrated operating environmental limits
under normal functional stresses?


As I stated before, I designed in 25 Ohm R's on the output, you can
short the output via the R to common with no problems. I tried to make
it idiot proof.

Ok, the load is inductive but I have TVS diodes with fast blow links
on the output. The fuse links are on the output side so that the TVS
will clamp and open the links. The links at that section were fine.

I intentionally caused a large induced voltage at the output leads originally just to test the abilities of the TVS diodes and links with
a HV scope attached on the sensitive side. The diodes clamped nicely with no sign of a pulse above the diode saturation level so i'm confident that isn't the issue.


How inductive is the load?

What happens when the input(s) are open-circuit or faulted otherwise?
Having to maintain a virtual ground, or driving one end of the bridge
when the other has slammed permanently to a rail under a signal fault
can produce stress that is unanticipated.
The bridge outputs are joined via 25 ohm R's and then into the load. Even if they slammed on in any manner between the R's and protection clamps I have, I don't see how that is irrelevant ?



And to make a another note I forgot about, the output didn't short to
any rails, only the Vcc and -Vee shorted together with no sign of short showing any where else on the chip.

I don't know if that means anything..


The mfr will be the one most likely to see relevance in the failure
mode.

How is power established in the circuit? What happens when a rail is
removed temporarily, arrives late, or sticks around too long at
power-down - simulate specific connector pin intermittency where
possible. Check connected assemblies, connectors and power sources
from the original failure situation.

If either one of the rails should fail to initiate, it wouldn't matter,

The power supply is a basic bridge rectifier as a full wave with a
CT xformer as the common. Also one thing to note. Both LM1875T's are
on the same supply rail. I would think if there is some psu issue, wouldn't both have a problem ?

In case I didn't state it before, I'm using a +/- 15 volt supply and
operating the bridge from the + & - side. with this config, I can either
get a ~ +/- 14 volts out to the load as a single output with respect to
common as a bipolar out or use both for a floating bridge which gives
me +/- 30 volts.


In answer to your question, devices like the venerable L149 and L165
(from pre-1984) have the same package and pin-out. There have to have
been a host of others since then.

RL
Ok, I'll take a looky to see how they would look for alternate replacements.

Excuse me, that was a LM1875T my mistake..


It's the only flavor of lm1875 they sell, so the t is irrelevant.

RL

AT the moment this happens to be a bring it home on the weekend project and I got a test load on it now performing random output. everything is looking fine at this point.

Thanks.

http://webpages.charter.net/jamie_5";

.

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