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Thread: MCF5375 Supply voltage sequencing
MCF5375 Supply voltage sequencing
user name
 2006-07-21 14:09:19 
Hello All,

I am a bit confused by the 5375 power sequencing
requirements, the
datasheet is contradictory.

Section 5.4.3 note 4: "Use 1 ms or slower rise time
for all supplies."
Section 5.4.3.1: "Use 1 s or slower rise time for all
supplies." "The
rise times on the power supplies should be slower than 1 s
to avoid
turning on the internal ESD protection clamp diodes."

I can't imagine a 240MHz part having anything with a 1ms
time constant
so I guess 1us is correct?

Section 5.4.3.1: "There is no limit on how long after
EVDD/SDV DD
powers up before IVDD must powered up."
Note 2: "IVDD/PLLVDD and EVDD/SDVDD should track up to
0.9 V, then
separate for the completion of ramps with EV DD/SD VDD going
to the
higher external voltages. One way to accomplish this is to
use a low
drop-out voltage regulator."

I would rather use a switching regulator than an LDO because
its more
efficient, most of the current being on the 1.5V rail. It is
easy to
sequence switching regs but not easy to make them track. The
first
statement implies sequencing is fine but the note below it
recommends
tracking. Why?

Another aspect that is not clear from the datasheet is the
internal
reset capability. I am led to believe it has an internal
reset
circuit. Is this true? In which case which rail does it
monitor and
what threashold does it apply?

Regards, Chris

JrzǧuƠz

]*.+-qdR{.n+zwZ,jb}lh
vx(Z+~+bp/ȭm?0hux(b
MCF5375 Supply voltage sequencing
user name
 2006-07-25 18:12:00 
1. The ramp time should be 1ms (not 1us, that is almost
certainly too fast).

2. The power supplies do need to be sequenced to avoid
reverse biasing the internal diodes, but the note saying the
power supplies need to track is incorrect. As long as you
get the order right (SDVDD/EVDD before IVDD) the timing
shouldn't matter.

-Michael

> -----Original Message-----
> From: ColdFirelists.wildrice.com 
> [mailto:ColdFirelists.wildrice.com] On Behalf Of nop head
> Sent: Friday, July 21, 2006 9:09 AM
> To: Norman Michael-ra6329
> Subject: [ColdFire] Fwd: MCF5375 Supply voltage
sequencing
> 
> 
> Hello All,
> 
> I am a bit confused by the 5375 power sequencing
requirements, the
> datasheet is contradictory.
> 
> Section 5.4.3 note 4: "Use 1 ms or slower rise
time for all supplies."
> Section 5.4.3.1: "Use 1 μs or slower rise time
for all supplies." "The
> rise times on the power supplies should be slower than
1 μs to avoid
> turning on the internal ESD protection clamp
diodes."
> 
> I can't imagine a 240MHz part having anything with a
1ms time constant
> so I guess 1us is correct?
> 
> Section 5.4.3.1: "There is no limit on how long
after EVDD/SDV DD
> powers up before IVDD must powered up."
> Note 2: "IVDD/PLLVDD and EVDD/SDVDD should track
up to 0.9 V, then
> separate for the completion of ramps with EV DD/SD VDD
going to the
> higher external voltages. One way to accomplish this is
to use a low
> drop-out voltage regulator."
> 
> I would rather use a switching regulator than an LDO
because its more
> efficient, most of the current being on the 1.5V rail.
It is easy to
> sequence switching regs but not easy to make them
track. The first
> statement implies sequencing is fine but the note below
it recommends
> tracking. Why?
> 
> Another aspect that is not clear from the datasheet is
the internal
> reset capability. I am led to believe it has an
internal reset
> circuit. Is this true? In which case which rail does it
monitor and
> what threashold does it apply?
> 
> Regards, Chris
>
Nn‰z™j txŠZrʽΉ±j±Ά•­|Ά•q˜~…™ʼ
²ΆZFr
> 
> 

JrzǧuƠz

]*.+-qdR{.n+zwZ,jb}lh
vx(Z+~+bp/ȭm?0hux(b
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