[SI-LIST] Re: Reset problem with power supply

Hello V S,

I understand that you have been able to fix your problem but I would like to 
say a few words anyway.

Bench supplies have power on ramps in the milli-second range. When you plug a 
power cable directly into a hot power supply, you are more likely to see 
micro-seconds, depending upon the power supply strength and amount of 
capacitance on the board.

There are two more phenomena apart from the ramp simply being fast, that can 
happen when you engage in hot plugging. When you turn on a bench supply you 
will commonly see a monotonic rising ramp. When you "plug" a wire you will go 
through "contact bounce" with a bunch of make-break sequences before the 
contact finally settles. This is when a big cap on the board is helpful. At the 
first "make" it gets charged and can hold the supply up at the few short 
"breaks". Just capture a power supply ramp with a digital scope to see what's 
going on.
The second phenomenon I wanted to mention is "ringing". The power wire is an 
inductor, the board has lots of capacitance and you feed it with a fast ramp. 
Classic 2nd order oscillation. Theoretically it can overshoot up to twice the 
power supply voltage. Even though it usually does not swing up this high 
(circuits turning on tends to dampen the ringing), it may still approach 
"absolute maximum" ratings for some chips where things like latching may occur.
Both phenomena can make the power supply move back down after the first move up 
but not all the way down. My experience is that this sequence of up, then 
partly (!) down and back up can be deadly.

Of course when you have POR's with enough time to pass over the power up 
carnage then you should be safe but only for the chips that have POR 
facilities. How about the 'simple' chips like zero-delay buffers or clocks that 
are not so simple anymore....?  That circuitry would be better off sitting 
behind a regulator with POR.

Eddy
Fremont CA


--- On Fri, 12/5/08, V S <for_si2003@xxxxxxxxx> wrote:
>
> From: V S <for_si2003@xxxxxxxxx>
> Subject: [SI-LIST] Reset problem with power supply
> To: si-list@xxxxxxxxxxxxx
> Date: Friday, December 5, 2008, 12:39 PM
> When I power my board using a bench power supply it boots.
> When I try to  plug in the cable, with the one end of the
> cable hooked to the same power supply in power on position
> it does not. My question is what difference cable inductance
> makes ? The failure looks like between processor and Flash
> core, as, the processor is always alive.
> 
> The processor is freescale MX27. I am using national Semi
> LM2717-ADJ IC to generate 1.8V for memory IO, 1.5V for core
> and PLL, 2.775V for processor IO and 3.15 for other IO. A
> POR is generated using Schimtt Trigger. The HRESET is
> generated using the same schmitt Trigger 74LCX07MX.
> 
> Any one faced similar situation ?
> 



      
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field


List technical documents are available at:
                http://www.si-list.net

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

Other related posts: