CU :-)
Georg
[email protected] wrote:
**************[Begin of situation]*************
I want to simulate the driver's output signal
> which have 10ns's Tr/Tf and 0 to 5V swing with
> 1000pF capacitive load.
> 1000pF loads are actually distributed having
> equal distance on burn-in board.
> (please ref. following config.)
>
> This driver has to drive 24 receivers(64M SD RAM)
> distributed in burn-in board.
> We don't use daisy chain system but bus stytle
> to distribute driving signals.
> (main net length : 50 in., stub length : 2 in.)
>
> Our driver's short circuit current is estimated
> about 170mA. This driver locate on seperated driver
> board.
>
> Estimated loading conditions for one driver is
> 496pF(min) or 568pF(max).
>
> Followings are loading condition.
>
> [Receiver's loading for 24 of SDRAM, PKG, socket]
>
> - SDRAM's input capacitance :
> 3~5pF/ea -> 24 X (3~5pF) = 72/120pF
> - PKG & socket :
> 3~4pF/ea -> 72/96pF
>
> [24 stubs and one main net]
> - stub : 4pF/in.(2in. for one stub)
> & nbsp; -> 24 X 8pF = 192pF
> - main net :
> 4pF/in.(40in.) -> 160pF
>
>
> [receiver] 1 2 3 4 5 6 7 8 9 10 11 12
> | | | | | | | | | | | |
> | | | | | | | | | | | |
> [Driver] --------+--+--+--+--+--+--+--+--+--+--+--+---R to Gnd
> Ron=29.4 | | | | | | | | | | | | (30ohm)
> Tr/Tr=10ns | | | | | | | | | | | |
> Voh/ol = 5/0V 13 14 15 16 17 18 19 20 21 22 23 24
> Isc = 170mA
>
> ***************[End of situation]**************
>
> ***************[Begin of question]*************
>
> Our earlier driver's short circuit current is
> 40mA when the driver operates at 30ns's Tr/Tf for
> same loading condition. The Zo of strip/microstrip
> is 50 around. This system operates correctly.
>
> Now we designed upper bus system by 30 ohm
> strip/microstrip lines with 10ns's Tr/Tf driver.
> But this system doesn't operate at this frequency,
> showing over/undershoot and steps in rise/fall
> edges.
>
> -----------------------------------------------
> [My guess for short circuit current]
>
> (referenced by "CMOS/TTL Digital System Design"
> pp.2 02 by James E. Buchanan)
>
> dV/dt = Isc / C
>
> where Isc is the output short circuit current
> of the output driver and C is the increase in
> load cap. beyond the specified load value.
>
> 5V/10ns = Isc / 568pF, 5V/10ns = Isc / 496pF
> Isc = 284mA Isc = 248mA
> -----------------------------------------------
>
> [Q1]
> If our driver doesn't have 284/248mA Isc
> characteristics.
> We estimated it has 170mA.
> What kind of problems could be occured?
> It couldn't drive the receiver located far
> end of main net?
> It could show large delay or steps in rising
> edge?
>
> [Q2]
> My short circuit current estimation is
> acceptable?
> I've solved the equat ion by one lumped
> capacitance for net and stub's transmission
> line capacitance.
>
> [Q3]
> Which effects could be emerged by designing
> the strip/microstrip's Zo to 30 from 50? We
> reduced the Zo from 50 to 30.
> And also I use bus system for 10ns's driver.
>
> [Q4]
> We tested CMD(California Micro Devices Co.)'s
> PAC DN005Q for our end(load) terminator. This
> terminator is consisted of shottkey diode which
> has reverse recovery time less than 400ps.
>
> (Please check for this terminator at
> www.clamicro.com/prod/spice/s_diode_sub.htm)
>
> But this terminator couln'd clamp any
> over/undershoot.
> Could anyone let me know why it doesn't
> operate correctly?
> Does the huge driving current make this
> problem?
>
> [Q5]
> Our driver has been specially designed
> by Analog Device.
> We developed driver module containing 8 of
> this driver and this module is attached to the
> driver board.
>
> One driver's Isc is 170mA. So, the simultaneous
> switching current in this module is about
> 170mA*8= 1.36A. The module size is 3 by 1 in.
>
> Is this switching current too big to this
> size of module? If we doesn't have ideal
> power/ground plane and short power/ground net,
> it could make ground bound or power droop
> problem? So, the driving characteristics could
> be heavily damaged?
>
> [Q6]
> We only checked the driver's Tr/Tf and
> voltage swing level.
> If the driving current(Isc) has major role
> to drive the 24 receivers and transmission load,
> how could we measure the driver's Isc correctly?
>
> *****************[End of question]***************
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