标题: Power Distribution System (PDS) Design! [打印本页] 作者: Allen 时间: 2007-9-17 09:11 标题: Power Distribution System (PDS) Design! Introduction: # o' Y# h u# b7 ?5 ]8 f- B FPGA designers are faced with a unique task when it comes to designing power distribution % S' i+ C+ Q0 `+ p9 M! Psystems (PDS). Most other large, dense ICs (such as large microprocessors) come with very$ [/ I9 O. @4 Y8 n$ A
specific bypass capacitor requirements. Since these devices are only designed to implement 4 ~0 }. {* ] s0 W" d* ispecific tasks in their hard silicon, their power supply demands are fixed and only fluctuate' Q' U* a0 ^5 K) {3 w
within a certain range. FPGAs do not share this property. Since FPGAs can implement an3 _# w. C# @# E( Q" o& F
almost infinite number of applications at undetermined frequencies and in multiple clock) K- F% P* R9 l* ]8 L: p! {
domains, it can be very complicated to predict what their transient current demands will be. O. P- |! A e
Since exact transient current behavior cannot be known for a new FPGA design, the only 2 e7 r' k( T+ \6 nchoice when designing the first version of an FPGA PDS is to go with a conservative worstcase : L% P2 ~0 g8 w1 V9 [% D# c; tdesign.1 `. q& K7 M* \8 g
Transient current demands in digital devices are the cause of ground bounce, the bane of highspeed3 r5 R+ i: q& C5 U8 q& ^& C1 z
digital designs. In low-noise or high-power situations, the power supply decoupling 6 d* b7 ^4 }& v1 V) K. ]network must be tailored very closely to these transient current needs, otherwise ground# P- d0 T3 a. _! F$ n, I
bounce and power supply noise will exceed the limits of the device. The transient currents in an / {" x/ m: s2 \FPGA are different from design to design. This application note provides a comprehensive4 v3 B/ \& [5 m" m
method for designing a bypassing network to suit the individual needs of a specific FPGA ! T- S9 r- W# L& Fdesign.$ V J; v* T9 O9 H
The first step in this process is to examine the utilization of the FPGA to get a rough idea of its & T8 Y* w% E1 P6 m ^2 g' gtransient current requirements. Next, a conservative decoupling network is designed to fit these 0 m- n' P' s% m0 m5 f- Vrequirements. The third step is to refine the network through simulation and modification of; p! P) b7 f8 }7 @
capacitor numbers and values. In the fourth step, the full design is built and in the fifth step it is 6 v$ Y' W3 m* X/ \measured. Measurements are made consisting of oscilloscope and possibly spectrum analyzer+ q+ L5 m9 z; p. V6 _
readings of power supply noise. Depending on the measured results, further iterations through 3 T1 Z: |" m) ]( T$ u6 O8 athe part selection and simulation steps could be necessary to optimize the PDS for the specific 0 R/ ~9 E# E o3 Bapplication. A sixth optional step is also given for cases where a perfectly optimized PDS is# W! J" \0 q# y. {
needed.
