LM2507 datasheet(17/22 Pages) NSC

17 / 22 page

Application Information (Continued)

Power and Ground - Bumped Package

Power and ground bump assignments are shown in Figure

17. The nine center balls must be connected ground on the

PCB. See also, National’s Application Note AN-1126, Ball

Grid Array, for information on land pattern recommendations

and escape routing guidelines.

FLEX CIRCUIT RECOMMENDATIONS

The three MPL lines should generally run together to mini-

mize any trace length differences (skew). For impedance

control and also noise isolation (crosstalk), guard ground

traces are recommended in between the signals. Commonly

a Ground-Signal-Ground (GSGSGSG) layout is used. Lo-

cate fast edge rate and large swing signals further away to

also minimize any coupling (unwanted crosstalk). In a

stacked flex interconnect, crosstalk also needs to be taken

into account in the above and below layers (vertical direc-

tion). To minimize any coupling locate MPL traces next to a

ground layer. Power rails also tend to generate less noise

than LVCMOS so they are also good candidates for use as

isolation and separation.

The interconnect from the Master to the Slave typically acts

like a transmission line. Thus impedance control and ground

returns are an important part of system design. Impedance

should be in the 50 to 100 Ohm nominal range for the

LM2507. Testing has been done with cables ranging from 40

to 110 Ohms without error (BER Testing). To obtain the

impedance, adjacent grounds are typically required ( 1 layer

flex), or a ground shield / layer. Total interconnect length is

intended to be in the 20cm range, however 30cm is possible

at lower data rates. Skew should be less than 500ps to

maximize timing margins.

GROUNDING

While the LM2507 employs three separate types of ground

pins, these are intended to be connected together to a

common ground plane. The separate ground pins help to

isolate switching currents from different sections of the inte-

grated circuit (IC). Also required is a nearby signal return

(ground) for the MPL signals. These should be provided next

to the MPL signals, as that will create the smallest current

loop area. The grounds are also useful for noise isolation

and impedance control.

PCB RECOMMENDATIONS

General guidelines for the PCB design:

•

Floor plan, locate MPL Master near the connector to limit

chance of cross talk to high speed serial signals.

•

Route serial traces together, minimize the number of

layer changes to reduce loading.

•

Use ground lines are guards to minimize any noise cou-

pling (guarantees distance).

•

Avoid parallel runs with fast edge, large LVCMOS swings.

•

Also use a GSGSG pinout in connectors (Board to Board

or ZIF).

•

Slave device - follow similar guidelines.

•

Bypass the device with MLC surface mount devices and

thinly separated power and ground planes with low induc-

tance feeds.

•

High current returns should have a separate path with a

width proportional to the amount of current carried to

minimize any resulting IR effects.

20186021

FIGURE 17. LM2507 PWR (V

DD) and GND (VSS) Bumps (TOP VIEW)

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