8.8.2.2.7 High-Speed Bypass Capacitors
TI recommends that a PDN/power integrity analysis is performed to ensure that capacitor selection and placement is optimal for a given implementation. This section provides guidelines that can serve as a good starting point.
High-speed (HS) bypass capacitors are critical for proper DDR2 interface operation. It is particularly important to minimize the parasitic series inductance of the HS bypass capacitors, processor/DDR power, and processor/DDR ground connections. Table 8-28 contains the specification for the HS bypass capacitors as well as for the power connections on the PCB. Generally speaking, it is good to:
- Fit as many HS bypass capacitors as possible.
- HS bypass capacitor value is < 1µF
- Minimize the distance from the bypass cap to the pins/balls being bypassed.
- Use the smallest physical sized capacitors possible with the highest capacitance readily available.
- Connect the bypass capacitor pads to their vias using the widest traces possible and using the largest hole size via possible.
- Minimize via sharing. Note the limites on via sharing shown in Table 8-28.
Table 8-28 High-Speed Bypass Capacitors
| NO. |
PARAMETER |
MIN |
TYP |
MAX |
UNIT |
| HS21 |
HS bypass capacitor package size(1) |
|
0201 |
0402 |
10 Mils |
| HS22 |
Distance, HS bypass capacitor to processor being bypassed(2)(3)(4) |
|
|
400(12) |
Mils |
| HS23 |
Processor HS bypass capacitor count (12) |
|
12(11) |
|
Devices |
| HS24 |
Processor HS bypass capacitor total capacitance per vdds_ddrx rail (12) |
|
3.4 |
|
μF |
| HS25 |
Number of connection vias for each device power/ground ball per vdds_ddrx rail(5) |
1 |
|
|
Vias |
| HS26 |
Trace length from device power/ground ball to connection via(2) |
|
35 |
70 |
Mils |
| HS27 |
Distance, HS bypass capacitor to DDR device being bypassed(6) |
|
|
150 |
Mils |
| HS28 |
Number of connection vias for each HS capacitor(8)(9) |
|
4 (14) |
|
Vias |
| HS29 |
DDR2 device HS bypass capacitor count(7) |
|
12 (13) |
|
Devices |
| HS210 |
DDR2 device HS bypass capacitor total capacitance(7) |
0.85 |
|
|
μF |
| HS211 |
Trace length from bypass capacitor connect to connection via(2)(9) |
|
35 |
100 |
Mils |
| HS212 |
Number of connection vias for each DDR2 device power/ground ball(10) |
1 |
|
|
Vias |
| HS213 |
Trace length from DDR2 device power/ground ball to connection via(2)(8) |
|
35 |
60 |
Mils |
- LxW, 10-mil units, that is, a 0402 is a 40x20-mil surface-mount capacitor.
- Closer/shorter is better.
- Measured from the nearest processor power/ground ball to the center of the capacitor package.
- Three of these capacitors should be located underneath the processor, between the cluster of vdds_ddrx balls and ground balls, between the DDR interfaces on the package.
- See the Via Channel™ escape for the processor package.
- Measured from the DDR2 device power/ground ball to the center of the capacitor package.
- Per DDR2 device.
- An additional HS bypass capacitor can share the connection vias only if it is mounted on the opposite side of the board. No sharing of vias is permitted on the same side of the board.
- An HS bypass capacitor may share a via with a DDR device mounted on the same side of the PCB. A wide trace should be used for the connection and the length from the capacitor pad to the DDR device pad should be less than 150 mils.
- Up to a total of two pairs of DDR power/ground balls may share a via.
- The capacitor recommendations in this data manual reflect only the needs of this processor. Please see the memory vendor’s guidelines for determining the appropriate decoupling capacitor arrangement for the memory device itself.
- For more information, see Section 8.3, Core Power Domains
- For more information refer to DDR2 specification.
- Preferred configuration is 4 vias: 2 to power and 2 to ground.