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ZHCS488K October 2011 – December 2018 AM3351 , AM3352 , AM3354 , AM3356 , AM3357 , AM3358 , AM3359

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ZCZ|324

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7.7.2.3.3.7 High-Speed Bypass Capacitors

High-speed (HS) bypass capacitors are critical for proper DDR3 interface operation. It is particularly important to minimize the parasitic series inductance of the HS bypass capacitors, the AM335x device DDR3 power, and the AM335x device DDR3 ground connections. Table 7-65 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.
Minimize the distance from the bypass capacitor to the power terminals 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 limits on via sharing shown in Table 7-65.

Table 7-65 High-Speed Bypass Capacitors

NO.	PARAMETER	MIN	TYP	MAX	UNIT
1	HS bypass capacitor package size⁽¹⁾		0201	0402	10 mils
2	Distance, HS bypass capacitor to AM335x VDDS_DDR and VSS terminal being bypassed⁽²⁾⁽³⁾⁽⁴⁾			400	mils
3	AM335x VDDS_DDR HS bypass capacitor count	20			devices
4	AM335x VDDS_DDR HS bypass capacitor total capacitance	1			μF
5	Trace length from AM335x VDDS_DDR and VSS terminal to connection via⁽²⁾		35	70	mils
6	Distance, HS bypass capacitor to DDR3 device being bypassed⁽⁵⁾			150	mils
7	DDR3 device HS bypass capacitor count⁽⁶⁾	12			devices
8	DDR3 device HS bypass capacitor total capacitance⁽⁶⁾	0.85			μF
9	Number of connection vias for each HS bypass capacitor⁽⁷⁾⁽⁸⁾	2			vias
10	Trace length from bypass capacitor connect to connection via⁽²⁾⁽⁸⁾		35	100	mils
11	Number of connection vias for each DDR3 device power and ground terminal⁽⁹⁾	1			vias
12	Trace length from DDR3 device power and ground terminal to connection via⁽²⁾⁽⁷⁾		35	60	mils

LxW, 10-mil units; for example, a 0402 is a 40x20-mil surface-mount capacitor.
Closer and shorter is better.
Measured from the nearest AM335x VDDS_DDR and ground terminal to the center of the capacitor package.
Three of these capacitors should be located underneath the AM335x device, between the cluster of VDDS_DDR and ground terminals, between the DDR3 interfaces on the package.
Measured from the DDR3 device power and ground terminal to the center of the capacitor package.
Per DDR3 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 DDR3 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 DDR3 device pad should be less than 150 mils.
Up to two pairs of DDR3 power and ground terminals may share a via.