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2.2.3
2.2.3.1
2.2.3.2
Hardware Design Guide
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Power Supply Regulation
Power-Up Power Supply Regulation
During power-up, after NRESET is de-asserted, the device will experience an increase in current
consumption after the PLL locks and the system clocks begin to operate at full speed. During this
time, a droop on the core power supply may occur due to this increase in current consumption. It is
acceptable that the core power supply droop a maximum of 100 mV to a minimum of 1.2 V during
this time. It is expected that the device is Idle during this time and that no instructions are executing
until the power is within the 5% regulation specification. The behavior is undefined if instructions
are executed while the power is not within the 5% regulation specification.
Power Supply Decoupling
To maintain supply regulation during power-up and during normal operation sufficient de-
coupling, mid-frequency and bulk capacitors should be used to account for transient power steps.
The number and size of decoupling capacitors required for a particular implementation depends on
the board stack-up and power-delivery circuitry, so a single solution will not work for all
implementations. Note that the worst case transient occurs when the device comes out of reset as
explained in
Section
2.2.3.1. The IXDP2800 decoupling design implements a via-sharing
mechanism that maximizes the number of decoupling capacitors that can be accommodated under
the device while minimizing the number of vias. This reduces the amount of holes in the power and
ground planes. The decoupling layout from the IXDP2800 Advanced Development System is
shown in
Figure
2.
The IXDP2800 development platform uses 63 0.22 uF 0402 AVX* capacitors for the core supply
decoupling. A 1uF 0402 capacitor is now available from AVX* which can be substituted for the
0.22 uF capacitor. This will provide better regulation stability during the power-on transient event.
Refer to
Section 2.5
for details of the power-on transient event.
IXP28XX Network Processor
Power Ratings and Requirements
25
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