A. Hardware Implementation
419
PowerSpan II User Manual
80A1010_MA001_09
Integrated Device Technology
www.idt.com
A.3.2
PowerSpan II External PLL Decoupling for New Designs
To provide the cleanest possible supply environment for the PLL, the supplies should be decoupled
externally. Isolation should be provided between the external core supply voltage on the board and the
supply that goes to the PLL. This can be done in the following ways:
•
A separate core voltage regulator can be provided and a separate trace run up to the PLL supply
pins.
•
An isolation and decoupling network can be provided for on the board to isolate and minimize
noise on the core voltage supply plane before it gets to the PLL supply pins.
For optimum PLL jitter performance, the PLL should be isolated and decoupled from the main core
power plane using a surface mount RF inductor and low ESR tantalum surface mount capacitor
network is recommended. The power supplies for PLLs on a device should come from a single point on
the board. The power trace should then be isolated from the main power plane using the network
shown in
.
The routing parasitic resistance of the trace route from any PLL supply pin to the decoupling capacitors
in the isolation network must be less than 0.1 Ohms. To minimize the transient IR drops across the
leads from the isolation network and the PLL supply device pins, the trace routes must be kept short.
The preferred layout is to have the cripple capacitors, shown in
, placed as close
to the device pins as possible; potentially on the backside of the board underneath the device.
Figure 42: PLL Decoupling
.0.5 Ohms(MIn)
4 Ohms(Max)
Cfilter
4.7uF(Min)
33uF(Max)
Lfilter
470nH(Min)
4.7uH(Min)
(RF SMT)
PowerSpan II
PLL_AVDD
PLL_DVDD
Cripple2
0.1uF
PLL_DVSS
PLL_AVSS
Core VDD
The VDD to VSS 0.1uF decoupling caps
must be as close to the device pins as possible.
Capacitors should be Low ESR (High Frequency)
ceramic chip capacitors.
The trace routing and
the Rdc of the inductor accounts
for this resistance and should be
in the range shown.
The trace routing
resistance must be
less than 0.1Ohms to Cripple1
and Cripple2.
Cfilter must be a
low ESR Tantalum
SMT capacitor.
Lfilter must
be a high SRF
SMT Wire wound
RF inductor.
Cripple1
0.1uF