Zynq-7000 PCB Design Guide
19
UG933 (v1.8) November 7, 2014
Chapter 3:
Power Distribution System
For example, if current demand in the device increases in a few picoseconds, the voltage at
the device sags by some amount until the capacitors can supply extra charge to the device.
If current demand in the device maintains this new level for many milliseconds, the
voltage-regulator circuit, operating in parallel with the decoupling capacitors, replaces the
capacitors by changing its output to supply this new level of current.
shows the major PDS components: the voltage regulator, the decoupling
capacitors, and the active device being powered (AP SoC).
shows a simplified PDS circuit with all reactive components represented by a
frequency-dependent resistor.
Role of Inductance
Inductance is the property of the capacitors and the PCB current paths that slows down
changes in current flow. Inductance is the reason why capacitors cannot respond
instantaneously to transient currents or to changes that occur at frequencies higher than
their effective range.
Inductance can be thought of as the momentum of charge. Charge moving through a
conductor represents some amount of current. If the level of current changes, the charge
X-Ref Target - Figure 3-2
Figure 3-2:
Simplified PDS Circuit
+
AP SoC
L
REGULATOR
L
DECOUPLING
C
DECOUPLING
Voltage
Regulator
V
UG933_c3_02_081312
X-Ref Target - Figure 3-3
Figure 3-3:
Further Simplified PDS Circuit
ltransient
AP SoC
UG933_c2_03_081312
Z
P
(f)
V
RIPPLE
+
-
+
V
