Datasheet

Chapter 20. Electrical Specifications 20-3
General Parameters
20.1.1 Supply Voltage Sequencing and Separation Cautions
Figure 20-1 shows two situations to avoid in sequencing the IV
cc
and EV
cc
supplies.
Figure 20-1. Supply Voltage Sequencing and Separation Cautions
IV
cc
should not be allowed to rise early (1). This is usually avoided by running the regulator
for the IV
cc
supply (1.8 V) from the voltage generated by the 3.3-V EV
cc
supply
(Figure 20-2). This keeps IV
cc
from rising faster than EV
cc
.
IV
cc
should not rise so late that a large voltage difference is allowed between the two
supplies (2). Typically this situation is avoided by using external discrete diodes in series
between supplies as shown in Figure 20-2. The series diodes forward bias when the
difference between EV
cc
and IV
cc
reaches approximately 2.1V, causing IV
cc
to rise as EV
cc
ramps up. When the IV
cc
regulator begins proper operation, the difference between supplies
should not exceed 1.5 V and conduction through the diode chain reduces to essentially
leakage current. During supply sequencing, the following general relationship should be
adhered to: EV
cc
≥ IV
cc
≥ (EV
cc
- 2.1 V). The PLL Vdd (PV
cc
) supply should comply with
these constraints just as IV
cc
does. In practice, PV
cc
is typically connected directly to IV
cc
with some filtering.
EV
cc
IV
cc
, PV
cc
Time
3.3V
1.8V
0
DC Power Supply Voltage
Notes:
1IV
cc
, PV
cc
rising before EV
cc
2EV
cc
rising much faster than IV
cc
, PV
cc
2
1
Supplies Stable