56855 Technical Data, Rev. 6
22
Freescale Semiconductor
Notes: 1. VDD rising before VDDIO, VDDA
2. VDDIO, VDDA rising much faster than VDD
Figure 4-2 Supply Voltage Sequencing and Separation Cautions
VDD should not be allowed to rise early (1). This is usually avoided by running the regulator for the VDD
supply (1.8V) from the voltage generated by the 3.3V VDDIO supply, see Figure 4-3. This keeps VDD from rising faster than VDDIO.
VDD 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 4-3. The series diodes forward bias when the difference between VDDIO and VDD reaches approximately 2.1, causing VDD to rise as VDDIO ramps up. When the VDD regulator begins proper
operation, the difference between supplies will typically be 0.8V and conduction through the diode chain
reduces to essentially leakage current. During supply sequencing, the following general relationship
should be adhered to:
VDDIO > VDD > (VDDIO - 2.1V)
In practice, VDDA is typically connected directly to VDDIO with some filtering.
3.3V
1.8V
Time
0
2
1
Supplies Stable
VDD
VDDIO, VDDA
DC
Power
Supp
ly
V
olt
age