SCF5249 datasheet(30/56 Pages) FREESCALE

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SCF5249 Integrated ColdFire® Microprocessor Data Sheet, Rev. 3

Electrical Characteristics

Freescale Semiconductor

9.1

Supply Voltage Sequencing and Separation Cautions

Figure 2 shows two situations to avoid in sequencing the CoreVdd and PADVdd (I/O) and PLLL supplies.

Figure 2. Supply Voltage Sequencing and Separation Cautions

CoreVdd supply should not be allowed to rise early (1). This is usually avoided by running the regulator

for the CoreVdd supply (1.8 V) from the voltage generated by the 3.3V supply (PADVdd). This keeps the

CoreVdd supply from rising faster than PADVdd supply.

Also CoreVdd, PLLGVdd, PLLCVdd supply 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. The series diodes forward bias when the difference between PADVdd and

CoreVdd reaches approximately 2.1V, causing CoreVdd to rise as PADVdd ramps up. When the CoreVdd

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: PADVdd, >=

CoreVdd, PLLGVdd, >= (PADVdd - 2.1 V).

The PLL core supplies (PLLGVdd and PLLCVdd) should comply with these constraints just as the

CoreVdd does. In practice, PLLGVdd and PLLCVdd are typically connected directly to the CoreVdd with

some filtering. Further, the PLL PAD supply (PLL1VDD) would be connected directly to the PAD supply

via some filtering.

SCF5249 Datasheet(PDF) 30 Page - Freescale Semiconductor, Inc