DSP56857E datasheet(25/53 Pages) FREESCALE

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Supply Voltage Sequencing and Separation Cautions

56857 Technical Data, Rev. 6

Freescale Semiconductor

4.3 Supply Voltage Sequencing and Separation Cautions

Figure 4-2 shows two situations to avoid in sequencing the VDD and VDDIO, VDDA supplies.

Note: 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.

Figure 4-3 Example Circuit to Control Supply Sequencing

3.3V

1.8V

Time

Supplies Stable

VDD

VDDIO, VDDA

3.3V

Regulator

1.8V

Regulator

Supply

VDD

VDDIO, VDDA

DSP56857E Datasheet(PDF) 25 Page - Freescale Semiconductor, Inc