Datasheet

Tracking Regulator and Power Sequencing

1-6

1.3 Tracking Regulator and Power Sequencing

To avoid potential problems with the processor and system ICs, designers can

apply three general techniques for power-up sequencing: sequential,

ratiometric, or simultaneous. Sequential power up, as the name implies,

powers up the two rails one after the other. Typically the second rail begins to

ramp up once the first rail reaches regulation. Alternately, the second rail may

begin its ramp after a set delay from the start of the first rail. Either method must

comply with the processor manufacturer’s restriction on the minimum and

maximum time one supply is not powered, or the duration and amount that one

supply exceeds the other. With the second or ratiometric method, the two rails

begin to power up and reach regulation at the same time. This requires a

higher slew rate for the rail with the higher final voltage, and results in the

maximum voltage differential occurring when regulation is reached. However,

some processors may not tolerate the instantaneous voltage differences that

occur before regulation is reached, or the processor may draw high current

from one supply during this period. The third approach eliminates

instantaneous voltage differences and minimizes the magnitude and duration

of stresses. A common way of implementing this method is simultaneous

power up, in which the voltage rails rise together and at the same rate, with the

higher or I/O voltage rail continuing after the lower or core voltage rail has

reached its final value.

The TPS54x80EVM-228 evaluation module is designed to demonstrate the

described power sequencing technique. The basic idea is shown in

Figure 1-1.

Figure 1-1. Different Power Sequencing Technique Selection

I/O Regulator

VSENSE

TPS54x80

Core Tracking

Regulator

VSENSE

TRACKIN

V I/O

(core)

Implementation of different power sequencing techniques is based on proper

resistor-divider ratio selection. The equations 1, 2, and 3 provide related ratios

for different ways of power sequencing.

– core voltage tracks IńO voltage (simultaneous power up)

– ratiometric relation between core and IńOvoltage

* core voltage rises first at power up and falls second at power down.

(1)

(2)

(3)