AN1769 Freescale Semiconductor / Motorola, AN1769 Datasheet - Page 36

• Manufacturer Part Number: AN1769
• Description: Designing a Minimal PowerPC System
• Manufacturer: Freescale Semiconductor / Motorola

It is virtually impossible to insure that all voltages ramp up to their steady state at an identical rate and at an
identical time. In , either requirement 2 or requirement 3 will be violated depending only on the load on the
3.3-V power supply. Because such tracking is difÞcult to achieve, PowerPC processors may be subjected to
a differential voltage between the V
cannot track within speciÞed limits within this period, other means must be employed to correct the
problem; otherwise, the long term reliability of the processor may be affected due to failure of internal
protection circuitry.
One means of keeping two supplies synchronized is to use a so-called ÒbootstrapÓ diode between two power
rails. An example is shown in .
The bootstrap diodes are selected such that a nominal V
until the V
diodes are connected in series; each has a forward voltage (V
1.2V drop, maintaining the 2.5V power line at 2.1V.
Once the core power supply is stable at 2.5V, then the bootstrap diode(s) will be reverse biased and only a
few nanoamperes of leakage current will ßow.
NOTE: It is essential that the forward voltage be effective at the current levels needed by the processor;
1Ð3 amps or so depending on the PowerPC device. Many diodes have only a nominal V
nothing at high current; such devices are not acceptable.
36
3.3V
2.5V
5V
DD
Minimal Load 3.3V Ramp
power supply becomes active. In the above example, a pair of MUR420 Schottky barrier
HAZARD
(requirement 2)
Freescale Semiconductor, Inc.
Main Power
Core Power
For More Information On This Product,
Figure 24. Power Supply Sequencing
Minimal PowerPC System Design
DD
Figure 25. Bootstrap Diodes
Go to: www.freescale.com
and OV
t
DD
power signals for up to 500 µs. If the power supplies
DD
MUR420
MUR420
will be sourced from the OV
3.3V
2.5V
F
5V
) of 0.6V at high currents, and so provides a
3.3V (OV
2.5V (V
Maximum Load 3.3V Ramp
DD
HAZARD
DD
(requirement 3)
)
)
F
which falls off to
DD
power supply
MOTOROLA
t