AN1769 Freescale Semiconductor / Motorola, AN1769 Datasheet - Page 35

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

MOTOROLA
This switcher has a 5-bit digital input which allows the output voltage to be set in 0.1/0.05V increments in
two ranges between 1.2V to 3.6V. This allows a single power supply to be easily programmed to meet
current and future PowerPC processor requirements. In addition, the digital settings match those used on the
PowerPC processor/cache module (interposer), which allows the processor to automatically select the
desired voltage.
6.3 Power Supply Sequencing
Once consequence of multiple power supplies is that when power is initially applied, the voltage rails will
ramp up at different rates depending upon the nature of the power supply, the type of load on each, and the
manner in which the different voltages are derived. This can present a problem because the power supplies
of a PowerPC processor have the following restrictions:
On most PowerPC processors, the OV
core) power, and the I/O cells are three-stated during reset, so a 3.3-V power supply may ramp up faster than
the core voltage. Alternately, with more devices now operating at 3.3V, including the PCI bus, that power
rail may be so loaded (from a system perspective) that the V
shows an example of two possible power sequencing waveforms.
+5.0V
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VIN must not exceed OV
during power-on reset.
OV
during power-on reset.
V
during power-on reset.
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NOTE: Not all details have been shown.
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/AV
2.5µH
Voltage
Encoding
Inputs
must not exceed V
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must not exceed OV
1200µF
x3
Freescale Semiconductor, Inc.
Figure 23. Simple Switching Power Supply
For More Information On This Product,
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Minimal PowerPC System Design
Raytheon/
Fairchild
RC5050
/AV
by more than 0.3V at any time including(requirement 1)
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Go to: www.freescale.com
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(+3.3V I/O) load is typically less than 10% that of V
by more than 1.2V at any time including(requirement 2)
by more than 0.4V at any time including(requirement 3)
MOSFET
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power will stabilize more quickly. Figure 24
1.3µH
6mW
1500µF
x3
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(+2.5V
+2.5V
35