User's Manual

Layout/Routing Guidelines
2-38 Intel
®
820 Chipset Design Guide
AGP 2.0 requires that these power planes be separate. In conjunction with the 4X data rate, the
AGP 2.0 Interface Specification provides for low-voltage (1.5V) operation. The AGP 2.0
Specification implements a TYPEDET# (type detect) signal on the AGP connector that determines
the operating voltage of the AGP 2.0 interface (VDDQ). The motherboard must provide either
1.5V or 3.3V to the add-in card depending on the state of the TYPEDET# signal (refer to
Table 2-9. The 1.5V low-voltage operation applies ONLY to the AGP interface (VDDQ); VCC is
always 3.3V.
Note: The motherboard provides 3.3V to the Vcc pins of the AGP connector. If the graphics controller
needs a lower voltage, then the add-in card must regulate the 3.3V VCC voltage to the controller’s
requirements. The graphics controller may ONLY power AGP I/O buffers with the VDDQ power
pins.
The TYPEDET# signal indicates whether the AGP 2.0 interface operates 1.5 volts or 3.3 volts. If
TYPEDET# is floating (no connect) on an AGP add-in card, the interface is 3.3 volts. If
TYPEDET# is shorted to ground, the interface is 1.5 volts.
As a result of this requirement, the motherboard must provide a flexible voltage regulator. This
regulator must supply the appropriate voltage to the VDDQ pins on the AGP connector. For
specific design recommendations, refer to the schematics in Appendix A, “Reference Design
Schematics: Uni-Processor” and Appendix B, “Reference Design Schematics: Dual-Processor”.
VDDQ generation and AGP V
REF
generation must be considered together. Before developing
VDDQ generation circuitry, refer to the AGP 2.0 Interface Specification.
Figure 2-31 demonstrates one way to design the VDDQ voltage regulator. This regulator is a linear
regulator with an external, low R
DS-ON
FET. The source of the FET is connected to 3.3V. This
regulator will convert 3.3V to 1.5V or pass 3.3V depending on the state of TYPEDET#. If a linear
regulator is used, it must draw power from 3.3V (not 5V) in order to control thermals (i.e., 5V
regulated down to 1.5V with a linear regulator will dissipate approximately 7W at 2A). Because it
must draw power from 3.3V and, in some situations, must simply pass that 3.3V to VDDQ (when a
3.3V add-in card is placed in the system), the regulator MUST use a low R
dson
FET.
AGP 1.0 modified VDDQ3.3
min
to 3.1V. Using an ATX power supply; the 3.3V
min
is 3.168V.
Therefore, 68 mV of drop is allowed across the FET at 2A. This corresponds to a FET with an
R
dson
of 34 mW.
How does the regulator switch? The feedback resistor divider is set to 1.5V. When a 1.5V card is
placed in the system, the transistor is off and the regulator regulates to 1.5V. When a 3.3V card is
placed in the system, the transistor is on, and the feedback is pulled to ground. When this happens,
the regulator drives to gate of the FET to nearly 12V. This turns the FET on and passes 3.3V - 2A *
R
DS-ON
to VDDQ.
Table 2-9. TYPDET#/VDDQ Relationship
TYPEDET# (on add-in card) VDDQ (supplied by MB)
GND 1.5V
N/C 3.3V