Voltage Regulator-Down (VRD) 10.1 Design Guide

Output Indicators

56 Design Guide

positive comparator terminal). As the thermistor temperature increases due to system loading, the

resistance will decrease. When the voltage drop across the thermistor falls below the trigger

reference voltage, established by R1 and R2, the comparator will change state and bias the bipolar

transistors. When biased, Q1 and Q2 provide the active low assertion of PROCHOT# and

FORCEPR# compliant to Table 7 signaling specifications. Q1 and Q2 must be selected to

adequately drive PROCHOT# and FORCEPR# VOL signaling values.

FORCEPR# is a processor signal with active-low input buffers terminated to the system Vtt (FSB

termination voltage). PROCHOT# is a processor signal that can be configured as i/o using open-

drain, output buffers terminated to the system Vtt (FSB termination voltage); This signal drives

and receives with active low signaling. In some processor configurations, this buffer is configured

as an output only signal To maintain reliable signaling, the bipolar transistor must be selected to

operate with a collector bias established by motherboard, processor, or chipset on-die termination

(See Figure 9-2). The bipolar transistors must be chosen to drive an the Vol levels identified in

Table 9-3 with an effective termination range defined in Table 9-3. Note that the termination

topology can take multiple forms; A generic representation is provided in Figure 9-3.

PROCHOT# and FORCEPR# may see this full configuration, or a subset including any

combination of the identified loads.

The values for R1, R2 and R3 in Figure 4 are included as an example and must be calculated

using specific design parameters. The value of R2 is adjusted to calibrate the comparator’s trigger

reference voltage (and assertion of the output signals) against the sensor voltage representing a

thermal violation.