For unidirectional clamping (higher voltage to lower voltage), the G_REF input must be connected to D_REF and both pins pulled to the higher-side V_CC through a pullup resistor (typically 200 kΩ). TI recommends a filter capacitor on D_REF. Pullup resistors are required if the chipset I/Os are open drain. The opposite side of the reference transistor (S_REF) is connected to the processor core power supply voltage. When D_REF is connected through a 200-kΩ resistor to a 3.3-V to 5.5-V V_CC supply and S_REF is set from 1 V to V_CC – 1.5 V, the output of each Sn has a maximum output voltage equal to S_REF.

Figure 9-3 Unidirectional Down Translation to Protect Low-Voltage Processor Pins