Design Ideas: March 3, 1994
The circuit in Fig 1 automatically senses when a user upgrades a Motorola 68040 µP system with a 68060 µP, changing the µP's supply voltage from 5 to 3.3V. This automatic voltage change is far safer than any manual scheme could be, and this technique lets you easily adapt the circuit to other applications that must switch between 5 and 3.3V.
The 68060 has two pins on its inner ring of pins called THERM0 and THERM1. These pins are for testing only and have no function during normal operation. A resistance of approximately 400(ohm) exists between these two pins at room temperature. The circuit uses these pins to detect which µP is plugged in.
In operation, when the figure's circuit powers a 68040, the THERM0 and THERM1 pins are open, and the gate of FET Q1 goes to 5V, turning the FET on hard. With Q1 on, resistors R1 and R2 are in parallel, programming IC1, a dual current-mode switched-mode power-supply controller, to run at 100% duty cycle and deliver the full 5V to the 68040.
If the user plugs in a 68060, however, the resistance between the 68060's THERM0 and THERM1 pins effectively grounds Q1's gate, ensuring that Q1 shuts off. With Q1 off, R1 is removed from IC1's programming pin, causing the circuit to supply 3.3V to the 68060.
IC1 suits this application because it can operate at a 100% duty cycle. Running IC1 at a 100% duty cycle means the load sees a drop in output voltage arising from only the resistance of the FET Q2, the inductor L1, and the sense resistor R3. For the components in the figure, the total drop is less than 200 mV at 2A. Thus, at 100% duty cycle, the circuit can deliver nearly its entire input voltage to its output. This characteristic obviates transformers or other complex arrangements. EDN BBS /DI_SIG #1372