Termination supply tracks one-half core voltage

Chester Simpson, National Semiconductor, Santa Clara, CA -- EDN, 4/27/2000

Modern µPs typically require core voltages of 2 to 3V. They also require "termination" voltages that, for best performance, should equal half the core voltage. The problem is, core voltages vary among CPUs, and some systems even incorporate variable-core-voltage supplies that allow systems to adjust the voltage on the fly, thereby increasing or reducing CPU operating speed to optimize power consumption. A termination-voltage power supply that maintains an output of one-half V_CORE over a range of core voltages is highly desirable, as the core voltage can vary without upsetting the termination-voltage set point. The circuit in Figure 1 is a 6A power-supply design that generates a termination voltage regulated to 1/2 V_CORE. The circuit targets applications in which the core voltage is approximately 1.8 to 3.6V.

An LM2636 synchronous-rectifier con-troller switching at 300 kHz provides an efficient power converter that operates from a 5V input. Because the LM2636 is designed to operate at a fixed output voltage (as determined by control bits 14 through 18), the circuit in Figure 1 uses a different control scheme to force the regulated output to track at one-half V_CORE. Resistors R₁ and R₂ halve the core voltage, and this voltage serves as the reference in error amplifier IC_1A. The amplifier compares the one-half V_CORE reference with the termination output voltage obtained through R₃ and adjusts its output to lock the termination voltage at one-half V_CORE. In this way, the core-voltage signal sets the termination voltage. IC_1B is a unity-gain inverter that corrects the phase of the feedback signal that goes to the input of the LM2636's internal error amplifier. R₄ sets the gain of the internal amplifier to unity. R₅ forces a soft start on turn-on and also eliminates overshoot. IC_1C amplifies the 1.23V internal reference to approximately 2.5V, which sets the operating point for the error amplifier, IC_1A.

The 2.5V also connects to the sense input of the LM2636, which would normally sense the regulated output voltage. Because the termination output voltage must be variable (to track the core voltage), a fixed 2.5V goes to the sense pin, and the control pins 16 and 18 are grounded. These connections program the internal DAC for a 2.5V output. This scheme prevents the LM2636's internal error-detection circuitry from shutting down the part in response to an under- voltage or overvoltage condition. R₃, R₆, C₁, and C₂ provide loop compensation. (DI #2517)