Dynamic voltage scaling to improve system efficiency and thermal performance

    The power consumption in a processor is expressed as P f * V 2. As system clock frequencies increase, approaching a state known as overclocking, efficiency suffers, and heat becomes a primary concern for designers. Excessive heat generated by the processor can lead to thermal shutdown, system power cycling, and/or permanent damage, ultimately shortening the processor's lifespan.

    Microprocessors, microcontrollers, DSPs, ASICs, FPGAs, and any other digital load variant don't require maximum power 100% of the time. In most applications, these digital loads, or "processors," spend the majority of their time idle. Even when idle, the processor may still be running at maximum voltage and generating unnecessary heat. One way to combat the negative effects of idle time is a technique commonly known as dynamic voltage scaling (DVS).

    When in idle state, the processor's frequency can be reduced to save energy. Core voltage and frequency are directly related. The core voltage can also be lowered during idle state. This requires the ability to change the processor's core voltage in real time. DVS requires a feedback loop between the processor and the voltage regulator that sets the core voltage. During idle state, the processor sends a signal to the regulator, telling it to lower its output voltage because the processor's lower frequency operation is sufficient. The regulator's output adjusts accordingly, lowering the processor's core voltage and reducing its power consumption.

    So how does a processor interface with a voltage regulator to adjust the output voltage in idle and overclocked states? There are several ways to do this, but one interface protocol is the VID, or voltage identification number. A VID programmer can be used with nearly any point-of-load buck regulator or controller to change the core voltage supplied to a VID-enabled processor, such as the Texas Instruments (TI) LM10010 and LM10011.

    Here's how it works: The LM10010 receives a 6-bit code from a VID-supported processor via a 4-pin VID interface (see Figure 1). This code contains information including the core voltage value required for the processor's current state. The LM10010 then converts this code into an output current in µA using its integrated DAC. This output current is then fed into the regulator's feedback node, whose output is the processor's core voltage. The additional positive current now flowing into the feedback node (highlighted in red in Figure 1) causes the amount of current flowing through R FB1 to decrease. This modification of the current through R FB1 adjusts the regulator's output voltage to match the value in the 6-bit code provided by the processor.

    The LM10010's 6 bits allow it to program the regulator output voltage to any one of 64 different voltages, with the voltage range set by the feedback resistors used externally to the regulator. The LM10011 adds the flexibility of achieving 4- or 6-bit resolution, both of which offer tighter output current accuracy than the LM10010 (1.25% versus 3%). The LM10011 is pin- and package-compatible with the LM10010.

    Figure 1: VID solution using processors such as the LM10010, LM21215A-1, and TI's C6000 DSP

    Another advantage of the LM10010-11 is its cost-effective approach, as it can be used with a low-cost controller or regulator of the designer's choice. If this immediately reminds you of your favorite regulator or controller, that's another big benefit—it can be used with the regulator you're already familiar with, assuming the regulator has an externally divided feedback node (most do). The flexibility of being able to use the LM10010-11 with nearly any regulator allows you to implement dynamic voltage scaling while optimizing for cost and/or performance.

    Texas Instruments' TMS3206000™ DSP with the SmartReflex™ interface is an example of a processor family that supports VID and can be used to implement the LM10010-11 VID solution. However, a VID solution with the LM10010-11 can be designed with any processor that has four spare GPIO lines. The option to use the LM10010-11 with virtually any regulator or processor offers a previously unknown dimension of flexibility in the VID world. Customers can retain their existing designs and add the LM10010-11 to immediately and simply implement dynamic voltage scaling and reap the efficiency benefits associated with this technology.