Understand how a power subsystem’s digital interface can benefit a specific electronic application.
Digital communication and control benefit power converter subsystems and the systems they are part of. A digital interface provides the power design engineer with greater flexibility to finalize and optimize certain parameters (e.g. sequencing of voltage rails, fault protection thresholds, and feedback compensation) after a system board returns from assembly.
Using the OSI model for digital communication, there are two major aspects of digital communication: the physical layer (PHY) over which communication is executed, and the data link layer, which defines the protocol or a command set that is used to communicate information.
The PHY for most power converters is the I2C serial interface, or its derivative, the SMBus, shown in Figure 1. The I2C interface defines a bidirectional clock signal (SCL) and a bidirectional data signal (SDA). The SMBus adds an optional alert signal (normally used to generate interrupts) and utilizes fixed logic levels for the signaling. Signal timing for clock and data are the same for both standards. These PHY choices allow multiple converter circuits to share a single interface with a host controller and are used in many server and PC subsystems, such as fan control and power/sleep functions.
The data link layer defines the information to be communicated between the host and voltage converter circuit. The data can be defined as a set of addressable registers with data bits uniquely defined for each IC, or the registers can follow an industry standard. One such standard for power management is the PMBus standard, which provides a defined set of registers/commands to communicate status. (For the distinctions between I2C, SMBus, and PMBus, refer to the PMBus website.)
Figure 1 The I2C/PMBus signals provide the physical interface most often used in communications to power converters.
The PMBus standard states that, to be considered compliant with the specification, a power device must implement at least one of the PMBus commands. Most PMBus-equipped devices implement the PMBus protocol for several common commands, such as setting the output voltage or reading die temperature. However, nearly all devices also implement MFG_SPECIFIC registers unique to that device.
Data and command formats
In addition, the PMBus standard calls out two methods to encode values into a digital word that can be communicated over the interface: direct and linear. For the direct method, the value in the command is the integer value the manufacturer has defined for the register in the device. Linear data format, on the other hand, is a form of floating-point value representation. In practice, most devices use either a Linear11 or Linear16 representation of data values.
The Linear11 data format has an 11-bit mantissa and a 5-bit exponent (Figure 2). Both the mantissa and the exponent are two’s-complement integers, meaning they can be positive or negative. You convert a real-world value to Linear11 format with Equation 1:
XREAL WORLD = Y · 2N (1)
Figure 2 The Linear11 data format has an 11-bit mantissa and 5-bit exponent, both expressed in two’s-complement.
The exponent N can be positive or negative, and 2N defines the size of the Y mantissa’s LSB. Thus, the smallest number that can be represented using Linear11 format is ±2-16 x 1 = ±15.3e-6. The largest number is ±33.5e6.
Since a signed, 11-bit integer goes from -1,024 to +1,023, the mantissa’s magnitude should be kept between 512 and 1,023 to maximize the data resolution during encoding. This suggests an approach to encoding a real-world value as a PMBus Linear11 formatted value: start with N = -16, then increment N until the mantissa’s magnitude is within the desired range.
The Linear16 format has a 16-bit mantissa and a 5-bit exponent, with both the mantissa and the exponent as two’s-complement integers. The data structure is like that of Linear11, except that Linear16 uses the full 16-bit I2C command packet for the mantissa and supplies the exponent separately (Figure 3), possibly mixed with a command code. For example, the MPQ4230 buck-boost converter from MPS uses the Linear16 format for VOUT_COMMAND and READ_VOUT, which set and read the output voltage, with the command code and exponent combined in one byte. You convert a real-world value to Linear16 format with Equation 2:
Xreal world = Y · 2N (2)
Figure 3 Linear16, like Linear11, uses two’s complement for the mantissa and exponent, but sends them in separate words that may also include a command code.
With a 5-bit exponent, the smallest number that can be represented using Linear16 format is still ±2-16 x 1 = ±15.3e-6. However, the target value for the mantissa is now between 214 and 215, or between 16,384 and 32,767. This gives more resolution with which to set and read the output voltage.
PMBus commands can set the output voltage of a converter, enable the device, determine whether a warning threshold has been exceeded, and report a fault. Some commands are commonly used by many manufacturers, for example the STATUS_WORD command for MPS’s MPM3695-25 power module, shown in Table 1.
Table 1 This STATUS_WORD command provides an indication of various system faults.
But manufacturing-specific commands unique to a given device are also in use. Consider the MPM3695-25’s MFR_CTRL_COMP command. While STATUS_WORD is a common command for many ICs, the constant-on-time compensation loop for the MPM3695-25 is unique to this chip and has its own unique command, as shown in Table 2.
Table 2 This MRF_CTRL_COMP (Addr/Command 0xD0) command is unique to the MPS MPM3695-25.
Memory options for converters with a digital interface
Converters that can communicate over a digital bus also differ in terms of available memory options. These options fall into three memory classifications:
Converter ICs with no permanent memory often do not follow PMBus standards for register definitions, which minimizes overhead for their configuration registers. With just a few optimized registers in such devices, the user’s effort to develop firmware to read and write to these non-standard registers is manageable. However, developing the firmware to interface to a device with dozens of unique configuration and monitoring registers can be daunting, so making the register definition follow PMBus standards can be very valuable.
Adding a digital control and communication feature to the power converter allows better integration of these devices into modern electronic systems by streamlining system design, and increasing flexibility and reliability. However, the best way to reap the benefits of these features is for a design engineer to familiarize themselves with different approaches to converter designs with the digital interface and control, as well as the specific nomenclature of these devices. Understanding the communications standards and the tradeoffs associated with following them will help designers choose the right combination of cost and flexibility in a power converter.
Mark Hagen is a Senior Field Applications Engineer for Monolithic Power Systems.
Oleg Volfson is a Senior Field Applications Engineer for Monolithic Power Systems.