Power Management IC Digitally Monitors and Controls 8 Power Supplies

Introduction

Today’s high reliability systems require complex digital power management solutions to sequence, monitor, monitor, and margin a large number of voltage rails. Indeed, it is not uncommon today for an application board to have dozens of voltage rails, each with its own unique requirements. Typically, power solutions for these systems require multiple discrete devices controlled by an FPGA or microcontroller to be spread around the board to sequence, monitor, monitor, and margin the array of supplies. In such solutions, developing the necessary firmware consumes a great deal of time, and the tendency to underestimate the complexity of this task and the cycle to complete it is common.

The LTC®2978 8-channel digital power manager with EEPROM provides power system designers with an integrated modular solution that reduces debugging time and effort compared to microcontroller solutions. The LTC2978 can sequence, monitor, supervise, margin and trim up to 8 power supplies. Multiple LTC2978s can be easily cascaded using a single-wire shared clock bus and one or more bidirectional fault pins (Figure 1 shows a typical application).

Figure 1: Typical application circuit for LTC2978

In addition, the LTC2978 uses a protected non-volatile memory to record system voltage and fault information when a critical system fault occurs. By saving critical system data in non-volatile memory, users can find faulty voltage rails and isolate the cause of board failures during system development, test debugging or failure analysis.

The LTC2978 uses the industry-standard PMBus command protocol to simplify firmware development. However, the most important feature of the LTC2978 is its precision integrated reference and 15-bit ΔΣ ADC that provide ±0.25% absolute accuracy when measuring or adjusting supply voltages.

The key to the success of the LTC2978 is Linear Technology's LTpowerPlay ^TM , a free, downloadable graphical PC interface that facilitates interaction with the device during the design and test process. LTpowerPlay provides an easy-to-use yet powerful configuration tool for using the features of the LTC2978. In addition, the LTpowerPlay design tool will also support future digital power management devices from Linear Technology.

Improving Manufacturing Yields with Accurate Margin Testing

Margin testing of system voltages is an effective method for rooting out early failures in high-reliability systems. Typically, voltage margining is at least ±5% to ensure that the system under test is rugged enough to operate reliably in the field. However, depending on the system tolerance, this approach can result in excessive test fallouts. Many of these test failures could have been avoided if the supply voltage tolerances described above were tighter.

The LTC2978 provides a relatively simple yet powerful solution to this problem with its high precision reference, multiplexed 15-bit ADC, 8 margin DACs and integrated servo algorithm. By simply writing an I2C command to trim or margin to a specific voltage, the LTC2978 regulates the DC/DC point-of-load converter to provide a commanded output voltage with an absolute accuracy of ±0.25% within specified software and hardware limits.

The margin DAC output is connected to the feedback node or trim input of the DC/DC POL converter through a resistor. The value of this resistor sets the limit of the margining range of the output voltage, which is an important limiting factor for software-controlled power supplies. Another significant advantage of the 10-bit margin DAC is its ability to achieve very fine resolution when margining voltages. This makes it possible to extract useful data from failure testing, as opposed to having a trash can full of failed boards without a thorough understanding of them.

Flexible Power Sequencing and Fault Management

Many traditional power sequencing solutions rely on comparators and daisy-chain PCB wiring. While this approach is relatively easy to implement for a small number of supplies, it quickly becomes more complex as the number of voltage rails increases, and it is relatively inflexible in the face of specification changes. It is also extremely difficult to implement shutdown sequencing using this approach.

The LTC2978 makes sequencing easy, regardless of the number of supplies. Using a time-based algorithm, users can dynamically sequence on and off in any order (see Figure 2). Sequencing of multiple LTC2978s can also be accomplished using a 1-wire shared clock bus and one or more bidirectional fault pins. This approach greatly simplifies system design because the channels can be sequenced in any order, regardless of which LTC2978 provides control. Moreover, more LTC2978s can be added later without worrying about system constraints such as power limitations on daughter card connector pins.

Figure 2: Sequencing and margining example

On-sequencing can be triggered by a variety of conditions. For example, when the intermediate bus voltage of the downstream DC/DC POL converter exceeds a certain turn-on voltage, the LTC2978 will perform automatic sequencing. Alternatively, on-sequencing can be initiated by the rising or falling edge of the control pin input. Sequencing can also be initiated using a simple I2C ^command . The LTC2978 supports any combination of these conditions.

The bidirectional fault pins can be used for various fault response dependencies between channels. For example, in the event of a short circuit, the turn-on sequencing of one or more channels can be interrupted. When a power rail is powered up, the undervoltage monitor function is enabled (the overvoltage function is always enabled). The overvoltage and undervoltage thresholds and response times of the voltage monitors are programmable. In addition, the input voltage and temperature are monitored. If any of these parameters exceeds its "over/under" limit, the customer can choose from a wide range of fault responses. Examples include immediate latch-off, debounce latch-off, and latch-off with retry.

The device includes an integrated watchdog timer for monitoring an external microcontroller. Two timeout intervals are available: an initial watchdog interval and a subsequent interval. This allows a longer timeout interval to be specified for the microcontroller immediately after the power good signal is asserted. If a watchdog fault occurs, the LTC2978 can be configured to reset the microcontroller within a predetermined time and then reassert the power good output.

Multifaceted Telemetry

The LTC2978 has a variety of remote sampling data prepared in its registers. The multiplexed 15-bit ΔΣ ADC monitors the input and output voltages and on-chip temperature and stores the minimum and maximum values ​​of all voltage and temperature readings. In addition, the ADC inputs for odd-numbered output channels can be reconfigured to measure the sense resistor voltage. In this mode, the ΔΣ ADC can resolve voltages as low as 15.3μV, which is valuable when trying to measure current using an inductor DCR circuit.

Although the LTC2978 can be powered directly from a 3.0V to 3.6V supply, the ADC can accept input voltages up to 6V - without worrying about body diodes or highly unstable standby supply voltages. And, using its internal regulator, the LTC2978 can also operate from a 4.5V to 15V input supply. A separate high voltage (15V maximum) sense input is provided to measure the input supply voltage of the DC/DC POL converter controlled by the LTC2978.

Black Box Data Logger

When a channel is disabled in response to a fault, the LTC2978’s data log can be dumped to a protected EEPROM. This 255-byte block of data is saved in NVM (non-volatile memory) until it is cleared by an I2C command. The block contains output and input voltage and temperature data for 500ms before the fault, along with the corresponding minimum and maximum values. In addition, the status register value and total uptime since the last system reset are also stored in the data log.

The contents of the data log can be viewed using the LTpowerPlay design tool. In this way, the LTC2978 provides complete instantaneous status information of the power system just before a critical fault occurs, allowing the cause of the fault to be well identified after the fault occurs. This is an invaluable feature for debugging first prototypes and field failures.

Graphical User Interface and PMBus

Linear Technology's easy-to-use PC-based LTpowerPlay graphical user interface (GUI) allows users to configure and program the LTC2978 online through a USB to PMBus interface board (see Figure 3). The GUI is free and downloadable, eliminating much of the code writing work during the development process and speeding up time to market by allowing designers to configure all device parameters within an intuitive framework. Once the device configuration is selected, designers can save the parameters in a file and send it to a contract manufacturer or authorized distributor along with a BPM Micro programmer for pre-programming.

Figure 3: LTpowerPlay interactive GUI

The LTC2978 uses the industry-standard PMBus interface protocol, which is an extension of the ^I2C -compatible SMBus standard. PMBus is an open and widely adopted standard that clearly defines a digital power management protocol for individual DC/DC POL converters. The LTC2978 supports a large number of PMBus commands. In addition, the device also has some DC/DC converter manufacturer-specific commands to keep complexity low and versatility high.

Conclusion

With its unprecedented parametric accuracy, rich feature set and modular architecture, the LTC2978 is an ideal solution for managing a wide range of DC/DC POL converters in a variety of end markets, from medical, industrial, computer, video and communications to consumer.

The industry-standard PMBus interface, powerful LTpowerPlay GUI and integrated EEPROM enable customization of the LTC2978 for all applications. Designers can configure the device using a PC-based graphical interface and upload the device configuration to Linear Technology, which can then provide ready-to-use pre-programmed devices customized for specific applications.

Other features of the LTC2978 include an integrated high precision reference, a multiplexed 15-bit ΔΣ ADC, eight 10-bit voltage buffered DACs, eight overvoltage and undervoltage 10-bit voltage supervisors with programmable thresholds and response times, and an integrated EEPROM for storing configuration parameters and fault log information. The LTC2978 is available in a 64-lead 9mm x 9mm QFN package.