How to simplify power sequencing

The ideal approach to multi-rail power design is to use only one IC from start to finish in a design, with no need to change the wiring throughout the life of the product. The IC autonomously monitors and sequences multiple power rails and works with other ICs to seamlessly monitor multiple power regulators in the system and provide fault and reset management. When the system is connected to the I ² C bus, designers can use powerful PC-based software to configure, visualize and debug the system in real time.

The LTC2937 fits the bill. It is a 6-channel voltage sequencer and high accuracy supervisor with EEPROM. Each of the 6 channels has two dedicated comparators that accurately monitor overvoltage and undervoltage conditions with ±0.75% accuracy. The comparator thresholds are individually programmable from 0.2V to 6V with 8-bit resolution. The comparators are fast, with 10μs peak-suppressed propagation delays. Each sequencer channel has an enable output that controls an external regulator or the gate of a pass FET. All aspects of supervisor voltage and sequencer timing are individually configurable, including up- and down-sequence order, sequence timing parameters, and fault responses. The built-in EEPROM makes the device fully autonomous, powering up in the correct state to control the system. In addition, multiple LTC2937s can work together to autonomously sequence up to 300 supplies in a system, all using a single communications bus.

The LTC2937's autonomous fault response behavior and debug registers allow control, visibility and management of power supply faults. The LTC2937 automatically detects fault conditions and can power down the system in a coordinated manner. The device can remain powered down or attempt to re-sequence supplies after a fault. In systems with a microcontroller and I2C/SMBus, the LTC2937 provides detailed information about the type and cause of the fault and the status of the system. The microcontroller can make decisions on how to respond or allow the LTC2937 to respond on its own.

Figure 1: LTC2937 sequencing six supplies

Table 1: Programmable 6-Channel Sequencer and Supervisor with EEPROM

A power cycle has three operational steps: power-up sequencing, monitoring, and power-down sequencing. Figure 2 shows these phases for a typical system.

• During power-up sequencing , each power supply must wait and then power up to the correct voltage within a specified time;

• During the monitoring phase, each power supply must remain within specified overvoltage and undervoltage limits;

• During power-off sequencing , each power supply must wait (often in a different order than the power-on sequencing) and then power off within a set time;

At any point in time, something could go wrong, causing a failure in the system. The design challenge is to design a system where all of these steps and all of the variables are easily configurable but must be carefully controlled.

Figure 2: Power supply sequencing waveforms

When the ON input transitions to active, power-up sequencing begins. The LTC2937 powers up one by one in an upward sequencing order, starting each supply in turn and monitoring to ensure that the supply voltage rises above the programmed threshold before the specified time. Any supply that fails to meet the programmed time requirement will trigger a sequencing fault.

Providing a sequencing position clock is a unique advantage of the LTC2937. Each channel is assigned a sequencing position (1 to 1023) and receives a start signal when the LTC2937 counts to a given sequencing position number. The channel with sequencing position 1 always starts before the channel with sequencing position 2. If the system specification is changed to require the two channels to be sequenced in a different order, the sequencing positions can be swapped, and the second channel is powered up when sequencing position 1 is counted, and the first channel is powered up when sequencing position 2 is counted. Multiple LTC2937s can share sequencing position information so that sequencing position N appears simultaneously to all LTC2937 chips, and channels controlled by different chips can participate in the same sequencing (see Figure 3).

Figure 3: Typical connections for multiple LTC2937s

When the last channel powers up and crosses its undervoltage threshold, the monitoring phase begins. During the monitoring phase, the LTC2937 uses its high accuracy comparators to continuously monitor the voltage of each input to see if it exceeds the overvoltage and undervoltage thresholds. The device ignores small disturbances on the input signal and triggers only when the voltage exceeds the threshold by a sufficient magnitude and for a sufficient period of time. When the LTC2937 detects a fault, it responds immediately according to the programmed supervisor fault response behavior. In a typical case, the device shuts down all supplies at the same time, asserts RESETB to the system, and then attempts to power back on in a normal startup sequence. This prevents a power supply from powering one part of the system while leaving others unpowered, or prevents the system from performing inconsistent fault recovery after a fault. Multiple LTC2937s in a system can share fault status information and respond to each other's faults, maintaining complete consistency between cooperating channels when recovering from a fault. The LTC2937 offers a myriad of programmable fault response behaviors to meet the needs of many different system configurations.

When the ON input transitions low, the power-off sequencing phase begins. The sequencing position clock begins counting again to power down the supplies, but all power-off sequencing parameters are independent of the power-on sequencing parameters. Channels can be sequenced down in any order, and multiple LTC2937 chips coordinate the sequencing of all controlled supplies. When sequencing down, each supply must fall below its discharge threshold within a specified time limit or a sequencing fault will be triggered. The LTC2937 can pull down the supply voltage with an optional current source to effectively discharge slow-switching supplies.

The sequencing position clock enforces an event-based sequencing order, with each event waiting for the previous event to occur before continuing. The LTC2937 also allows time-based sequencing, which can be used in systems where power rails are started at predetermined points in time. Reconfigurable registers can operate in both time-based and event-based sequencing modes.

The LTC2937 has an extensive and powerful set of registers, and controlling these registers is simple. The LTpowerPlay graphical user interface (GUI) displays all information from the status registers and debug registers in one simple interface. The GUI communicates over I2C/SMBus with any of ADI's power system management ICs, including the LTC2937. Configuring one or more LTC2937s is as simple as a few clicks of the mouse.

LTpowerPlay saves settings on the PC and can write settings to the LTC2937’s EEPROM. The GUI also displays all debug information for system failures. LTpowerPlay shows when any supply has experienced an overvoltage or undervoltage, or if a supply failed to sequence. After a failure, the GUI allows complete control of the system restart. At every stage of the design, start-up, configuration, tuning, and operation, LTpowerPlay is an indispensable window into system performance.

Figure 4: The LTpowerPlay graphical user interface (GUI) displays all information in the status and debug registers in one convenient interface. Configuring one or more LTC2937s is as easy as a few mouse clicks. LTpowerPlay saves settings on the PC and can write settings to the LTC2937’s EEPROM.

The LTC2937 simplifies power system sequencing and supervision. The device takes up very little board space to complete a complete system. The LTC2937 is flexible, reconfigurable, and autonomous through EEPROM registers. The device can operate independently or be used with other chips in a larger system to seamlessly coordinate the operation of up to 300 power supplies.