What is circuit board power management?
Circuit board power management often is associated with different aspects of powering a circuit board. Some of the common associations are:
- Selection of various DC-DC converters to power the board
- Power Supply sequencing/tracking
- Voltage monitoring
- All of the above!
In this article, power management is defined simply as the management of all power sources (including DC-DC Converter, LDOs, etc.) on the circuit board. Power management includes the following functions:
- Managing circuit board DC-DC controllers - e.g. hot-swap, soft start, sequencing, tracking, margining and trimming
- Generation of all relevant power supply status and control logic signals; e.g. reset signal generation, supply fault indication (supervision) and voltage measurement
Figure 1 illustrates typical power management functions on a circuit board using a CPU or a microprocessor
Figure 1 - Typical Power Management Functions on a Circuit Board
The Hot-swap/ Soft-start Control Function is used to limit the current inrush to reduce the start up load to the power supply. This is an important function in circuit boards that plug into a live backplane.
The Power Supply Sequencing and Tracking function controls how multiple power sources are turned on/off while meeting the sequencing requirements of all the devices on a circuit board.
All supply voltages are monitored for faults (both over voltage and under voltage) to warn the processor of an impending power supply failure. This function also is referred to as the Supervisory Function.
The Reset Generation Function provides a reliable start-up for the processor upon power on. Some processors require the reset signal to be active for an extended period of time after all power supplies to the processor are stable. This is also called Reset Pulse Stretching. The function of a reset generator is to hold the processor in the reset mode during power supply fault conditions in order to prevent inadvertent on-board flash corruption.
Limitations of the traditional power management solution
Traditionally, each of the power management functions on a circuit board is implemented using individual single function ICs. These ICs have a separate part number for each power supply voltage combination. Consequently, hundreds of single function IC part numbers are available, from various vendors, to address multiple power supply management needs.
For example, to select a Reset Generator IC part number, the following information must be provided:
- Number of supply voltages that the Reset Generator IC will monitor
- Combination of supply voltages (3.3,2.5,1.2 or 3.3,2.5, 1.8, etc.)
- % Fault detection voltage (3.3V-5%, 3.3V-10%, etc)
- Accuracy (3%, 2%, 1.5%)
- Reset pulse extension capability programmed with an additional capacitor
- Manual reset input
In order to address all the possible permutations of these variables, there can be hundreds of part numbers just for a Reset Generator IC...and those from just one of several vendors. And if during the design process, as is likely, the engineer needs to add another voltage to monitor, then yet an additional and different part number has to be selected. Similarly, there are many single function ICs with part numbers for each variation of the single functions: hot-swap controller, supply sequencer and voltage supervisor/ detector. A system with multiple boards will require different sets of these singe function ICs for each board, increasing the Bill of Material (BOM) cost.
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