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Building blocks for power supervision with SOC

01 Aug 2013  | Archana Yariagadda

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The Pgood signals of the hardware fault detector and the power monitor can be combined to obtain the Pgood signal for the voltage sequencer, as shown in figure 4.

Figure 4: Adding hardware fault detection.

All the blocks discussed so far were to turn ON/OFF the power lines if they deviate from the desired voltage/current. There are situations, such as debugging or adapting to new requirements, when the voltage output of the DC-DC convertor needs to be adjusted to a new level. This feature is called Trimming/Margining (TM) of rails. The trim function is used in systems where adjustment is required in the field after production. This function might be used in real-time after the product is manufactured. Margining is a feature that is used to test the system itself, during development or before production. It is a feature used to test, characterize, or validate the system performance under different voltage limits.

Implementing functions
These functions are implemented by driving the Vadj (voltage adjust or voltage feedback) of the DC-DC convertor up or down based on the requirement. The current value of voltage is measured by the power monitor and the new value is adjusted by the trim/margin circuit. The trimming voltage output is obtained by a PWM-DAC, i.e a PWM output filtered with a low-pass RC circuit (external). For example, say we want the Vout to be lower than its current value, we can increase the PWM duty cycle, thereby increasing the Vadj value. This will cause the DC-DC convertor to lower its output voltage Vout, based on its Vadj pin voltage. This functionality can be achieved by using DACs if available, or can be achieved by using PMW and adding an RC filter outside, as shown in figure 5. The accuracy achieved by implementing this feature with a PWM-DAC is 0.60%.

Faults that are detected by the system can be made available by storing them in non-volatile memory. In this example, EEPROM is used to save all the faults and Flash is used to save user settings.

All the data and settings discussed thus far can be made accessible and configurable to a host device through different forms of communication. The preferred protocol in the power supervision field is the Power Monitoring Bus (PMBus) protocol. PMBus is a system management protocol communication built on the I2C communication interface. The PMBus can be used to obtain information about the status of different power lines, the settings on the DC-DC convertors and many other detailed instructions that are pre-defined by the protocol. For additional commands that are not part of the pre-defined PMBus protocol, additional custom commands can be added. PMBus can be used to read all the status and component parameters, as well as overwrite any of the other component parameters. This makes the real-time system upgrade easy for the customer, with any reprogramming. In this example, the PMBus protocol is build over the existing I2C block, as shown in figure 6. In addition to the PMBus component, a PMBus host emulator GUI is provided with the complete power supervision solution to support the design/debug of the system.

Figure 5: Adding trim and margining.

Figure 6: Adding PMBus communication to the power supervision system.

All these blocks are available as components, as shown in figure 7.

Figure 7: These blocks are available as components.

This article shows the building blocks that are required to build a power supervisor system. Depending on the end application, some or all of the blocks might be required. You can build your own power supervisor system, with the SoC of your choice with the blocks mentioned in this article. Advanced features available when using a SoC-based system include temperature measurement, fan control, Single Event Upset (SEU) handling, and bootloader.

About the author
Archana Yarlagadda has her Masters in Electrical Engineering from University of Tennessee, Knoxville. She is Field Applications Engineer Staff at Cypress in Bay area. She is interested in analogue and mixed signal designs and has developed many analogue sensor interfaces with PSoC.

To download the PDF version of this article, click here.

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