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April 2009
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Differential Sensors Provide Superior Accuracy for Supervisory and Trimming ApplicationsDifferential signaling has emerged as a mainstream connection method for data communication channels because of several benefits, including:
Differential signaling (or sensing) can also be used for accurate PC board voltage supervision and power supply trimming. Lattice has employed differential input buffers within the analog monitors of the Power Manager II family to minimize measurement error in noisy environments and mask differences in ground potential. This accuracy is imperitive for the voltage tolerance requirements of processors (microprocessor, DSP, FPGA, or ASIC) using the trim mechanisim of power supplies. Voltage Supervision Using Differential SensingVoltage supervisor ICs are designed to detect supply rail failures and issue a reset or interrupt so the processor can gracefully initiate a safe shut down. For reliable fault detection the designer must account for the power supply output voltage range, the processor’s supply voltage tolerance, and the accuracy of the supervisor. Traditional supervisor ICs that use single-ended inputs rate voltage threshold accuracy as much as +/- 3%. This variability forces designers to lower the reliable range of system operation to account for the supervisor error. Variation in the PCB ground plane also contributes to voltage supervisor IC monitoring error. For example, if the ground voltage difference between the supervisor and a processor at another location on the PC board is 25mV and the core rail monitored is 1V, the result is a 2.5% error. No matter how accurate the supervisor is, it cannot compensate for the ground voltage difference. This implies dedicating a supervisor to each processor load to compensate for the ground difference. Figure 1 illustrates a supervisor solution with the ispPAC-POWR1220AT8 Power Manager II device. Its analog monitors use differential sensing that provides VMON accuracy 0.2% typical / 0.7% maximum accuracy of any voltage trip-point. 
The ground voltage difference becomes common mode voltage for both the VMON and VMONGS pins. The input differential amplifier cancels out the common mode voltage such that the VMON accuracy is guaranteed up to ground voltage difference of -200mV to +300mV. This allows the designer to integrate multiple supervisor ICs with a single, centrally-placed Power Manager II. Power supply trimming circuits can also employ the differential sensing technique to improve the accuracy and performance of DC-DC converters. About Power Manager II Voltage Monitor BlocksThe Power Manager II provides differential input comparator blocks that are independently programmable with a variety of trip points (see figure below). The VMON input pins themselves are a pair of differential inputs to minimize measurement error in a noisy environment. The differential pair for each VMON allows the designer to monitor the voltage at a remote point on the board with a pair of pins, one the positive VMON and the other the sense line or ground sense (VMONGS). In Figure 1, the ispPAC-POWR1220AT8 Power Manager II controls and monitors the 1.5V supply for a 1.5V Load Side Circuit. 
Note the remote sense line and location of the VMON1 voltage monitor line. The physical location on the net where the voltage is to be measured is critical if there is high current for the supply rail or noise on the board. Each VMON has a ground sense line is connected in all cases. The ground sense lines can have a maximum voltage of -200mV to +300mV with respect to the ground of the Power Manager II. By allowing for this variability in the ground sense line allows the Power Manager II to be placed in a central location on the board and minimize the number of supervisor ICs required to monitor processor supply rails. The Power Manager II provides additional features that make it applicable far beyond the traditional voltage supervisory role. 368 trip points can be programmed over a range of .664V to 5.734V which allows the device to be adapted to a variety of supply rail requirements and detect over-voltage or under-voltage conditions. A built-in 10-bit ADC block monitors the VMON input value and makes the data available as a status register accessed via an I2C slave interface. This allows an external microprocessor running supervisory firmware to manage the power system based on actual supply rail values. Improved Power Supply TrimmingCalibration of power supplies is typically accomplished by a trimming control circuit to vary the power supply output voltage up or down depending on the load demands of the PC board. For simple designs a manual potentiometer (pot) and a screwdriver might be used to dial in the correct set point then glued into place by the manufacturer. The downside to this approach is that manual pot settings contribute to PC board fabrication cost and once set can’t compensate for temperature effects and aging components that can cause the supply to drift. More sophisticated designs use programmable digital resistors and a microcontroller to automatically calibrate the supply. This approach adds overhead to the power management firmware and depends on the accuracy of the measurement circuitry. Ground voltage variations will contribute to accuracy error in the same way they did in the voltage supervision scenario. To improve the accuracy and lower cost of trimming solutions, Lattice uses differential analog monitors and an embedded analog to digital converter (ADC) block in the Power Manager II family. Both the ispPAC-POWR1220AT8 and ispPAC-POWR6AT6 provide trim cells that maintain the output voltage of the DC-DC converter by periodically measuring the supply output voltage and applying corrective current to the power supply feedback node. Power Manager II devices reduce output voltage error to <1%. 
The Power Manager II digital closed loop trim feature continuously compares the voltage set point of a given power supply with the output of the on-chip ADC that is monitoring that power supply voltage. The error signal that results automatically increases or decreases the DAC voltage, maintaining the power supply voltage at a constant value. Further, an external microcontroller can monitor the power supply voltage through the on-chip ADC and directly control the corresponding DAC through the I2C interface. The trim cell also can store four different DAC code settings or configurations that can be selected using hardware pins dedicated to voltage profile selection. ConclusionTo increase PC board reliability, voltage supervisory circuitry must be highly accurate. Traditional voltage supervisor ICs, while accurate, must be placed close to the each load circuit to compensate for ground voltage effects. With differential sensing techniques you can monitor all the supplies on the board accurately from an integrated central location. With the Power Manager II device family, PCB designers and system engineers now have a reliable way to to measure load circuits distributed across the board. Differential sensing by the Power Manager II device family provides highly accurate (0.2% typical / 0.7% maximum) voltage supervision and DC/DC trim circuits (<1%) that traditional discrete ICs cannot achieve with single-ended inputs. By integrating multiple supervisor devices and DC-DC trim circuits into a central Power Manager II helps reduce component count and PC board fabrication cost. To Learn MoreThe following application notes cover voltage monitoring topics as well as techniques to extend the sensing ability of the VMON inputs of Power Manager and Power Manager II family devices:
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