Power Monitoring Functionality at the Hardware-Level

Before looking at how the power monitoring functionality is accessed within the software, it is worth taking a look at the underlying hardware, by which such power monitoring of the system is made possible.

On the NB2DSK01 motherboard, current monitoring is in place for each of the following power lines:

• 1.2V, 1.8V, 2.5V, 3.3V and 5V lines routed to the 'PERIPHERAL BOARD A' connector

• 1.2V, 1.8V, 2.5V, 3.3V and 5V lines routed to the 'PERIPHERAL BOARD B' connector

• 1.2V, 1.8V, 2.5V, 3.3V and 5V lines routed to the 'PERIPHERAL BOARD C' connector

• 3.3V and 5V lines routed to the corresponding connectors for the daughter board

• 3.3V or 5V line selected for use with 'User Header A' connector

• 3.3V or 5V line selected for use with 'User Header B' connector.

A total of 19 monitored power lines.

In terms of hardware, this monitoring is achieved through use of a MAX4372T high-side current sense amplifier (from Maxim). Each power line is routed to its destination connector through a 47mΩ resistor, which serves as the external sense resistance for the associated MAX4372T device. Powered by the NB2DSK01's 9V supply, this device provides a voltage output that is proportional to the voltage across this sense resistance, and has a fixed gain of 20.

Figure 1. Schematic fragment showing the current-sensing circuitry in place for the daughter board
power rails on the NB2DSK01.

For current passing through the 47mΩ resistor, the voltage across the device's input is 47uV/mA. The output of the device is therefore 940uV/mA (applying the x20 gain).

Two low-power, 12-channel, 12-bit ADC devices (MAX1229, from Maxim) collect together the analog voltage outputs from all current sense amplifiers, for transmission back to the NB2DSK01's Xilinx Spartan-3 Controller FPGA (the NB2DSK01 - NanoTalk Controller), over an SPI bus.

Figure 2. Schematic fragment showing the two ADC devices, which pass the subsequently converted data to the NanoTalk Controller over the SPI bus, and subsequent
processing by the firmware.

The output of each ADC, which has an internal reference voltage of 2.5V is:

ADC output = (2.5V/4096) / 940uV = 0.64931mA/bit (=2.65957A full scale)

See Also

For more information on the MAX4372T and MAX1229 devices, refer to the datasheets (MAX4372-MAX4372T.pdf and MAX1227-MAX1231.pdf) available at www.maxim-ic.com.