i.MX Power Profiling System: Smart Current Sensor and Aggregator Shield

Document created by David G Dicarlo Employee on May 11, 2018Last modified by David G Dicarlo Employee on May 14, 2018
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The i.MX Power Profiler system consists of one to fourteen "smart" current sensors, an aggregator shield, and a Kinetis FRDM board (the FRDM-KL25 has been used in prototyping but the FRDM-K64F and FRDM-K66F should also be fully compatible). One of the biggest improvements of this system over its preceeding dual-range measurement system is that the microcontroller on each sensor board allows near-simultaneous measurement of all instrumented rails on a board. The dual range profiler has only a single MCU for all sensors, so only one measurement can be made at a time. 

 

It is intended to be used to instrument one to fourteen rails of a target i.MX appliation board. Ideally, the target board will have been designed with a matching/mating power sense footprint for each rail to be measured. 

Each smart sensor can sense current in three ranges with three current sense amplifiers. They are "smart" because each sensor board has a Kinetis KL05Z on it to control the switching FETs and to digitize the analog signals (the sense amplifier outputs and the target's power supply rail voltage). A 1% voltage regulator on each smart sensor provides a good voltage reference right next to the KL05Z to ensure better ADC accuracy. Each smart sensor board communicates via I2C.

 

The aggregator shield has three I2C bus extenders (PCA9518) which essentially provide a dedicated I2C bus for each of the connected smart sensors. The FRDM board's I2C is also connected to one of the bus extenders ports. Individual GPIO lines are routed to each smart sensor's connected along with a ganged reset and trigger line for all of the connected smart sensors. A boost regulator generates almost 12V from the FRDM board's 5V supply, which is used for all the switching FETs on the smart sensor boards. The FRDM board's 5V rail is also routed to each smart sensor, which is regulated down to 3.3V locally on each connected smart sensor.

 

Here is a photo of the very first prototypes after moving to 10-pin 0.05" spaced headers and ribbon cables instead of FFC:

 

The smart sensor is intended to mate with through-hole current sense tap points on the target i.MX application board. Three holes spaced at 0.05" each. When not instrumented with sensor, a short needs to be placed across the outer two pins so that the board will function normally. The through hole connections provide physical protection to the target board, keeping traces from getting ripped off. The ground connection in the center provides a reference for meauring the rail voltage on the target board. A partial layout example of the implementation of the current sense footprint is below, where two 0805 shorting resistors in parallel are placed on each side of the holes. The top trace connects to the regulator output and the bottom to the load, usually an i.MX power supply rail.

 

To include the current sense footprint into a board during the design phase, it should be configured as in the following partial schematic: 

Every effort should be made to place the feedback on the i.MX side of the sense points so that the regulator compensates for the additional series resistance of the smart sensor, which effectively eliminates the additional series resistance the smart sensor adds. 

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