Using current monitors to accurately measure system power parameters - Embedded.com

Using current monitors to accurately measure system power parameters

Many modern electronic systems now require some form of currentmeasurement to improve power dissipation, efficiency and reliability.These systems range from LED driving to portable equipment and powersupplies of all sizes.

To maximize high-power LEDs' lifetime, accurate regulation of theLED current is required. Most regulators, however, are voltageregulators using 2.5V or 1.25V references to maintain high-performanceregulation.

Unfortunately, when programmable voltage regulators are used ascurrent regulators, the voltage drop across the currentsensingresistors gives too large a power loss, as the voltage drop across theresistor is equal to the reference voltage.

So, for a 3W LED, an additional 2.5W would be dissipated in thecurrent-sensing resistor – be it a linear or switching regulator. Thiscreates large levels of self-heating and reduces efficiency to 50percent, a major impact on any DC/DC converter solution.

Figure 1 below shows asimple and cost-effective solution to this problem. Using a currentmonitor to measure the LED current and to amplify it to match thereference voltage reduces the voltage drop across the current sensingresistor, typically less than 100mV. This provides great power savings.

Figure1: Using a current monitor to measure the LED current and to amplify itto match the reference voltage reduces the voltage drop across thecurrent sensing resistor, typically less than 100mV.

When used with switching regulators, the overall performance andversatility can be improved by using a current monitor on the high sideof the LED to measure the LED's current.

This moves the sensing from being ground-referred, reducing noisesusceptibility. Another benefit of high-side current measurement usingcurrent monitors is that it can be used in buck-boost and boost as buckconfigurations.

Overcurrent situation
For increased reliability, many power supplies incorporate some form ofovercurrent protection/ sensing of supply rails. For single outputs,the current can be measured on the ground side. But this has thedisadvantage of disturbing the ground plane.

This can be overcome by measuring the current on the rail itself,thus allowing multiple rails to be measured. Many op amps can measurethe current referred to ground.

However, most cost-effective op amps are either not capable ofmeasuring a signal referred to their supplies or their power-supplyrange is not great enough to allow them to be used in theseapplications. Figure 2 below compares the traditional configurationwith that of using a current monitor.

Figure2: Current monitors have specifically targeted measuring high-sidereferred currents and derive their bias from the rails being monitored.

Current monitors have specifically targeted measuring high-sidereferred currents and derive their bias from the rails being monitored.

This means that they do not require a separate supply pin and thatthey require only two resistors. This allows them to substantiallyreduce PCB area and component count, and to improve performance overgeneral- purpose op amps. More recent devices have integrated areference and comparator, providing an integrated overcurrentprotection solution.

In Figure 3 below ,integration condenses the amplifiers, referencesand transistor into one device, thus saving PCB area and not disturbingthe ground.

Figure3: Integration condenses the amplifiers, references and transistor intoone device, thus saving PCB area and not disturbing the ground.

Battery-life estimation
A growing number of portable applications are demanding costeffectiveimprovements in battery- life estimation and increase of active time byadvanced system power management.

Traditional battery capacity measurement has largely relied on themeasurement of battery voltage to give a simple estimation of batterylife, since a decline in battery charge results in a decline in batteryvoltage.

However, this is proving to be inadequate in many applications dueto the cell voltage continually changing during the discharge of thecell.

Moreover, the cell voltage is very dependent on the temperature ofthe cell, the discharge rate and the temperature at which the cell wascharged.

Using battery voltage alone as a measure for battery capacity can bemade worse by false low battery readings, which can be caused by largeincreases in load current. This causes extra voltage to be droppedacross the effective battery impedance.

For example, a mobile phone with IrDA, Bluetooth connectivity anddigital camera with flash could confuse the battery monitoringcircuitry into giving a low battery warning. It can cause the system toswitch off parts of the system – potentially the part that wasdemanding the current increase from the battery – to increase batterylife.

For very high discharge rates (1,200mA from a 600mA-hr cell), thebattery life can be 20 percent lower than nominal but have a muchsofter discharge knee than that occurring at very light dischargerates.

This phenomenon greatly limits the accuracy of the measurement ofbattery life remaining. It means that using the same voltage for thelow battery flag, for all temperature and discharge rates, can producevery large errors.

The performance and accuracy of battery capacity can be increased bymeasuring the discharge current. This enables an estimation ofremaining charge to be calculated, which can be used to displayremaining battery capacity. It also enables the system to switch offsystem parts that are not being used to improve battery life.

A further advantage of measuring discharge current is that it can beused to protect the battery from too large discharge currents, whichcould shorten battery life or even damage the battery.

Notebook computer batteries have used dedicated gas gauge ICs tomeasure battery life. In many smaller cost-sensitive applications,however, these ICs have proven to consume too much power and be tooexpensive.

A simple solution for smaller portable equipment such as mobilephones is to use a micropower op amp or current monitor to measure thedischarge current via a small series resistor. They will normally beused with the existing system's power management system that measuresbattery voltage and temperature, thus removing the need for extraexpensive components and increase in PCB area.

Figure4: The only additional components required are the current monitor, alow-value series current sensing resistor and the gain-setting resistor.

Micropower current monitors are well-suited to these applications,as they can work with one or multiple Li-ion/polymer cells and do notinterfere with the ground connections. Also, they derive their powerfrom the battery rail being monitored.

A current output monitor uses one external resistor to set its gain,providing a simple way of one component to be used in multiple systemsto match the dynamic range required.

In Figure 4 above , the onlyadditional components required are the currentmonitor, a low-value series current-sensing resistor and thegain-setting resistor. In all the examples shown, current monitorsprovide a simple and cost-effective solution to current measurement.

The currents are measured by adding a small resistor value in serieswith the load, which causes minimal voltage drop and power dissipation.In most applications, they provide an increase in performance with areduction in overall footprint.

Simon Ramsdale is Product MarketingManager at Zetex Semiconductorsplc. To read a PDF version of this article go to “ Usecurrent monitorsfor accurate measurement.”

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