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Posted on 01 November 2019

Dedicated Current Monitors Boost System Performance

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To improve power dissipation, efficiency and reliability, electronic systems are increasingly depending on some form of continuous current measurement. Compared to alternative discrete and integrated solutions, dedicated current monitor ICs provide a simple and yet very accurate means of current measurement for a broad range of applications.

By Simon Ramsdale, Standard ICs Marketing Manager, Zetex Semiconductors

 

In all of the applications reviewed by this article standard current monitors provide a simple and cost effective solution to current measurement. The currents are measured by the addition of a small value resistor in series with the load which causes minimal voltage drop and power dissipation. In most applications they provide an increase in performance with a reduction in overall footprint.

Image 1

LED driving

To maximize the lifetime of high power LEDs accurate regulation of the LED current is required. Most regulators however are voltage regulators and use a 2.5V or 1.25V reference to maintain high performance regulation. Unfortunately when programmable voltage regulators are used as current regulators the voltage drop across the current sensing resistors produces too large a power loss – as the voltage drop across the resistor is equal to the reference voltage.

So for a 3W LED an additional 2.5W would be dissipated in the current sensing resistor – be it a linear regulator or a switching regulator. This creates large levels of self heating and reduces efficiency to be at best of the order of 50% - having a major negative impact on any dc-dc converter solution.

Figure 1 shows a simple and cost effective solution to the problem; by using a current monitor to measure the LED current and amplify it to match the reference voltage, the voltage drop across the current sensing resistor is reduced, typically less than 100mV. This provides great power savings.

LED current regulation

When used with switching regulators the overall performance and versatility can be improved by using a current monitor on the high side of the LED to measure the LED’s current. Since sensing is no longer ground referred, noise susceptibility is reduced. Another benefit of high-side current measurement using current monitors is that it can be used in buck, boost and boost as buck configurations.

Power supply over-current measurement

For increased reliability many power supplies incorporate some form of over-current protection/sensing of supply rails.

For single outputs the current can be measured on the ground side but this has the disadvantage of disturbing the ground plane. This can be overcome by measuring the current on the rail itself and allows multiple rails to be measured. While there are numerous op amps capable of measuring the current referred to ground, most cost effective op amps are either not capable of measuring a signal referred to their supplies or their power supply range is not great enough to allow them to be used in these applications.

Figure 2 compares the traditional circuit configuration to that using a current monitor. Current monitors have specifically targeted the measurement of high side referred currents and derive their bias from the rails being monitored.

Overcurrent sensing

This means that they do not require a separate supply pin and require only 2 resistors. This allows them to substantially reduce PCB area and component count and improve performance over general purpose op amps.

The more recent current monitoring devices have integrated a reference and comparator providing an integrated over-current protection solution. As shown in Figure 3 this integration brings the amplifiers, references and transistor into one device, so saving PCB area and avoiding disruption of the ground plane.

Overcurrent protection

Portable equipment battery life estimation

A growing number of portable applications are demanding cost effective improvements in battery life estimation (‘gas gauging’) and an increase in battery lifetime through advanced system power management.

Traditional battery capacity measurement has largely relied on the measurement of battery voltage to give a simple estimation of battery life - since a decline in battery charge results in a decline in battery voltage. However, this is proving to be inadequate in many applications due to the cell voltage continually changing during the discharge of the cell and is very dependent on the temperature of the cell, the discharge rate and the temperature at which the cell was charged.

Using battery voltage alone as the measure for battery capacity can be made worse by false low battery readings being caused by large increases in load current causing extra voltage to be dropped across the effective battery impedance. For example, a mobile phone with IrDA, Bluetooth connectivity and a camera flash all active at the same time could confuse the battery monitoring circuitry into giving a low battery warning. This can cause the system to switch off certain functions to increase battery life. Potentially the function that was demanding the increase in current from the battery!

For very high discharge rates (1200mA from a 600mAHr cell) the battery life can be 20% lower than nominal but has a much softer discharge knee than that occurring at very light discharge rates. This phenomenon greatly limits the accuracy of the measurement of the battery life remaining and means that using the same voltage for the low battery flag, for all temperature and discharge rates, can produce very large errors.

The performance and accuracy of battery capacity can be increased by measuring the discharge current. This enables an estimation of remaining charge to be calculated which can be used to display remaining battery capacity, as well as enabling the system to switch off parts of the system not being used in order to improve battery life.

A further advantage of measuring discharge current is that it can be used to protect the battery from too large a discharge current which could shorten battery life or even damage the battery.

Notebook computer batteries have used dedicated gas gauge ICs to measure battery life but in many smaller cost sensitive applications these ICs have proven to be too expensive and consume too much power.

A simple solution for smaller portable equipment such as mobile phones is to use a micro-power op amp or current monitor to measure the discharge current via a small series resistor.

They will normally be used in conjunction with the existing power management system that measures battery voltage and temperature, thereby removing the need for additional expensive components and an increase in PCB area.

Micro-power current monitors are well suited to these applications as they have the ability to work with one or multiple Li-Ion/polymer cells and do not interfere with the ground connections and will derive their power from the battery rail being monitored. A current output current monitor uses one external resistor to set its gain, providing a simple means by which one component can be used in multiple systems to match the dynamic range required. In Figure 4 the only additional components required are the current monitor, a low value series current sensing resistor and a gain setting resistor.

Cost effective micro-power gas gauge

 

 

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