Many modern electronic systems require some form of current measurement” title=”current measurement”>current measurement method to improve the power consumption, efficiency and reliability of the system, these systems include LED drivers, portable equipment” title=”portable equipment ”> Portable devices and power supplies of various sizes, etc.

In order to maximize the lifetime of high-power LEDs, precise regulation of the LED current is required. Most regulators are voltage regulators with a reference voltage of 2.5V or 1.25V, these regulators can achieve high performance regulation, but unfortunately, when a programmable voltage regulator is used as a current regulator, the current sense” title= “Current Sense” > The voltage drop across the current sense resistor will cause a large power loss because the voltage drop across the resistor is equal to the reference voltage.

For a 3W LED, regardless of whether a linear regulator or a switching regulator is used, an additional 2.5W of power will be dissipated in the current sense resistor, so it will cause a large degree of self-heating, and even in the best case the efficiency can only reach 50%, which makes a big difference to any DC-DC converter solution.

Figure 1 presents a simple and efficient solution to the above problem. Use the current monitor” title=”current monitor”>current monitor to measure the LED current value, and match the reference voltage after amplification, which can reduce the voltage drop on the current detection resistor (generally less than 100mV), so it can greatly save power .

Using a current monitor on the high side of the LED to measure the LED’s current can also improve the overall performance of the system when using a switching regulator. This detection method is no longer ground-referenced, thereby reducing susceptibility to noise. An additional benefit of the high-side current measurement method using a current monitor is that it can be used in buck, boost, and boost-buck configurations.

Simple and efficient current measurement solution with current monitor
Figure 1: LED current regulator.

Power overcurrent detection

  To improve reliability, many power supply products incorporate some form of supply rail overcurrent protection/detection circuitry. For a single output, the current can be measured on the ground side, but this has the disadvantage of interfering with the ground plane. This problem can be overcome by measuring current on a voltage rail, and it is possible to measure multiple rails.

Figure 2 compares the traditional configuration with a configuration with a current monitor. The current monitor is designed to measure the current referenced to the high side and can obtain the bias signal from the voltage rail being monitored. This means they do not require separate power pins and only need two resistors, which can significantly reduce PCB area and component count, and provide higher performance than general-purpose op amps.

Simple and efficient current measurement solution with current monitor
Figure 2: Overcurrent sensing circuit.

Many newer devices have integrated reference sources and comparators to provide a complete overcurrent protection solution. The integration scheme shown in Figure 3 combines the amplifier, reference source, and Transistor into a single device, saving PCB area without disturbing the ground plane.

Simple and efficient current measurement solution with current monitor
Figure 3: Overcurrent protection circuit.

Battery level estimation for portable devices

  More and more portable devices require efficient methods of battery level estimation and extended usage time through advanced system power management.

The traditional battery power measurement methods mostly provide a simple power estimate by measuring the battery voltage” title=”battery voltage”>battery voltage, because a drop in battery power leads to a drop in battery voltage. However, this method is proven in many applications It is less than ideal because the voltage of a battery cell changes continuously during cell discharge and is highly dependent on cell temperature, discharge rate, and the temperature of the cell while charging.

Using only the battery voltage as a measure of battery capacity makes things worse, as a Sharp increase in load current can cause an additional voltage drop across the battery’s effective internal resistance, resulting in erroneous low-voltage measurements. For example, a mobile phone with infrared, bluetooth and camera functions will cause the battery monitoring circuit to falsely give a low battery warning when all functions are turned on, and cause the system to turn off some circuits in order to prolong the battery life.

At high discharge rates (1200mA from a 600mAHr cell), the battery capacity will be less than 20% of nominal, but the discharge curve will be smoother than at low discharge rates. This phenomenon greatly limits the remaining battery measurement accuracy, which means that using the same voltage can produce large errors for the low battery flag and for all temperatures and discharge rates.

By measuring the discharge current, the battery capacity measurement accuracy can be improved, so that the estimation of the remaining capacity can be calculated, the remaining capacity can be displayed more accurately, and the system can also turn off the unused part of the circuit without error to prolong the battery life.” title= “Battery Life” > Battery Life.

An added benefit of measuring discharge current is that it protects the battery from excessive discharge current that can shorten battery life or even damage it.

Laptop batteries typically use dedicated gas pressure chips to test battery life, but in many small cost-sensitive devices, these chips are too expensive and consume a lot of power. For small portable devices such as cell phones, a simple solution is to use a micropower op amp or current monitor and measure the discharge current through a small series resistor. These circuits are often used in conjunction with power management systems that measure battery voltage and temperature, so no additional expensive components are required and no additional PCB area is required.

Micropower current monitors are ideal for these applications because they work in conjunction with one or more Li-Ion/Polymer cells without disturbing the ground connection and can draw power from the voltage rail being monitored. The current monitor requires an extra resistor to set its gain, which provides an easy way to match the required dynamic range using one component in multiple systems. The components that need to be added in Figure 4 are a current monitor, a low-value series current sense resistor, and a gain-setting resistor.


Figure 4: Cost-effective micropower fuel gauge circuit.

In conclusion, the current monitor provides a simple and efficient solution for current measurement. The current measurement can be achieved by adding a small resistor in series with very little voltage drop and power dissipation across the resistive load. In most applications this approach can not only improve system performance, but also reduce overall size.

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