Add flexible current limiting function

Add flexible current limiting function

Author: Frederik Dostal, Field Application Engineer, Analog Devices

problem:

Can I easily and accurately limit the current according to the load?

Answer:

Current limiting IC can be used for current limiting.

In some power management applications, the current needs to be accurately limited.Whether you want to protect the power supply (for example, the middleCircuitThe voltage needs overload protection in order to be able to reliably provide power to other system components), or to protect the load that may be damaged by overcurrent in the event of a fault, it is necessary to accurately limit the current.

When looking for a suitable DC-DC point-of-load regulator to meet this requirement, we found that voltage converters with adjustable current limiting functions are rare on the market.Adjustable current limit functionControllerIt is more common in design, and all integrated solutions rarely provide such features. Moreover, the accuracy of the adjustable current limiting function is usually not very high. In addition, the current limiter in the DC-DC converter IC generally only limits the inductor current of the power supply, and does not limit the input or output current. This type of integrated current limiting function is only designed to protect the switching regulator itself from damage under fault conditions. The current limit value is higher than the rated maximum output current, and sometimes the accuracy is relatively low, which is enough to protect the switching regulator, but it is usually not enough to be used as an adjustable current limiter.

To flexibly solve this problem, an adjustable current limiting function can be added through an additional component (such as LTC7003). The accuracy can reach about 15%, depending on the application. The LTC7003 is a high-end N-channel MOSFET static switch driver. Because of its adjustable current-limiting function and current monitoring function, it is very suitable for adding current-limiting function to common DC-DC converters. Figure 2 shows the use of the LTC7003 current limiter to monitor the output current of the ADP2370. ADP2370 is a step-down DC-DC converter.

Generally speaking, high-end current sense amplifiers can also be used to sense current through the power pathresistanceMeasure small pressure drops. They can measure current very accurately. However, for most of them, the allowable voltage difference between the two current sensing connections is very small. If the power supply may be short-circuited due to the load, when using this general-purpose current sense amplifier, the voltage on the sense resistor may soon exceed the allowable range. In this case, it is best to choose a solution approved for use in the power supply, such as LTC7003. According to the design, LTC7003 allows a large voltage difference at the SENS input. When the current reaches the set threshold, the LTC7003 also supports an optional N-channel MOSFET Q1 to interrupt the power path. Figure 3 shows the LTC7003 solution with an external N-channel MOSFET that interrupts the power path when the current reaches the set threshold.

Through the IMON output, a voltage proportional to the current flowing through the sense resistor can be provided. This voltage is in terms of system grounding, which is equivalent to multiplying the voltage on the detection resistor by 20 times. The voltage value is between 0 V and 1.5 V. This voltage can be used with an additional external operational amplifier to feed into the feedback circuit of the switching regulator. In this way, the output voltage of the DC-DC converter can be reduced in proportion to the current level detected by the LTC7003. This option is shown in the gray part of the circuit in Figure 3.

With its meaningful functions, LTC7003 is very suitable for monitoring, limiting and disconnecting power lines in a large number of different systems.